INSTRUMENTATION FOR SEISMIC EXPLORATION FORGROUND WATER IN HAWAII

by

Leonard A. Palmer

Technical Report No .6

April 1967

Project Completion Report

for

FEASIB ILI TY OF SEISMIC EXPLORATION FOR GROU NDWATER

OWRR Project No. A-009-HI Grant Agreement No. 14-01-0001-781

Principal In vest igators : Leonard A. Palmer , Doak C. Cox, WilliamM. Adams

Project Period : July 1, 1965 to June 30, 1966

The programs and activities described herein were supported i n part by fundsprovi ded by the Unite d Stat e s Department of t he Interior a s authorized undert he Wat e r Resour ces Ac t of 1964, Publ ic Law 88- 379.

ABSTRACT

This study developed i nstrumentation and t echniques t o ·be use d

for seismic exploration of groundwat er in Hawaii. A t hree-stage field

test of instruments and methods was conducted to determine their

capability of recording and reproducing seismic data. Reproduci­

bility was the main criterion to permit usi ng certain signal analysis

techniques.

Multichannel explosive tests utilizing dynami t e shots were car­

ried out in waimanalo, Oahu during the first stage of t esting.

The second stage was the use of a two-channel magnetic t ape r e­

corder converted to receive voice and up-hole geophone si gnals on one

channel and seismic signals on the other channel.

St age three tested the use of a "thumper" acoustica l source~

seismic filters~ and timers deve l oped f or the pro j ect .

The signals generated by explosives and recorded photographically

were very reproducible~ but this is a re lat ively expe nsive met hod and

analysis is slow.

Sei smi c data recorded on magnetic tape allows versati lity i n

analysis of recorded signals reproduced ei t her in wiggle or i nt ensity

contrast analog form. Sections can be selected fo r digi t al computer

analysis.

The findings from this pi lot phase of seismi c exp Lorat.ion for

groundwater does not indicate that seismology would not be an appropriate

tool for studying geological structure . Success in f ur t her work iai l. ]: be

implemented by this preliminary work i n solving i ns trument ation problems

and the number of personnel of the Water Resource s Research Center and

the Hawaii Institute of Geophysics who have been trained.

iii

CONTENTS

INTRODUCTION .................................................•..... 1

OBJ ECTI VES 2

PROCEDURES 3Field Operations 5Stage 1: Multichannel-Explosive Seismic Field Test, Waimanalo,

Hawaii 5

Stage 2: Magnetic Tape Seismic Data Recording Development 8Stage 3: Thumper Seismic Signal Source l?

TRAINING AND EDUCATION 23

SUMMARY AND CONCLUSIONS ..........•................................ 24

ACKNOWLEDGEMENTS 25

BIBLIOGRAPHY 26

LIST OF FIGURESFigure

1

2

3

4

5

6

?

8

9

10

"012

Location Map 6A Multi-Channel Galvanometer Type Seismic Recorder 9Seismic Recording at the University of Hawaii ExperimentStation in Waimanalo 10

Manual Digitization of the Seismic Records 11Utilization of Acoustical Energy to Test the MagneticTape Recording of Seismic Signals 12

Block Diagram of Seismic Signal Recording Apparatus 13Multiple Trace Time Analog Oscilloscope and Oscilloscope

Camera 14Schematic Diagram of Simple Seismic-signal Filter 15Design of Tripping Hook for Weight Dropping 16Seismic Control Timer 19Schematic Diagram of Thumper and Recording Equipment 20"Thumper" Weightdropping Equipment 21

v

2

·OBJECTIVES

The applicability of seismic techniques to the problem of detail­

ing the structure of Hawaiian basaltic lavas at and below the water table

and the advancement of the state-of-the.art in seismology were the

principal concerns of this study . Multiple impulses and signal-averaging

techniques are being used to develop exploration capabilities at depths

of several thousand feet in other areas of the world. The utilization

of such techniques in the exploration for groundwater in the Islands was

evaluated. Extrapolation of porosity and permeability data from existing

wells by seismic techniques is contemplated.

3

PROCEDURES

The original procedures included (i ) construction of a weight

dropper or vibrator, CiiJ recording of repeated drops on the Enhancatron

or equivalent, (iii) application of the techniques over known structures

having high velocity contrast such as tunnels and (i v) application of

the techniques over aquifers having known porosity contrast.

