How effective is geophysical survey? A regional review

Archaeological ProspectionArchaeol. Prospect. 16, 77–90 (2009)Published online 3 March 2009 in Wiley InterScience

348
(www.interscience.wiley.com) DOI: 10.1002/arp.
* Correspondence to: D. Joversity of Bern, Langasse SE-mail: david.jordan@arch

Contract/grant sponsor: Ament of English Heritage.

Copyright # 2009 John

HowEffective is Geophysical Survey?ARegional Review

DAVID JORDAN*

Archaeology Institute, University of Bern, Langasse Strasse 10, 3012 Bern, Switzerland

ABSTRACT GeophysicsissuchanacceptedpartofBritisharchaeology that itseffectivenessseemsobvious.Yet ifthere is no reason to doubt the benefits of geophysics why do some experienced archaeologistsuse it so rarelyandwhy is it little used in some countrieswhich, in other ways, havehighly developedprofessionalarchaeologyservices? Thereare, often cheaper, alternatives forarchaeological survey.Yet since the performance of different surveymethods has rarely been studied systematically thereis no objective basis onwhich to test which choices best meet archaeologists’needs.Moreover thegeophysicists’ understandable desire to present successful rather than unsuccessful surveys, andtodiscussresultsingeophysicalrather thanarchaeological terms,makessuchassessmentmoredif-ficult. Thus although geophysical surveyors have strong grounds to claim that their work benefitsarchaeology, thosewhopay for surveycanreasonablyask that thesebenefitsbe clarified, quantifiedwhere possible, and compared with alternatives, such as aerial photography or surface artefactsurvey, so that they canmake the best choices about its use.Thispaper summarizes a studyof all the geophysical surveys carried out in the northwest of Eng-

land before 2006. The study assessed the performance of geophysical surveys in archaeologicalterms and was centred on a detailed analysis of 35 sites for which there is good comparativeexcavationdataorwhichhaveparticularlyillustrativecasehistories.Thestudyconcludesthat, despitethe doubts in this area, geophysics serves archaeologistswell andprovidesgreater certainty in bothidentifyingwheresitesexist andwhere theydonotexist thanhasbeengenerallyassumed.It thereforedeservesmore extensiveandmore rationaluse.Geophysics is, however, beingunderusedbecause,although abundant, surveys are formulaic and commercial surveyors are rarely able to fit method-ologies to sitesbya programme of reflective project development.Thus, although currently effective,geophysicsmight be evenmore so if surveyorshad the time and resources to do this and to answermorecomplexandspecificquestions.Thepaperconsidershow thesefindingsrelateto theuseofgeo-physical survey in other countries.Copyright# 2009 JohnWiley & Sons, Ltd.

Key words: review; geophysical survey; England; planning

Introduction

We have half a century of evidence thatarchaeological geophysics ‘works’. Thousandsof surveys, carried out all over the world and in avast range of conditions, have mapped buriedremains. The wide use of geophysics in the

rdan, Archaeology Institute, Uni-trasse 10, 3012 Bern, Switzerland..unibe.ch

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Wiley & Sons, Ltd.

cut-throat environment of British commercialarchaeology shows that some archaeologiststhink it provides what they need against toughcompetition from other methods of prospectionand site evaluation. Yet the relative rarity ofgeophysical survey in many countries with well-developed archaeological services, and the infre-quency of its use by some major British archae-ological consulting organizations, suggest thatothers remain sceptical.

It is striking that professional archaeologistscan disagree so much about the value ofgeophysics. This disagreement is a result of the

Received 24 October 2008Accepted 17 December 2008

78 D. Jordan

complexity of the judgements archaeologistsmust make about the methods they use and ofthe persistent lack of objective measures ofsurvey effectiveness available to support thosejudgements. Moreover, surveyors tend to con-sider their results in geophysical terms whereasthe users of survey results, overwhelminglyarchaeologists, are concerned with relative costand confidence in results as much as with thesimple detection of remains. Surveyors musttherefore answer some hard questions. Couldthe same results have been better achieved, orachieved more cheaply with other approaches –aerial photography for example? What doesgeophysics not tell us about the buried remainsof a site? What will it fail to find? The issues atstake can be significant. For example an archaeo-logist might forbid the construction of a factoryon the eastern half of a field, where geophysicshas found a Roman villa, but permit it on thewestern half where geophysics has failed tofind the ephemeral – but archaeologically vital –traces of a Neolithic settlement. The key matterhere is not what geophysics can find but whatit cannot, because, in many such matters ofplanning, it is the risk of failure to find remainsthat is more significant.

This paper is the result of a review ofarchaeological geophysics in the northwest ofEngland commissioned by English Heritage. Itwas prompted by the concerns of some pro-fessional archaeologists, consultants and curators(regional archaeology managers usually workingfor local government), that geophysics did notgive them what they needed – in particular that ittoo often failed to find remains for them to haveconfidence when making complex planningdecisions. As a result some had doubts aboutwhether it should be used at all and many were,in fact, making a conscious decision not to use itdespite its abundant use by colleagues workingnearby. These doubts seem widespread, bothnationally and internationally. Some countriesmake abundant use of geophysics and othersvery little.

