the use of electromagnetic induction in locating graves and mapping cemeteries: an example from...

* Correspondence to: D.Baldwin Hall, Athens, GA

Copyright © 2012 John

Archaeological ProspectionArchaeol. Prospect. 19, 31–39 (2012)Published online 16 January 2012 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/arp.1416

The Use of Electromagnetic Induction inLocating Graves and Mapping Cemeteries:an Example from Native North America

DANIEL P. BIGMAN*

University Of Georgia, 250 Baldwin Hall, Athens, GA 30602, USA

ABSTRACT One of themost popular applications of geoph
ysics to archaeological problems has been to locate unmarked graves andmapcemeteries. Although electromagnetic (EM) induction was one of the early techniques used in such applications, alternativetechniques such as ground-penetrating radar and resistivity have become more popular in recent years. Despite some ofthe method’s drawbacks the EMmethod still presents numerous advantages such as speed in data collection and collectionin a variety of survey environments and ground cover. A case study from Ocmulgee National Monument, USA is presentedthat identified numerous anomalies that may be interpreted as Native American burials. Small anomalies having low apparentconductivity were distributed near the Funeral Mound (Mound C); a known cemetery. The conductivity survey redefined thespatial extent of the cemetery and increased our understanding of burial density. The data collected during the 2010 fieldseason will aid in conservation efforts and help the US National Park Service avoid these sensitive materials during futurearchaeological work and park management. Copyright © 2012 John Wiley & Sons, Ltd.
Key words: Electromagnetic induction; conductivity; NAGPRA; graves; Native American burials; historic preservation

Introduction

One of the most popular applications of geophysics toarchaeological problems has been to locate unmarkedgraves and map cemeteries. Early mapping of historiccemeteries that used electromagnetic (EM) inductionand ground penetrating radar (GPR; Bevan, 1991) pavedthe way for many future studies. Electromagneticinduction has fallen out of favour as archaeologists beganto rely on alternative techniques to locate burials (i.e.Davenport, 2001). During the past two decades, electricalresistivity, magnetometry and GPR have become theessential tools for investigating and mapping bothhistoric and pre-historic cemeteries (Nobes, 1999;Conyers, 2006; Jones, 2008; McKinnon, 2009). There hasbeen a general absence of published case studies relyingon EM induction in the past two decades.Here I review the fundamental principles of the EM

induction method and briefly explore its history, use,benefits and drawbacks in archaeology. Second, I willpresent the methods and results of a conductivity survey

P. Bigman, University Of Georgia, 25030602, USA. E-mail: [email protected]

Wiley & Sons, Ltd.

carried out in 2010 at the Ocmulgee Funeral Mound; aNative American cemetery located in Georgia, USA.

The electromagnetic induction method

Ampere’s and Faraday’s laws are the fundamental princi-ples underlying the EM inductionmethod. Ampere’s lawstates that when electrical current flows through a coil ofwire it produces a magnetic field that is perpendicular tothe plane of the coil. Maxwell later expanded Ampere’slaw to include time-varying EM fields. Faraday foundout that the converse is also true. Faraday’s law states thatwhen a conductive object is placed into a moving mag-netic field, a current will be induced in that conductivebody (Daniels et al., 2008, pp. 109–113).The EM inductionmethod combines these principles to

measure variation in apparent soil conductivity. Manysoil conductivity meters consist of two coils: a transmitterand a receiver. An electrical current is applied to the trans-mitter coil, which in turn produces a primary magneticfield perpendicular to the coil. This magnetic field theninduces an electrical current in the soil. This electricalcurrent flows in a circular path and is typically referred

Received 11 August 2011Accepted 2 December 2011

32 D. P. Bigman

to as an eddy current. Eddy currents themselves produce asecondary magnetic field that is perpendicular to thecircumferential direction of the current (i.e. it approxi-mates the behaviour of the transmitting coil). Finally, thesecondary field induces a current flow in the receiver coillocated at the opposite end of the soil conductivity meter.It is the strength of the currentflow in the receiver coil thatis measured and recorded. A time-varying current isapplied to the transmitter coil. The strength of the time-varying current flows in a cosine wave and has regularpeaks and troughs. Receiver coil measurements basedon the cosinewave are said to be in-phase. Delayed valuesmeasured by the receiver coil that follow in a sine wave(i.e. shifted 90�) are said to be out-of-phase or phase shifted(Witten, 2006, pp. 166–170).Factors influencing conductivity measurements in-

