Exploration Archaeology [John M. Stanley]

8/10/2019 Exploration Archaeology [John M. Stanley]

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TREASURES sought after by archae-ologists differ between Europe andAustralia due to the nature of therespective civilisations.

European communities produced last-ing hardware of baked clay or metals.They built cities of permanent materi-als with considerable use of stone andbricks, and they often fortressed thesewith substantial walls. Although theircivilisations have decayed, they leftmany remnants now submerged beneath

windswept sands or buried by alluvialflood plains. Yet others have been builtover by later communities. In common,these folk considerably altered the land-scape where they built their cities. They

left permanent relics of their handcraftand they frequently left written evi-dence of their existence.

The scene in Australia is very differ-ent from this. The aborigines rarelyaltered their habitat with permanentconstructions, and rarely if ever, madeuse of bricks or metals. Consequently,the only lasting remains of their camp-sites are fireplaces, shell concentrationsand humus-rich deposits where wander-ing tribes made seasonal camps when

food was abundant. These middens,as they are generally termed, do how-ever contain small items, usually ofchipped stone, which are of interest toour natural historians.

These differences require new exploration procedures. In the pursuit foremains of a highly developed community, it is logical firstly to searchhistoric writings for clues as to where atownship may have been situatedAerial photography may then disclosesurface formations not normally visiblefrom the ground.

In the past it has been necessary tofollow these activities with tedious drilling and trenching, but a great deal othis laborious work may now bereplaced by the refined use of geophysical methods, and the final excavations

Exploration archaeology

searching for our past

John M. Stanley


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commenced with greater confidence ofsuccess.

Our aborigines left no writings orphotographable surface features indicat-ing the whereabouts of their campsites.Fortunately for the archaeologist, muchof the countryside in Australia has notchanged very much since aboriginaloccupation and it is logical that mid-dens be associated with featuresproviding a regular source of food.Lakes, river estuaries, rocky shorelines,natural springs and waterholes are ourclues to past occupation. They are virtu-ally the only means we have ofconfining the area of our search. In factexcavations have only been made inAustralia in places where surface evi-dence of middens has been observed.But if geophysics can be employed suc-cessfully, then much older middens maybe located buried at greater depths. Theauthor is at present concerned with thispossibility.

Geophysical methods in present

use

There are three principal geophysicalmethods which have been applied toarchaeological studies. They are: seis-mology, resistivity and magnetics. Theiruse depends upon the nature of the par-ticular environment, the amount offinance available for equipment, and

upon the experience of the operatingcrew.

Seismology

The principle of seismology is thatshock waves travel at particular andwell-defined velocities through mate-rial of different types. The denser thematerial, the faster the speed that shockwaves will travel through it. The veloci-ties vary from as low as 600 ft/sec inlight and dry top soil, to 20,000 ft/sec inunseamed granite.

If the speed of the shock wave ismeasured, then the type, hardness anddepth of the various strata can accu-rately be determined. This is relativelyeasy to do, for when a shock wave

strikes an interface between two differ-ent types of material it will be refractedalong that interface.

With the simplest types of seismo-graphs the shock wave is initiated bystriking the ground with a hammer. Fig-ure 1 shows how the shock wave thusgenerated (at point S), travels out inhemispherical wavefronts. If a detect-ing instrument is at point D adistance of X feet from S then theshock wave travelling horizontally

through the top material (the direcwave) will reach the receiving instrument before any other wave as longas X is small. For longer distances, thewave travelling along the lower strata(which has a higher characteristic velocity) will arrive at the receiver before thedirect wave.

Angle Ic is the critical angle awhich the shock wave is refracted alongthe interface. It is in fact the anglewhere Sine is V0/V1.

The most convenient way to represent this data is to measure and plot thearrival time of the first refracted wavevs the short distance X. For examplewith two layered stratum (Figure 1) wewould have the plot shown in Figure 2

From the gradient of the first arrivasegments we can deduce the velocitiesV0 and V1 and hence calculate the

depth to the interface. Figure 3 showsthe experimental data plotted over atrench buried under a layer of silt.

In the far more complex situation oidentifying echoes from irregulaarchaeological objects, interpretationbecomes a job for the expert. Howeverthere are many cases when seismologyis quite practical to use. These include

Combining technology with the classical arts, todays archaeologist isa refined crossbreed of historian and geophysicist.


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buried tombs and building sites contain-ing walls or similar large structures.Seismology has been successfully usedto locate underground passages andtomb cavities within the Egyptian pyra-mids, and is ideal for sounding thedepth of deposits in caves and rockshelters.

Portable instrumentation has recentlybecome commercially available, but at a

cost of about $3600! Quite prohibitivefor the amateur treasure seeker! Such asignal enhancement seismograph isbattery operated, weighs only 17 lbs andis exceedingly accurate and easy to use.The seismic disturbance is made bysimply hitting the ground with a 10 lbhammer.