The project therefore set out to identify all ofthe surveys that had been carried out in theregion and to analyse those with sufficientevidence to see how effective they had been. Inaddition, discussions were held with almost all of

Copyright # 2009 John Wiley & Sons, Ltd.

the senior professional archaeologists – sur-veyors, curators and consultants – working inthe region to find out how they view the use ofgeophysics and its future potential. On theunderstanding of anonymity some strong viewswere expressed and it became clear that very feware comfortable with some key aspects of theprofessional practice of geophysics in England.This is of wider significance because geophysicsis deeply integrated into routine archaeologicalservices in England and English survey practiceis influential elsewhere, not least through theEnglish Heritage Guidelines for geophysicalsurvey (English Heritage, 2008). This paperdistils both the study of survey outcomes andthe subsequent discussions to consider whetherarchaeological geophysics is doing its job, at leastin this region, and how it might be better used.

The effectiveness of archaeologicalgeophysics

Concerned archaeologists in the north west ofEngland refer to key cases, at Welsh Row,Nantwich for example (Connelly and Power,2005), where geophysics did not find the sub-stantial remains that were later proved inexcavation. Discussions with archaeologists else-where in England, and the wide variations in theuse of geophysics nationally, suggest that thesedoubts are widespread. Yet the use of geophysicsin English archaeology is well established.Indeed statistics in the AIP Database (Archae-ological Investigations Project, 2007) and theexperience of those involved (A. David, personalcommunication) show that it is increasing both infrequency and area of surveys, which suggeststhat it has earned its place despite these doubts.On the other hand, although English curatorialarchaeologists may need to defend their choicesof survey methods before a legal enquiry asreasonable, enforceable and fair, the largedifferences in the choices they make, undersimilar circumstances, suggests that this hasdone little to consolidate a common view of whatmethods should be chosen and why. This allowswide variations in practice, among the effects ofwhich is to undermine the confidence ofobservers in the decisions archaeologists make

Archaeol. Prospect. 16, 77–90 (2009)

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How Effective is Geophysical Survey? 79

about the methods they use. Thus a manager oftwo gravel quarries in adjacent counties may berequired to pay for very different methods ofarchaeological evaluation in each – one with andone without geophysics, for example – because ofthe different preferences of the county curators.

The review began by creating a list (Jordan,2007) of all the geophysical surveys carried outfor archaeology in the northwest of Englandbetween 1990 and 2006, and then analysing anumber in greater detail to assess and evaluatethe extent to which they proved effective. Surveyand excavation reports were obtained from allthe principal commercial archaeologists andgeophysical surveyors and from public archives,including Historic Environment Records. Infor-mation on the method, instrument, date andthe state of the ground were obtained wherepossible. Topographic, geological and soil maps,digital elevation models and aerial photographswere obtained for each site and land-useidentified. Survey and excavation data were thencompared to see how well features found inexcavation had been detected by geophysics.Interviews were held with almost all of thoseinvolved, who were asked about their experienceof the effectiveness of geophysical survey in theregion, and of the way it is applied.

Only 87 of the 188 geophysical surveysidentified had been previously recorded innational databases. Thus the project greatlyimproved our record of work in the region andwould be worth repeating elsewhere for thispurpose alone. Thirty-five sites were selected formore detailed study, 17 of these because therewere good, comparable survey and excavationrecords and a further 18 because they illustratedparticular issues of wider significance.

The use of geophysical surveyin the region

The northwest of England contains a wide rangeof upland and lowland landscapes with variedgeology, hydrology and soils. The region isdominated by two groups of hills, the LakeDistrict and the Pennines, which reach heights upto 1000 m. Much of the south, and the far north,are lowland and low hills. Precipitation over the


hills is high, averaging around 2000 mm per year,but the lowlands receive less than 1000 mmand the climate is generally mild. The geology isvaried. Limestone and sandstone underlie muchof the region, but there are large areas of coalmeasure rocks, saline and calcareous mudstonesin the south and a mix of metamorphic andigneous rocks of widely varying chemistry andphysical properties in the north. Over most of theregion the bedrock is covered by Devensian tilland outwash deposits, the complex and variedcomposition of which reflects the wide varietyof its source rocks and the complex history ofglacial reworking (Taylor, 1971; BGS, 1977, 2007;Aitkenhead, 2002).

The soils are likewise diverse with largeareas of wet, acid upland heath and peat butwith extensive lowland gleys and well-drained,productive soils on the upland fringe and acrossthe lowlands (SSEW, 1984a,b).

This vast diversity of environments imposesvery variable background geophysical propertiesthat contrast with those of southern England.Set within these diverse landscapes are a widerange of archaeological remains, from ephemeralprehistoric ditches to stone-built Roman forts andMedieval abbeys.

Geophysical surveys have been relativelysparse in the northwest compared with therest of England (Table 1, Figures 1 and 2; Darvilland Fulton, 1995; Archaeological InvestigationsProject, 2007). The new records show that themajority have been carried out by magneticfluxgate gradiometry (67%) at a reading spacingof 1� 0.5 m. At the time that the records weregathered (2006) only two surveys had beencarried out using a caesium vapour magnet-ometer, one of which was a small, experimentalsurvey, atypical of commercial practice. Theproportional area surveyed by magnetometeris even greater (86%) because of a relatively smallnumber of very large unrecorded magnetometer‘scan’ surveys (7% by numbers of surveysbut 43% by area surveyed). The average standard(recorded) magnetometer survey covered 3.1 ha,whereas the average magnetometer scancovers 23.8 ha. In practice, unrecorded ‘scan’surveys have been accompanied by smaller –often very much smaller – areas of recordedsurvey.


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Figure 1. Thedistributionofallgeophysicalsurveyssince2000(Archaeological Investigations Project data for England only).Although incomplete, thismap indicates the relative density ofrecent surveys.