The initial procedure emphasized processing seismic signals in

electrical form on an Enhancatron. The Enhancatron is a single-purpose

digital computer. It performs time-averaging by breaking the time axis

into 1,024 segments and striking an average within each segment. Al­

though it works very well, its inflexibil ity was soon apparent . Analo g

magnetic tape data recording was selected, ut ili zing an appropriately

modified Ampex tape deck for recording seismic signals. (The modifica­

tion is described in Stage 2, Magnetic Tape Seismic Data Recording

Development .) The data recorded on the magnetic tape can be processed

either by analog or digital methods. Analog processing is done with a

data presentation system developed in cooperation with several groups of

the Hawaii Institute of Geophysics at the University of Hawaii.

For experimenting with various analysis techniques , the analog in­

formation is digitized and processed by a digital computer. This project

is contributing the format interface between the analog-to-digital con­

verter and the digital tape recording units. The interface permits

analog tape seismic data obtained in the field on the Ampex magnetic tape

recorder to be inserted into the data-reduction system and obtain output

on magnetic tape suitable for direct input in the IBM 7040, or IBM 360

computer.

4

To permit development of the digital computer programs for pro­

cessing the seismic data, i. e., summation, cross-correlation" spectral

analyses, etc., some of the paper seismograms have been hand-digitized.

This process is extremely tedious and results only in marginal repro­

ducible data. Although it does permit test data analysis for computer

program development, hand-digitizing will not be extensively used.

There are numerous reasons for the switch from the on-line Enhanca­

tron processor doing only straight summation. Conversion to analysis by

digital computer has added unlimited flexibility of processing. For

instance, seismic signals can be 'evaluated in each acoustical band pass

spectrum. Seismic data are recorded in a form suitable for direct input

to an Enhancatron processor, if one is available, and if such signal sum­

mation is desired.

The analog playback equipment will permit a quick look at the data

to indicate zones which should have more intense study. The corresponding

data from these zones can be processed on the digital computer after an

analog-to-digital conversion and the results plotted back on a digital

plotter for interpretation.

The decision to change from the Enhancatron processor does not divert

from a basic policy of using analog equipment for processing when the op­

timum technique has been selected. In the current situation, the pro­

cessing procedure is still in a state of flux. Once the study is able to

fix upon the optimum processing method, then purchase of the necessary

analog hardware would be appropriate.

The approach of this effort is verified, to a great extent, by that

used by the U. S. Geological Survey for the National Aeronautics Space

Administration (Watkins, et aZ.~ 1966).

5

Field Operations

Field operations progressed through a series of stages during

which changes and improvements in instrumentation wer e required. Pre -

liminary tests were carried out with conventional multichannel s ei smi c

equipment available at the Hawaii Institute of Geophysics. In later

stages, the need for less expensive and more versatile methods led to

the development of magnetic tape data recording from a thumper source.

Description of the various field operations are gr ouped into stages ac-

cording to the type of equipment and procedure used.

STAGE 1: MULTICHANNEL-EXPLOSIVE SEISMIC FIELD TEST,WAIMANALO, HAWAII

I ntroduct i on. Preliminary seismic field investigations were carri ed

out at Waimanalo, Oahu. This site was chosen not only for its geol ogi c

setting, but because the seismic work would contribute new information

concerning the underground structure of the area. Convent ional s eismic

equipment available at the Hawaii Institute of Geophys i cs of the Univer -

sity of Hawaii was utilized. Reflection and refraction profiles were

made in order to obtain informat ion to be used in the signal-averaging

method.

Location. The location of the town of Waimanalo and the site of the

field test is shown in Figure 1. The first seismic array was set up on

Waikupanaha Street between Kumuhau and Kakaina Streets , the second, on

the University of Hawaii Exper iment Station property.

Repetitive reflection shooting was done at the center of the array

with 3 geophones on each side. From 10 to 30 explosions were used wi t h

the same shot point and geophone to obtain repeat data for signal averag-

....9 1900 20PO 3~00 40100

5~Oqmile

FEET

Figure 1. LOCA TION MAP

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I,,,, ,<, / Univ. of Hawa ii..... •"-1., Exp.erimental Slo.w ....,.

J. •, ..SEIsMIC • ***

/ FIELD TEST NO.2,,,,.• •

LEGENDGeophone Spread

Shot Points

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7

ing. The geophone array in both instances was set up with one geophone

every 100 feet. The six geophones extended over a distance of 500 feet.

