A PRELIMINARY ASSESSMENT OF THE USE OF ROUTINE GEOPHYSICAL TECHNIQUES … · physical techniques...

IntroductionThe routine application of archaeologicalgeophysical survey techniques is wellestablished as a means of locating, map-ping and aiding the interpretation ofarchaeological sites in many areas of theworld, over a variety of landscapes andgeology (David, 1995; Boucher, 1996).However, one area where such prospec-tion has been neglected to date is Iceland,

probably as a result of a number of con-tributing factors, including the type andcharacter of both the archaeology andgeology, in addition to the inhospitablenature of many parts of the country.

This paper describes a preliminaryassessment of the integrated use of mag-netometer and earth resistance surveysundertaken in Iceland during the summerof 1999, as part of a wider study of the

TIMOTHY J. HORSLEY & STEPHEN J. DOCKRILL

A PRELIMINARY ASSESSMENT OF THE USE OFROUTINE GEOPHYSICAL TECHNIQUES FOR THE

LOCATION, CHARACTERISATION AND INTERPRE-TATION OF BURIED ARCHAEOLOGY IN ICELAND

Geophysical survey techniques are well established to provide information about archaeo-logical sites in many parts of the world, over a variety of landscapes and geology. However,one area where the application of systematic prospection has been neglected is Iceland,where a particular set of geomorphological and archaeological problems are present. Thispaper presents the results of the first systematic and integrated application of geophysicalsurveys in Iceland. High-resolution magnetometer and earth resistance surveys have beenundertaken at eight archaeological sites, to obtain geophysical results for a range of featuresand geological conditions. The results of these geophysical surveys have been integrated with other sources of archae-ological evidence to allow a proper assessment of their success for not only locating buriedremains, but also for their characterisation and interpretation.Despite the potential limitations of igneous thermoremanent effects, tephra deposits andperiglacial phenomena, the project has successfully demonstrated the ability of these rou-tine techniques for archaeological prospection.From this evaluation it is possible to provide strategies for the future use of such geophys-ical techniques as part of an integrated study of archaeological remains in Iceland, and topinpoint areas where further research is necessary.

Timothy J. Horsley and Stephen J. Dockrill, Department of Archaeological Sciences, University of Bradford, UK

Keywords: church; earth resistance; evaluation; farm; geomorphology;gradiometer; igneous; magnetometer; medieval

ARCHAEOLOGIA ISLANDICA 2 (2002) 10-33

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archaeological monuments and land-scapes.

General principles of geophysicalprospectionFor the prospection of archaeologicalsites a great number of different geo-physical techniques and instrumentsexist, each with their own capabilitiesand limitations. Most techniques work bydetecting a contrast in the material prop-erties of the subsurface, producing a geo-physical anomaly. On an archaeologicalsite each method can have differentpotential depending on the nature of theburied archaeology, geology and possiblyclimate and land-use.

Following data-collection it is thennecessary to determine the causativebody, and to interpret this as an archaeo-logical feature (Horsley 1998, 17).

Geophysical surveys suffer from aninherent ambiguity in the conclusionsthat can be drawn, as many different sub-surface configurations could reproducethe same observed measurements(Kearey and Brooks 1984, 8-9). It isoften the case that processing is requiredto aid in the identification and interpreta-tion of anomalies in a survey, but alteringthe raw data in any way must be under-taken with care.

Much has been written about the differ-ent geophysical techniques routinelyemployed in archaeological surveying, inparticular the two methods employed forthis evaluation: fluxgate gradiometry andearth resistance methods (see Clark,1975; 1990; Keary & Brooks, 1984;

Scollar et al., 1990, Telford et al. 1976,among others). A full background to thetechniques and theoretical details will notbe given here, and the reader is advisedto look at these works, and to the guide-lines for the use of geophysical tech-niques in archaeological field evaluationsprovided by David (1995) and Gaffney etal. (1991).

Archaeological prospection in IcelandIn recent years, Fornleifastofnun Íslands(FSÍ) has undertaken an interdisciplinaryinvestigation of the settlement of Iceland,including topographical surveys of extantearthworks, chemical surveys and exca-vation (Friðriksson & Vésteinsson,1998a, 1998b). However, geophysicalprospection as an additional techniquefor site location and interpretation hasnever been systematically applied inIceland, either on its own or as part of anintegrated strategy.

Archaeological features are notalways represented on the surface, buteven where earthworks do exist, theirmorphologies may appear ubiquitous anddefy attempts at qualification or charac-terisation (Dockrill and Gater, 1992).Geophysical surveys would be an effec-tive and unique tool in aiding this multi-disciplinary research in Iceland, in thenon-destructive characterisation andinterpretation of archaeological sitesidentified by other methods, or in locat-ing new sites in its own right.

Before this can happen however, aproper and systematic assessment of thetechniques in this new environment isnecessary to allow an understanding of

A PRELIMINARY ASSESSMENT OF THE USE OF ROUTINE GEOPHYSICAL TECHNIQUES FOR THE

LOCATION, CHARACTERISATION AND INTERPRETATION OF BURIED ARCHAEOLOGY IN ICELAND

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limitations of the methods, and the con-ditions that make a site suitable for sur-vey work.

Iceland presents a particular set of geo-morphological and archaeological prob-lems for the case of geophysical prospec-tion, and it was the aim of this project toevaluate the success of such methods forthe location and interpretation of buriedarchaeology.

This research has investigated what,at present, appear to be the most impor-tant of these limiting factors for Icelandicgeophysics: soils, geology, geomorphol-ogy and archaeology, although it is acombination of these and more whichproduce the complex result obtained.

