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GEOPHYSICS, VOL. 51, NO. 3 (MARCH 1986); 533-537

Archaeological prospection: An introduction to the Special Issue

Jeffrey C. Wynn*

ABSTRACTArchaeological prospection, as the use of geophysicalmethods in archaeology is known in Europe, is aboutfour dec ades old (seven decades, if aerial photo graphy ofarchaeolog ical sites is included). Virtually the entire

range of geophysical methods, perhaps excluding onlyborehole tech niques, has found app lication in the searchfor archaeologica l sites unseen or pa rtially known . Pres-sures by developers, and the pu blics growing sensitivitytoward the preservation of historic and prehistoric cul-tural artifacts and sites, has led to an accelerating use ofhigh-resolution geophysical methods in the archae-ological sciences.The archaeogeophysical articles in thisSpecial Issue are reasonably representative of the devel-opment of this specialty field of geophysics.

INTRODUCTIONThis Special Issue discusses he use of high-resolution geo-physical exploration methods in archaeological applications,which is called archaeolog ical prospection in Europe. Wo rk inthis field began at least in the 1940s but is not well-known,particularly in North America. Most practitioners have beenrelatively small groups of individuals with geotechnical back-

grounds who were fascinated by archaeology. They have neverbeen well-funded; archaeology h as traditionally been one ofthe least funded of academic research disciplines.Perhaps becau se of earlier recognition of ancient man-mad estructures in Europe and the Middle East, geophysical workm archaeo logy has been concentrated there. Practitioners inthis field have communicated informally with ea ch other buthave not foun ded any sch olarly associations, and no journa ldedicated solely to archaeological prospection exists; one spe-cialty journal, Prospezioni Archeoloyiche, was published onlyfrom 1966 to 1973. Several journals, such as Archueometry,Bonner Jahrhucher, Journul ofField Archurology, and the Jour-nal ef Archaeological Science have published occasional paperson the subject since 1973. This Special Issue is one of the fewcollections of articles on this subject in the last deca de.

Every profession has its own un ique jargon, usua lly a short-hand or set of abbreviations created to facilitate communi-cation. Unfortunately, jargon can isolate practitioners fromcolleagues outside the immediate specialization. Although thisabbreviation process can lead to the (often condemned ) use ofacrunyms, perhaps~ he- mcJstch&rrgerous nse~quence~s usi?ofcommon expressions to com municate ideas that have subtlydifferent meanings depending upon the background of thereader. To avoid confusion, terms need to be defined carefullybefore misleading usages become widespread as archaeologicalprospection becom es better known and more widely practiced .The first problem encountered is the need for a word todescribe the subspecialization high-resolution ground geo-physical methods used to measure many physical propertiesuseful in archaeological map ping applications. An abbrevi-ation m ight be useful here. The term archaeophysics mightbe acceptable, but it is already used to describe a somewhatbroader field that includes archaeomagnetic and isotopicmethods to date and eve n suggest provenance (source) of arti-facts. Archaeogeophysics is perhaps the most descriptiveterm, but like archaeological prospection, it is cumbe rsome.Although the term archaeological prospection is morecommonly used in Europe to mean geophysical methods usedm archaeology, the most commonly used term am ong archae-ologists in North America is remote sensing. This is anexample of the possible confusion previously alluded to, be-cause remote sensing for a geophysicist usually conveys theidea of using aircraft- or spacecraft-acquired image ry of theEarths surface; digital enha ncement an d geometric correctiveprocedures are assumed to be necessary adjuncts. To a N orthAmerican archaeologist, howeve r, remote sensing is used pri-marily to describe the use of high-resolution ground geophysi-cal and geochemical techniques; in fact, the archaeologicaltarget is indeed sensed remotely, that is, nondestructively.Another term wrestled with during encounters between geo-physicists and archaeologists is rescue archaeology, that is,an archaeological survey of a tract of land before it is exten-sively modified and probably buried by modern development.Usually a large tract cannot be surveyed effectively without

Manuscript eceivedby the Editor September , 1985; evisedmanuscript eceivedSeptember 0, 1985.*U.S. GeologicalSurvey,927 National Center, Reston.VA 22092.Paper preparedby an agencyof the U.S. Governm ent.533

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534 Wynnassistance from geophysical methods in the v ery short timeallowed.Archaeomagnetism refers to dating sites by comparisonof the magnetic p roperties of oriented samples from kilns orhearths with carefully reconstructed records of magnetic secu-lar variation to date the last time of firing (when the magneticdomains were reset). Nondestructive archaeology refers tothe use of geophysical methods to provide three-dimensionalinformation abou t potential archaeological targets withoutdisturbing them . This ide a is similar to the concept of nonin-vasive tomograp hic evaluation techniques used in the medicalprofession, such as CAT (comp uter assisted tomog raphy)scans, and is important to archaeologists who wish to pre-serve the cultural heritage as well as study it.