Figure 2. The distribution of geophysical surveys in the north-west of England recordedby thisproject.

80 D. Jordan

Electrical resistivity surveys have been com-mon (27%) and much smaller, averaging only1.3 ha. Almost all have been carried out using theGeoscan RM15 resistance meter and a twin-electrode array at a fixed electrode spacing of0.5 m and a reading spacing of 1� 1 m. Finallythere had been only five ground-penetratingradar (GPR) and two magnetic susceptibilitysurveys up to 2006.

Thus the cheapest and fastest techniques, themost widely available instruments and moststandard field methods dominate, as we mightexpect. The proportion of resistivity surveys may


be a little higher than elsewhere in Englandbecause the northwest has benefited from moreresearch and conservation surveys over stone-built monuments, especially along Hadrian’sWall, where resistivity survey might be favoured.The proportion of GPR and magnetic suscepti-bility surveys may be a little lower, perhapsreflecting some conservatism in project design.

Survey success and survey design

The ‘success’ of survey is difficult to define and asensitive issue for commercial surveyors, whoselivelihoods depend on the degree to which theyare thought competent. Almost all of the surveysexamined for this project produced interpretableimages (although not necessarily of archaeo-logical remains) and appear to have beenperformed with professional care. Only a fewhad noticeable striping, zig-zagging or otherartefacts resulting from poor field technique.Where remains existed but were not detected by


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the geophysical survey, excavation subsequentlyshowed that there was some property of the sitethat made it unlikely to have been detectablegiven the technique, instrument and readingspacing used. In most cases buried remains werenot detected either because the anomalies theygenerated were too weak, were detectable overtoo small an area at the ground surface or werein some way obscured by material above orclose by. Thus failure to detect remains appearsto be largely the result of the nature of the sitesthemselves and the choice of methods, sampledensity, instruments and area covered, not of thesurveyor’s field or processing skills.

The project found that the range of methods,instruments and spacings being used is narrow,implying that surveys are not being optimised forthe conditions of the site or the aims of thearchaeologist, a situation with which many feeluncomfortable. Surveyors’ skills are comprom-ised by commercial pressures, which means thatthey often have little time and backgroundinformation with which to design surveys. Thereis rarely the opportunity, in particular, tocombine knowledge of site conditions with adescription of the remains an archaeologist needsto be able to detect in order to model explicitly themethods, instruments and reading spacings thiswill require. Surveyors said that, all too often,those who commission surveys expect them toapply a standard approach prescribed by com-mon practice and published guidelines, evenwhere these might not be optimal. Likewise thedate when survey takes place tends to be setby the requirements of the client, which may,for electrical resistance and GPR survey, beinappropriate.

Some of this conservatism in survey design isself-imposed. Several surveyors said that do notwish to challenge survey briefs they thinkinadequate because they fear that the workwould simply be given to a competitor. Thismight suggest that there is a vicious circle inwhich curators (principally the County Archaeo-logists and their teams) do not adequatelychallenge the designs that surveyors and con-sultants produce in response to the archaeologi-cal need the curators themselves define.

Assessments of survey ‘success’ are compli-cated by the fact that survey results, in creating


new archaeological knowledge about areaswhere we may initially know very little, changethe questions that might be asked. Thus asingle cycle of prospective survey might oftenbe usefully followed by a second cycle whichclarifies detail. Few of the surveys in the north-west were repeated in this way, although somehad been extended, suggesting once again thatgeophysics is being used without sufficientdesign or resources, and as a formula to beapplied rather than as an analytical tool.

Geophysical survey is often part of a packageof data from many sources and its effectivenessmust therefore be considered in terms of theefficiency with which it adds new information.Even where we can be confident that geophysicalsurvey will produce clear images of buriedremains there will be cases where other tech-niques will be just as effective. Thus, for example,both magnetometry and aerial photography areeffective at finding new sites on the gravel soilsaround Oxford, in central southern England, as atRadley Barrows (English Heritage, 1983, 1985;Barclay and Halpin, 1997). Since it is muchcheaper to trace crop-marks from aerial photo-graph archives it is reasonable for archaeologiststo ask why geophysics is needed. In practicegeophysical surveys in the northwest are oftensupported by very little other data and suchcheaper options are not so often available.

Archaeologists said that, as a result, it is crucialthat geophysical survey is effective in the north-west because there are large areas where the onlyalternative to geophysics, in looking for buriedremains, is excavation. The abundance of pas-ture, in particular, often makes aerial photogra-phy and field-walking ineffective. Moreover oneof the County Council Curators said that therelative economic weakness of some parts of thenorthwest can make the cost of evaluationexcavation unacceptable, and thus that geophy-sics is often the only survey method available.Thus the confidence that users have in geophy-sical survey results is crucial to the protection ofremains because there may be no practicalalternative. Surveyors and curators confidedthat geophysics is sometimes commissionednot because they think it will allow them tosuccessfully evaluate a site, but simply becausenothing else is economically possible.


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One might therefore expect that the lack ofalternatives would mean that geophysical sur-veys would be both more extensive and betterresourced than elsewhere in England, takingplace in stages with gradual refinements, exten-sions and repetitions at higher density and withdifferent instruments. This does not, however,seem to be the case.

Case histories

To learn more about the reasons why specificsurveys produced good, or poor, results thestudy looked at some surveys in further detail.Those summarized here illustrate key influenceson survey outcomes seen repeatedly during thereview.