Igneous geology and tephraThe principle constraint identified witharchaeological prospection in Iceland isthe nature of the geology of the island:being volcanic and hence igneous, thiswill have a effect on magnetic surveysundertaken over such bedrock (Clark1990, 92-4; David 1995, 10).

A serious complicating factor withigneous material is the presence of a ther-moremanent magnetisation, acquiredwhen the rock first cooled (Burger 1992,412, 438; Clark 1990, 92). This geologi-cal thermoremanance will produce anintense response, far greater than that dueto archaeological deposits.

Igneous rock will also be present inglacial erratic material and, having beendisplaced, the magnetic directions of therocks are randomly jumbled, which canproduce 'noisy' background signals thatobscure archaeological anomalies (Clark

1990, 94).However, prior to this study no mag-

netometer data has been systematicallycollected over archaeological sites inIceland. Little work in general has beendirected into the effect of igneous parentmaterial on archaeological prospectionanywhere, and to date there have been nocomprehensive studies into the effects oftephra deposits on magnetic surveys.

Geomorphic processes in IcelandWhen compared to Britain a number ofunusual geomorphic processes are activein Iceland that have implications forarchaeological prospection. This field-work has revealed that for the results ofgeophysical surveys, geomorphologicalinfluences are as important as geologicalfeatures.

When the soil temperature dropsbelow 0ºC, the transformation of soilwater into ice, results in a markedincrease in the overall soil volume. Theresulting stresses bring about fragmenta-tion, compaction and deformation of thesoil constituents. These mechanical dis-turbances generate specific featureswhose nature and degree of developmentare related to the intensity of the frost, thewater content and the characteristics ofthe soil materials (e.g. soil texture andporosity) (Courty et al. 1989, 160). It isclear that several periglacial processestend to separate coarser and finer parti-cles in soils, and repetitive freezingfavours the fragmentation of coarse ele-ments; after some time the initial charac-teristics of archaeological soils may havebeen irreversibly altered (Ibid. 1989,161; Bird 1974, 720).


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Currently no work has been done toassess the effect of such periglacial phe-nomenon as patterned ground, involu-tions and frost hummocks (thufur) ongeophysical prospection.

Icelandic soilsFor this project the available informationregarding Icelandic soil types was fairlylimited. This lack of pedological andgeological information does not preventgeophysical survey, but is crucial in theunderstanding and interpretation of theresults. It is hoped that details relevant togeophysical surveys might have beenrecorded during vegetation surveys, andthat this resource could be made use of infuture work.

Climatic conditions have a seriouseffect on the soils in Iceland where theyslow down the rate of soil formation(Gerrard 1985, 81). Coupled with preva-lent soil erosion this obviously presents aproblem for the preservation of archaeo-logical remains, and also for geophysicalprospection. In one place, soil erosionmight be so severe that wind erosion hasexposed the archaeology or removed italtogether. Elsewhere, this soil will havebeen deposited, possibly burying remainsbeyond the detection limits of mostprospection techniques.

Icelandic archaeologyIceland additionally provides an unusualarchaeological situation: geologicaldeposits may both predate and post-datearchaeology remains, since buried fea-tures may be sandwiched between parentmaterial and tephra deposits. It is thus ofinterest to learn the effect of these cir-

cumstances on archaeological prospec-tion.

There is also the nature of the archae-ology to consider. Many excavationshave revealed structures not built instone, but of turf sods, sometimes withstone facings.

Is geophysical prospection capable ofdetecting buried turf remains within thecollapse and back-fill of more turf andsoil? Surveys might only be able to iden-tify areas of activity, such as hearths ormiddens, but if combined with other evi-dence such as earthwork analysis, thiswould still be new and useful informa-tion for site interpretation.

As stated in the introduction, only verylimited and certainly no systematic,assessment of geophysical surveys forthe prospection of buried archaeologyhas been conducted in Iceland. Indeed,such methods have only been employedin a handful of instances prior to thisstudy, in each case focussing solely onanswering specific archaeological ques -tions, not to fulfil a systematic assess-ment of the methods. One of the weak-nesses of these targeting approaches hasbeen the lack of proper understanding ofthe geophysical techniques employedtogether with a high working knowledgeof geophysics and a sound appreciationfor the archaeological and geologicalanomalies likely to be encountered.

It is important that the factors out-lined above are fully understood beforesuch geophysical methods can effective-ly be provided routinely and as a service.

It was the aim of this investigation toprovide a preliminary assessment of the



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use of routine geophysical prospectiontechniques as part of an integrated studyof the archaeological monuments andlandscapes of Iceland.

Site selectionThe opportunity to investigate sites inIceland was the result of collaborationwith the FSÍ, to whom acknowledgmentis made for their enthusiastic inclusion ofgeophysical surveys with their own sea-son of work. Working closely with theFSÍ has allowed geophysical prospectionto be integrated with their own archaeo-logical evaluations; in some instancesparts of the survey areas were subse-quently excavated, providing direct feed-

back for this assessment. In total, eightsites were investigated in this study, thelocations of which are shown in Figure 1.Three of these, (Skálholt, Gásir andHofstaðir), have been selected as casestudies for discussion in the context ofthis paper, as they illustrate the problemsand potentials of such surveys.

Fieldwork methodsAt each location an area of archaeologi-cal remains was targeted for survey,known either from surface evidence inthe form of earthworks, or from docu-mentary sources. A grid of 20m x 20msquares was then established over theinvestigation area and, where time


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Figure 1. Sites investigated in 1999.1. Neðri Ás; 2. Þingvellir; 3. Nes; 4. Hólar; 5. Skálholt; 6. Silastaðir; 7. Gásir; 8. Hofstaðir.

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allowed, this grid was surveyed with bothmagnetometer and earth resistance tech-niques.