clude the material properties, size, shape, orientation ofa conductive object and porosity/compaction (Bevan,1991, p. 1310; Witten, 2006, p. 157). In a survey designedto locate pre-Hispanic hearths, decayed wooden postholes, burials and pits, the phase shifted values are ofmost importance. Most geophysicists believe that theout-of-phase value is the closest approximation ofapparent conductivity. Apparent conductivity can bedefined as ‘the electrical conductivity measured for abulk volume of soil’ assuming the true electrical con-ductivity is heterogeneous (Allred et al., 2008, p. 383).The internal composition of any sample of earth shouldvary and the average conductance of this varied samplecan be considered its apparent conductivity.Electromagnetic induction was introduced to archaeo-

logical contexts during the 1960s. Early field work andcontrolled experiments focused on the detection of postholes (Tite, 1961; Colani, 1966; Colani and Aitken, 1966;Howell, 1966; Tite and Mullins, 1970). The benefits ofthe method quickly became apparent. It drew a lot ofattention early on because it eliminated the need for probecontact as opposed to electrical resistivity techniques(Bevan, 1983). This made data collection much moreefficient and allowed research to be conducted in placesthat were inaccessible to resistivity meters. The samecomparison can now be made with GPR, which requiresexcellent site conditions (Jones, 2008).Conductivity became a standard in locating large

archaeological features such as the remains of woodenpalisade walls (Dalan, 1989, 1991), large plazas (Holleyet al., 1993), stone walls (Osella et al., 2005), stone houseplatforms (Sweely, 2005), drainage systems (Rogerset al., 2010), filled in ditches, palaeochannels and lakebeds (Bevan, 1983; Hildebrand et al., 2007; Conyerset al., 2008), refuse pits (Bevan, 1983), stratigraphiclevels (Dalan, 2006; Dalan and Goodman, 2007) andburied occupational layers (Dalan, 2006; Dalan and

Copyright © 2012 John Wiley & Sons, Ltd.

Goodman, 2007). Advancements in data collectionmethods and instrumentation now allow mapping ofentire sites or communities in relatively short periodsof time (e.g. Kvamme and Ahler, 2007; Witten et al.,2000, 2003), and streamlined data processing softwareis letting more archaeologists exploit this geophysicaltechnique (Auken et al., 2006).Low resolution and slow continuous sampling are

the two most significant drawbacks in conductivitymapping. Conductivity meters often collect data upto a rate of two readings per second (Clay, 2001,2006). This rate is fast enough to distinguish variationbetween the soil matrix and large features such asearthworks or ditches if the surveyor is walking at apace of 1m s�1, but it may not be a close enough inter-val to adequately map small features such as burials.This can be contrasted with fluxgate gradiometersand caesium vapour magnetometers that can collect10 readings per second, or GPRs that can accuratelyrecord traces every 5 cm with a survey wheel.I attempted to circumvent the resolution problem in

the survey at Ocmulgee’s Funeral Mound cemeteryby recording only a single frequency from a multifre-quency induction instrument. Multifrequencyinstruments theoretically provide instant depthsounding by producing EM fields at both high andlow frequencies. Higher frequencies have shorterwavelengths and prospect to a shallower depth.Lower frequencies have longer wavelengths andprospect deeper into the ground. There is somedebate surrounding the theoretical basis of multifre-quency instruments in performing depth soundingof apparent conductivity (for an excellent summaryof the arguments see Auken et al., 2006), but the factis that more frequencies force the instrument to recordat slower paces. By reducing the number of recordedfrequencies to one I was able to record eight readingsper second. This also allowed greater flexibilityduring post-acquisition processing. Data can still beplotted at a 0.5m sampling interval, but an averageof three to five points can be taken to smooth the dataor account for instrument instability. The other optionis plotting the data at closer intervals while reducingthe number of averaged data points, ultimatelyproviding higher resolution.

Case study: Ocmulgee National Monument

Background

Ocmulgee is the fourth largest (70 ha) mound site inthe eastern USA (Figure 1) and there is evidence of

Archaeol. Prospect. 19, 31–39 (2012)DOI: 10.1002/arp

Figure 1. Aerial view of the Ocmulgee National Monument. This figure is available in colour online at wileyonlinelibrary.com/journal/arp.

33Use of EM Induction to Locate Graves and Map Cemeteries

human occupation dating from the Paleo-Indianperiod (approximately 11 000 BC) to post-AmericanCivil War. One of the more famous aspects of Ocmulgeeis the Funeral Mound that sits at its southwest corner(Figure 2). Burials dating to two periods have beenunearthed there: the Mississippian (AD 900 to AD 1100)and historic Creek (AD 1680 to AD 1720) occupations.The beginning of the Mississippian period in Native

Figure 2. Photograph of the Funeral Mound (looking north). This figureis available in colour online at wileyonlinelibrary.com/journal/arp.


