288 Everyday Practical Electronics, April 2003

main field, as you will see presently fromFig.2.

The overall current flow between theprobes is thus not just governed by theresistance of one direct horizontal path, butby the total resistance of innumerablepaths effectively in parallel within a givenvolume of soil, and each experiencing dif-ferent values of resistance. Despite thecomplexity, though, as far as the readingon a current meter is concerned, theanswer is a single value, and from it an

assessment of the soil’s relativedensity can be made.

connected across them, current will flowbetween them, just as it does through anordinary resistor.

The amount of current that flowsdepends on how much resistance the soilinterposes between the two electrodes. Thevalue depends on several factors, the soil’swater content and chemical make-up (i.e.the impurities the water contains), and thepresence (or absence) of non-conductiveobjects. The relationship is complex, andwill not be discussed in detail here,although some experiments which shouldgive an insight into it are suggested in thetext file supplied with the software. It isdiscussed more fully by Anthony Clark inhis book.

The current flow through soil is alsocomplicated by the fact that it is not flow-ing in a straight line, as it does (in effect)through an ordinary resistor. The currentcan simultaneously flow through a multi-tude of paths, not only horizontally, butthree-dimensionally, as illustrated inFig.1. It also radiates outwards beyond the

JANUARY and February 1997 saw thepublication in EPE of Robert Beck’sEarth Resistivity Meter, an electronic

tool to assist amateur archaeological soci-eties “see beneath the soil” in their searchfor ruins and other hidden features.

The design presented here is based uponthe same concept as used in Robert’s cir-cuit, but it has been considerably simpli-fied in terms of the components count andtheir ready-availability. Significantly, ithas also been put under the command of aPIC microcontroller and provided withdata logging facilities. The principal fea-tures of this design are outlined in Table 1.

Before going any further, though, the

author wishes to “put his cards on the sur-vey grid”. He is not an archaeologist andhas approached this design purely as anelectronic problem to be solved – transmita signal, retrieve it at a distance and storeit for later analysis.

Along the path to this end, he hasresearched a fair bit, chatted with a localarchaeological society and with EPE read-ers who have knowledge in this field. Mostimportantly, Nick Tile, EPE reader andfriend of the author, has spent severalmonths successfully using the prototype foractive archaeological survey work. More onthis in Part 2. Further reference to Nick’ssurveying will be made during this article.

A list of useful references is quoted atthe end of Part 2, to which readers arereferred for more information on survey-ing techniques. The main reference sourceused by the author has been AnthonyClark’s Seeing Beneath the Soil.

For the sake of readers who have not yet

been enticed into joining their localarchaeological society in search of knowl-edge about our ancestors and how theylived, it is appropriate to outline how elec-tronics can help us see subterranean fea-tures without ever touching a spade ortrowel.

When two conductors are placed inmoist soil with a d.c. voltage source

Part One

GROUNDLEVEL

PROBES

SECTIONTHROUGH

SOIL

PLAN VIEW

Fig.1. Current paths set up by probearray.

Prototype Earth ResistivityLogger, housed in a plasticcase with transparent lid.

What is being looked for in an electron-ic survey is reliably monitored variations inreadings across a site, the pattern of whichindicates where different sub-soil featuresexist.

A problem arises, however, in that not

only does the soil have resistance, but italso has capacitance and additionallyexhibits various electrolysis effects as thed.c. current continues to flow, and mostsignificantly, a polarization process takesplace, resulting in progressively changingvalues on the meter.

To be able to take meaningful readings itis necessary to counteract the polarizationeffect. This can be done by passing analternating current through the soil insteadof a direct one. With each of the a.c. cur-rent’s phases, the polarizing effects of thepreceding phase are reversed, thus causinga more consistent current flow to occur inboth directions.

Whilst the soil’s electrolysis process willnot be reversed, its effect is likely to be sominute in relation to the polarization effect,that it can be ignored during the relativelybrief time during which current flow read-ings are taken.

The capacitance effects are also largelyovercome by using an alternating current ata suitable frequency.

The question then arises: at what fre-

quency should the current direction berepeatedly reversed? Too high a frequencywill cause the soil’s capacitance effects to“mop-up” and attenuate the alternating sig-nal’s amplitude. Too low a frequency willagain cause variation in the monitoredreadings, albeit smaller than would occurthrough using a d.c. signal.

It appears that the optimum rate at whichthe signal phases must be changed hasbeen established at around 137Hz(Anthony Clark quotes 137·5Hz but alsosays that 67Hz is used in some equipment).These frequencies assist in not only theelimination of the polarizing effects, butalso in reducing the affect of other alternat-ing electrical fields which might be presentin the site being surveyed, such as a 50Hzmains frequency, for instance. EPE contributor Aubrey Scoon has

researched into this latter aspect and hasreported the presence of many other fre-quencies in some locations he has exam-ined, some emanating from a nearby“supercomputer” in one instance.

The frequencies of 67Hz and 137Hz (thelatter is used in this Logger), are not a multi-ple of 50Hz, nor of the 60Hz mains cycleused in some countries, such as the USA.Thus, by performing rectification orsampling that is synchronised with the trans-mission signal, the effects of these extrane-ous fields can be reduced. They are also min-imised by the use of a differential amplifier,which will be discussed presently.

It is worth pointing out, however, that inthe suburban garden where the author’s tri-als with this Logger were performed inconjunction with an oscilloscope, residual50Hz mains currents were not evident.

The discussion so far has been in rela-

tion to the current flowing between twoprobes in series with a meter. Over themany years that geophysicists have been

Everyday Practical Electronics, April 2003 289

TABLE 1. WHAT IT DOESThe PIC microcontroller performs the following functions:

Generates 137Hz square wave ground-penetrating transmission signalConverts the received and amplified analogue signal to a 10-bit digital valueStores each converted value to user-specified non-volatile (EEPROM) memory

address representing specific site plotting coordinatesContinually displays immediate real-time data and coordinates on alphanumeric

liquid crystal display (l.c.d.)On request, outputs stored data via serial link to Windows 95/98/ME PC for storage

to disk and subsequent analysis

Other features of the logger include:Switchable output resistance to vary transmission currentSwitchable amplifier gain, x1, x10, x100Pushswitch selection of survey site row and column coordinates allocation in memoryMemory capacity for 16384 10-bit samples, representing a survey site grid of

128 x 128 squaresData storage action under complete user controlData locations may be overwritten with fresh data if requiredSampled data stays in memory indefinitely, even after power switch-offRecall of last used survey coordinate when next switched on, allowing survey to be

spread over several days or weeksIndividually stepped push-button recall and display of recorded samples and their

coordinatesTotal clearance of memory to zero value upon request, with security feature to help

prevent erroneous useOperable from any d.c. supply between about 9V and 15V, consuming about 25mA.

It is equally suited for use with a 9V PP9-size battery (rechargeable types areavailable), or a 12V car battery (see later)

Software features for the downloaded memory samples include:Program written in Visual Basic 6 (VB6)Disk storage under unique dated and timed file nameGraphical display of data on PC screen as waveform graphs and value-related

coloured or grey-scale grid squaresFour screen slider controls allow data to be processed for best visual contrast

to aid analysisFacility to invert data values for viewing as “valleys” or “peaks”Main screen display as 20 x 20 samples block, with vertical and horizontal panning

across full 128 x 128 gridSecondary screen displays of separate grid or graph data for full 128 x 128 samples

blockZoom facility for closer examination of separate graph and grid dataReloading of previous survey files via dedicated file selection screenDownloaded files stored in format suited for analysis and graphical display via

Microsoft Excel (found on most PCs)Data may be downloaded to PC as often as required without disrupting its existing

on-board storage (allowing on-going visual display of site progress across longperiods)

Suited to survey monitoring using any of the standard probing techniques (Wenner,Schlumberger, Twin-Probe, etc).

Typical example of one of the three analysis screens used by the Earth ResistivityLogger’s PC software. The other two show full-screen displays of grid or graph datafor a 128 x 128 samples survey site, with zoom facilities.

290 Everyday Practical Electronics, April 2003

electrically probing the soil in theirsearch for minerals and oil deposits(since 1946 says Robert Beck), it hasbeen found that there are better probingtechniques than just using two probes.Some of these have been adopted byarchaeologists.

Most of the favoured ones all use fourprobes – two for transmission (TX), andtwo for reception (RX). The righthand sec-tion of Fig.2 shows one way in which thesecond pair of probes can be used. AnthonyClark says that there are also some tech-niques that use five probes – with push-pullTX across two and the fifth becoming agrounded reference perhaps?

There are several ways in which four

probes are used in relation to each other,and each with its own merits. Their use isoutlined later, but no quality judgement isoffered here on their appropriateness to

various survey situations – but it is worthnoting that Clark considers the Twin-Probetechnique to be the most favoured forarchaeological surveying, although theWenner technique is said to provide moredetailed results. Nick in his extensive useof the prototype adopted the Twin-Probetechnique.

The Twin-Probe and Wenner techniqueswere outlined in Robert Beck’s article andwere used in the author’s garden tests withthis Logger. They will be discussed in Part2 in a bit more detail. Suffice to say for themoment, both involve placing in the soil areference probe that is connected to the cir-cuit’s 0V line (common ground). This isregarded as one half of the TX probes pair.

To the other TX probe is fed the alter-nating voltage or current, evenly swingingas a square wave above and below the 0Vreference value. The function of the TXprobes is to set up a field of potential gra-dient in the soil, which is then sampled bythe RX probes.

The RX probes are positioned at dis-tances away from the TX probes as dictat-ed by the probing technique being used.They are connected to the twin inputs of adifferential amplifier, whose output signalamplitude is determined by the differencein the two input levels. It is this signalwhich is then monitored by the controlcircuit.

It is not even necessary to use specialprobes, any metal object that does not cor-rode and can be inserted into the soil with

a wire attached willdo. The probes don’teven need to beinserted very far, justenough to penetratethe soil to makeelectrical contactwith its moistness.

It will be obvious,of course, that drysoil will be lesscapable of passing acurrent than moistsoil. Keep in mindthat the surface ofthe soil can dry outfaster than thatbelow it, and so areasonable amount

of penetration should be allowed. RobertBeck allows 200mm with his probe struc-tures discussed in Part 2.

With some sites it may be necessary toevenly damp the soil with water beforeadequate probing can begin.

The PIC-controlled processing circuit is

almost irrelevant to the main aspects of soilmonitoring! So first let’s look at the powersupply requirements, and the simple trans-mission circuit, both illustrated in Fig.3.