A refraction seismic traverse was made with the same geophone array. Shot

points were located every 200 feet extending beyond one end of the geophone

array. "Dupont" Boosters and "Nitromon" explosives provided the energy

source for the refraction shooting.

Equipment. Equipment used included: seismic truck with a multi-channel

galvanometer-type seismic recorder with signal monitoring on photographic

paper, developing equipment, two-way radios, and one operator for record­

ing and developing seismic records; a blasting truck carried the explosives

and a shot box with all accessory explosive materials. Radio communica­

tion between the shockpoint and the seismic truck was used for timing

(Figures 2 and 3). Analyses of the data from these seismic shots re­

quired manual conversion of the photographic seismic records to digital

data on IBM cards. Photographic data were recorded at 4 inches per second

with timing marks each 1/100 of a second and a darker time line marked

every tenth line. By use of the IBM OSCAR, the photographic records were

read each 1/200 of a second for 1.5 seconds after the shot instant (three

inches of record on the photographic paper). Digital data on IBM cards

are punched by the OSCAR which are processed by computer averaging to pro­

duce a composite signal trace on which signal noise ratio is considerably

improved (Figure 4).

The results of the Waimanalo seismic operation were utilized by the

Division of Water and Land Development of the Department of Land and

Natural Resources of the state of Hawaii in their consideration of the

construction of an infiltration well. The effort has provided data con­

cerning the depth of the coastal sedimentary fill underlying the Waimanalo

8

area. Geologic data on the depth of sedimentary formations at Waimanalo

is also valuable basic information to the scientific community at large.

STAGE 2: MAGNETIC TAPE SEISMIC DATA RECORDING DEVELOPMENT

The magnetic tape data recording capability was tested at the

University of Hawaii by utilizing a seismic source generated by dropping

a SOOO-pound foundation exploration cable drill tool rig from a height of

24" (Figure 5). Two channels of data were recorded by placing one geo­

phone at the acoustical source and the other away from it. The former

provided the time source for the seismic data; the latter recorded reflected

signals.

The seismic signals were satisfactory on the geophone placed away

from the drilling rig. However, the up-hole geophone produced distorted

signals due to overdriving and drilling-rig vibrations.

A block diagram of the seismic signal recording apparatus is shown

in Figure 6. Geophones with a natural frequency of 4.5 cycles per second

fed signals into a simple seismic filter consisting of balancing potentio­

meters. An isolation transformer was added because of the radio frequency

hum and spark interference. The filtered signal was amplified to a level

suitable for recording on magnetic tape by an Ampex model 860 dual

capstan drive unit with built -in audio amplifiers.

Playback of the seismic magnetic tape data on the same Ampex 860 tape

deck is monitored on a Tektronix Type 453 oscilloscope which contains time

delay capability (Figure 7). Reproducibility of the seismic signals was

judged as excellent. Since overdriving of the up-hole geophone destroyed

the usefulness of the timinggeophone, signal averaging was not possible.

The filter circuit is shown as Figure 8.

FI ~URE 2. A MULTI-CHANNEL GALVANOMETER TYPE SEISMIC RECORDER WAS UTILIZED TO PRODUCE REPEATEDREFLECTION RECORDS ON PHOTOGRAPHIC PAPER.

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FIGURE 3. SEISMIC RECORDING AT THE UNVERSITY OF HAWAII EXPERIMENT STATION IN WAIMANALO. THE EXPLOSIVESTRUCK IS IN THE FOREGROUND AND THE SHOTPOINT IS IN THE BACKGROUND.

.....o

FIGURE 4. MANUAL DIGITIZATION OF THE SEISMIC RECORDS. THE OSCAR AUTOMATICALLY PUNCHES IBM CARDS FORUSE IN COMPUTER AVERAGING.

~

~

FIGURE 5. ACOUSTICAL ENERGY FROM A CABLE TOOL DRILL RIG WAS UTILIZED TO TEST THE MAGNETIC TAPE RECORD­ING OF SEISMIC SIGNALS.

......N

GEOPHONE SEISMIC FILTER AUDIO AMPLIFIERMAGNETICTAPE RECORDER

.. .. 0_0~ II~

V

FIG . 6. Block Diagram of Seismic Signal Recording Apparatus

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FIGURE 7. MAGNETIC RECORDING OF SEISMIC DATA IS ANALYSED ON A MULTIPLE TRACE TIME ANALOG OSCILLOSCOPEAND RECORDED BY AN OSCILLOSCOPE CAMERA.

.......,.