THE ARCHAEOLOGICAL NEED

Anthropology is a large field, traditionally 90 percent basedin the academic world, and includes many subcategories suchas physical anthropology, ethnology, archaeological geology,paleopatholog y, an d provenan ce studies, among others. Allthese subcategories depend upon source materials gathe red byfield archaeologists. Tradition al archaeolo gical excavationswere carried out with careful trenching and coring into siteshaving historical doc umen tation, surface artifacts, or surfacemorphology suggesting earlier human occupation. The objec-tive of the excavation process was to acqu ire cultural a ndbiological artifacts for subsequ ent laboratory and statisticalstudies and to docum ent the three-dimensional (3-D) relation-ships among artifacts by using an accurate, on-site recordingprocedure as the work progressed.

During the last several decades, the emphasis in archae-ology has shifted toward use of laboratory instrumen tation toanalyze human cultural artifacts and performing cultural siteevaluations in advance of land development. time required toexcavate entire sites became a luxu ry th at archaeologists couldnot afford. If no surface expression led the archaeo logist on,the time-intensive nature of trenching usually precluded anywork being initiated beyond random shallow coring or augerwork. A faster method to evaluate the hidden dimensions of asite was desperately needed.Becaus e the trenching process exposes previously p reservedremains to surface weathering, oxidation, and van dalism, andthe excavation of a site irrevocably destroys the spatial inter-relationships, archaeologists have always wanted fast and ef-ficient reconnaissance methods tha t are also nondestructive.Recently, a heightened desire to preserve our human cul-tural heritage from the advances of development has created asubstan tial political pressure, typically expressed by law s man-dating environmental impact evaluation of a site before it isforever lost. These laws are very stringent in Europe, particu-larly in G erman y (Scollar et al., 1986 , this issue). The U.S.Government has more than 500 archaeologists who arecharged with preserving prehistoric and historic buildings andlandmarks, and with conducting rescue archaeology (D. Sco-vill, Chief Anthropologist, U.S. National Park Service, pers.comm .). Because of the pressure to release the land for devel-opmen t, archaeologists often are forced to limit their evalu-ation to a careful examination of surface artifacts and perhapssome random coring, usually to depths of less than 0 .5 m.Geoph ysical methods, and to a lesser extent geoche micalmethods (Eidt, 1977 ), provide the rapid subsurface, nonde-

structive exploration technology the archaeologist must have.Geophysical methods also provide an added benefit: withrapid, uniform reconnaissance of an entire site now available,a synoptic view of the interrelationships within a large site canbe obtained with relative ease (Weymouth and Huggins, 1 985).Archaeologists using geophysical methods now routinelymap hearths as well as soils magnetically altered by campfires(Weymouth and Huggins, 1985). They also map building foun-dations, midden s (trash heaps), burials, and soils compac ted orexcavated by previous human activities. Remarkably, how-ever, geophysical methods are still only infrequently used.Most archaeologists, while awa re of the need for geophysicalsupport, are largely unaware of applicable geophysical tech-nology.