As said in Table 1, the power can origi-nate from any d.c. source (e.g. battery)ranging between about 9V and 15V. This isinput via diode D1 to the +5V voltage reg-ulator IC1. The diode prevents distress tothe circuit in the event of the battery beingconnected with the wrong polarity.

The regulated +5V output from IC1powers the main PIC-controlled circuit,which must not receive a supply signifi-cantly greater than +5V. It also providesthe positive power to the TX and RX cir-cuits. Both of these circuits additionallyneed an equivalent negative supply. This isgenerated from the +5V line by the voltageinverting chip IC2, which outputs a voltageof close to –5V.

Op.amp IC3 is the device which feeds the

137Hz alternating signal to one TX probe(the “active” TX probe). As previously said,

the other TX probe is connected to the 0Vpower line. IC3 is configured as a compara-tor whose inverting input (pin 2) is tied tothe potential divider chain formed by equal-value resistors R1 and R2. The resistors areconnected across the +5V and 0V lines andthe voltage at their junction is thus 2·5V.

The non-inverting input (pin 3) of IC3 isconnected to one of the PIC microcon-troller’s output pins (RA2) and is fed witha 137Hz square wave, generated by thesoftware, and which alternates between+5V and 0V. As this square wave repeated-ly crosses above and below the 2·5V refer-ence voltage, IC3’s comparator actiontakes place and its output (pin 6) alternatesbetween the device’s upper and lower volt-age limits, i.e. swinging between about+4V and –4V.

Note that the op.amp to which the TXprobes are connected (IC3) is short-circuitprotected internally and is unlikely to suf-fer if the probes accidentally come intocontact with each other while the power isswitched on. However, do not sustain suchcontact since it could cause regulator IC1to get hot, and it will shorten the batterycharge life.

Depending on the probing technique

used, experienced geophysicists can deter-mine not only the subterranean density, butalso its possible composition. This isapparently achieved by pre-setting the cur-rent which flows between the two TXprobes.

Robert discussed this in the ’97 text,referring to the technique as providing a“constant current”. It would appear,though, that his circuit did not provide aconstant current in the literal sense – samecurrent flowing irrespective of resistiveconditions – but rather it provided a currentlimit. It is the same limiting approach thathas been taken in this Logger design.

The output from IC3 can be switched byS2 to the active TX probe via one of fivepaths. These comprise a direct unlimitedpath, and four limiting paths via resistorsR3 to R6, in order of 10, 100, 1k and10k.

Readers are referred to the publicationslisted in Part 2 for information on resis-tive path use. The field tests performed by

B19V

RESISTANCEOUTPUT

7660IC2 N.C.

N.C.

N.C. OUT

LV

OSC

+VE

C

GND

C+

N.C.

78L05IC1IN OUT

COM

+

+

+

ON/OFF

S1

C122µ

C2100n

C3100na

kD1

1N4001

C422µ

C522µ

R2100k

*

*SEE TEXT

R1100k

+IC3TL071

2

3

7

4

6

R610k

R51k

R4100Ω

R310Ω

TO RA2

S2TO SK2

*

(C1, WHITE)(FREQUENCY)

SK1(C2, BLACK)

0V

5V

+5V

2

4

1

8

7

6

5

3

Fig.3. Power supply and transmission interface circuit for the Earth Resistivity Logger.

LINES OFEQUAL POTENTIAL

CURRENT FLOWLINES

CURRENTSOURCE

A) B)

mA

MEASUREDPOTENTIAL

V

Fig.2. How current flowing between two probes is detected bya second pair.

the author and Nick Tile were carried outvia the direct TX path (Nick says he hasnot found the switchable resistance facil-ity to be useful). In this role, the signalamplitude across the TX probes is pickedup by the RX probes simply as an alter-nating signal whose amplitude variesaccording to the soil density it has to passthrough.

The receiving circuit is shown in Fig.4.

The twin RX probes and their received d.c.coupled signals are connected via bufferingresistors R7 and R8 to the respective inputsof the differential amplifier, formed initial-ly around op.amps IC4a and IC4b and hav-ing a gain of three. The outputs from theseop.amps are summed, still as d.c. signals,by op.amp IC4c, which provides unity gain.

The resulting signal represents thedifference between the two input signallevels. It is now a.c. coupled via capacitor

C6 to the amplifying stage around IC4d.Here the gain can be switched by S3between ×1, ×10 and ×100. In the proto-type’s garden tests, the ×1 gain wassatisfactory across the maximum probeseparation distance that the dense gardenflower beds would allow (11 metres)! Nicksays he prefers the ×10 setting.

At this stage the signal is swingingabove and below 0V. It has to be shifted sothat it only swings between 0V and +5V atthe maximum extremes, to suit the PICmicrocontroller’s limits. This is achievedby a.c. coupling the signal via capacitor C7to the level-shifting potential dividerformed by resistors R22 and R23. DiodesD4 and D5 limit the maximum voltageswing then fed to the PIC, preventing itfrom swinging above or below the PIC’slimits of acceptance.

It will be seen that two additional signalpaths are provided from the output ofIC4a/b and consist of resistors R16 and

R17 plus diodes D2 and D3. These are notpart of the required analogue processingcircuit but were included for use duringsoftware development. Their function willbe described presently.

The PIC-controlled processing circuit is

shown in Fig.5. At its heart is a PIC16F876microcontroller, IC5, manufactured byMicrochip. It is run at 3·6864MHz, as setby crystal X1. The frequency may seemunusual, but crystals tuned to it are stan-dard products. Its choice provides greateraccuracy of the baud rate at which thelogged data is output to the computer.

The software-generated 137Hz squarewave pulse train is output via pin RA2, andfed to the TX op.amp IC3 in Fig.3.

Pin RA3 is the pin to which the level-shifted signal output from IC4d is input.The pin is configured by the software as ananalogue-to-digital converter (ADC).

Everyday Practical Electronics, April 2003 291

R71k

R81k

R10100k

R9100k

R11100k

R1610k

R1710k

R12100k

R14100k

R15100k

R13100k

R1810k

R211M

R20100k

R1910k

+

+

++

IC4aTL074

IC4bTL074

IC4cTL074 IC4d

TL074

9

10

8

11

5

6

7

4

2

3

1+C622µ

TO RA0

TO RA1

a aa

a

k kk

k

D21N4148

D31N4148

D51N4148

D41N4148

0V 0V

5V

+5V

TO SK3(P1, YELLOW)

TO SK4(P2, GREEN)

13

12

14

C7470n

R23100k

R22100k

VOUTTO RA3

GAIN

S3

Fig.4. Differential amplifier that receives, amplifies and conditions the RX probes signal prior to sending to the ADC input of thePIC microcontroller.

RA0/AN0

RA1/AN1

RA2/AN2/VREF-

RA3/AN3/VREF+

RA4/TOCK1

RA5/AN4/SS

OSC1/CLKIN

OSC2/CLKOUT

MCLRGND

+VE

GND

T1OSO/T1CKI/RC0

T1OSI/CCP2/RC1

CCP1/RC2

SCK/SCL/RC3

SDI/SDA/RC4

SDO/RC5

TX/CK/RC6

RX/DT/RC7

INT/RB0

RB1

RB2

PGM/RB3

RB4

RB5

PGCLK/RB6

PGDA/RB7

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28PIC16F876

14

3

2

1

4

5

6

7

8

9

10

11

12

13

D7D7

D6D6

D5D5

D4D4

D3

D2

D1

D0

EE

R/WGND

+VE

+VE

CX

CX

X2L.C.D.

MODULE

RSRS

A0

A1

A2

GND

+V

WP

SCL

SDA

IC624LC256

UP DOWN MODE DOWNLOAD

N.C.

N.C.

N.C.

N.C.

N.C.

N.C.

N.C.

CONTRAST

S4 S5 S6 S7

SAVETEST

S9 S8

0V VPP DATA CLK

TO IC7 PIN 11RS232

N.C.

IC5

3.6864MHzX1

1N4148D6

0V

0V

TB1

1

2

3

4

5

6

7

8

R2810k

R3010k

R2910k

R2410k VR1

10k

k

a

C810p

C910p

R2610k

R2710k

R251k

+5V

0V

TO R16

T0 R17

F OUT

TO D4/D5

TB2 *PROGRAMMER

R3110k

Fig.5. PIC-controlled processing, display and data storage circuit.

The PIC repeatedly converts the inputsignal to a 10-bit binary value which it out-puts for display on the 2-line × 16-charac-ter l.c.d. X2, as a decimal number. As usualwith the author’s designs, the l.c.d. is con-trolled in 4-bit mode (and its pinouts on theprinted circuit board are in his standardorder). Its screen contrast is adjustable bypreset VR1.

Pressing switch S8 causes the PIC tostore (Save) the ADC’s 10-bit binary out-put value to the 32 kilobyte (32768 bytes)serial EEPROM chip, IC6, at the addressset by the user via switches S4 to S6. Thischip is another Microchip device, and wasfirst demonstrated by the author in hisPIC16F87x Data Logger of Aug/Sep ’99.Its device number, 24LC256, indicates thatit has 256K single-bit memory locations.These are accessed as 8-bit bytes.

In other applications, the 24LC256 iscapable of being multiplexed with sevenothers of its type, using its A0 to A2 inputsto set each device’s multiplexed address. Inthis application they are left unconnected,leaving them biased internally. ResistorR31 is essential to the correct reading ofthe device’s retrieved data output value.

The 24LC256 data sheet can be down-loaded from Microchip’s web site(www.microchip.com).

Data stored in the 24LC256 can beretrieved and downloaded serially to a PCvia the RS-232 interface device (IC7) andsocket SK5, in Fig.6. Transfer is initiatedby pressing switch S7. Once started, all32K bytes are sent to the PC in consecutiveaddress order.

The software controls the output of a

train of square wave pulses at the 137Hzrate. Data sampling takes place on eachphase of the output pulse (high and low).On each complete cycle, the minimumvalue received is subtracted from the max-imum (to establish the received signal’samplitude) and the result stored to a 32-byte temporary memory block. So thatmaximum peak-to-peak values of thereceived square wave have stabilised, thesynchronous sampling takes place at theend of each peak.

About once a second, the pulse trainstops while the 32 sample values are aver-aged, and the l.c.d. display updated. Thepulse train then recommences for anothersecond. This gives the soil time to respondto the re-application of the a.c. waveform,and for the effects of any d.c. currents to beover-ridden.