GEOPHONE

INPUT

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.01

LINE TO GRIDTRANSFORM:R

AUDIOAMPLIFIEROUTPUT

a:.,~' . "".'.-.. ..;.;.._.. ,. c -,. ......""....= ;. .""'"' -~--

Figure 8. Schematic Diagram of Simple Seismic - signal FiIter Found Effective in Reducing

Radio Frequency Spark and Hum from the Recorded Signal

......U1

16

Figure 9. Design of Tripping Hook for Weight Dropping

17

STAGE 3: THUMPER SEISMIC SIGNAL SOURCE

A surplus jeep was renovated into a wei ght-dropping vehi.c l e with

a steel top installed for protection of the seismic equipment. A winch

with power takeoff from the jeep and a two-pole A-frame with a pulley

at the top was attached to the front of the j eep wi t h a reinforcing frame­

work at the rear of the jeep. Voltage outlets were provided on the jeep

for 12 volt power sources to the electronic equipment . A 300 lb. weight

was constructed in 50 lb. sections so that it could be handled more eas i­

ly. The jeep, winch, and A-frame, however, can lift several thousand

pounds more than 10 feet above the ground surface. Release of the 300 lb .

weight from the A-frame was done by a specially constructed release hook

styled after marine "pelican" hooks (Figure 9). The wei ght was dropped

on a steel plate and the contact impact started the time signal for the

seismic record.

Seismic signals generated by the weight impact were picked up by

either one or an array of geophones and recorded on magnetic tape. The

signals were directed through the filter described in the previous sec­

tion (Stage 2 , Magnetic Tape Seismic Data Recording Development ). The

filtered seismic signals were put into the tape recorder amplifer. The

dual-capstan tape-drive of the Ampex tape r ecorder mechanism improves

tape transport velocities and stability with reasonable audi o frequency

response and comparable transport. Hence, the inexpensive Ampex 860 ,

originally a four-track stereo system in which two tracks recorded in

each direction on a magnetic tape, has been ideal for this study. In

initial tests, one track was utilized to record voice data and seismic

timing signals and the other track on the magnetic tape was used to re­

cord seismic data.

18

A special in-line four track head was purchased from Applied Mag­

netics Corporation of Santa Barbara, California for installation on the

tape recorder to enable it to record voice data of field operations and

record time signals on separate channels, and seismic signals on the re­

maining two channels.

An oscillator has been constructed so that the timing signals

triggered by the contact of the weight striking the metal plate will

start and run for approximately two seconds and stop. It is reset and

triggered when another seismic impulse is initiated. The timing signal

itself is a 300 cycle oscillator available from Accutronics, Incorporated

in Batavia, Illinois. Circuit for the oscillator and trigger mechanism

is shown as Figure 10. The limited duration of the time signal permits

utilization of this signal by digitizing equipment in the laboratory to

analyze only those portions of the magnetic tape which contain the timing

signal. The wave form of the timing signal not only indicates the time

of arrival of seismic signals, but also is used to set the sampling rate

at which the seismic wave form is converted to digital form for analysis

with digital computers. The weight dropper, the geophone, seismic filter,

magnetic recording, and accessory timing device are shown diagramaticall y

in their operational configuration in Figures 11 and 12.

Magnetic tape recordings of seismic signals allow versatility in

the analysis of the seismic signals. The quality and reproducibility of

the signals can be monitored on time delay oscilloscopes. Either individ­

ual traces or composite traces can be viewed and photographed on an oscil­

loscope screen by using intensity modulation or the standard vertical

deflection of the oscilloscope scanning spot . Thus, seismic signals can

be examined by either intensity contrast on the oscilloscope screen or as

FlIP·FLOP ONE-SHOT

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·This capacitor controls time interval

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TRANSISTORS - 2N2925DIODES - lN198OSCILLATOR - MODEL JC 15-22Z

ACCUTRONICS INC.BOTAVIA, ILL.300 cps! 0.0251 at 25- C.

FIGURE 10. SEISMIC CONTROL TIMERSHOT TONEOUT

TUNING FORKOSCILLATOR

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No

VOICE DATA

.'AMPLIFIER

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TIMER

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GEOPHONE

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Figure 11. Schematic Diagram of Thumper and Recording Equipment.

21

FIGURE 12. ACOUSTICAL SI;ISMIC SIGNALS CREATED BY A "THUMPER" WEIGHT­DROPPING SOURCE WERE RECORDED ON MAGNETIC TAPE USING AMODIFIED AMPEX 860 TAPE TRANSPORT. THIS THUMPER IS USEFULIN INHABITED AREAS NOT PERMITTING THE USE OF EXPLOSIVES.