Earth resistance surveysEarth resistance surveys were conductedusing a Geoscan RM15 resistance meterusing the twin probe configuration with amobile probe separation of either 0.5m or1.0m. For surveys with a mobile probeseparation of 0.5m, readings of apparentresistivity were collected at 0.5m inter-vals along traverses spaced 0.5m apart.For the grids surveyed with a 1.0mmobile probe separation, readings wererecorded at 0.5m intervals along travers-es 1.0m apart. These high resolutionswere chosen so as to gain the maximuminformation within time constraints.

Fluxgate gradiometer surveysThe magnetometer employed for thisstudy was a Geoscan FM36 fluxgate gra-diometer. Readings were recorded at thehigh resolution of 0.25m intervals alongtraverses spaced 0.5m apart, to provide ahigh level of detail. This was made pos-sible by the use of a sample trigger con-nected to the instrument, which automat-ically records a pre-set number of meas-urements at regular intervals.

It was decided to collect data within agrid by walking in the so-called 'zigzag'fashion, as opposed to 'parallel'. In thisway the direction of travel alternatesbetween adjacent traverses to maximisesurvey speed, allowing a greater area tobe covered. However, the magnetometeris always kept facing in the same direc-tion, regardless of the direction of travel,to minimise heading error (Horsley,

1998). It is acknowledged that collectingmeasurements in this way can producedefects in the data. 'Bunching' within thedata is caused by differences in the han-dling and orientation of the instrument inalternate traverses causing a pronouncedstriped effect parallel to the traverses(David 1995, 19).

Data processingGeophysical data often benefits from theapplication of pre-treatment and process-ing techniques, specifically applied toremove background noise or to correctfor collection defects. For this purpose adedicated computer program is oftenused to process a survey data set and aidin its interpretation. The packageemployed for processing and display ofsurvey data for this assessment was thededicated Geoplot software, of whichboth Geoplot 2.02 (© 1994 GeoscanResearch) and Geoplot 3.0 (Beta versionGPW3045R © 1999 Geoscan Research)were used.

The SurveysGásir, Eyjafjörður Gásir, near Akureyri on the west coast ofEyjafjörður, was an important tradingcentre in medieval Iceland, dating tobefore 1400 according to documentaryevidence (Friðriksson 1994, 95). Visitorsto the site today are presented with agreat number of earthworks - the remainsof trading booths that stretch along a nat-ural harbour. Another circular earthworkto the west, at a distance from the booths,may indicate the site of a church andchurchyard, although the exact nature of



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these remains has never been confirmedthrough archaeological evidence (Ibid.1994, 96-7).

In 1907, Daniel Bruun and FinnurJónsson excavated four of the booths andin the building interpreted as the proba-ble church (Bruun & Jónsson, 1908;Jónsson, 1908; and Bruun, 1928). Morerecently, in 1986 Margrét Hermanns-Auðardóttir excavated four trial trenchesin different areas of the site, including aninvestigation of the church (Hermanns-Auðardóttir, 1987).

The remains at Gásir present an idealsituation for the assessment of geophysi-cal techniques since the surface featuresallow the anomalies detected to be readi-ly compared with known archaeology.Gásir poses many archaeological ques-tions and it was hoped that these surveysmight also provide new informationabout the site. Aerial photographs of thesite clearly reveal the large survivingearthworks, however the question ariseswhether the limit of these remains accu-rately indicates the limit of buriedarchaeology. Geophysical surveys mightreveal if activity continues in the areasoutside the earthworks, in addition toproviding archaeological information forthe amorphous earthworks.

The bedrock in the region consists ofbasalt strata formed during the Tertiaryperiod, 5-10 million years ago(Hallgrímsdóttir 1997, 2), which out-crops on the higher ground at the site.The land in which the remains are situat-ed is given over to horse pasture and iscovered with well-established thufur, upto 0.5m in height.

A grid, 40m x 100m, was established

to included the churchyard, a number ofbooth remains, and an apparently'archaeologically quiet' area in between.In this way the geophysical responses toa variety of features could be assessed.

Magnetometer resultsThe results of the magnetometer data(see Figure 2b for a processed plot) haverevealed a number of interesting anom-alies. The most striking of these are visi-ble at the western end of the survey area,where a circular positive anomaly con-taining a rectilinear positive anomaly hasbeen detected, coinciding with theremains of the church and churchyard.Other intense anomalies in this regioncorrespond to the areas where the basaltbedrock outcrops, and are interpreted asbeing of geological origin.

The area of booths also reveals itselfas an area of broader effects, with someintense positive anomalies following reg-ular shapes. This contrasts to the zonebetween the booths and churchyardwhere it is still quite magnetically noisy,but the anomalies are less intense andsmall scale.

Figure 2c presents an interpretation ofthe main anomalies recorded. The bankof the churchyard boundary appears as apositive linear magnetic anomaly, insome places there are two of these linearanomalies parallel to each other. Thesemight be the response to magnetic rockswithin the bank, possibly as stone fac-ings, which have not previously beenrecorded. Similar anomalies are seen inthe area corresponding with the churchstructure. As concluded by Bruun andHermannsdóttir, the church was con-


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structed over a stone foundation (Bruun,1928; Hermannsdóttir, 1987), and theseresults indicate that the magnetometer isresponding to individual, near-surfaceigneous rocks.

The results from the area over boothsare at first sight not entirely clear,although with comparison to the aerialphotograph (Figure 2a), many of the pos-itive anomalies coincide with the highbanks.

The data also indicates quite a cleardifference between the booth area and theregion between this and church earth-works where no surface features exist.This might confirm that booths were notpresent in this area.