Resistors R16 and R17, mentioned pre-

viously, allow the PIC to monitor the volt-age on the outputs of IC4a/IC4b for testpurposes, via its ADC inputs RA0/RA1.Diodes D2 and D3 prevent the PIC fromreceiving damaging negative voltages.

Originally, these outputs were intendedpurely for development use. However, theiruse has also proved beneficial in the out-door monitoring environment and has beenretained. The monitored values are dis-played in decimal on the l.c.d. and provideindication of relative probe signalstrengths, and of the loss of connection toone or more probes.

In relation to this test-motivated option,a second signal strength display option hasbeen included via the software. The second

mode displays theupper and lowerpeak values of thesignal applied to thePIC’s RA3 input.The two modes areselected by toggleswitch S9.

In common with

many other PIC de-signs, the facility hasbeen provided to pro-gram the PIC in situ,via connector TB2.Diode D6 and resis-tor R25 prevent dis-tress to the +5V lineduring programming.

Software, including source code files,for the PIC unit and PC interface is avail-able on 3·5-inch disk from the Editorialoffice (a small handling charge applies –see EPE PCB Service page) or it can bedownloaded free from the EPE FTP site.The latter is accessible via the top of thehome page of the main EPE web site atwww.epemag.wimborne.co.uk. Click on“FTP Site (downloads)”, then in turn onPUB and PICS, in which page the files arein the folder named EarthRes.

This month’s ShopTalk page providesinformation about obtaining pre-pro-grammed PICs.

The PIC program (ASM) was written inTASM, although the run-time assembly issupplied as an MPASM HEX file, which hasconfiguration values embedded in it (crystalXT, WDT off, POR on, all other values off).

Regarding the PC interface, if you haveVisual Basic 6 already installed on yourmachine, you only need to use filesEarthRes.exe and INPOUT.DLL. Copythem into a new folder named C:\EARTHRES, or any other of your choosing onDrive C (the usual hard drive letter).

The ability to install to another drive let-ter, e.g. Drive E on a partitioned drive, hasnot been provided with this program.Although the author has previously offeredthe option with other VB6 programs, feed-back from readers has indicated that theoption is not always reliable with somesystems. Consequently, it has beendropped from this program. Readers whoknow how this option can be reliablyimplemented with VB6 are invited to tellthe author at EPE!

292 Everyday Practical Electronics, April 2003

ResistorsR1, R2, R9

to R15, R20,R22, R23 100k (12 off)

R3 10R4 100R5, R7, R8,

R25 1k (4 off)R6, R16 to

R19, R24,R26 to R31 10k (12 off)

R21 1MAll 0·25W 5% carbon film or better

PotentiometerVR1 10k min. preset, round

CapacitorsC1, C4 to

C6 22 radial elect.25V (4 off)C2, C3 100n ceramic, 5mm

pitch (2 off)C7 470n ceramic, 5mm pitchC8, C9 10p ceramic, 5mm pitch

(2 off)C10, C11 1 radial elect. 16V (2 off)C12 to C14 10 radial elect 16V (3 off)

SemiconductorsD1 1N4001 rectifier diodeD2 to D6 1N4148 signal diode

(5 off)IC1 78L05 +5V 100mA

voltage regulatorIC2 ICL7660 voltage inverterIC3 TL071 f.e.t. op.ampIC4 TL074 quad f.e.t. op.ampIC5 PIC16F876

microcontroller,preprogrammed (seetext)

SeeSSHHOOPPTTAALLKKppaaggee

IC6 24LC256 256 kilobitserial EEPROM

IC7 MAX232 RS-232interface driver

MiscellaneousS1, S9 s.p.s.t. min. toggle switch

(2 off)S2 2-pole 6-way rotary

switchS3 4-pole 3-way rotary

switchS4 to S8 min. push-to-make

switch (5 off)SK1 to SK4 4mm single-socket,

1 each black, white,yellow, green (seetext)

SK5 9-pin D-type serialconnector, female,chassis mounting

TB1, TB2 pin-header strips to suit, or1mm terminal pins (2 off)

X1 3·2768MHz crystalX2 2-line, 16-character

(per line) alpha-numeric l.c.d. module

Printed circuit board, available from theEPE PCB Service, code 388; plastic casewith see-through lid, 190mm x 110mm x90mm (see text); 8-pin d.i.l. socket (3 off);14-pin d.i.l. socket; 28-pin d.i.l. socket;knobs (2 off); 4mm plugs, colours to match4mm sockets (4 off); heavy-duty crocodileclips, with coloured covers to match 4mmsockets (4 off); robust cable for probes(see text); 9V PP3 battery and clip (seetext); p.c.b. supports (4 off); nuts and boltsto suit l.c.d. mounting style (4 off each);internal connecting wire; solder, etc.

Approx. CostGuidance Only ££4455

excl. batts & case

MAX232IC7

N.C.

N.C.

N.C.

OUTPUTSERIAL

SK5

FROM IC5 PIN 17

+VE

GND

R2 IN

R1 IN

T2 OUT

T1 OUT

V-

V+

R2 OUT

R1 OUT

T2 IN

T1 IN

C2-

C2+C1-

C1+ +

+

+

+

+

1

3

4

5

11

10

12

9

15

16

2

6

14

7

13

8

C101µ

C111µ

C1210µ

C1410µ

C1310µ

SERIALOUTPUT

+5V

0V

1

5

6

9

Fig.6. RS-232 interface circuit.

Everyday Practical Electronics, April 2003 293

If you do not have VB6, you need threeother files, comdlg32.ocx, Mscomctl.ocxand Msvbm60.dll, held on our 3.5-inchdisk named Interface Disk 1, and in theInterface folder on the FTP site (they arealso included with the Toolkit TK3 soft-ware). These files must be copied into thesame folder as the other Earth Resistivityfiles.

Details of the component and track lay-

outs for the printed circuit board (p.c.b.)

are shown in Fig.7. This board is availablefrom the EPE PCB Service, code 388.

Assemble in any preferred order, ensur-ing that all the on-board link wires areincluded, and that all polarity-consciouscomponents are the correct way round.The use of sockets for all the dual-in-line(d.i.l.) i.c.s is recommended; it is essentialto use one for the PIC, IC5. Treat all i.c.sas static sensitive and discharge static elec-tricity from yourself before handling them,by touching the bare grounded metal of anitem of earthed equipment, for example.

Double-check the perfection of yoursoldering and component positioningbefore applying power. Do not insert anyof the d.i.l. i.c.s until the correctness of the+5V output from regulator IC1 has beenproved.

To provide a degree of waterproofness,the prototype was mounted in a robustplastic box with a see-through lid. Thel.c.d. was mounted below the lid on theinside. If a metal box with a see-throughlid can be found, it would provide evengreater durability.

4.3in (109.2mm)

2.8i

n (7

1.1m

m)

R3

R4

R5

R6

D5R23

D3 D2R26

R27

C3

R28

R29R30

IC1

IN

COM

OUT

T.P. T.P.

R17

R16

R11C2

C1+

+

++

+

+

++

+R13

R12

R9 R7

IC4

R8

R14

C6

R10

R18

R19R20

R21

C7

R22

R15

D4

IC3

R1

R2

C4

C5

IC2R25

D6

IC5

D1

R24

R31

C9

C8

X1

C10

C11

C13

C14

C12

VR4

IC6

IC7

a

a

a

a

a a

k

k

k

k

k k

32

1

5

641413

12

11

0V

CX

+5V

0V(R/W)

E

RSD7D6

D5

D4

TB1

S8

S9

TB2

UP DOWN MODE SAVEDOWNLOAD TEST

S4 S5 S6 S7 S8 S9S1

SK5

SERIALOUTPUT

A

A

1

1

2

2

3

3

4

5S2

OUTPUTRESISTANCE

TO SK2

S3

GAIN

TO RA4

TO RA5

TOBATTERY+9V

+5V

0V

0V

ON/OFF

TO SK3

TO SK4

TO SK1

MCLRDATARB7

CLKRB6 0V

TO PROGRAMMER (SEE TEXT)

OUT

REAR VIEWOF PINS

1

5

6

9

388

Fig.7. Printed circuit board component layout and full-size copper foil master track pattern for the Earth Resistivity Logger.

It is recommended that a case of at least50 per cent larger than used in the proto-type should be employed to allow a large9V to 12V battery to be adequately housed.

Probe sockets were 2mm types on theprototype, simply because the author hadthem in stock. It is recommended that 4mmtypes should be used. These providegreater robustness of the plugged connec-tions and allow them to be removed readi-ly. Nick recommends the use of restraintsnear the sockets to prevent the connectionspulling out during a survey.

The probe sockets should be colourcoded, as should their respective plugs.Colour suggestions are shown in the circuitdiagrams of Fig.3 and Fig.4, but may bechanged to suit availability. It is importantNOT to duplicate the colours – doing socould result in leads being incorrectlyallocated to probes.

The use of crocodile clips with colour-coded plastic covers was found to facilitatethe connection of leads to the probes them-selves. Heavy-duty crocodile clips are rec-ommended for ease of use (especially incooler or wet weather!).

When testing the prototype, it did notappear to matter whether the probe leadswere screened or not. Consequently, stan-dard lighting or low current cable could beused. Twin-core mains cable was used bythe author and Nick, but in long term sur-veys it might prove more convenient tohave a mix of cable arrangements, of

differing lengths and cores. Obviously thethicker it is, the lower the loss over longlengths, but 50m (say) of such cable isexpensive, and heavy to drag about.

Details of constructing customisedprobes are given in Part 2, but in simpleapplications four thin metal rods of thetype used in gardens as flower supports canbe used.

Having established that +5V is present

on the output of regulator IC1, plug in thevoltage inverter chip, IC6, and check thataround –5V is present on its output.Naturally, always disconnect power beforemaking component changes.

If all is well, the remaining i.c.s can beinserted and the l.c.d. connected. Typical

pinouts for the latter are shown in Fig.8. Itwill probably be necessary to adjust itscontrast using VR1 before a display will beseen.

With power switched on again, checkthat +5V and –5V are still present wherethey should be. Switch off immediately ifthey are not, and correct the cause ofmalfunction.

On line 1 of the l.c.d., the message“SOIL RESISTIVITY” will be displayedbriefly before being replaced by somenumerical values, with more on line 2.

With Test switch S9 switched on, the firsttwo values on line 1 show the monitored val-ues present at the outputs of IC4a/IC4b, asdetected by the PIC’s ADC conversions.Respectively, they are suffixed by the lettersB and A, indicating the op.amp to which theyrefer (as given in the circuit diagram Fig.4).