Typical, successful surveys that accuratelyidentified the presence or absence of remains

Middlewich Eastern BypassThis site lies in undulating pasture on poorly

drained soils formed in till over saline marl.Excavation demonstrated that magnetometersurvey had comprehensively located ditchesbeneath 25–30 cm of plough-soil on the top ofan interfluve.

Ulgill, CumbriaThis site is on wet, valley-side pasture,

overlying till derived from Coal Measures.Caesium magnetometry at 1� 0.25 m and twin-electrode resistivity survey at 1� 0.5 m identifiednothing archaeologically significant. Excavationconfirmed agricultural and natural soil ano-malies found by the geophysics and also showedthat there were no archaeological structures thatthe geophysics had missed. Other surveys ofroad routes in the region, along sections of theA66 for example, likewise found large areasapparently without archaeological remains,the absence of which was then confirmed inexcavation.

Maryport Roman fort and VicusThis gently sloping grassland site overlies till

formed principally from the underlying iron-richSt Bees Sandstone. Measurements on nearby


exposures showed a high topsoil magneticsusceptibility, decreasing down the soil profile.Fluxgate magnetometer survey was carried out at1� 0.25 m. The clear and comprehensive surveyimages produced show the fine structure of thesite as strong magnetic anomalies and form anexcellent basis for future conservation.

Unsuccessful or partly successful surveys

Barker House FarmThis site lies in pasture, on poorly drained

lowland gley soils formed in stony till thatproduces a background magnetic field withmany, very localized natural dipole anomalies.Low magnetic susceptibility enhancement in thearchaeological strata coincided with a variablenatural background field to obscure small, butarchaeologically important remains. Thus thefluxgate magnetometer survey failed to locatea narrow ring-ditch known from previousexcavation but did find a second, broader ringditch to the southeast, confirmed in furtherexcavation. Simple, approximate modellingshows that at the standard reading spacing of1� 0.5 m, approximately 280 magnetic gradientreadings will have been recorded above thebroader enclosure ditch and only about 36 withinany likely narrow ring ditch anomaly – nearlyfive times fewer per linear metre of ditch (4.5 m�1

as against 0.96 m�1). This illustrates how thedensity of survey determines, in any givengeophysical environment, not just whetherremains will be detected but what kind ofremains.

Buckleys Field, MiddlewichThis site lies in pasture on wet soils derived

from mudstones. Fluxgate magnetometry wascarried out at 1� 0.25 m, and resistivity surveyusing a 0.5 m twin-electrode array at 1� 1 mintervals. Neither survey identified the abundantremains found in excavation, but, instead, weredominated by large anomalies due to recentdebris in the surface soil and to metal structuresaround the site.

Welsh Row, NantwichThis site lies in an area of flat grassland, on

poorly-drained soils derived from mudstone till.


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Unrecorded magnetic ‘scanning’ was followedby 6 ha of recorded fluxgate magnetometryat 1� 0.5 m intervals. Nothing archaeologicallysignificant was found. Excavation, however,revealed abundant remains, some very large,especially of Roman salt-making industry. Thiswas, however, buried beneath at least 50 cm oftopsoil and alluvium. The largest remains,moreover, were wooden brine tanks containingdeposits very similar to the surrounding alluvia.There was probably, therefore, little magneticsusceptibility contrast between the tanks and thesoil to each side, and a larger contrast withthe soil above, which is likely to have been ofhigher susceptibility. The result would thereforebe a weak anomaly due to the tanks – perhapsdetectable in itself but very difficult to dis-tinguish within the more varying magnetic fieldbackground produced by variations within thetopsoil above.

New Cowper Farm, CumbriaFluxgate and caesium magnetometry, at read-

ing densities up to 0.5� 0.25 m, were carried outon a quartz-sand ridge, and revealed few of thepits and ditches later found in excavation.Analysis showed that the relatively high andvariable magnetic susceptibility of the deeptopsoils masked the much weaker contrastsbetween the archaeological remains and thequartz-dominated sand parent material in whichthey lay.

Manchester AirportFluxgate magnetometer surveys, in pasture

overlying till and Holocene alluvia, foundnothing definitely archaeological. Anomalies inone area, however, that might have had anarchaeological origin, were excavated andfound to be natural former stream courses,potentially valuable sources of palaeoenviron-mental evidence.

Survey outcomes

Of the 35 sites for which detailed records weregathered, 24 produced results that fully met theneed for which they were commissioned. Amongthese were surveys of major, protected sites that


produced clear, comprehensive geophysicalimages on which conservation and managementdecisions could be based. The remaining 11 eitheridentified remains, or the absence of remains,which were subsequently confirmed in excava-tion. Six of these 11 surveys produced equivocalresults. Five of these six missed some archae-ologically important remains, whereas one ident-ified apparently archaeological remains thatexcavation showed to be shallow geologicalstructures. The remaining five of these 11 surveyswere demonstrably unsuccessful. They failed tolocate extensive archaeological remains laterproved in excavation despite the surveys havingbeen carried out to standards and with methodsin line with currently accepted practice.

Thus a high proportion of surveys achievetheir aims. The few outright failures can beattributed to a limited range of circumstances.Further analysis suggests that these often can beidentified in advance and thus might be avoided.