American history marks a transition to intensifiedmaize agriculture, higher population densities at thetown and polity level, institutionalization of leadershiproles, and the integration of multiple households,villages and towns into a single decision-making body.The Creek nation was formed during the historic periodas a response to hostilities from European colonies andsettlers. It has been stated in Creek oral tradition thatOcmulgee was the settlement place of their ancestorswho migrated east and was the place of up to two creeksettlements in the seventeenth and eighteenth centuries.The Funeral Mound is a flat-topped cylindrical

structure with seven construction episodes. The highestdensity of burials at Ocmulgee was discovered in thisstructure and the area immediately surrounding it. TheOcmulgee National Monument Geophysical SurveyProject undertook an EM conductivity survey near theFuneral Mound in 2010. More details about methods,larger research questions and additional results havebeen published elsewhere (Bigman, 2010, 2011).

Previous investigations at the Funeral Mound

The earliest evidence of skeletal removal at the FuneralMound was documented by C. C. Jones in 1873 (Jones,1999) following the destruction of the northern third of


34 D. P. Bigman

the mound by a railroad cut. The observations made byJones suggest that at least several skeletons were locatedjust beyond the northern periphery of the FuneralMound and that several skulls, dating to both theMississippian (AD 900 to AD 1100) and Creek (AD 1680to AD 1720) occupations, were placed within thenorthern limits of the mound. The mound began toerode and was subject to looting over the subsequent60 years. Professional excavations began in the 1930s aspart of the Civil Works Administration (CWA) andWorks Progress Administration (WPA) initiativescarried out during the Great Depression as part of aprogram to put people back to work. These investiga-tions focused on what remained of the mound andcarried out limited testing in the village level surround-ing the mound (Kelly, 1938; Fairbanks, 2003 [1956]).Ninety-four burials (114 individuals) were exhumedfrom the mound and its vicinity to the south, east andwest (Powell, 1994, p. 116). Sixteen of these burials werelocated at the village level (Fairbanks, 2003 [1956], pp.34–35) all within approximately 2m of the surface. Nofurther fieldwork was carried out in this area for over50 years. The US National Park Service eventuallyshovel tested several areas around the Funeral Moundin response to compliance issues such as the re-chainingof a fence (Cornelison, 1992) and modifications to aparking lot curb (Cornelison, 1993; Halchin, 2004). Noneof the recent work has identified any additional graves.

Geophysical expectations

In a survey conducted by Bevan (1991) at KetteringShaker cemetery, two distinct anomalies were identifiedas possible graves. Both provided a lower apparentconductivity compared with the surrounding soil matrix.Bevan suggested that ‘the most distinctive feature of agrave may be the disturbed soil in the filled excavation.Through the l–2m depth of a grave shaft, the soil may

Figure 3. Enlarged image of burial anomalies from survey block 2. This figu


change markedly’ (Bevan, 1991, p. 1310). Mixing oftopsoil with subsoil, differences in compaction and aircavities may also contribute to variation in apparentconductivity (Bevan, 1991, p. 1310).More recently, a forensic simulationwas conducted on

pig cadavers at several test sites in Great Britain usingelectrical resistivity (Juerges et al., 2010). They found thatthose pigs left exposed to the geochemical processes inthe soil were less resistive (more conductive) than thesurrounding soil whereas the pigs that were wrappedwere more resistive (less conductive) than the surround-ing soil. The limited exposure of the wrapped pigs togeochemical processes in the soil may limit decay andultimately the effect of organic material on the soil. Thepigs that were wrapped may approximate secondaryburials such as many that were excavated at Ocmulgee.Secondary burials, which refer to a mortuary practicewhere bodies are initially buried elsewhere and thenreburied after the flesh has decayed, should be moreresistive/less conductive.I expected Mississippian and historic Creek burials to

yield low conductivity values (see Figure 3 for an enlargedplan viewmap of some anomalies suggested to be burialsand Figure 4 for conductivity traces from each surveyblock). Soils within the park boundaries consist of 3–6 ftof red sandy soil overlying Georgia red clay (NPS, 1982,p. 14). Clay is generally a conductive material withminimal porosity and high water retention. Burialsinterred into this clay layer should have greater porosity.These pores are filled with air, a highly resistive material.Despite the impermeability of clay, the amount of rainfallin a given day should drain quickly through theexcavated-out and redeposited soil of a pit or burial. Simi-lar findings were confirmed at Cahokia, a contemporaryof Ocmulgee located in Illinois. Dalan (1989, 1991) investi-gated the palisade wall surrounding the main plaza atCahokia. The post holes were interred into a wall trench,an excavated trench refilled with the same soil. These

re is available in colour online at wileyonlinelibrary.com/journal/arp.