With S9 off, the values are the upper andlower peak values resulting from the ADCconversion of the output of IC4d. They aresuffixed by the letters H and L (High andLow). Any value between 0 and 1023 couldappear at this time for all four readings.

294 Everyday Practical Electronics, April 2003

The final prototype board prior to installation.

Fig.8. The two “standard” l.c.d. modulepinout arrangements.

Interior of the case showing the relative positioning of the components. The p.c.b.is the first prototype which did not include the RS-232 device, IC7. The latter canbe seen on its own sub-board to the left of the push-switches. It is recommendedthat a larger case is used to allow a heavier-duty battery to be inserted.

L.C.D. display following switch-on.

Example display when carrying out soilmonitoring with S9 switched on to testmode.

At the top right of line 1 is anothernumber, suffixed by a hash symbol (#).This is the processed value that, whenSave switch S8 is pressed, is stored to theserial memory as a grid value for thecoordinates on line 2. Switching betweengain settings using S3, the value willchange. (During a survey always keep S3at the same setting.)

Note that if too strong an input signal isamplified, the op.amp’s output may satu-rate (reach its maximum obtainable level).In practice, keep the value at the right ofline 1 well below about 500. A value of1023 is the maximum that can result froman ADC conversion, indicating that theADC has received an input voltage equal tothe power line voltage of +5V. This is animprobable event as the op.amp output isunlikely to swing that high.

At the left of line 2 are shown the col-

umn and row values which represent thesurvey grid coordinates, and thus the loca-tion in the serial memory at which theprocessed IC4 value is stored. They aresuffixed C and R respectively. An asterisksymbol (*) will be seen to the right of oneor the other of these coordinate values(more on setting coordinates in a moment).

At the right of line 2 is shown the valuethat is currently stored at the specifiedmemory address. During the survey it willnormally show 0 as each new coordinate isselected. When the Save switch S8 ispressed the display will change to repeatthe number that has just been saved to thememory as a 2-byte value. At any time dur-ing the survey, the coordinate switchesmay be used to recall the values that arestored for each grid location.

There are three switches for coordinatesetting. Two of them, S4 and S5, respec-tively increment or decrement the valuebeside which is shown the asterisk. Therange is 0 to 127, rolling over to 0 after

incrementing beyond 127, or rolling overto 127 after decrementing below 0.

Pressing Mode switch S6 changes theposition of the asterisk, thus allocating the+/– switches to that aspect of the grid, i.e.vertical (column) or horizontal (row).

Pressing Download switch S7 causes the

PIC to send the contents of the serial mem-ory to the PC at a rate of 9600 baud. Aspreviously said, the values for each of the16384 possible grid coordinates are storedas two bytes – the MSB and LSB of the 10-bit ADC values.

No attempt has been made to be selec-tive about which set of values is sent to thePC. All 32768 values are sent on eachoccasion that S7 is pressed. The transfertakes about 30 seconds.

During transfer, the top l.c.d. line showsthe message “SENDING TO PC”, withline 2 blank. Upon completion of the trans-fer, line 2 shows “SENDING FINISHED”,and line 1 briefly displays the “SOILRESISTIVITY” message again, beforeclearing to once more show the valuesbeing sampled.

Line 2 remains with its last messageshown until the asterisk (Mode) switch S6is again pressed, to once more show thecoordinate values.

Check that all the switches perform asintended. It is not necessary to have probesconnected at this time, and it does not mat-ter that the serial download will not be des-tined anywhere – the PC’s data receptionside of things will be covered in Part 2.

Incidentally, experiments were made

using a graphics l.c.d. instead of analphanumeric one, to see if survey datacould be illustrated by the unit as an in-built 20 × 20 grid display. However, the

ability to display values as different inten-sity grey-scales was found to be too limit-ed to justify the extra expense (at leastanother £30) and so the facility wasdropped.

Had the result been acceptable, aPIC16F877 would have been used with thescreen, in a manner similar to the author’sUsing Graphics L.C.D.s with PICs articleof Jan ’01.

The contents of the serial EEPROM

can be reset to zero when required. As asecurity measure (to avoid resetting inap-propriately!), the reset routine can onlybe called at the moment that the power isbeing switched on. With the power off,press and hold down Save switch S8,then switch on the power. When the mes-sage CLEARING EEPROM is seen,release S8.

On line 2 will be a progress count dis-play as the software writes zeros to all32768 EEPROM data locations. It is asomewhat lengthy process, taking aboutthree and half minutes. This is due tonumerous essential delays that are builtinto the writing procedure.

The software for the EEPROM writingand reading was originally downloadedfrom Microchip’s CD-ROM for use in thePIC16F877 Data Logger referred to earli-er. It is recommended that you do notattempt to modify Microchip’s coding tospeed the resetting process!

On completion of the resetting, whichalso resets the column and row values, thescreen briefly shows the SOIL RESISTIV-ITY message and proceeds in the normalway as described earlier.

In the final part next month, the PC-

compatible Windows software is describedand probing methods discussed.

Everyday Practical Electronics, April 2003 295

Example of display when Save switchS8 is pressed. In this case saving 28 toEEPROM location 41.

Example display when downloadingstored data to a PC-compatiblecomputer has been completed.

Example display during serial memoryresetting.

Everyday Practical Electronics is published on the second Thursday of each month and distributed S.O.R. by COMAGMake sure of your copy of EPE each month – cut out or photostat this form, fill it in and hand it to your newsagent.

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360 Everyday Practical Electronics, May 2003

There are six screens associated with theLogger’s VB6 program:

Main screen as shown below, throughwhich sectional analysis of the surveydata is performed

Full graph screen on which all 128 ×128 download amplitude values aredisplayed graphically, in oscilloscopefashion (bottom photo on next page)

Full grid screen on which all 128 × 128download values are displayed as gridsquares having amplitude-related huesor greys (top photo on next page)

Download screen through which dataretrieval from the Logger is initiated

Directory screen through which previ-ously recorded survey logging files canbe loaded for on-screen analysis

Error Message screen – which hope-fully you will never see! This comesinto action if the VB6 software detectsvarious types of error (such as trying toload a named file which does notexist). It does not intercept errorswhich occur outside VB6’s specified

IN Part One last month we discussed theprinciples of earth resistivity monitor-ing and described the construction of a

circuit through which this could readily beaccomplished and the data stored for com-puter analysis. This month we detail thesoftware that can help in this analysis, andthen examine some of the soil probingtechniques. The latest updates to the soft-ware are then discussed, followed bybriefly considering the ethics of surveyingand some practical advice and a list of fur-ther reading.

The Earth Resistivity Logger’s PC soft-

ware is written in Visual Basic 6 (VB6). Ithas been proved under Windows 95, 98and ME. It has not been tested withWindows NT, XP or 2000 as the authordoes not have these systems.

Readers who wish to try running thesoftware under the last three systems mayfind benefit from reading Mark Jones’ arti-cle Running TK3 under Windows XP and2000, published in Oct ’02.

error interception repertoire as pro-grammed by the author – the PC itselfwill report any such unlikely events.

The main screen offers several options

to enable you to analyse the data receivedfrom the Logger. It must be said, though,that the facilities offered through theWindows Excel software supported bymost PCs probably exceed what thisscreen can offer – more on Excel later.

There are two main areas on this screen,as seen in its screen-dump photograph.

To the right is a 20 × 20 grid block ofsquares, arranged so that the vertical axisrepresents the survey site columns, and thehorizontal axis the site rows. The site datavalues determine the colour or grey-scaleappearance of each of these squares. Twoscroll bars are provided which allow thegrid data coordinates to be panned verti-cally and horizontally so that all 16384values of a 128 × 128 survey grid can beviewed in 400-sample blocks, seamlesslyjoined.

The range of coordinates from the gridmatrix displayed is stated below it. Toknow the precise coordinate of any square,add the values (numbered 0 to 19) indicat-ed alongside the edges of the matrix.

There is a choice of four options regard-

ing the colour shade range, as shown at theleft of the screen. The lefthand bargraphdisplay shows the grey-scale range avail-able, from white to black, 36 shades in all,representing values from 0 (white) to fullblack (35).

The second bargraph shows a 36-valuerange of “rainbow” colour shades arrangedin the order that VB6 offers them in thesystem’s own (peculiar!) numerical order.They are allocated by the program to rep-resent 0 (top) to 35 (bottom).

Bargraph 3 is a monochrome scale ofcolours essentially in the green range butwith varying intensities of red added. The36 shades are again numbered from top tobottom as 0 to 35.

The 8-colour bargraph shows the “pri-mary” colours offered by VB6, numbered0 to 7, top to bottom.

The quality and definition of the scaleshades may vary depending on the qual-ity of your VDU.

The scales are selected by clicking onthe “radio” buttons above them. InExample of the prototype’s revised main screen displays and facilities.

Part Two

practice, the greyscale and monochromebargraph provide the clearest indication ofsample value relationships.

The values which are actually obtainedfrom the survey site could, as said previ-ously, fall into the range 0 to 1023. Twoslider controls are provided so that the val-ues logged can be suitably displayed ascomparative values within the grid squares.They are to the left of the grid squares,jointly captioned Graph, with sub-captionsof / (forward slash symbol) and minus.Clicking the sub-captions with the mousecursor toggles them to show X (multiply)and add, and back again on the next click.

With the lefthand control, moving theslider causes the basic sample values to beeither multiplied or divided by the slider’svalue, according to its sub-caption mode.Similarly with the second slider, addingor subtracting the slider’s value.Multiplication/division take place first in thesoftware routine, followed by add/minus.

These two controls allow the optimumshades or colour to be shown that bestillustrate the sample value relationships.Even seemingly similar readings can havetheir values manipulated to increase thecontrast.

Above the two sliders is a Show Valuestick box. When unticked, just the colourshades are shown. When ticked, the equiv-alent numerical value of the scale shade,from 0 to 35, or 0 to 7 as appropriate, isdisplayed within the squares as well.Clicking the box alternates the two modes.

If a particular shade is too dark to readthe value, move the mouse cursor over itand a “Tool Tip Text” box will appear, stat-ing the value. Tool Tip Text box messagesappear for various functions on screen ifyou move the cursor over them.

To the bottom of the screen below thegrid are two text lines. The first shows theactual range of the sample values, the sec-ond shows the range after correction.