Earth resistance resultsOnly a small area over the churchyardwas surveyed with the earth resistancemeter, however the results show greatpotential for future use of this method atGásir. The bank of the churchyard hasbeen detected as a low resistance anom-aly, although such a feature might beexpected to produce a high resistanceresponse due to the better drainage. Lowresistance anomalies over turf banks andmounds were detected elsewhere inIceland in this survey (e.g. Neðri Ás -midden, Nes - farm mound), and impliesthat the turf used in their construction iswater retentive. This would result in thethick turf deposits found in such featuresto be better electrical conductors than thesurrounding soil. Curiously, there arebands of very low resistance linearanomalies within the bank, roughly coin-ciding with the positive magnetic anom-alies thought to be due to rocks. However

there are no positive resistance anomaliesindicating buried stones. These lowerresistance anomalies might be due tobands of more-retentive turfs, althoughthe underlying cause for these results canonly be confirmed by excavation.

Both survey techniques have been influ-enced by the presence of frost hum-mocks, or thufur, a periglacial phenome-non that distorts the ground surface. Theyare visible on the plots as areas of back-ground noise, although the effect is moreintense in the earth resistance data. Atpresent the degree to which these hum-mocks affect the results of an earthresistance survey is not known.

ConclusionGeophysical surveys at Gásir have onlybeen undertaken over a small sample ofthe site, yet demonstrate the potential ofthese method for providing archaeologi-cal information for this site and for simi-lar features elsewhere.

Such prospection could be used toprovide evidence for the use of some ofthese booths, and possibly locate areas ofindustrial activity. By surveying a muchgreater area, it should be possible todetermine the extent of the booths, andhence define the full extent of this impor-tant site.

Skálholt, ÁrnessýslaSituated in the lower part of theBiskupstungur valley between the riversHvítá and Brúará, Skálholt is one ofIceland's places of special historicalinterest. Within two centuries of the set-


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Figure 3: Plan of the Cathedral and farm at Skálholt, dated 1784 (After Ágústsson 1989, 296).

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tlement of Iceland, the first bishopric wasfounded at Skálholt in 1056 for SouthIceland, and soon a second at Hólar in1109 for North Iceland.

When the modern cathedral buildingwas constructed in 1954 there was greatarchaeological interest in the remains ofthe medieval Cathedral (Ólafsson 1988,7-9), yet very little archaeological workhas been undertaken into the associatedfarm at Skálholt. A 'map' of the Cathedraland farm complex dating to 1784 sur-vives (see Figure 3), although compari-son with the few features that are still vis-ibly today reveals that it is not an accu-rate plan. However, this plan does pro-vide an indication as to the farm size,number of buildings, and even details theuse of each room at that time.

Geophysical surveys were undertak-en immediately to the south of the mod-ern cathedral building, over land ownedand maintained by the Skálholt ChurchTrust.

The bedrock of this region isPleistocene lava (Hallsdóttir 1987, 3),however it is not clear which tephra lay-ers are present at the site.

The documentary evidence for thefarm makes Skálholt another ideal sitefor an assessment of archaeologicalprospection techniques. Today only thetwo sunken streets and huge middenmounds are visible on the surface, yetparticular buried remains can be expect-ed allowing a useful evaluation with theadditional benefit of providing newinformation about the history of Skálholt.

Magnetometer resultsFigure 4 shows that in general, a high

level of small-scale magnetic noise hasbeen detected, interpreted as being due toindividual buried rocks, possibly as arubble spread from buildings which oncestood in this area. Not visible in the dataare the intense regular anomalies detect-ed at many other sites (see Gásir (Fig. 2b)and Hofstaðir (Fig. 6a)) and attributed asthe magnetic response to the strong geo-logical thermoremanence. This may bedue to a thick layer of wind blowndeposits on top of the bedrock, therebycausing a greater distance between theparent material and the surface and dra-matically reducing the anomaly intensi-ties. Off-site augering had not hitbedrock at the auger's maximum depth of1.15m. It is believed that this reducedgeological input has allowed a number ofpositive magnetic anomalies to stand outfrom the background noise and look dis-tinctly like the expected responses toburied archaeological features. Thesemagnetic anomalies are the first seenduring this assessment in Iceland, whichdiffer from the responses to geology orrocks, and are therefore very interesting.Many dipole anomalies are still present,but there are positive and negative linearanomalies in addition to broader areas ofpositive magnetisation. Due to the factthat the general level of magnetic noise isstill fairly high, it might be expected thatstrongly magnetic features, such as accu-mulations of burnt material, are the causeof these anomalies. This hypothesis isbacked up by a soil sample collected at adepth of 5cm in one of the positive fea-tures. This consisted almost entirely ofpeat ash, and later laboratory measure-ments produced an enhanced magnetic


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susceptibility. (594 compared to a back-ground value of 170 x10-8m3kg- 1).

Linear anomalies, composed of manydiscrete magnetic dipoles and runningalong and parallel to the southern edge ofthe survey area, coincide with the stone-lined sunken lane in this area, and aretherefore interpreted as the responses toindividual rocks used in its construction.

Earth resistance resultsThe earth resistance survey has been verysuccessful, clearly locating a great num-ber of linear high resistance anomaliesinterpreted as being due to the buriedstone foundations for a great number ofbuildings.

The results, presented in Figure 5,clearly show a wealth of archaeologicaldetail in this small 60m x 60m area. Alarge number of rectilinear anomalies arevisible, the majority of which are inter-preted as the responses to buried structur-al remains. Other less intense anomaliesmay be due to compacted floor featuresor paths

The mound on which the Cathedralsits is seen as an area of higher resist-ance, probably indicating its compactednature, yet there are anomalies visiblewithin it, which might be due to its con-struction. (The intense linear anomaly isdue to the re-roofed tunnel, the top ofwhich is only a few centimeters belowthe surface.)

The southern survey grids are also ofa higher resistance, coinciding with thearea of long grass, although it is not clearif this is related in any way.