Resistivity

Another characteristic of differingstrata is electrical resistivity in factthe range of electrical resistivities is

enormous. It extends from 10-1 ohm/

metre to 1019 ohms/metre. It followsthat if we can measure vertical and hori-zontal resistivity profiles of the ground,we must be able to detect changes incomposition, and hence deduce theexistence of buried objects. There aremany ways of doing this, some involv-ing ac measurements and others usingdc. Generally, the resistivity is far fromuniform and so the measurement used isone of apparent resistivity in effectit is a mean value depending on the dis-tribution of rocks and their individual

resistivities.One of the most common electrodearrangements for measuring apparentresistivity is that known as the WennerArray (illustrated in Figure 4). Using aWenner Array (with electrode separa-tion a) on the surface of a semi-infinite solid with uniform resistivity p,then p= 2a V/I = 2aR (where R is the

resistance between the inner electrodes).There are two applications of this for-

mula. We may perform electricaldrilling or electrical trenching. In theformer, a vertical profile of the resistiv-

ity may be measured by plotting p asthe separation of electrodes a is var-ied. The depth at which pis measured isapproximately 0.6a. Apparent resistiv-ity profile curves may be generated by acomputer for different models of groundstructure. Volumes of standard curvesof this type have been published andthese facilitate the interpretation ofresistivity drilling.

Electrical trenching is achieved byselecting an electrode separation corre-sponding approximately to the depth ofinterest, and moving the whole arrayalong the traverse line. Figure 5 shows atypical set of results plotted over a bur-ied wall.

Resistivity methods are applicable tosimilar situations as the seismic method.The field skills and interpretation com-plexity are comparable to those requiredfor seismology but the cost of equip-

ment is very much less. A quiteeffective ac resistivity meter may bepurchased for less than $500) and a dcoperated meter such as that describedimmediately following this article

may be home assembled for very muchless.

Magnetics

The Earths natural magnetic field iperturbed by the magnetic properties ofmaterials within its influence. If theEarths field may be measured to anaccuracy of the order of 1 part per 1000this perturbation can be detected. Information concerning dimension, locationand composition of the perturbing bodymay be extracted from carefully compiled maps of anomalies in the magneticfield.

During the mid 1950s, a team aCambridge University developed amagnetometer, having a sensitivity of 1part per 100,000, specifically forarchaeological work. This instrumenmeasured the frequency of protons in anorganic fluid as they precessed abou


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the Earths field. The precession fre-quency was linearly related to theintensity of the magnetic field. Theproton precession magnetometer isavailable now at a cost of about $500.More recently an instrument has beendeveloped which measures the electron-nuclear spin of atoms in an alkali metalvapour. This spin frequency is also lin-early related to the magnetic field, butyields an accuracy of 1 part in 1 million(ETI Jan, 1973). At present these instru-

ments are expensive in excess of $1000 but as refining developmentsprogress, this cost may be expected todecrease substantially.

The magnetic field on the Earths sur-face is almost entirely (95%) due tostable sources within the core. Theremaining 5% originates from variablecauses, and may be divided into tem-poral (time) or special (position)variations. The temporal changes result

principally from solar-induced currentsin the Earths crust, and magnetic pulsa-tions in the magnetosphere. They rangein frequency from a fraction of a sec-ond to diurnal. The amplitude of suchvariations is typically a few gammas butunder severe conditions magneticstorms of several hundred gammas maybe encountered.

Special variations arise principally

from the degree of magnetism inducedin materials of the Earths crust. Differ-ent rocks and minerals exhibit a rangeof susceptibilities to magnetisation inthe Earths field and this magnetisationcan readily be detected with moderninstruments. A second very significantcause of special anomalies results fromremnant magnetism exhibited byobjects containing ferromagnetic miner-als which have been heated strongly atsome time. Within the crystals of themineral are small, randomly orientatedregions of uniform magnetisations,

called domains, which become mobileabove the Curie temperature of about600C. During cooling, many of thedomains align themselves parallel to theEarths magnetic field and are thus fro-zen in this alignment. Since they areparallel to the Earths field they are alsoparallel to each other, thus creating anet magnetic effect. Pottery, kilns,hearths and baked rocks will frequentlyexhibit a measurable remnantmagnetism.

If the archaeologist is to distinguishbetween temporal and special anoma-

lies it is usual to use twomagnetometers. Both will respond totemporal changes simultaneously so ifthe difference in field value between thetwo is measured while one instrument iskept stationary, then only the spacialchanges will be recorded. Since thedevelopment of the extremely high res-olution alkalai vapour magnetometersit has been possible to use such twoinstruments as a gradiometer. Both

field sensors are mounted with a fixedseparation on a vertical staff. Againboth respond simultaneously to temporal changes and so the field valuedifference between the sensors yieldsthe vertical spacial field gradient.

This data is of particular value to thearchaeologist who is usually looking forobjects buried under a quite shallowlayer of sediments. This is because i

effectively filters out background magnetic anomalies that originate in thedeeper underlying geologic strata. Idoes this because the magnetic field of adipole is inversely proportional to thecube of the distance from it. The significance of the inverse cube factor iapparent if we compare the anomalousintensities, at each of two sensors, froma buried wall overlying a geologic magnetic disturbance. Let us suppose thathe two sensors are directly above thewall at distances of one and two metresand that the wall overlies the geologic

source at a distance of 10 metres. Thenif the geologic anomaly were even aslarge as the wall anomaly at the site ofthe lower sensor, the differential anomaly of the wall would be almost fourtimes that of the geologic strata.

The interpretation of magnetic fieldand gradient data is certainly a task forthe expert if full value is to be extractedfrom the data. The nature of the anomaly will depend upon a large number ofactors such as size, shape, depth, magnetic susceptibility of the object, and itorientation relative to the Earths field

Mineral and oil exploration researchhas developed computing prowess inthis field and it is now possible toachieve exciting successful results if theright skills are applied to the data. Figure 6 shows an actual magnetic contourmap over a corner of a stone wall buried at a depth of 5 m. This data wasmeasured with a differential magnetometer pair during the search for the loscity of Sybaris in southern Italy.

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Exploration Archaeology [John M. Stanley]