Figure 4. Conductivity data traces from survey block 1 and surveyblock 2 with interpretation of low conductivity burial anomalies.


trenches produced lower apparent conductivity readings,which Dalan (1989, 1991) suggests are the result of quickdrainage and air filled pores.In addition to variation in soil compaction and por-

osity, the contents of prehistoric and historic graves atOcmulgee consisted of generally non-conductive mate-rials such as bone, ceramic vessels, caches of shell,caches of glass beads, large ground stone, and in rarecases preserved logs. Although ‘individual bones maynot be directly detectable’ (Bevan, 1991, p. 1310), theirassociation with abundant grave goods may increasethe contribution to lower apparent conductivity values.Shell is known to be highly resistive (Thompson et al.,2004), and in some cases thousands of shell beads werelocated in a single Ocmulgee burial. Eleven of the 94burials were multiple burials (Powell, 1994, table11.1).The agglomeration of more bone should also be moreresistive and make it easier for the soil conductivitymeter to differentiate between burials and the sur-rounding soil matrix. The contribution of a burial andits associated contents to lower conductivity valuesmay vary depending on depth. Burials just below thesurface should have a greater contribution to lowervalues. Differential grave shaft fill is probably a moreimportant factor, but the contents of a grave may havean influence on burial detection.


Data collection and processing

Two survey blocks were collected near the FuneralMound in 2010 as part of a larger project to mapOcmulgee using non-invasive techniques. A compassand tapes was used to establish survey grids and atotal station was used to spatially reference theblocks. The survey was carried out using a GEM-300conductivity meter manufactured by GSSI, Inc. (coilspacing of 1.67m) at a frequency of 12 150Hz. Depthof investigation at this frequency is approximately2m (GSSI, Inc.). The in-phase, quadrature phase andapparent conductivity measurements were recorded.The apparent conductivity data mirrored the quadraturedata suggesting that the in-phase had little impact onapparent conductivity readings. All data in this reportare presented as apparent conductivity and aremeasured in mS m�1.Data were collected in a ‘snake line,’ where the

surveyor traversed neighbouring transects in alternatingdirections. The instrument collected data readings con-tinuously every 1/8 s and the surveyor attempted towalkat a constant pace of 1ms�1. The sampling interval ran-ged from between six and nine readings per metre andtransects were spaced 1m apart. The direction of burialsand other archaeological features were not known priorto survey. Survey direction was chosen based on thelandscape and ease of data collection. The conductivitymeter was held at waist height and was orientedhorizontal to the ground and perpendicular to thetransect direction. Following Bevan (1998, p. 34) thedirectional alignment of the transmitter and receivercoils was not readjusted every alternating transectbecause changes in coil orientation should not affectconductivity readings. Constant readjustment may alsounbalance the instrument over time (Clay, 2006).Data were downloaded to MagMap2000, which was

used to orient transect lengths and position. Data wereinterpolated using the Kriging algorithm in Surfer 9.0 tosmooth the data. All survey data were processed usingArcheoSurveyor 2.0 by D. W. Consulting. Processingprocedures generally followed Gaffney and Gater (2003)and Kvamme (2006), where all data were filtered firstand enhanced second.

Results and discussion

Both survey blocks contained randomly distributedanomalies of lower conductivity that are the size andshape of Native American burial units. Many of theseanomalies are generally oriented in the same direction,


36 D. P. Bigman

which also may suggest that they are possible burials.However, the data range between the two blocks variedsubstantially. This is most likely due to the differences incondition when each was surveyed. Electromagneticinduction is an active geophysical technique and dependson soil conditions. Block 1 (8750m²; Figure 5) wassurveyed in the summer when there was moisture inthe soil. Thismay have created a greater contrast betweenthe non-conductive materials that make up a burial andthe surrounding matrix. Block 2 (3750m²; Figure 6) wassurveyed in the winter. The conductivity meter stillidentified probable burials, but the contrast was not asdramatic because a resistive layer of frost (probablyseveral centimetres thick) was present during thewinter survey. This resistive layer may have made itmore difficult to induce a current in the surroundingmatrix and minimized the variation (See Figure 4 tocompare the conductivity values from each block).Differences in seasonality may also have contributedto variation of the anomaly shape. Carrying out thesetwo surveys at the same time would have been anideal situation. Unfortunately, scheduling difficultiesmade this impossible.In total, over 60 additional possible graves were

identified in the conductivity data (Figure 7). Theconductivity meter probably did not identify all of