Note that if an original sample value of0 is found, a dash line (–) is shown in placeof a numerical value. This allows recogni-tion of any survey site squares for which asample has not been taken.

The large vertical display area towards

the left of the screen shows the sample val-ues plotted as graph waveforms. There are20 lines (each numbered) representing thenumbered grid rows to their right.Horizontally, the lefthand end of each linecorresponds with the lefthand side of thegrid row.

The two sliders to the left of the grapharea allow the plot values to be varied inthe same way as with the grid, with thesame multiply/divide and add/minusoptions. Thus the display can again bemanipulated to show the survey site fea-tures to best degree, in this case as differ-ing amplitude waveforms.

The range of sample graph values ischanged at the same time as the grid’scoordinate range is set via its scroll and pansliders.

Below the graph area is another tick box,Fill Graph Blanks. When the box isunticked, any zero values in the original(unmodified) samples are not plotted onscreen, indicating any survey site squaresthat have not been sampled. With the boxticked, the zero values are plotted so that acontinuous graph line is shown. Clickingthe box alternates the modes.

To the left of the graph display are 20numbered tick boxes. These allow selectedgraph lines to be hidden (no tick) to makethe viewing of the data in the other linesclearer. As with all tick boxes, clickingthem again alternates between on and off.

Below the Show Values tick box is

another tick box, Invert Values. When sur-veying, less-dense soil produces highervalues than dense soil. High values pro-duce darker shades on the grid squares andlower troughs on the graph lines.

The Invert Values tick box allows thevalue relationships to be swapped, highbecoming low, low becoming high. Thisallows denser soil conditions to be dis-played more darkly on the grid than forless-dense soil, and the graph lines to showpeaks rather than troughs (valleys).Clicking the box alternates between thetwo modes. The default is for inversion(tick on).

The VB6 program allows the main

screen to be minimised and shifted in theusual Windows-type fashion. Because VB6does not regard the graph lines as being“permanent”, these can be fully or partial-ly erased by the act of minimising or shift-ing. To restore the graph lines on-screen,click the Refresh button.

It is also necessary to use the Refresh onthe Full Grid and Full Graph screens toaction various value selection changes.

There are two buttons, marked Full Grid

and Full Graph. They respectively causethe selected full screen mode to be dis-played. On both, value manipulation and

Everyday Practical Electronics, May 2003 361

Example of the full screen grid display, using a zoom value of x9. With zoom at x1all 16,384 grid squares are shown. The contrast will show more clearly on screenthan it may on this printed page.

Example of the full screen graph display. The zoom is at x9 to emphasise the con-tour lilnes. The samples cover a maximum area of 16 rows x 26 columns, eachsample representing one square metre. The data is the same as in the full grid dis-play and is in eight colours on the screen.

362 Everyday Practical Electronics, May 2003

inversion are available as on the mainscreen. So too is colour mode selection.

Above and to the left of the grid andgraph areas are two sliders. Whenclicked, these display the survey site gridcoordinates to which their arrows point.Their position is also used by the zoomslider facility at the top left.

There are 10 values of zoom selectableaccording to the ratio Zoom / 2 + 0·5, witha range of ×1 to ×5·5. The slider arrowpositions determine the origin point onwhich the enlargement is made. Interceptsare included in the program to keep the dis-play within the bounds of its frame.

You require a standard serial cable, of

the type used with normal modems (D-range 9-way male to female, straightthrough), for data transfer from the Logger.It should have a connector suited to yourPC at one end, and a 9-pin male plug at theother. Adaptors (25-pin to 9-pin) are avail-able if an existing modem lead has a 25-pinmale plug.

To download data serially from theLogger, first click on the Download Databutton at the bottom of the main screen.This causes a message screen to be dis-played, asking if you want to continue withthe download, or cancel the call and re-show the full main screen.

If the OK button is clicked the smallDownload screen is displayed. As advisedon the previous message you now haveabout 30 seconds in which to press theLogger’s Download switch S7. During this30 seconds or so, a bargraph shows the

elapsing time before a time-out erroroccurs.

If the time-out occurs before data isreceived, you are offered the options tocancel the download, for the PC to trydownloading via its other COM portaddress (there are two allowed for, COM1and COM2, at addresses 2F8h and 3F8h),or to retry downloading from the sameCOM port address.

If you choose that the other COM portaddress should be tried, this address isstored to the EarthResSettings.txt file,which resides in the same folder as the restof the Earth Resistivity software. It is thenrecalled next time you run the program.

It is permissible to change the COM portaddress within this file if you wish (viaWindows Notepad for instance) – it is thefirst entry in the file. Take care not to upsetthe positions of the other lines in the file.These lines set various parameters for theprogram each time it is loaded and run.

When the Logger starts to send databefore the time-out ends, and the PCbegins to receive it, the countdown

bargraph halts and a confirmation thatdata is being received is displayed. Thefull 32K block of Logger data (16384samples) is downloaded at 9600 baud, andinitially stored into temporary memorylocations.

During this process, another time-outcountdown of about one second per databyte is monitored. If this period is exceed-ed the program assumes that the downloadis complete (the PIChas stopped sendingdata), or that the seriallink has been broken.

Because the down-load is asynchronous(i.e. no handshaking),an error checking rou-tine has been includ-ed. When the PICstarts transmitting, itfirst sends severalzeros followed by themessage RESISTY.

When the PC pro-gram finds that theone-second serial time-out has occurred, itchecks through the first20 downloaded bytesto see if these valuescontain the ASCIIcoded RESISTY mes-sage. It also assesseswhether the downloadquantity is correct.

If neither fact iscorrect, the screendisplays a messagebox stating so,

offering the option to try again or cancelthe download.

Occasionally, the PC software thinksthat data is arriving immediately followingthe click of OK in the Download messagebox, even before the Logger’s Downloadswitch S7 has been pressed. The reason hasnot be found. It is a rare situation, though,and in this event the PC software almostimmediately experiences a time-out as datadoes not continue to arrive, and then offersyou the option to try again.

It is worth waiting a couple of secondsafter OK has been clicked before pressingswitch S7, in case this situation is about tooccur. Once the Logger has started to senddata the process must run its full courseand cannot be halted. The same applies tothe PC routine, it too cannot be interrupted,and will continue until a time-out has beenexperienced.

It had been hoped to provide a bargraphto graphically show the progress of thedownload. Regrettably, it was found thaton slower PCs the software is incapable ofsimultaneously updating the bargraph (orother visual forms of timing) and reliablyinputting the serial data. Consequently, thisoption has not been provided. It takesabout 30 seconds to download the full32768 bytes. A starting time is displayedon screen below the primary time-outbargraph.

When the download has been success-fully finished, a routine combines all thedouble-byte values into single 16-bit bina-ry values. These are converted to decimaland combined into lines of text data, eachvalue separated by a comma. Each lineholds the data for one survey site row (128values). There are 128 lines, representingthe number of survey columns.

This data is then output to disk, to a filewhose unique name is in the form of thefollowing example:

EarthRes 12JAN03 10-27-35.TXT

in which the date and time (hh-mm-ss) isthat applying at the moment that the file is

Download option screen.

Screen displayed in the event of datanot received due to COM port failure.

Countdown bargraph while waiting fordata to start coming from PIC.

Example of the folder directory screen through which filesfrom any folder path can be selected according to a filteredprefix option.

Screen displayed if synchronisation isnot correct.

created. (The Logger itself has not beenprovided with date or location recordingoptions – it is up to you to record this infor-mation in some other way.) The file is heldin the same folder that holds the rest of theEarth Resistivity software.

Having saved the file, the software splitsthe recombined values into a matrix of reg-isters whose coordinates correspond withthose used during the site sampling.

It is these values that are used for displayvia the main screen’s graph and grid areas.They are plotted there immediately theDownload screen closes. Simultaneously,the grid matrix location coordinate slidersare reset to zero. The value correction slid-ers are left as previously set, allowing var-ious sets of file data to be recalled fromdisk for viewing under the same correctiveconditions.

On return to the main screen after theDownload, the name of the current fileloaded (in this instance that just saved) isdisplayed in bold towards the screen’s bot-tom right.

To load the program with data from a

previously saved file, click on theDirectory button. This displays a multi-function screen through which files in anyfolder on any installed disk can be selected.It is a modified version of the Directoryscreen originally designed for use with theauthor’s PIC Toolkit TK3 software (Nov’01), and since used in modified form withother VB6 programs as well.

It will not be discussed in detail here asthe screen has a NOTES button which callsup a Windows Notepad text windowthrough which you can read the detail ofthe Directory screen’s use.

In brief, you can change drives andfolder paths, set a “filter” option to onlyshow files having a specified prefix intheir name, and recall previously select-ed paths through a History box. To selecta file, double click on its name in therighthand display area. This causes it tobe loaded and split for grid matrix allo-cation in the same way that the down-loaded file just discussed was split anddisplayed.

One of the author’s files is included withthe software (but with fewer that 400 sam-ples), plus a much longer one produced byNick during his survey work. Experimentwith them and the screen’s manipulativecontrols.

When the downloaded survey data is

output to disk, it is formatted to suit itsanalysis and display via Windows Excel, afacility that should be on any PC runningWindows 95 and later (search yourWindows CD-ROM for it if it is notalready on your system).

As well as offering graphing facilities,Excel provides for mathematical expres-sions to be computed, making it capable ofbeing set-up to calculate true earth resis-tivity in relation to known resistance andcurrent factors. Study Excel’s Help facili-ty, and read Anthony Clark’s book. (Assaid last month, though, Nick’s surveyswere done in relation to signal amplitudevalues and not the actual resistance, butsee later.)

The formatting simply entails usingcommas to separate the sample values,which are expressed as normal text charac-ters (e.g. 1234).

Inevitably, there are many versions ofExcel and specific use details that apply toall of them cannot be given. The chancesare, though, that the use will be similar tothat on the author’s main PCs. The follow-ing is the procedure he uses for Excel 97:

Load Excel, using Windows’ Find buttonto locate it if necessary – on the author’sPCs it is at

C:\MSOffice\Excel\EXCEL.EXE

Now follow the path File (in top tool-bar), Open, Select folder, set File Type toText Files, then double click on therequired EarthRes file name to load it. AText Import Wizard – Step 1 of 3 windowis now shown, with the first several import-ed values on display. Select the Delimitedoption as the active “radio” button.

Click Next to show the Text ImportWizard – Step 2 of 3 window. Click the“Delimited Comma” box to reveal a tick.Click other ticks to become unticked (ifnecessary). Ignore the “Text Qualifier”box.