A full and detailed archaeologicalinterpretation will be included in the

forthcoming geophysics report forSkálholt.

While these results can be compareddirectly with the site plan of 1784 (Fig.3), it should be remembered that the geo-physical techniques are not detecting asingle phase, as represented in the map.The survey results will detect all thephysical contrasts in the soil within thelimitations of the technique, which maybe due to many archaeological phases.However, comparison with the plan doesreveal a remarkable correlation betweenthe buildings and the high resistanceanomalies, although on a slightly differ-ent orientation to that implied by theplan.

Geophysical surveys at Skálholt havedemonstrated that these techniques arevery successful in detecting buried fea-tures at the site, for which there is no sur-face evidence. It appears that intensegeological and geomorphological effectsseen at many of the other sites in Icelandare less dominating here, allowing themore subtle archaeological anomalies tobe identified.

Interpreted on their own, the earthresistance data reveals a number ofanomalies typical of stone wall founda-tions, and when combined with the docu-mentary evidence for the approximatelocation of farm buildings, this interpre-tation can be confirmed.

The results of the fluxgate gradiome-ter are less clear, however a number ofmagnetic anomalies have been detectedwhich are attributed to archaeologicalsources. Not only does an area of mag-netic noise roughly coincide with the


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building remains revealed in the earthresistance survey, but linear positiveanomalies also reveal buried features intheir own right. Some of these anomaliescan be compared with anomalies in theearth resistance survey and demonstratethe complementary nature of these twosurvey methods.

Hofstaðir, MývatnssveitThe farm of Hofstaðir in North EastIceland has been a focus of archaeologi-cal interest throughout this century, inparticular on the Viking Age great hall(skáli) long considered a temple or 'tem-ple-farm' site (Friðriksson andVésteinsson, 1997a).

Since 1996, a Field School has beenheld at Hofstaðir, run jointly by the FSÍand NABO (The North AtlanticBiocultural Organisation), and in 1999provided the opportunity for geophysicalsurveys to be fully integrated with theirown investigations.

Hofstaðir is located within the valleyof the Laxá river in Mývatnsveit, NEIceland, centred on Grid Ref.461488/568107 (Lucas, 1998). Theregion around Lake Mývatn lies on theÓdáðahraun lava fields (Hjálmarsson andAstridge, 1998). The proximity of thefarm to Mývatn and the Mid-AtlanticRidge means that this basalt will be quiterecent, less than 10,000 years old (Ibid.1998).

Although the area of the presenthomefield is generally level, both exca-vation and geophysical evidence atHofstaðir show that the depth of soildown to the geology is quite varied. Inone place, augering revealed that solid

rock was only 0.2m below the surface,while the 1999 excavation of the pit-house only c.60m away, extended to adepth of almost 2m and had not hitbedrock.

Archaeological deposits at Hofstaðirare sealed between layers of aeoliandeposits, including sands and tephras(Sigurgeirsson, 1998; Simpson et al.,1998). The main tephra layers associatedwith cultural deposits at the site includethe Landnám tephra, formed in 869-873AD, H-1104/1158, V-1477 and V-1717(Sigurgeirsson 1998), and a summary ofthe tephrochronological studies at the sitemay be found in this reference.

“Today, the farmland at Hofstaðir isused for growing hay, usually cut anddried in the first weeks of August. Turfcutting has taken place on the site inrecent years” (Jónsson and Jónsson, pers.comm.).

A variety of surface features exist atHofstaðir providing some evidence forthe subsurface archaeology. Theseinclude the walls of the Viking-Age long-house, the farm mound, turf boundarybanks and other slight earthworks indi-cating the sites of former structures asso-ciated with the farm. Geophysical inves-tigations were undertaken over many ofthese features, although only the resultsfrom the survey over the farm mound andthe area immediately to the east are pre-sented here.

Still in use, the present day homefieldis relatively flat and free of thufur,although in places there are bands of par-allel ruts, similar to those caused byploughing, but may be artefacts of turf-


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cutting (Friðriksson, pers. comm.).Around the eastern perimeter of the farmmound the ground is quite disturbed bymany well-formed thufur, and at the timeof survey this area stood out due to thebright yellow flowers of numerous but-tercups. The rest of the farm mound isnot mown due to the uneven ground, andas a result is colonised by well-estab-lished grasses. The western edge of thefarm mound roughly coincides with themodern track that passes over the top,although it is not known whether buriedstructures exist on this western side.

One of the archaeological aims of thegeophysical surveys was to attempt tolocate the remains of a church known tohave existed on the eastern side of thefarm mound. There are no surface indica-tions for the location of this or a church-yard.

Magnetometer resultsA processed plot of the fluxgate gra-diometer survey is presented in Figure 6.The data was collected in the 'zig-zag'fashion and as a result the plots sufferfrom quite severe bunching effects. Thisis especially noticed around the intenseanomalies caused by the backgroundgeology. The data would be free of thisdefect had a 'parallel' approach beenadopted, but it is the small-scale jumblednoise detected which is of interest, andthe striping does not detract from thisarchaeological information. Walking'zig-zag' allowed a greater survey area tobe covered at Hofstaðir, however it isrecommended that future magnetometersurveys be conducted in a different man-ner.

As the previous surveys have shown,the intense geological anomalies limit thetype of archaeological features that canbe detected, and it is often the smallerscale jumble of magnetic dipoles due toindividual rocks that provide usefulinformation. This is certainly true ofthese results. During data processing, alow-pass (Gaussian) filter was employedin an attempt to remove the bunching dis-cussed above, however it was found thatthis also reduced the small-scale detail,resulting in a loss of information.