A SURVEY OF GEOPHYSICAL METHODSUSED IN ARCHAEOLOGY

A distinction must be made among geophysical methodsused in archae ology between exploration methods (prospec-tion) and related, but none xploratory, techniques such asprovenance or dating methods (including archaeomagnetism).Prospection methods can be further grouped as site explora-tion and as intrasite mapping. The h igh-resolution geop hysicalmethods outlined here almost always apply to intrasite map-ping: that is, they are used to guide excavation programswithin already discovered sites. Site exploration, the search forundiscovered sites, can sometimes be accom plished with aerialphotographs or digital imagery from aircraft-borne instru-ments. High-resolution spacecraft-borne systems (such asthose on the French SP OT satellite and the Landsa t thematicmapper) and advanced technologies now available (such as thelarge-format camera) may hold the promise of major growthin the field in the next decade.The oldest prospecting technique in use is aerial photo-graphy. A erial photograph y was successful n Eng land, whereextensive Rom an and pre-Rom an structures were found vir-tually everywhere (Beazeley, 1919). The usual practice was toacquire photographs, both in the visible and near-infraredwavelengths, from a balloon tethered above a site before ex-tensive ground work was initiated. S ubseque ntly, multispectralimagery from aircraft-borne instruments and imagery frominstruments on space craft including Sk ylab and I andsa t havebeen evaluated; this imagery is not as useful as airphotosprimarily because of the low resolution ava ilable (Stringer andCook, 1974; Lyons and Avery, 1977; Ebert and Lyons, 197 8).The ad vent of the large-format camera , to be used on U .S.space shuttle missions (Doyle, 1985 ), will u ndoub tedly c hangethis situation.Reports of work using tempera ture probes to search forburied structures (stone and wo od) hav e been published(Benner and Brodkey, 1984). Although not documented,various military agencies have operated therm al infraredimaging systems for years. Little work on therma l imagery inarchaeological applications has been published. A notable ex-ception is a paper demonstrating how aerial thermographywas used to identify S inagua-c ulture agricultural fields aban-doned more than 700 years ago in northern A rizona (Berlin etal., 1977). Therm al infrared (IR) or therma l inertia methodswould be particularly useful over targets that have consider-able hidden structure or over targets that are inaccessible be-cause of logistical or political reasons.

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Archaeological Prospection 535Seismic-refraction methods have been used occasionally

since the late 1950s (Carson, 1962), but with only m arginalsuccess. A more archaeolog ically successful variant is thesonic spectroscope used principally in the search for cavities(Carabelli, 1966). This method uses a transmitted frequencyranging from 20 to 3 000 Hz; reflected returns are monitoredfor reso nant frequencies (after a calibrated response is re-moved). Acoustic-reflection techniques (such as sidescansonar) have been used in m arine en vironments, mainly inEurope , where sunken cities in the Aegea n and shipwreckspartially or co mpletely buried in the sea-floor sediments havebeen identified (McGh ee et al., 1968; Edgerton, 1972 ).Because of the cost, and difficult and complex field pro-cedures, reflection seismic methods ha ve no t been use d in ar-chaeological applications until recently. High-resolution sur-veys (of the kind used to identify po tential hazards to d rillingplatforms in the Gulf of Mexico) were recently used in thenear-offshore environment to exam ine paleobea ch deposits forpotential sites of ancient human occupation (Stright, 1986 , thisissue).These kinds of surveys could significantly affect archae-ological studies of the B ering S traits land bridge, where m ostpotential occupation sites of interglacial human migrationolder than 5 000 years are now underwater (Kontrimavichus,1976; Dikov, 1983; McManus et al., 1983).Magne tic surveying, the geophysical tool m ost frequentlyused by the archaeologist, was first tried in 195 8 in England(Aitken, 1974 ). It is routinely us ed to m ap buried stone foun-dations and to outline the locations of forges and kilns,hearths, and cam pfire sites (Gibson, 198 6, this issue).The util-ity of magne tic-susceptibility measurem ents of topsoil in themapp ing of anthrosols (soils modified by h uma n activity) wasdescribed n Colani and Aitken (1966).Recent magnetic surveying innovations in archaeolog y havecentered largely around microcomputer-based magnetic data-gathering systems to a cquire the ve ry large da ta b ases neces-sary DOTEwho&site evaiuatron (Weymouth, 1986, this issue).Considerations of speed and reso lution in m apping large areashave led som e archaeologists ?o switch from use~of a proton-precession magne tometer to a tluxgate gradiometer includingautomated recording systems.Gibson (1986, this issue) eportsuse of replicative model experiments to permit more preciseevaluation of field data.Archaeom agnetism (Tarling et al., 198 6, this issue), thoughtechnically not prospection, is a d eveloping science that seeksto obtain site-occupation dates from the orientation of theremanen t magn etic field found in stones lining an cient kilns.The history of the E arths magne tic field is currently beingcalibrated and refined using historical records as well as radio-carbon dates, tephrochronology, and dendrochronology (tree-ring) for control.Use of electrical-resistivity methods in archaeology predatesthe use of magnetic methods. First used in 1946 in England(Atkinson, 195 2; Aitken, 197 4), the technology has been fur-ther refined and is now frequently used in E urope to maplarge areas rapidly, up to several hectares per day using l-mgrids. Electrical resistivity methods permit the discernment ofburied ditches and stone foundations.The induced polarization (IP) method was first used inEngland in 1968 (Aspinall and Lynam , 1968, 1970) with m od-erate successbut is not widely used now. F ew anthropogen icmaterials have an inherent induced polarization, the principalexception being unoxidized metal burial artifacts which areusually too sm all to cause more tha n noise-threshold signals