Click Next to show the Text ImportWizard – Step 3 of 3 window. Ignore theoptions offered, but click Finish.

The main Excel screen will now beshown, with the survey values allocated tocolumn and row boxes.

Left click on one of these boxes, say thefirst one at top left, to select the startingcoordinate of the matrix area you wish toshow graphically. With the left mousebutton still held down, move the mousedownwards and to the right, causing theselected boxes to show white text on a blackbackground as the area is increased. Thefirst box, though, remains as black on white.

Release the mouse button when you’ve

selected the required area. Now click onthe Chart Wizard icon on the top tool bar (itlooks like several vertical rectangles, withan elongated diagonal shape above them –a chimney falling onto a factory?). Themouse cursor becomes a similar (but notidentical) symbol plus a cross, representingthat graphing mode has been selected.

Move this cursor anywhere over thedarkened area and left click to reveal theChart Wizard – Step 1 of 5 window. Thedarkened area reverts to normal black onwhite, surrounded by a dotted box, possi-bly “shimmering”.

Ignore the options offered and just clickNext, to show the Chart Wizard – Step 2 of5 window. Select (left click) one of thegraph type options offered, the “3-DColumn” option is suggested.

Click Next to show the Chart Wizard –Step 3 of 5 window. Select one of the charttype variants on offer, the one numbered 6,perhaps.

Click Next to show the Chart Wizard –Step 4 of 5 window, and an illustration ofthe Sample Chart selected will be seen.Ignore the right hand option boxes andclick Next, to show the Chart Wizard –Step 5 of 5 window. Now just click onFinish.

The graph type selected will now be dis-played on the main Excel screen, with thevalue boxes still visible behind it. It can bemoved around the screen and sized in theusual Windows style. A small (mobile)Chart selection window will also be dis-played, allowing different options of dis-play to be selected and manipulated.

Save the file and its graphical displays(more than one can be generated on thescreen at any time and placed at differentpositions) as an Excel-type file with anyname of your choosing. Alternatively, sim-ply Exit unsaved if you prefer.

It is now up to you to experiment withExcel’s numerous options, calling up itsHelp files for more information. It is anamazing package with many uses, andseemingly ideal for the sort of analysis thatarchaeological survey data calls for. In aword – experiment!

The construction of the probe assem-

blies will be discussed once some of theprobing techniques have been examined.

There are several probing methods avail-able through which to obtain grid dataabout a survey site. The author makes noattempt to recommend any one in particu-lar. You must do your own research intothat, through the references given later, andby chatting to those in archaeological soci-eties who know about such things.

Everyday Practical Electronics, May 2003 363

Example of using Windows Excel to display data graphically.

There are numerous archaeological websites with bulletin boards and “chat-zones”on-line if you search through the excellentwww.google.com, or other quality internetsearch engines. It is worth noting, though,that Anthony Clark considers the Twin-Probe technique to be that most suitable toarchaeological work, and is the one usedby Nick with his surveys.

With all techniques, the area to be sur-veyed is first marked out as a grid withtapes or similar, to form squares havingsides of, say, one metre in length (this is acommonly quoted distance in this context),and probably forming a 20 × 20 matrixedarea, see Fig.8.

Anthony Clark comments that plasticcovered clothes line is also useful for set-ting out a grid matrix. He cautions, though,that it can be difficult to untangle and onone site he knows of, it had to be “guardedin the presence of sheep, by whom it wasregarded as a rare delicacy”!

The Twin-Probe technique is apparently

more suited to initial surveying of a site,establishing whether or not it is worth car-rying out a more detailed survey.

With this method, the two probes C1 andP1 are inserted into the ground, sufficientto make electrical contact with it (see earli-er), centrally to and somewhat outside thearea to be surveyed. Anthony Clark dis-cusses the best distance in his book.

Probe C1 is the transmitting probe con-nected to the comparator IC3, via switchS2. Probe P1 is one of the receiving probes,connected to the input of op.amp IC4a.

Probes C2 and P2 are inserted into theground, to a similar depth, at the far cor-ners of the first square to be monitored, saytop left, coordinate R0/C0 (row 0, column0). Probe C2 is the 0V reference probe, andP2 is connected to the input of op.ampIC4b. The respective leads from the Logger

are then connected to the probes, usingheavy duty crocodile clips seems theeasiest method.

The Logger’s storage coordinates are setto suit the square number, i.e. to R0/C0 inthis case, and a reading saved to theLogger’s serial EEPROM by pressingswitch S8.

The C2/P2 probes are then moved to thetop corners of the next square, to the rightfor example, to be monitored and its coor-dinates set into the Logger, in this caseR0/C1. Again a reading is stored to theEEPROM.

The process continues fully across hori-zontally for the width of the marked surveyarea, e.g. R0/C19 (the final column of thisrow in a 20 × 20 grid). The probes are thenmoved down by one row, and the processrepeated, to the left this time, back toR1/C0. And then down by another row, andso on for all 400 squares.

Note that the relative order of all probe

connections must be maintained during thesurvey. Differences in reading can result ifthe order is changed, hence the earlier rec-ommendation that the plugs and crocodileclips should be colour-coded.

In practice, it does not matter in whichdirection you move the probes, or whetheryou start the survey from the top of the gridor the bottom. Note that the PC screenregards location 0,0 as being at top left ofthe screen.

“Be methodical and consistent” seems tobe the key phrase, though – this helps youto establish a routine that becomes secondnature, which the author soon found whenstarting his own mini surveys!

It was also soon found that it is not nec-essary to move both probes on each occa-sion. Since one is already at the corner ofthe next square, it is only necessary tomove the probe from the corner now

finished with, putting it in the next square’sopposite top corner, and swap the probeleads to retain the correct order.

The author surveyed his garden severaltimes in different ways during designdevelopment, and on each occasionbecame faster at doing so. On the final sur-vey, on an 11 × 7 grid (77 samples) it tookabout an hour and half.

Of course, during the process of doingthe test surveys, several methods for speed-ing it were imagined. For a solitary survey-or, perhaps the most efficient in terms ofspeed would be to insert separate probes ateach corner of the matrix prior to takingreadings. It would then only be required torepeatedly change the lead connections – aseemingly much faster “conveyor belt”system. No doubt, though, having an assis-tant would probably make the moving ofjust two probes a speedy alternative.

A perhaps less practical method was(bizarrely?) thought up too – using amotorised vehicle like a golf buggy withprobes attached to the wheels in Boadiceafashion. This would then be driven backand forth across the grid, the probes auto-matically inserting themselves, and trigger-ing the storing of each reading at thecorrect coordinates! (Well – a chap candream, can’t he?!)

Another seemingly useful technique is

known as the Wenner configuration. In thismethod the four probes are arranged in astraight line, equally spaced apart, say ametre between them. Fig.8b shows theorder of arrangement.

This method is apparently better suitedto doing a more detailed survey of thematrixed grid site. The principle is that theTX probes are the outer two. The RXprobes are in line between them. The cur-rent flows across the TX pair and is pickedup across the RX pair, the received signalvalue varying with the resistance in serieswith the probes in a more direct fashionthan with the twin-probe technique.

A variant of this technique, theSchlumberger, in which the probes are notequally spaced, is discussed in AnthonyClark’s book. But he regards it as not ide-ally suited to archaeological surveying.

Another method is known as the Square

Array in Anthony Clark’s book. With thismethod, the TX and RX pairs are placed atthe corners of the one metre squares, asshown in Fig.8c. The four probes aremoved as a set from square to square.

The transmission signal flows betweenthe TX pair as before. This time the RXpair pick up the radiated signal at the samedistance from the TX probes. If the soilresistance between the TX and RX pairs isuniform, so too will be the amplitude of thesignal received by both RX probes.

Tests showed that because the probes areconnected to a differential amplifier, if thetwo input amplitudes are the same, theywill cancel each other out at the final com-bining stage (IC4c).

If, on the other hand, the amplitudes arenot the same, the difference between themis that which will be finally output fromIC4c. In this case, what would be lookedfor is any difference values, indicating theedge of a subterranean feature.

364 Everyday Practical Electronics, May 2003

BASIC GRID LAYOUT, ALSOSHOWING TWIN-PROBEEXAMPLE POSITIONS

THESE PROBES ARE MOVEDBETWEEN COORDINATE NODES

00

1

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19ROWCOLUMN

C2 P2AT COORDINATES

R19/C3

SEE TEXTFOR DISTANCE

C1 P1THESE PROBES REMAIN IN FIXED POSITION

A GRID MATRIX OF SQUARES IS COMMON TO ALL NORMAL SURVEY METHODS.THE ABOVE ALSO SHOWS EXAMPLE POSITIONS FOR PROBES IN THETWIN-PROBE TECHNIQUE.

PLACING PROBES AT THE GRID INTERSECTIONS ASSISTS REGIMENTATIONOF THE SURVEY TECHNIQUE.

SQUARES TYPICALLY HAVE SIDES MEASURING 1 METRE.

C1

C1

P1

P1

P2

P2

C2

C2

WENNER ARRAY

SQUARE ARRAY

PROBES EQUIDISTANT

PROBES C1 AND P1 REMAIN IN FIXED POSITION.PROBES C2 AND P2 TRAVERSE THE GRID.

Fig.8. A 20 x 20 grid layout with Twin-Probe example positions, and the positioningof the probes in Wenner and Square arrays.

Everyday Practical Electronics, May 2003 365

It is evident, however, that balanced(zero) readings, when the two input valuesare equal, would only indicate the unifor-mity of the terrain in that grid. It would notindicate whether that uniformity was dueto a highly resistive feature or a highlyconductive one. Nonetheless, the detectionof only outlines might in itself be a desir-able situation.

A variant of this technique would be toplace the two TX probes at one end of acolumn, and the RX pair at the other, tak-ing a reading and moving both pairs to thenext column, still at the top and bottompoints. This could perhaps yield initialinformation about whether or not a site isworth examining more closely. Not beingan archaeologist, though, the author can-not comment on the validity of this.

Anthony Clark discusses the abovenamed probing techniques in more detail,and describes others.

During garden tests with the prototype

Logger, individual metal rods measuringabout one metre long by 5mm diameter,and with a right-angled bend at the topwere used as the probes. These were pur-chased inexpensively from a garden cen-tre, their intended use being to supportplants.

A recently observed, but not tried,possibility was in the form of long inex-pensive barbecue skewers – seen in a localsupermarket.