The interpretation (fig. 6b) simplyindicates the areas of small-scale mag-netic noise, and the identifiable anom-alies of the modern track and two buriedpipes. The general area of magnetic noisecorresponds well to the area of the farmmound, and it is interpreted that the moredense areas are due to clusters of rocks,and might therefore indicate the sites ofburied structural remains. Some rectilin-ear features can be made out within thenoise, but it is difficult to make any firmconclusions.

A large area to the east of the farmmound is free of this noise, and while thismight be indicative of a lack of looserock debris in this area, it cannot be con-cluded that it is free of any archaeologi-cal activity. Subtle anomalies might bepresent but are overwhelmed by theigneous geology.

Earth resistance resultsThe results of the earth resistance surveyover the farm mound are presented inFigure 7. The survey was conducted atthe high resolution of 0.5m x 0.5m torecord a maximum level of information.


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Figure 7 reveals that many of theseanomalies have a regular form, withsome linear and rectilinear features visi-ble. These are interpreted as being theresponses to buried stone foundations forstructures originally in these locations.

As stated above, one of the aims ofthe surveys in this area was to locate anyfeatures associated with a church, andthis has been achieved. A high resistanceanomaly has been detected to the east ofthe main cluster, and in the area wherethe church was expected. Almost rectilin-ear, this anomaly could be structural inorigin, but with anomalies only detectedon three of the four sides. Although ori-ented east-west and about 6m x 4m, onits own this rather amorphous anomalycannot be confidently identified as achurch. However this anomaly is seen tobe situated at the centre of a circularanomaly of low resistance, about 30m indiameter. This ring of low resistancemight be due to an infilled boundaryditch or bank, and is consistent with amedieval Icelandic church. This was laterconfirmed by excavation when a numberof graves were discovered within the area(Gestsdóttir 1999, 44). It does not appearthat either method has detected thesegraves.

A very subtle positive linear anomalycan be made out in the northeastern cor-ner of the survey area, and might be dueto the buried remains of a bank, possiblya field boundary. Two linear low resistance anomalieshave been detected to the west of thetrack, which could be interpreted asinfilled ditches. However, these coincidewith the positions of the ferrous pipe

anomalies seen in the gradiometer sur-vey, and can be positively identified asthe response to these modern pipe trench-es.

Within the survey area to the east ofthe track, a number of linear high andlow resistance stripes are visible. Theseare real anomalies, not survey defects,and can be seen to be on a slightly differ-ent orientation to the survey grid. Duringthe data collection, a number of lineardepressions were noticed on the farmmound, probably be caused by treadmarks of a bulldozer employed to levelbuildings on the farm in the 1970s. Thesewould certainly produce anomalies likethose detected.

When the results of both geophysicalsurveys over the farm mound are com-pared, it can be seen that the gradiometersurvey has successfully detected thechurch anomaly as one of the areas ofintense dipole anomalies. Other anom-alous areas can then also be confidentlyinterpreted as being due to buried struc-tural remains, as they too coincide withareas of high resistance.

Geophysical surveys within the farmof Hofstaðir have proved successful notonly for the detection of anomalies, butalso for the interpretation and assessmentof buried remains at the site, thereforeimproving the archaeological under-standing of the site.

Anomalies detected during the earthresistance surveys and subsequently con-firmed by excavation indicate that thistechnique has the potential to locate andidentify cut features into sediments, in


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addition to the more clearly defined stonefeatures. Both these sets of evidence havethen been used to confirm the results ofthe gradiometer survey for the detectionof structural remains, and demonstratethe potential for this technique.

At this stage in the development of anunderstanding of such methods inIceland, the results from Hofstaðirdemonstrate the necessity for both posi-tive and negative geophysical results tobe backed up by trial excavation.

Discussion of resultsEarth resistanceEarth resistance has been shown to posi-tively identify buried features where nosurface indications remain, and to beapplicable in many situations althoughsome limitations caused by periglacialphenomenon and wet ground conditionsneed further investigation. The mainadvantage of this technique over the flux-gate gradiometer is that it is not severelyoverwhelmed by geological effects.

At Skálholt and Hofstaðir this methodhas been used to detect and correctlyidentify known archaeological remains,where the precise positions of buried fea-tures was unknown. In addition, theresults from Gásir and other sites notreported here demonstrate the potentialfor this technique as an aid in the inter-pretation of known features, where thevisible surface remains may be amor-phous and unclear.

A problem of surveying on wetground was observed in the earlier sur-veys, most likely due to 'geometriceffects', although it is not entirely under-

stood yet. Once this effect had beenrecognised, earth resistance surveys wereconducted when the conditions were dry,and successfully avoided these surveydefects.

Periglacial effects caused other limi-tations, where a number of unusualanomalies were recorded. Most influen-tial were surface deformations such asthufur. These produce intense localisedanomalies and create a noisy backgroundagainst which anomalies due to buriedfeatures can become less clear.

The fine sampling interval adopted inIceland, 0.5m x 0.5m, has allowed a highlevel of detail to be recorded, however itcan be seen that a coarser interval of1.0m x 1.0m can still be used to success-fully locate sites. This choice will dependupon the aims of the survey, as well attime constraints.

MagnetometryAlthough in many cases the magnetome-ter data is overwhelmed by the intensebackground geological signal, it has beendemonstrated that it is often possible todistinguish discrete rocks in an archaeo-logical deposit from bedrock, even wherethese natural anomalies dominate thedata.

At Gásir it is possible to identifyanomalies attributed to buried stonewalls, despite the near-surface igneousgeology producing strong thermorema-nent effects. There is also a distinguish-able variation produced by the boothearthworks that might allow the extent ofthe archaeological remains at the site tobe defined. However, the much deeperaeolian deposits overlying the bedrock at



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Skálholt reduce this thermoremanentinput, allowing more subtle archaeologi-cal anomalies to be detected.