There is one particularly significant finding.Some surveys identified large areas wherenothing, or nearly nothing, archaeological couldbe found. In almost all cases excavation showedthat this correctly reflected an absence or avery low density of remains. For planners,identifying where development will not impingeon archaeology is often a key goal. It is widelyassumed that geophysics cannot be used for this– providing reliable evidence only for the presenceof remains. Thus, in contrast to Hey and Lacey(2001), this study suggests that archaeologistsmay be able to put more weight on negativeevidence in this region than has been thought.Hey and Lacy (2001) argue that geophysicalsurvey should be verified by excavation yet theneed for this must depend on the reliability ofgeophysics as a means to achieve a particularpurpose. It is clearly unwise to rely on any onesurvey approach, where alternatives are avail-able, particularly where commercial pressureto reduce costs encourages archaeologists to cutcorners. Yet where, as is frequently the case, theprimary purpose of survey is to identify if there isa risk of archaeological remains being present,the possibility that geophysics alone may providereliable evidence for the absence of remains issignificant, even if this is only true in specificcircumstances. Although a combination of data


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sources is clearly best – and necessary for criticalplanning decisions – there are many occasions,especially early in the planning process, wherethis is not feasible and thus where the reliabilityof geophysics alone is crucial. Archaeologistsneed not restrict their choices artificially togeophysics or to some alternative, yet wherethis restriction is forced upon them by circum-stances, such as soil type, land-use or economics,their confidence in geophysics becomes key.The crucial factor in the reliability of negativeevidence on the sites studied here is the absenceof reasons why geophysics will not work at aparticular site, and thus for results to beunreliable. This project identified a limited rangeof ‘risk factors’ due to which surveys had notfound remains and which usually can bepredicted by a little preliminary map and fieldstudy and avoided by choosing an appropriatesurvey approach, which may not include geo-physics. It might be expected that the reliabilityof such negative evidence will vary considerablywith the regional geomorphology, parentmaterial types and density of past and presentoccupation. The sample of surveys gathered bythis project is too small to draw firm conclusionsand it would be very interesting to conduct awider study to find out if this is more widely true.

Discussion

The results show that geophysical surveys in thenorthwest of England largely fulfil their purpose,despite the complex geophysical environment.Important remains have been missed in only afew cases (five out of the 35 reviewed). This maygive us confidence that geophysics is alsoproving effective in less complex environmentsacross much of the rest of Britain. The studyfound that most failures result from a combi-nation of a very few, well-understood circum-stances. Thus survey might usefully be precededby a simple map and field study to identify ifthese circumstances are likely to be present.

To summarize, geophysical survey only failedto meet the archaeologists needs where:

(i) R

Copy

emains lay too deeply buried to bedistinguished – more than 50 cm at Welsh

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Row, Nantwich and at New Cowper Farm,Cumbria for example. More deeply buriedremains can be detected geophysically butthe weaker and more dispersed anomaliesdetected are increasingly obscured by othervariations in the soil around them.

(ii) T
he remains are overlain by, or embeddedin, soil containing natural or man-madedebris with a strong geophysical contrast toits surroundings, such as brick fragments origneous stones in finer till.
(iii) T
he geophysical anomalies due to remainsare masked by anomalies caused bynearby surface structures such as (formagnetometry) metal-clad buildings, fencesand vehicles.
(iv) T
he remains and natural soil do not contrastgeophysically. This appears rare. Despite thewide range of geologies, mineralogies andsoil types in which surveys have taken placein the northwest, only on the clean quartzsands of the New Cowper ridge and in thelarge brine-tanks of Welsh Row, Nantwichwas magnetic susceptibility contrast toosmall to allow remains to be found and,even here, alternative methods proved, ormight prove, effective.
(v) T
he remains are too small, in spatial extent,to be detected, given the spacing of geophy-sical readings and the sensitivity of theinstrument used.
In practice, as the above examples show,remains are often obscured by combinations ofthese effects. Thus, although the more deeplyburied remains at Welsh Row and New CowperFarm might be distinguished in a geophysicallyquiet environment, the presence of strongeranomalies, forming complex distributions inthe soil above, can make the pattern they formindistinguishable and thus uninterpretable asarchaeological remains.

Discussions with surveyors suggest ways inwhich we might reduce the risks of such failures,and improve survey practice, with a few simplemeasures.

Preparation

Most of the risks above could be anticipated.Those sites found in this study to be too deep to


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be detected by standard survey methods eitherlay within the flood range of a river, which hadburied them in alluvium, or in coarse, easilyeroded soils, where they were masked bycolluvium or deep cultivation. Sites with suchrisks can be identified from existing mapsincluding the new BGS Parent Material map(Lawley and Smith, 2008) and brief preliminaryfieldwork, given a few geomorphological andsoil criteria and a simple check-list of riskfactors. Likewise a little research on site history,using historical and modern maps, would haveidentified those sites in this study where remainswere obscured by surface debris or nearbystructures.

Sites that contain natural debris (suchas igneous cobbles), which creates a disturbedgeophysical background, are more difficult toidentify in advance from maps alone. Most ofthose in northwest England are likely to lie ongravels or till (as at Barker House Farm) with aproportion of larger stones that have a raisedmagnetic susceptibility and electrical resistivity.Areas of gravel are restricted and may beidentified from existing geological and aggregatemaps. Stony till, however, is common andundistinguished on British geological maps. Suchsites can, however, be identified by an initialwalk-over survey, digging a few spade holes andfilling in a standard check-list on the way. Suchpreliminary surveys have much else to recom-mend them, in identifying the wide range offactors that affect how a survey might best bedesigned. It is not clear, however, who shouldperform them. It might be best if they wereperformed by an experienced geophysical sur-veyor employed, by the project consultant, todesign an appropriate brief. On the other hand,such an additional step in the process of surveycommissioning might prove too costly or causetoo much delay to be acceptable in the context ofBritish commercial archaeology.