If you wish to construct purpose-builtprobe structures of more durability, andperhaps greater ease of use, the probe

assemblies describedby Robert Beckshould be considered.Schematics of theoriginal figures illus-trating these probeshave been redrawnand are repeated here.Other than the fol-lowing details, noadditional informa-tion can be offered.

Robert’s rigidframe assembly fortwo probes is shownin Fig.9. Details ofhis single probe aregiven in Fig.10.

His original textstates that the Twin-Probe assembly wasspecially developedand that its top mem-ber is a wooden bat-ten, 30mm × 50mm ×1050mm, the ends ofwhich are bound withself-amalgamatingtape to form handgrips.

An aluminiumplatform is attachedto the centre of thisbatten to carry thecase that holds theelectronics, secured by rubber bands. Thebottom member is a similar wooden bat-ten, but this piece must have good insulat-ing properties (to prevent current leakagebetween the probes). He suggests that you

either dry and coat the batten with varnish,or devise insulating collars of Tuffnol orsimilar material, and fit them where theprobes go through the batten.

The top and bottom battens are heldtogether by metal conduit pipes, threadedat each end and secured by lock-nuts.

In describing the construction of theother probes, he says that none of theirdimensions are critical and may be dictat-ed by what is to hand. In Fig.10a is showna substantial probe made out of stainlesssteel tubing with a brazed on T-handle andtip which assist soil penetration. Thisprobe is designed to be used by the opera-tor in the standing position.

A smaller version of Fig.10a is shown inFig.10b. This has a 4mm screw terminaladded, an alternative method of wireconnection.

Probes may be constructed of materialother than stainless steel, which is expen-sive and a little difficult to obtain, he says(provided it is corrosion resistant ofcourse).

An extremely simple probe is shown inFig.10c and which may be constructedfrom 6mm diameter metal rod, i.e. brazingor uncoated welding rod, mild steel, silversteel, etc. A depth guide consisting of aband of paint or insulating tape is addedand connections are made to the top usinga crocodile clip.

The depth stop in Fig.10d is adjustableby means of an Allen key. The materialneed not be insulating, and could be ofmetal if desired.

Since finalising the Earth Resistivity

Logger Part One for publication lastmonth, reader Joe Farr has provided EPEwith a specially written SerialIO.OCXprogram that allows legal access to VisualBasic’s own serial control I/O facilities.

170mm 100mm

15mm

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250mmTUFNOLCOLLAR

TUFNOLCOLLAR

A) A)

930mm

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B)

SCREW TERMINALWITH 4mm SOCKET

BAND OFPAINT OR

TAPE

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HOLE FOR PROBETO PASS THROUGH

120

0BA TAPPED HOLESTO RECEIVE SOCKETHEAD GRUB SCREWS

D)

Fig.10. Construction details of Robert Beck’s probes.

NOTE: THE UPPER AND LOWER HORIZONTAL RAILSARE OF WOOD. tHE LOWER RAIL SHOULD BE DRIED ANDVARNISHED. tHE L-SHAPED ALUMINIUM BRACKET IS TO

SUPPORT THE RESISTIVITY METER.

4mm PLUGS TO CONNECTTO RESISTIVITY METER

METAL CONDUIT PIPETHREADED AT BOTH ENDS

SOLDER TAGS

THIS AREATAPED TO

FORMHANDLE

THIS AREATAPED TO

FORMHANDLE

L-SHAPEDALUMINIUMBRACKET

500mm

200mm

800mm

PROBE C2 PROBE P2

30mm

30mm

1000mm

1050mm

Fig.9. Support frame for the Twin Probe configuration used by Robert Beck.

This option has previously only been avail-able to readers who have a registered ver-sion of MSCOMM (as Robert Penfold hasdiscussed many times through his Interfaceseries).

Joe’s serial OCX facility will be pub-lished in full at a later date – probably theSeptember issue. However, a section ofJoe’s program has been built into this EarthResistivity (ER) program and is availableto readers who are using theEarthResist.EXE standalone version.

To use Joe’s option, though, severalchanges need to be made to ER’s p.c.b.,without which the facility cannot function.They are:

1. Cut the track (0V) connecting to IC7(MAX232) pin 13.

2. Connect IC7 (MAX232) pin 13 to 9-pin serial socket SK3 pin 3.

3. Connect IC7 (MAX232) pin 12 to IC5(PIC) pin 18 (RC7).

This action allows the PIC to receivehandshake data from the PC.

To set the PIC program to respond to thecorrect serial data transmission routine, ini-tiate the following procedure:

1. Before switching on power, press andhold down the Mode switch, S6.

2. The screen will go into serial pathchange mode, alternating at about one-sec-ond intervals between a display saying“SERIAL PORT NORM” (original ver-sion) and “SERIAL PORT OCX “ (Joe’sOCX).

3. Release switch S6 when the mode yourequire is shown. This mode becomes theactive path mode and is also stored into thePIC’s data EEPROM, to be recalled nexttime the program is run.

4. On release of switch S6, normal run-ning of the PIC program resumes. The ser-ial path mode may be changed wheneveryou choose.

Ensure that the PC is also set for the

chosen mode, as follows:Click the on-screen button labelled

Please Read. Accept the option that thenfollows to read the text. Having read it, exitthe text reading screen to reveal anotheroptions screen. This allows you to choosebetween the new OCX option and the orig-inal (normal) serial mode. Click YES forJoe’s OCX, NO if you want to use the nor-mal serial download as originally writteninto the ER program, or CANCEL to exitwithout making a change to the serial pathused.

Your choice is recorded to disk andrecalled next time the program is run. Youmay change your mind at another time ifyou wish, re-entering via the Special Notebutton to do so.

Note that the same mode must be select-ed for the PIC and the PC.

The advantage of Joe’s program is that itallows a bargraph to display the progress ofthe data input procedure. It is also likely tobe better at detecting input data problemsas it uses a handshake procedure to com-municate with the PIC, inputting the 32768bytes of data in blocks of 256 bytes.

Whilst the original program inputs datathat is usually 100% accurate, there is theoccasional loss of synchronisation, which

is reported on screen, allowing you to re-download if you prefer, although minor“first aid” is provided by the program toregain sync after that point. It is rare,though, for more than one loss of sync tooccur. Such loss should not occur withJoe’s OCX program.

It should be noted that readers who wishto make their own changes to the ERsource code cannot make use of Joe’s OCXinput option. For that to be used, the instal-lation of Joe’s full OCX facility is required.For copyright reasons this will not becomeavailable to readers until its publication.Attempting to examine the ER source codewill generate an error condition because ofthe presence of Joe’s program. Until Joe’sfull serial program becomes available, theER program can only be recompiled ifJoe’s sub-program (EarthResOCX) and allreferences to it in the main program areremoved.

Also be aware that this version of ERwith Joe’s OCX has not yet been proved ona wide variety machines. If it will not workon your PC, revert to using the normal ser-ial download option on PIC and PC. Pleaseadvise us at HQ if this is necessary, tellingus the PC and its operating system type.

Another feature added to this version isthe ability to monitor the current flowingbetween the transmission (TX) probes. Ittoo requires a small change to the PCB:

1. Cut the track between resistor R16and pin 7 of IC4.

2. Connect the now-open end of R16 tothe pole of switch S2.

With switch S2 in the R5 (1k resistor)position, current flows from the switchpole through the 1k resistor and to 0V viathe resistance of the soil. These two resis-tances form a potential divider. The squarewave voltage at their junction is bufferedby R16 and half-wave rectified by diodeD2. The resulting peak positive voltage ismonitored via PIC pin RA0 operating inanalogue (ADC mode). The peak voltagedepends on the resistance of the soil, andfrom this voltage value the equivalent rela-tive current through the resistance path canbe calculated.

To establish an initial reference valueprior to any survey, switch on the unit.Then set switch S2 to the setting thatdirectly connects the pole to IC3 outputpin 6. Do not connect transmission probeC1 to socket SK2 at this time. Pressswitch S6 (Mode) and hold it pressed,then press switch S8 (Save) and hold itpressed until the message REF SAVEDappears, preceded by a value. Release S8,then release S6. The value shown is nowstored to the PIC’s EEPROM for presentand future use. Then switch S2 to the 1kresistor (R5) path.

During active surveying, the voltageat the pole of S2 is subtracted from thereference value and stored as a 6-bitnumber into bits 1 to 6 of the MSB of thesurvey value recorded to the externalserial EEPROM IC6. The range of cur-rent values acceptable is from 0 to 63,and the actual value is displayed as thesecond value on l.c.d. line 1 when in TestMode (S9 on). It is followed by the letter

A. The first value shown (followed by B)is that monitored from IC4b pin 8, asdescribed in Part One. The output fromIC4a is no longer monitored via thel.c.d.

If current values greater than 62 areencountered, they are limited to 63, and theword MAX is displayed on the l.c.d.Switch S2 may be used to select one of theother resistors (R3 to R6) in the event thatthe site being surveyed has greater or less-er resistance than appropriate to a 1k fixedresistor value. Do not change the resistorvalue during a survey.

The PC program stores the full 2-bytesurvey value to disk. On re-input the cur-rent value is extracted from the MSB, andthe MSB is then limited to one active bit(bit 0). The range of survey values is thenfrom 0 to 511. During surveying, the gainsetting via switch S3 should be chosen tokeep the values below 511, favouring amiddle range centred on 256. If a valuegreater than 511 is encountered by the PIC,it is limited to 511 and the word MAX isshown on the l.c.d.

All three display screens of the PC pro-gram now have an extra tick box markedCurrent. When it is ticked, each surveyvalue is multiplied by its associated currentvalue divided by 10. The theory is thatslight differences in the transmission cur-rent value at each survey grid square affectthe actual value of the received voltage sig-nal from the receiving probes. By relatingthese voltages to their prevailing transmis-sion current, compensation is made forvariations in the latter. The current valuesare not actual milliamp values, but simplynumbers representing the relative currentflowing.

It is suspected, however, that in practicethe variations make little difference to theinterpretation of the displayed results. Torepeat the statement made in Part One, theaim of this Logger is to show relative dif-ferences in signal amplitude across a sitebeing surveyed. It is the differences thatthen indicate different sub-soil features.

If there are significant differences theyare worth physical investigation. If thereare no significant differences, then the siteis probably not worth examining further,unless such techniques as magnetometry orground-penetrating radar reveal differently.A magnetometer design is currently beingworked on and will be published in EPE atsome time in the future, but not yetscheduled.