An important discovery from thisproject concerns the data collectionmethod for magnetometer surveys. Manyof the surveys revealed an enhancedeffect of 'bunching' produced when datais collected in the 'zig-zag' fashion, possi-bly exaggerated by the strong geologicalanomalies. The only solution is to under-take carry out gradiometer surveys in the'parallel' fashion. Unfortunately this willincrease the time taken to survey eachgrid, but will produce clearer results.

It has been demonstrated that themost information of archaeological valuewas produced by the small-scale noisecaused by individual rocks associatedwith structural remains. Being smallanomalies, these are best identified at ahigher resolution of surveying than mightordinarily be undertaken. Consequentlyit is advised that a high resolution of datacollection, i.e. 0.5m x 0.25m, isemployed for most effective and inform-ative results.

Magnetic susceptibilityThroughout this assessment it was possi-ble to collect only a limited number ofsoil samples for measurement of magnet-ic susceptibility at Bradford.

The preliminary results imply that,despite very high natural backgroundvalues (generally in the range 100 to 250SI x10 - 8m3kg-1), archaeological sedi-ments associated with burning producestrong enhancements, with values above400 SI x10- 8m3kg-1, and even as high as1800 SI x10- 8m3kg-1(midden material

from Neðri Ás). However, it is notknown how effective this method wouldbe for reconnaissance surveying, as inmany areas aeolian deposits overlay thearchaeology and the natural mixingprocesses in the soil might not beextreme enough to bring enhanced mate-rial to the surface. Instead, this techniquemight prove valuable by aiding the inter-pretation of deposits during excavationby distinguishing between samples withnatural and anthropogenic enhancement.

Detection of archaeological remainsAs a preliminary evaluation, it was nec-essary to target archaeological remainsknown from other sources to allow aproper and confident assessment of thesetechniques to be made. Surveys at a num-ber of sites, and over different remainsmake it possible to talk generically aboutcertain feature-types.

Many sites presented visible earth-works constructed of turf. As discussedearlier in this paper, a lack of good build-ing stone has meant that many structuresand field boundaries are constructed ofturf. Where geophysical surveys includedsuch remains, they were shown to appearas low resistance anomalies. This mightbe surprising since such extant remainscould be expected to be better drainedand therefore possess a higher resistance.These results therefore seem to indicatethat the turf is water retentive, and itmight be possible to use this characteris-tic for the future prospection of now-buried turf remains.

Buried stone walls have been detect-ed in earth resistance and magnetometersurveys, despite the intense geological


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effects. Discrete magnetic dipole anom-alies clearly distinguish individual rocksfrom the solid bedrock, and alignmentsof these anomalies can reveal buriedwalls. Comparison of the magnetometerresults with resistance data has con-firmed this interpretation.

The geological input is dramaticallyreduced on sites where deeper aeoliandeposits lie above the bedrock, as atSkálholt. In such situations, the geologyis at a greater distance from the instru-ment, allowing the subtler archaeologicalanomalies to be detected. It should there-fore be possible to assess the potential ofa magnetometer survey at a site by firstmeasuring the soil depth with an auger,although this has yet to be tested.

At both Gásir and Þingvellir, boothremains have produced distinct magneticand resistance anomalies, although thesmall survey area at both sites preventstheir full understanding at present.

ConclusionThis preliminary study has successfullydemonstrated the effectiveness of routinearchaeological prospection techniques inIceland, particularly when undertaken aspart of an integrated study as discussedhere. The benefits of combining earthresistance and magnetometer surveys hasalso been illustrated, and that where pos-sible earthwork surveys, aerial photogra-phy and trial trenching should also beused to better understand the geophysicalresults and therefore the archaeology.

Future WorkThis assessment of geophysical tech-niques in Iceland is the first step in inte-grating geophysical surveys with otherforms of archaeological evaluations.While a number of important questionshave been answered during the course ofthis study, many more have been asked,and further research is necessary to allowthe maximum information to be extractedfrom geophysical surveys.

A better understanding of the limita-tions caused by geology and geomor-phology is required, so that effects pro-duced by these features may be discrimi-nated from archaeological anomalies.

When geophysical surveys are inte-grated into archaeological evaluations,the time scale of most projects posessevere constraints on field geophysicists(Gaffney et al. 1991, 1). It is thereforenecessary that a full understanding of theviability and limitations of the use ofgeophysical techniques in such evalua-tions be obtained before contract field-work can be undertaken efficiently.

AcknowledgementsThis project was carried out by T.J.Horsley for an ArchaeologicalProspection MSc Dissertation under thesupervision of S.J. Dockrill, at theDepartment of Archaeological Sciences,University of Bradford. The MSc coursewas funded with a NERC studentship,and the author is grateful to NERC for itshelp with costs towards the fieldwork.Fieldwork was undertaken in collabora-tion with the FSÍ and thanks are due totheir staff, in particular to Orri



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ReferencesÁgústsson, Hörður (1989). “Húsagerð á

síðmiðöldum”. In Líndal, Sigurður (Ed.) Saga Íslands, Vol. IV, 261-300.

Bird, J.B. (1974). “Geomorphic processes in the Arctic”. In Ives, J.D. and Barry, R.G. Arctic and Alpine Environments .707-710.

Boucher, A.R. (1996). “Archaeological feed-back in geophysics”. In Archaeological Prospection, 3: 129-140.

Bruun, D. & Jónsson, F. (1908). “Det gamle Handelssted Gásar”. Videnskabernes Selskabs Forhandlinger No.3. København.

Bruun, D. (1928). Fortisminder og nutidshjempaa Island. København.

Burger, H.R. (1992). Exploration Geophysics of the Shallow Subsurface. Prentice Hall, New Jersey.