22

wiggle "u" lines. By photographing repeated seismic signals derived from

the same set of field conditions and location, signal averaging occurs by

the weakening of noise signals due to their erratic nature, while the re­

petitive seismic energy signals are reinforced.

A Tektronix Type 32lA oscilloscope is being used for monitoring

signals in the field, and a Type 453 oscilloscope is used for laboratory

evaluation and photography of seismic data (Figure 7).

23

TRAINING AND EDUCATION

Considerable outside interest was activated by the utilization of

seismic instrumentation for determining geologic groundwater conditions.

Besides Water Resources Research Center personnel, other research per­

sonnel and graduate students at the Hawaii Institute of Geophysics, and

city and state ground water agencies followed the development of the

instruments and techniques closely. Considerable volunteer field assis­

tance was obtained in almost all field operations. For these reasons, the

training and education of personnel extended far beyond those directly

employed in the project. A later study, exploring four techniques for

geophysical exploration for ground water was a direct result of interest

developed during execution of this preliminary phase.

24

SUMMARY AND CONCLUSIONS

Initial generation and recording of seismic signals were carried

out with conventional explosives and photographic recording. Develop­

ment of magnetic tape seismic data recording has been accomplished with

accessory filters and a timer adapted to an inexpensive magnetic tape

transport. Inexpensive and rapid generation of seismic signals was

developed with a weight-dropper.

Versatility in handling and analysis of seismic data is provided

by both analog and digital input of magnetic tape seismic-recording. High­

ly reproducible results have been obtained.

This project has been primarily involved in procuring, assembling,

and debugging equipment, training student helpers and technicians on field

procedures, and determining laboratory analysis techniques. No final an­

swer has been obtained, however, concerning the feasibility of seismic

exploration for structure relevant to groundwater control and movement.

Interpretation of the seismic data does not indicate that seismology

would not be an appropriate tool for studying geological structures. There ­

fore, funding has been obtained for ~nother project to continue this work.

Success in that effort is assured due to the extensive ground work in in­

strumentation and the personnel training afforded by this initial project.

A significant accomplishment of this study is the development of

equipment for seismic exploration using inexpensive and readily available

components which produce good quality records and of the versatility in

analysis techniques. The reasonable cost and availability of the equip­

ment enables it to be readily duplicated for groundwater exploration in

areas where availability and costs might otherwise prevent seismic studies.

2S

ACKNOWLEDGEMENTS

Assistance in design and construction of instrumentation by the

personnel of the Hawaii Institute of Geophysics greatly aided in achiev-

ing the objectives of this project. Although many individuals assisted,

special appreciation is due Noel Thompson, Griffith Woodruff, and per-

sonnel of the electronics and machine shops.

Graduate students who assisted gratuitously in field seismic tests,

include Loren Kroenke, Frisby Campbell, Don Hussong, Mel Caskey and Don

Walker. The assistance of Ron Tracy as graduate research assistant on

the project is appreciated. Professor Augustine Furumoto has been gen-

erous in giving guidance and help.

26

BIBLIOGRAPHY

Swartz, J .H. 1937. Resistivity studies of some salt-water boundariesin the Hawaiian Islands. American Geophysical Union Transactions 3

pt. II, 387-393.

Swartz, J.H. 1939. Geophysical investigations in the Hawaiian Islands.American Geophysical Union Transactions 3 v. 20, pt. 1, 292-298.

Swartz, J.H. 1940a. Resistivity survey of Schofield plateau, p. 56­59. In H.T. Stearns, Supplement to the Geology and GroundwaterResources of the Island of Oahu3 Hawaii. Hawaii Division, Hydro­logical Bulletin, v. 5.

Swartz, J .H. 1940b. Geophysical investigation on Lanai, p. 97-115. I nH.T. Stearns, Geol ogy and Gr oundwater Resources of the Island ofLanai and Kahoolawe3 Hawaii. Hawaii Division, Hydrological Bulletin,v. 6.

Watkins, J.S., DeBremaecker, J.e., Loney, R.A., Whitcomb, J.H., andGodson, R.H. 1966. Investigations of in situ physical propertiesof surface and subsurface site materials by engineering geophysicaltechniques. Annual report, fiscal year 1965. U.S. Geol. Surv.