Design

Survey briefs stipulating ‘standard’ methods aretoo limiting. A magnetometer reading spacingof 1� 0.25 m is now recommended (EnglishHeritage, 2008), which is a considerable improve-ment over the reading spacing of 1� 0.5 m


commonly used in surveys in the northwestbefore 2006. This was adequate, for example,for detecting typical 1-m-wide ditches againstsimple geophysical backgrounds but inadequatewhere remains are small, anomalies weakand the background complex. The question ofsurvey orientation in relation to the orientationof remains has become more critical as the newstandard for narrower spacings of readingsalong the survey traverse direction means thatsurvey sampling patterns are yet more stronglyorientated. Some of these remains are likely to bearchaeologically important but undetectable atcurrent sampling densities. Thus it might be wiseto abandon the use of ‘standard’ survey readingspacings and, instead, for survey briefs to stateexplicitly what scale of structures surveyors areexpected to be able to find and thus the spacingthis implies in a particular soil environment, witha given level of background variation againstwhich remains must be resolved. Where we haveprior knowledge of the archaeology at a site thisshould already have been made explicit since it iscurrent practice, in England, that all relevantinformation on known remains should beincluded in the survey brief (ACAO, 1993). Thusthe required survey design could be based onwhat more the survey needs to tell us. Where wedo not have prior knowledge of remains (as inmost cases of commercial site evaluation) surveydesigns could be based on local researchrequirements. In England these could be draftedfrom regional research frameworks which pro-vide an agreed list of research priorities. Detect-ing sufficiently small remains to meet a particularresearch need then requires that the surveyorresponds by setting an appropriate surveydensity and uses a sufficiently sensitive surveymethod and instrument identified through anadditional stage of numerical forward anomalymodelling. There is no need for this to becomplex, lengthy or costly if it is made routine– although its precision is bound to be limited byprior knowledge of the site. Indeed, the wider useof modelling has much to recommend it, at thedesign and interpretative stages, particularly ifthe testing of anomaly interpretations againstforward models were, as a common part of thereporting process, able to show how interpret-ations had been arrived at. It remains unclear,


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86 D. Jordan

however, how the geophysical expertise of thesurveyor can be used in this way to design briefsthat are then issued by the curator, who normallylacks the expertise required. Although modellingis bound to be imprecise it may be of particularbenefit if it provides a means by which geophy-sicists and archaeologists can interact, helpingarchaeologists to see their sites in geophysicsterms – as the geophysicist must – by makingexplicit how archaeological stratigraphy is ‘trans-lated’ into distributions of geophysical proper-ties. This might avoid both a misunderstandingof the limits of geophysics and an insufficientappreciation of its potential as a tool for theanalysis of remains, not just their bare depictionas grey shades on a plan.

The narrow range of survey methods andinstruments used in northwest England makes itdifficult to determine how much alternativesmight have contributed to better survey out-comes. Such a calculation is complex. A recentpaper (Linford et al., 2007) indicated that theability of caesium magnetometers to resolvesmaller variations in magnetic field increasesthe range of environments in which magneto-metry can be successful and the range of remainsdetected. Moreover the total field measurementsmade by such instruments offer considerableadvantages in understanding the origins ofthe anomalies detected. The advantages ofresolution might be even greater with the newgeneration of extremely sensitive instrumentssuch as portable SQUID magnetic gradiometers(Schultze et al., 2008). Although this high degreeof resolution will give benefits in some areas ofthe northwest of England, the abundance ofsites formed in magnetically variable substrates(such as stony till) suggests that the limitingfactor in detection will often be the spatial andamplitude scales of the magnetic ‘noise’ thisgenerates, in comparison with the density ofmeasurements taken. The difference between aninstrument able to resolve 0.001 nT and oneonly capable of 0.1 nT is clearly less significantwhere background variations are one or twoorders of magnitude higher at the spatial scale ofnormal survey sampling. Increasing the densityof survey, to better depict the distribution ofsoil ‘noise’ (which is actually unassimilatedinformation), may be fruitful but will not always


help to distinguish the archaeological anomalieswith which such ‘noise’ is melded. Theselimitations may be more widely true. Thespatially complex geology, varied mineralogyand dense pattern of glacial remains impose amuch more varied geophysical background onmuch of the northwest than on the southeast ofEngland. Thus the same issues of redundancy ininstrument sensitivity may well apply to othergeophysical methods and might, likewise, needto be overcome by greater sampling densityrather than more sensitive instruments. This isnot, however, to say that the greater geophysicalresolution of newer instruments is not valuablein geophysically quieter environments whereit can be used effectively. Despite its demon-strable benefits, the time-costs of increasingsample density are considerable, especially ifthis requires the surveyor to cover more groundby reducing the spacing between survey tra-verses. Thus one of the most interesting chal-lenges facing archaeological geophysicists is todesign survey strategies that optimise therecovery of archaeological anomalies by varyingthe spatial density of readings. This is analogousto the optimization schemes that can, forexample, be applied to electrical resistancetomography data gathering (Stumme et al.,2004; Wilkinson et al., 2006), where an initialsample of measurements is used to determinewhat further measurements will add mostinformation. The analysis required in archae-ological survey is rather different, however,because the time-costs of covering ground morethan once are high. Therefore, one of the futureroles of rapid, low-resolution survey may be tomap the distribution of geophysical propertyvariance in order to provide a basis for adaptivebut detailed survey designs.