Clark, A. J. (1975) “Archaeological prospecting:a progress report”. Journal of Archaeological Science, 2: 297-314.

Clark, A. J. (1990). Seeing Beneath the Soil. Batsford, London.

Courty, M .A., Goldberg, P. and Macphail, R. (1989). Soils and Micromorphology in Archaeology. 1st Edition. Cambridge University Press, Cambridge.

David, A. (1995). Geophysical survey in

archaeological field evaluation. English Heritage, London, Research & Professional Services Guideline No 1.

Dockrill, S.J. and Gater, J.A. (1992). “Tofts Ness: Exploration and interpretation in a Prehistoric landscape”. In Spoerry, P.S., (ed.). Geoprospection in the Archaeologi-cal Landscape. Oxbow, Oxford.

Friðriksson, Adolf (1994). Sagas and Popular Antiquarianism in Icelandic Archaeology. Avebury, Aldershot.

Friðriksson, Adolf & Vésteinsson, Orri (1997a). “Hofstaðir Revisited”. Norwegian Archaeological Review, Vol. 30:2, 103-112.

Friðriksson, Adolf & Vésteinsson, Orri (1998a).“Fornleifaskráning - Brot úr íslenskir vísinda-sögu”. Archaeologica Islandica 1, 14-44.

Friðriksson, Adolf & Vésteinsson, Orri (1998a).“Isleif - A database of archaeological sites

in Iceland”. Archaeologica Islandica 1, 45-46.

Gaffney, C.F., Gater, J.A. and Ovendon, S.M. (1991). The Use of Geophysical Techniques in Archaeological Evaluations.Institute of Field Archaeologists, Technical Paper No. 9, Birmingham.

Gerrard, (1985). “Soil erosion and landscape

Vésteinsson, Adolf Friðriksson, HildurGestsdóttir, Garðar Guðmundsson andMjöll Snæsdóttir. The authors would alsolike to thank Dr. Roger Walker for loan-ing the Geoplot packages for data pro-cessing undertaken throughout this study.

The survey locations discussed with-in were made available through the gen-erosity of the landowners: Skálholts-staður (Skálholt); Friðrik GylfiTraustason and Guðrún Björk

Pétursdóttir (Gásir); and Ásmundur andGuðmundur Jónsson (Hofstaðir).

Helpful discussions and contributionswere also provided by Oscar Aldred,Andy Dugmore, Throstur FreyrGylfason, Guðrún Kristinsdóttir, GavinLucas, Tom McGovern and NABO,Karen Milek, Dr. Armin Schmidt,Magnús Sigurgeirsson, Ian Simpson andall those at the 1999 Field School inArchaeology at Hofstaðir.

2issue.qxd 29.11.2002 12:13 Page 32



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stability in southern Iceland: a tephrochronological approach”. In Richards, K.S., Arnett, R.R. and Ellis, S. (eds.). Geomorphology and Soils. 79-95.

Gestsdóttir, Hildur (1999). “The Medieval Churchyard”. In Lucas, G. (Ed.) Hofstaðir 1999 Framvinduskýrslur/Interim Report.44-54.

Hallsdóttir, Margrét (1987). Pollen analytical studies of human influence on vegetation in relation to the Landnam tephra layer insouthwest Iceland. Lundqua Thesis 18, Lund University, Department of Quaternary Geology.

Hermannsdóttir, Margrét (1987). “Fornleifa-rannsóknir að Gásum og víðar í Eyjafirði árið 1986”. Súlur 27:3 39.

Hjálmarsson, J.R. & Astridge, C. (1998). “Lake Mývatn”. Special Iceland Review Supplement. Iceland Review.

Horsley, T.J. (1998). “Nine Ladies Stone Circle, Stanton Moor, Derbyshire: Report on Geophysical Survey 1998”. Ancient Monuments Laboratory Report Series,68/98.

Ives, J.D. and Barry, R.G. (1974). “Arctic and Alpine Environments”. Methuen, London.

Jónsson, Finnur (1908). “Hinn forni kaupstaðir‘at Gásum’". Árbok 1908.

Kearey, P., and Brooks, M. (1984). An Introduction to Geophysical Exploration.Blackwell Scientific Publications, Oxford.

Lucas, G. (1998). “Prehistory at Hofstaðir: An

Introduction to the 1996-1997 Excavation”. Archaeologia Islandica, 1: 119-122.

Lucas, G. (Ed.) 1999. Hofstaðir 1999 Framvinduskýrslur/Interim Report. Forn-leifastofnun Íslands FS102-91017, Reykjavík.

Ólafsson, Guðmundur Óli (1988). Skálholt. Kirkjuráð íslensku þjóðkirkjunnar.

Richards, K.S., Arnett, R.R. and Ellis, S.(Eds.) (1985). Geomorphology and Soils. George Allen & Unwin, London.

Scollar, I., Tabbagh, A., Hesse, A., and Herzog, I. (1990). Archaeological Prospecting and Remote Sensing. Cambridge University Press, Cambridge.

Sigurgeirsson, Magnús Á. (1998). “Gjóskulagarannsóknir á Hofstöðum 1992-1997”. Archaeologia Islandica, 1, 110-118.

Simpson, I.A., Milek, K.B. & Guðmundsson, G. (1998). “Archaeological Sediments and Site Formation at Hofstaðir, Mývatn, NE-Iceland”. Archaeologia Islandica, 1, 129-142.

Spoerry, P.S., (ed.) (1992). Geoprospection in the Archaeological Landscape. Oxbow, Oxford.

Telford, W. M., Geldart, L. P., Sheriff, R. E. and Keys, D. A. (1976). Applied Geophysics. 1st Edition. Cambridge University Press, Cambridge.

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