in a field survey. One notable exception is the cast-iron cofftnused infrequently in the late 18th and early 19th centuries 6.Potter, U.S. Na tional Park Service, pers. comm .; Habens teinand Lam ers, 196 2). On a larger scale, ocomotives and cannonlost in shallow water during the US. Civil W ar a re relativelysimple targets for magn etometer surveys (C. G alvin, pers.comm.).Electromagnetic (EM) methods have been used since about1960 @collar, 1962; Foster, 1968; Tite and Mullins, 1969,1970: Tabbagh, 1974; Bevan, 1983). One of the most usefulprospection instrumen ts still in use today is the inductive soil-conductivity meter; surveys can be carried ou t almos t as fastas the operator can walk, making this (along with radar)amon g the fastest ground g eophysical methods now available.Metal detectors, which work on a similar principle, were fre-quently used to search for ancient metallic objects not yetcompletely oxidized, and are also used to sort out moderncultural pollution (cans and other ferrom agnetic trash) frommagn etic anomalies deriving from older sou rces such ashearths (Lancaster, 1966). Use of Slingram EM systems withcarefully des igned spatial and frequency param eters now per-mits archaeolog ists to ob tain bo th electrical conductivity andmagnetic susceptibility data at the Same time without havingto establish physical contact with the ground (Ta bbagh , 1986 ,this issue).Probably the most important technical innovation in ar-chaeology in the last decade is ground-probing radar. Firstused in the U.S. in the early 1970 s. t was originally designedto serve as an aid to engineering geology studies (Moffat,1974; Morey, 1974; Cook, 1974; Vickers and Dolphin, 1975;Ulriksen, 1982). It is rapidly becoming the most essentialsingle instrument in intrasite mapping for archaeologistsmakin g g eophysical field measuremen ts. The hard-copy geo-electric section provided by the ground-probing radar is rela-tively easy to interpret in the field. Examples of ihe~vatiety or*ground-probing radar applications reported in this issue arethe search for shallow g raves and w halebone of a 17th centuryBasque whaling station (Vaughan, 1986, this issue; see alsoTuck, 1985; Laxalt, 1985) and the buried foundations of his-torical fortifications in Quebe c (Vaugh an, 198 6, this issue).Ground -probing radar is of limited us e where either the soilconductivity or loss tangent are larg e because a clay layermakes almost anything below it opaque to the radar. Untilrecently, the cost of a rada r system was prohibitive, limiting itsuse in arch aeology. Full paper-copy systems now on themarket cost as little as $15,000 US (1985).Radiometric methods were experimented with briefly,mainly because biological ma terials (human skeletons, wastes,and middens) harbor significant levels of calcium phosphatethat contain tiny but detectable amounts of radio-nuclides(Peschel, 1967). Only extremely shallow phosph ate-rich an-throsols can be detected, however. Ne utron scattering has alsobeen tried to search for buried walls and cavities (Alldred andShepherd, 196 3), but significant radiation hazards make thismethod one of last resort,

The gravity m ethod ha s seen limited use, principally be-cause of the enormous time and energy involved in makingadequ ate elevation and terrain corrections (Linington, 1966 ).Along with o ther methods, however, it was used to ou tlineancient Carthage in Tunisia (Kolendo et al., 1973) and a grav-ity grad ient method was used in Po land to search for under-ground cavities (Fajklewicz et al., 1982).