We shall be interested to learn if youfind that the current-monitoring featureenhances the results of your survey. Let usknow via EPE HQ.

A further option added to the PC pro-

gram since Part One is the dot-matrix dis-play facility, operative when the Matrixtick-box on the Full Grid screen is ticked.This draws small squares on the displaywhose dimensions are relative to the signalamplitude.

The principle is a bit like the dots thatmake up a B&W photograph in a newspa-per (known as half-tone). It will be moreuseful with a large amount of survey dataon screen than with a small quantity.

366 Everyday Practical Electronics, May 2003

A few other “tweaks” have also beenadded since Part One.

The text and demo circuit for someexperiments referred to on Part One page 1are now accessible via buttons at the bot-tom left of the screen.

Two other buttons allow you to examinethe survey data as text files, one showingthe twin-byte values separately, the otheras the full combined value. These valuesinclude the current values as well.

All three display screens have also beengiven a “pre-subtract” box, allowing you tosubtract, say, the minimum value receivedfrom all other values, enabling relevantdata to be extracted from any overall biaslevels.

Because this PC software will be usedwith the Magnetometer currently underdevelopment, two “radio” buttons allowselection of whether Earth Resistivity orMagnetometer data will be processed.Ensure that the Resistivity button is the oneselected.

It was said in the opening paragraph in

Part One that the original Earth ResistivityMeter published in EPE was an electronictool to assist amateur archaeological soci-eties. So too is this Logger design.

Whilst there is nothing to stop anyonefrom carrying out surveys on their ownproperty, there are considerable ethicalissues regarding the surveying of otherland.

First, other land is not your land, and soany surveying of it requires the permissionof those who own it. Remember that allland in the British Isles is owned bysomeone. Find out who it is and gain theirpermission before you proceed.

Secondly, do not dig without anarchaeologist’s involvement. If you havelocated through your earth resistivity sur-vey something that proves to be a site ofany importance, your unsupervised diggingwill certainly destroy information that isnecessary to fully interpret the site.

Earth resistivity surveying is essentiallynon-invasive except for the slight intrusionmade by the probes just into the surface.Many landowners could well be as inter-ested as an archaeological society in know-ing what history might lie beneath theirland, especially if they are approached in apolite manner and it is explained to themthat the resistivity surveying is just a mat-ter of sticking some shallow probes in thesoil.

Remember that some locations are des-ignated as Scheduled Ancient Monumentsand that permission to carry out any formof research on them requires officialapproval. Experienced local archaeologicalsocieties will know where these sites areand whether or not surveying permission islikely to be granted, and if so, by whom. Ifsuch information is not already known to asociety, enquiries at the local town hallshould provide answers.

It really is in your interests to join an

archaeological society if you are notalready a member. It is also in the society’sinterests if you join them and they thenhave the use of your Logger!

To find a local society, look in the tele-phone directory, or ask at the library. Theauthor’s local library building even has adisplay of the artefacts found by the societyin his area. It is a region once heavily pop-ulated by the Romans, with many artifactsthat have been found on display, and eventhe ruins of two Roman villas (but leftwhere they were found!).

Only a few hundred yards from theauthor’s house a Roman corn drier wasrecently found by his local society. 400yards further on are the ruins of a Romanbath house. It is quite probable that hisgarden is on a site where Roman’s oncetrod.

Although his survey graphics did notshow anything other than known modernfeatures, and probably including builder’srubble of recent decades, perhaps he’ll oneday do a more detailed survey and then callin the archaeologists to uncover an amaz-ing find – one way to get the garden dugfor him!

Nick was fortunate enough to be permit-

ted to survey a site made famous byEnglish artist Constable (before he waspromoted to Sergeant says reader andfriend John Waller – quoting an old GoonShow line!).

Constable painted several pictures ofsites at and around Flatford Mill in Essex.One of them is his Boat Building NearFlatford Mill, which is reproduced here. Itis near that site that Nick surveyed and hisresults are those illustrated earlier in thefull graph and full screen illustrations.They reveal very clearly the sub-surfacefeatures that could have been bays cut intothe ground where boats might well havebeen tied up. Much of the site, though, isnow overgrown with trees, preventing ade-quate survey.

The primary area covered in the surveyis approximately 16m × 26m at maximumdimensions. Most of it was covered in oneday, but then rain “stopped play” forseveral weeks.

From his experience with the prototype of

this Logger, and from his general surveyingactivity, Nick offers the following advice:

For extensive survey work the batteryneeds to be bigger than PP3 size

The case should be larger than in theprototype and a better shape to carryabout

Do not use small plugs and sockets The sockets need to be solidly mounted,

possibly on a metal base of somedescription, and include strain relief, it’ssurprising how hard you have to pull50m of cable laying on wet grass!

Lay the survey out accurately, based ona 3, 4, 5 triangle to get the lines perpen-dicular. Bamboo canes make goodmarkers for the 1-metre grid intersec-tions. If using clothes line with metremarks, beware that rope stretches.Survey tapes (typically 30m) need to becarefully cleaned after use, or they getfull of dirt and can jam

Keep perfect track of what you have sur-veyed, it is horribly easy to lose track ofthe grid section that you have justrecorded

Probe around the site at random beforeyou start to make sure that you are setup to keep the Logger’s values roughlyaround 250

Try and get it all done in a day – a show-er overnight throws in a step functionbecause you are then working in thearea that the rain will have penetrated

Coil everything neatly – controlling 50metres of cable across a plot is tricky

Everyday Practical Electronics, May 2003 367

Nick’s survey was done not far from where John Constable painted this BoatBuilding Near Flatford Mill scene. The contours in the full graph screen shownearlier clearly indicate a trench comparable to that in this painting. Illustration courtesy of

www.excelsiordirect.com/constable.htm.

Buy high visibility cable in case some-one tries to trip over it!

Colour code the probes – you need to beconsistent

Ask permission, most people will bechuffed to bits that you want to do thesurvey – but not everybody, and makesure that you are not somewhere whereyou should not be

Make contact with your local archeolo-gy group, they will be very helpful andinterested, and may well bite your armoff to get you helping them

Be prepared to talk to people, youwill cause interest if you aresomewhere public, and they will besurprisingly knowledgeable – andprobably have all been watching TimeTeam

Keep your ears open for local stories ofold ruins, you might be the one that re-discovers something lost to historybecause you happened to take the timeto listen to the ramblings of the old guyin the pub

You can do a survey on your own, but itis much easier with two of you

Keep a note book that notes the time,place, date, etc of the survey and thingslike weather conditions which couldexplain odd results, for example if itstarted to rain half way through thework

Any survey must have a repeatable basepoint, or base line so that if you do findsomething interesting, you can be surewhere it was without having to repeatthe survey!

Use compass bearings, fixed physicalfeatures, corners of buildings, drain cov-ers etc, or triangulate from fixed pointsif the survey is in an open area. Mostarchaeologists work north to south.

The author offers very grateful and

appreciative thanks to Nick Tile for carry-ing out extensive field tests with the proto-type, for discussing at length many aspectsof its use, for lending Seeing Beneath theSoil and vetting the script.

The author also thanks those EPE read-ers who provided him with informationduring the development of this design (inalphabetical order!):

Dave Allen for sending an ancientissue of ETI containing a rudimentaryER circuit using d.c. probing (and yesDave, this design could be used for mon-itoring relative impurity content levels inwater).

Peter Barnes, for vetting the script andfor several useful email exchanges ofthoughts and circuits, plus comments fromhis archaeologist acquaintance Derekabout using Robert Beck’s design.

Robert Beck, for the original inspiration.Aubrey Scoon, for comments about

stray electrical currents in the soil.ODAS, the Orpington and District

Archaeological Society, and Alan Hart inparticular.

Applied Geophysics, W.M. Telford, L.P.

Geldart, K.E. Sheriff, D.A. Keys. CambridgeUniversity Press. ISBN 0521-20670-7.

Applied Geophysics, Griffiths and King,Pergamon Press. 1965. (ISBN unknown).

Seeing Beneath the Soil, ProspectingMethods in Archaeology. Anthony Clark.Routledge. 2000. ISBN 0-415-21440-8.This is a revised edition of the title refer-enced in EPE Feb ’97, and having a differ-ent ISBN and publisher. It is the mostinformative source used by the author dur-ing the design of this Logger.

It additionally covers other earth survey-ing techniques, including magnetometry,and provides several further referencesources. It is known to be available for on-line ordering from www.Amazon.com andwww.BOL.com, current price around £25.

www.archaeology.co.uk. Various aspects

of the subject, including further links, accessto the magazine Current Archaeology, and tothe Council for Independent Archaeology.

www.geop.ubc.ca. Source of semi-mathematical tutorial on earth resistivityand a link to a site called Introduction toExploration Geophysics.

www.google.com. Excellent searchengine.

The ER software placed on the EPE ftp

site on 17 March ’03, was version V1.2.Look in on the site occasionally to see if anyfurther updates have been introduced.

CORRECTIONCrystal X1 should be 3·6864MHz (as in

Fig.5), not 3·2768MHz as in the compo-nents list.

368 Everyday Practical Electronics, May 2003

EEPPEE BBIINNDDEERRSSKEEP YOUR MAGAZINES SAFE – RING US NOW!

This ring binder uses a special system to allow the issues to be easily removed and re-inserted without any dam-age. A nylon strip slips over each issue and this passes over the four rings in the binder, thus holding the mag-azine in place.

The binders are finished in hard-wearing royal blue p.v.c. with themagazine logo in gold on the spine. They will keep your issues neatand tidy but allow you to remove them for use easily.

The price is £6.95 plus £3.50 post and packing. If you order morethan one binder add £1 postage for each binder after the initial£3.50 postage charge (overseas readers the postage is £6.00 eachto everywhere except Australia and Papua New Guinea which costs£10.50 each).

Send your payment in £’s sterling cheque or PO (Overseas read-ers send £ sterling bank draft, or cheque drawn on a UK bank orpay by card), to Everyday Practical Electronics, WimbornePuublishing Ltd, 408 Wimborne Road East, Ferndown,Dorset BH22 9ND. Tel: 01202 873872. Fax: 01202 874562.

E-mail: editorial@epemag.wimborne.co.uk.Web site: http://www.epemag.wimborne.co.ukOrder on-line from:www.epemag.wimborne.co.uk/shopdoor.htm

We also accept card payments. Mastercard, Visa, Amex, DinersClub or Switch. Send your card number and card expiry date plusSwitch Issue No. with your order.