Analysis

Many surveyors complained that they are notroutinely given reports on excavations thatfollowed their surveys, and therefore they donot have the opportunity to find out how welltheir survey performed, or how it might havebeen better carried out. It seems clear that ifsurveys are to be evaluated and improved thebest people to do it are surveyors. Obviously it


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therefore would be worthwhile for a summary ofexcavation results to be sent to surveyors andthat surveys be routinely evaluated againstremains found in excavation as a required partof archiving the excavation records. This feed-back is particularly important where furthersurvey in adjacent areas might be designed andinterpreted with reference to this previouscomparison of datasets.

Most other professions accumulate evidenceabout what methods to apply in differentcircumstances – which is why we trust surgeonsand airline pilots – using formal methods that areintegrated into routine practice and feed back,directly, into that practice. Replication and selfanalysis is, after all, a defining characteristic ofscience. By contrast, very little archaeologicalfieldwork, geophysics included, gathers objectiveevidence of its own effectiveness. Equallysurprising, perhaps, is that geophysical sur-veyors themselves rarely include in their sur-veys, or describe in their publications, a phaseof measurement replication at different scalesand with different controls, in order to gatherevidence from which the characteristics of thedata itself can be assessed. Thus at presentmost surveys give us no comparable means tomeasure data quality and analyse the sources ofvariability. Where archaeologists have tried toevaluate methods they have usually done so as aspecial exercise and the result, therefore, tends toinvolve few cases – as with the study publishedby Hey and Lacy (2001). Yet the great diversity ofarchaeological remains, and the environmentsin which we find them, requires that we mustformally analyse very many cases in order toextract trends. This suggests that we need to doas other professions and make the analysis of theeffectiveness of our methods, and the sources ofvariance within our data, a routine part of ourpractice. This, in turn, requires that we adoptsurvey standards that include a formal stage ofreplication and analysis.

Mapping the background

Another good reason to make analysis a routinepart of practice is that we have a lot more tolearn about the archaeological and geophysicallandscape. Some of the soil-physical mechanisms


behind archaeological geophysical anomaliesare well understood but our knowledge of thedistribution of the soil properties that determinethese physical behaviours across the landscape isusually too coarse to confidently predict the bestgeophysical survey approach at any particularsite. In looking to improve both the spatialscale and physical specificity of the soil andgeological mapping this requires, archaeologistshold a key advantage; there is perhaps no otherprofession that gets to see so many sites, and somuch soil, at such close quarters. Soil scientistsand geologists are interested in objective obser-vations that archaeologists might make about thesoil horizons and rocks they see because this mayallow them to improve their maps and analyses,and to do so on sites and in areas to which theyhave no access. Moreover geologists are increas-ingly recognizing the importance of Quaternarygeological features that archaeologists oftenreveal in excavation. Archaeological geophysicalsurvey data itself has a value beyond archae-ology, which gives archaeological surveyors afurther resource to exchange. On the one hand,geologists and soil scientists themselves need tounderstand soil geophysical properties, as doarchaeological surveyors, because this is of muchwider relevance to their clients – in the electricitydistribution industry, for example. On theother hand much archaeological survey datacontains evidence of soil property distributionsof an extent and at spatial resolutions that soilsurveyors are rarely able to achieve. Thus thereare good reasons why other earth scientistswant archaeological survey data. There isthe basis for a symbiotic relationship in whicharchaeologists provide soil scientists and geo-logists with all the data they can (simply butcorrectly gathered during routine operations).These earth scientists share the maps andunderlying interpretations they produce, withthis help, to guide the planning and interpret-ation of archaeological studies – geophysicsand aerial photography in particular. Thecreation of the British National Parent Materialmap, now in progress (Lawley and Smith,2008), could form a good focus on which suchsymbiosis might develop in Britain and form atemplate for such mapping collaboration in othercountries.


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88 D. Jordan

Research

In Britain at least there is little research inprogress to improve our key archaeologicalsurvey methods – geophysics, aerial photo-graphy and fieldwalking. Stagnant fundingreflects neither the needs, nor potential, ofprofessional geophysical survey. Although thegrowing output of commercial surveys and thesuccess of journals such as Archaeological Pro-spection give the impression of feverish activity,most commercial reports remain unpublishedand many publications of surveys are descriptiverather than self-analytical and thus do less thanthey might to advance the discipline. This is apity because the abundance of surveys in Britainmeans that we could learn a lot, and learnquickly, if even modest research funding becameavailable.

The wider picture

In the absence of similar reviews it is not possibleto say how relevant this study is beyond thenorthwest of England. Many of the planningissues apply nationwide, indeed Europe-wideand worldwide under international conventions.The diversity both of the archaeological remainsand of the natural environment in the northwestsuggest that the practical lessons are widelyapplicable. In particular the five factors limitingsurvey effectiveness are common worldwide,although their frequency and degree of influencewill vary with local geology, soils, archaeologicalremains and planning context. Thus there arereasons to think that much of the foregoingdiscussion is relevant more widely. There canhardly be a region or a type of monumentanywhere in the world where archaeologicalgeophysical survey has not been successful, yetthe difference in the degree to which it is routinelyused, and thus its impact on professionalarchaeology, in different countries is striking.Some of this is the understandable product ofdifferences in survey effectiveness and practi-cality in different geologies and landscapes.Magnetometry is less able to resolve smallanomalies in ‘noisy’ Scottish gravel soils full ofigneous cobbles as on the A74 survey (Banks,