The self-potential (SP) method was firstsused in a m arine

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536 Wynnarchaeolo gical application in 196 7 to identify a U.S. Civil Warera (sunk in 1 864) ironclad ship (Corwin, 1973), and in a landarchaeolo gical application in 1 980 , with surprising success(Wynn and Sherwo od, 1984 ). SP field work is relatively slow(the method requires stringent noise-reduction field pro-cedures), although it may be near y as fast as a m agnetometersurvey if two or three people participate. SP is, nevertheless,the least equipment-intensive of all available geoph ysical sys-tems used in archaeology, and, therefore, it is the method mostavailable to amateurs. The hardware requirements are hardlymore than a high-impedance digital voltmeter, three or fourAg-AgCl electrodes, a trowel, and some wire cannibalizedfrom a stereo.This discussion would not be complete witho ut reference tothe application of sophisticated new data-processing technol-ogy which brings out subtle features otherwise missed in ar-chaeogeo physical data sets. Spatial filtering and enhancem entalgorithms for images are, in particular, now b eing utilized toenhance weak linear features in the presence of strong anoma-lies of geologic origin (Scollar et al., 198 6, this issue). Ulti-mately, sop histicated new graphic package s becoming a vail-able for microcomp uters will beco me an essential part of anystudy of the 3-D interrelationships within a site. Mostarchaeogeophysical data acquired now are processed onmicrocomputers to enhance the visibility and the initial detec-tion of the subtle features typical of archaeological artifacts.

FUTIJRE DEVELOPMENTS IN ARCHAEOGEOPHYSICSThe application of geophysical methods to archaeology has

increased with environ mental and political p ressures mand at-ing assessmentsbefore land development, especially in Europeand on Federally owned lands in the United States. There ha salso been a relatively sudden growth in the number of geo-physicists and archa eologists working together (often in theirspare time) in the ie d.

I can confidently p redict that there will be an increase in theuse of rapid reconnaissance methods, as well as signal-processing, image-enhancement. and graphic-representationtechnologies in archaeology. Sites that cannot be exploredphysically for political or logistic reasons will inevitably leadto increased use of multispectral an d thermal imagery a c-quired from aircraft and spac e platforms.

Remarkably, almost no work has been published on thegeophysical signatures of anthropom orphic materials. There-fore, I am certain that the pressure to analyze ever-increasingvolumes of geophysical and geochemical data from field sitesworldwide will lead to an increase in laboratory petrophysicalanalysis. These analyses are an inevitable consequence of theneed to understand better the wide variety of geophysical sig-natures (and the influence of environme ntal controlling fac-tors) found in archaeological sites.

In the previous discussion, I only briefly me ntioned the fail-ures of geophysical methods to solve archaeological problems.Unfortunately, none of the methods discussed will work all ofthe time for instance, magnetic methods will not work incertain unfavorable geologic or cultural situations (nearpowerlines, etc.). This kind of situation leads to the inventionof new m ethods o r techniques . In a sense, failure is the mostimportant stimulus of all in archaeogeo physical (or any otherkind of) research.

It is possible, therefore, that the next decade will witness theadvent of another new exploration method, similar to the

recent introduction of ground-probing radar. Perhaps themost satisfying prediction of all, however, is that physical,cultural, and social research will become more integrated asinteraction increases between scientists of widely divergentbackgro unds and sp ecialties. The ex citement and fun thatcome from croscscultural communication make the sweat andoccasional failures inherent in this work worthwhile. The real-life detective work, resulting in homes and lifestyles from5 000 years ago coming alive on a com puter monitor, is areward in itself.

ACKNOWLEDGMENTS

1 have a very large number of people to thank for their helpand scholarly advice during the preparation of this manu -script, including in particular, Lam bert Dolphin, Brun o Froh-lich, Ray W atts, and Mar k Gettings. The au thors of the Spe-cial Issue have inadvertently played a large role in my educa-tion because serving as Special Editor of this issue, I read eachmanu script several times while considering the reviewers com-ments. I also must thank these authors for trying so hard tomeet all the deadlines. More than 40 different reviewers evalu-ated manuscripts in a most professional manner, and ulti-mately, the quality of this Special Issue on Archaeology andGeophysics derives from their efforts.

The growth of archaeogeop hysics owes almost everything tothe early precomputer age contributions of those individualsalluded to in the Introduction. These include the late RichardLinington and C. M. Lerici of the Fondazione Lerici, as wellas Martin A&ken, A. Hesse, Alain Tabbagh, Tony Clark, D onTarling, Irwin Scollar, and many others too numerous toname here.

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