Figure 5. Plan view map of block 1 conductivity data. This figure is availab


the possible burials surrounding the Funeral Mound.Many of the excavated burials were single skulls oras in one case, only teeth. The conductivity meter isnot sensitive enough to distinguish between theseand the soil matrix. Alternatively, some of the burialanomalies may actually be pits absent of any burials.Several of these were recovered from the FuneralMound and area surrounding it, but burials out-weighed non-burial pits ten to one.There were many more examples of low conductive

anomalies grouped together with anomalies of higherapparent conductivity that were not identified aspossible burials in the interpretation (Figure 7). Theseclusters probably represent historic Creek structuresor disturbance from the redeposition of soil andmaterial during the construction of the railway. If theyare the remains of Creek structures, then it is possiblethat some anomalies of lower conductivity in theseclusters represent additional burials. These eithercould be associated Creek burials or Mississippianburials below the house floor. It is also possible thatthey represent disturbed baked clay floors, whichwould also produce agglomerations of high and lowconductivity values.The EM conductivity survey mapped the Funeral

Mound cemetery rapidly (Block 1 was collected in

le in colour online at wileyonlinelibrary.com/journal/arp.


Figure 6. Plan view map of block 2 conductivity data. This figure is available in colour online at wileyonlinelibrary.com/journal/arp.


just over 3 h and Block 2 was collected in 1.5 h aftergrid set-up) and the data suggest that the cemeteryis denser and spatially larger than previously

Figure 7. Digitized map with transcribed interpretations of conductivity data


believed. The cemetery appears to have extendedto the north beyond the railroad cut and there is afall off in the density of burial anomalies to the

from survey block 1 and survey block 2.


38 D. P. Bigman

southeast of block 1 and the north east of block 2.Although a few isolated anomalies of lower con-ductivity do appear out in these margins the surveyseems to have located the general boundaries of thecemetery.

Conclusion

Despite a preference for alternative methods forgrave prospection, conductivity still provides severaladvantages. The most significant drawback (speedof recording) was overcome by finding the mostappropriate frequency in a multifrequency systemand then mapping with that single frequency. Theinstrument collected eight readings per secondwhich provided high resolution and post-processingflexibility for weighted averaging or interpolation.This level of collection speed and data set resolutionis similar to other methods such as magnetometryand the speed of collection exceeds that of GPR andtypical resistivity equipment. The purpose of thispaper is not to dissuade surveyors from using otherinstruments. It is to propose EM conductivity as aneffective tool in the archaeologist’s toolkit for locatinggraves and mapping cemeteries. I do not believe thatthe success of the survey was dependent on usingthe GEM-300 specifically. Any EM induction instru-ment capable of collecting data at a similar samplingspeed should create a successful situation acrosssimilar contexts.Electromagnetic induction was an effective tool for

locating and identifying pre-historic and historicNative American burials at Ocmulgee NationalMonument and the results have comparative valuefor similar contexts around the world. Over 60 possibleburials were identified around the Funeral Mound.The limits of the Funeral Mound cemetery werediscovered; it extends to the north of the mound andadditional burials appear to the southeast. Burialspresent themselves in a very distinct manner with aregular signature of low apparent conductivity. Whilethe issues surrounding the excavation of native bur-ials are still sensitive, grave density and distributionremain important data sets for answering anthropo-logical and archaeological questions. Conductivityhas helped address some of these questions withoutdisturbing the subsurface. The geophysical datacombined with more traditional lines of archaeo-logical evidence that has already been recovered canhelp the archaeologist manoeuvre through theseshaky waters.


Acknowledgements

I would like to first express my gratitude to LawrenceConyers. His effort as an editor vastly exceeds therequirements of such a position and the paper owesmuch to his dedication. Steve Kowalewski’s andDavid Hally’s comments on an earlier version of thepaper were very useful, and critical discussions withRobert Hawman were irreplaceable. I would also liketo extend my appreciation to the Park Service, Ocmul-gee National Monument, and the Muscogee Nationfor their continued support. Equipment was providedfor the project by the University of Georgia’s Labora-tory of Archaeogeophysics and Archaeometry, andLaboratory of Archaeology. University of Georgiagraduate students Yanxi Wang and Stephan Brennanhelped collect data from block 2. Dong Dong Li andBen Shirley helped collect data from block 1. Finally,Amy Hanenberg, designer at Stacy Garcia Inc., editedthe final versions of several figures.

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