2006) than in the uniform loess plains ofGermany, as at Xanten (Scollar, 1971) and othercentral European sites (such as examples inNeubauer, 2001), for example. It is less practicalto survey in jungle than in an English pasture.Needs likewise vary. In regions such assome parts of Spain – Jaen, for example (Zafrade la Torre, 2006), archaeological remains andrecent development largely coincide intown centres. Here those geophysical techniquesdominant in Britain, which excel in physicallyuncomplicated rural sites, are less useful, produ-cing ambiguous images in circumstances whereclarity is particularly necessary. By contrast, incountries going through periods of rapid infra-structure development – such as motorways inIreland or gas pipeline networks in Britain to taketwo recent examples – there is a period duringwhich rapid evaluation survey by geophysicsover large areas can adopt a central role incircumstances to which it is well suited. Yet theplanning and commercial context of archaeolo-gical control is clearly vital in determining wheregeophysics is actually applied. In France andSwitzerland, for example, the rapid developmentof the motorway network over recent decadeshas not been accompanied by the widespread useof geophysics, at least in part because it has beenargued there that the total excavation of suchroutes makes geophysics redundant. In Switzer-land geophysics may now find a new role as, withthe completion of the motorway network, thefocus of professional services swings backtowards enhancing the wider archaeologicalrecord and focuses less on specific, high-impactprojects.

A further constraint may be a lack of surveyorsand, in less developed regions, of equipment.This may be compounded by an associated lackof university departments providing trainingand support. In Britain this is rarely an issuebecause equipment is available and geophysics isfamiliar to almost all professional archaeologists,through university training, personal experienceand television programmes. Elsewhere, despitethe abundance of geophysical expertise inuniversities and industry, archaeologists havemuch less personal experience of its use andaccess to the equipment and skills is moredifficult – circumstances that are self-perpetuat-


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Table 1. Summaryofgeophysical surveys in thenorthwestof England to 2006 ^ types of surveys and areas covered(values rounded towhole numbersof hectares)

Technique Numberofsurveys

Areasurveyed (ha)

Magnetic scanning 9 214Magnetic susceptibility 2 22Magnetometry 77 217Resistivity 34 46Ground-penetrating radar 5 1Total 127 500


ing. Thus, if archaeological geophysics becomesmore widely taught in university and equipmentis made more widely available, the growingfamiliarity of archaeologists with its potential,worldwide, may lead to its wider use.

Yet, as in the northwest of England, thereremain significant doubts as to the value ofgeophysics even where archaeological servicesare sophisticated and well funded, in Britain andbeyond. What lessons may we therefore drawfrom this study of relevance to archaeologicalgeophysics elsewhere? The first, perhaps, is thatthese widespread doubts about the value ofgeophysics may originate, at least in part, fromthe same lack of appraisal – both of geophysicalsurvey itself and of its performance in compari-son with alternatives – in other countries as wellas in Britain. An objective appraisal of pastgeophysical surveys in France, for example, andthe great depth of relevant geophysical expertise,breadth of research and innovation in relatedfields among French geophysicists, might per-suade an independent observer that geophysicsdeserves a much larger role in French archae-ology. Might it be, therefore, that the wider use ofgeophysics will benefit from a similar appraisalof its potential, extending that undertaken by thisstudy? Might not sceptical archaeologists bemore willing to make selective use of geophysicsif surveyors could assess previous surveys toidentify where it will and will not producewhat archaeologists need – and do so, at leastsemi-quantitatively and with relative costs, incomparison to alternatives such as excavation,fieldwalking and aerial photography?

Beyond this, the same constraints on theobjective use of geophysics that this studyidentifies in the northwest of England appear


to be widespread. Thus the same benefits mayaccrue from the wider use of design based onforward models, of replication to assess sourcesof variance in data and of a formal evaluationof the effectiveness of survey and the causesof anomalies. Close collaboration between geo-physicists and geoarchaeologists is thereforeessential.

Acknowledgements

This project was funded by the Aggregates Levyunder the management of English Heritagewhich is gratefully acknowledged. Particularthanks are due to the English Heritage geophy-sics and geoarchaeology staff and the manyCurators, Consultants, Survey Contractors andDevelopers who gave very generously of theirtime: Matt Canti, Neil Linford, Andrew David,David Batchelor, Sue Stallibrass, Jim Williams,Ingrid Ward, Jennie Stopford, Sarah Cole, TonyWilmott, Mark Leah, Jill Collens, Sarah-Jane Farr,Mark Adams, Ron Cowell, Norman Redhead,Mike Nevell, Rob Iles, Peter McCrone, JeremyParsons, Mike Morris, David Mason, RichardNewmann, Jeremy Parsons, Jo Mackintosh, JamieQuartermaine, Alan Lupton, Emily Mercer, KarlTaylor, Brian Grimsditch, Phillip Day, MarkNoel, Anne and Martin Roseveare, Peter Barker,Alister Webb, Alan Biggins, Ian Brooks, RussellColman, Peter Stephenson, Nick Edwards, BobBewley, Alister Bartlett, Frank Harkness, FrankGeicco, Tim Malim, Ben Stephenson, MarkFletcher, Chris Gaffney, John Gater, John Dear-ing, Frances Tattum, Patricia Crompton, ArminSchmidt, Charles Parry, Chris Jones, Roy Can-ham, David Knight and Simon Buteux. Apologiesto any that have been missed.

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How effective is geophysical survey? A regional review