Graves and Recover Other Information About Historic Period Cemeteries
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64 Lawrence B. Conyers unmarked graves and recover other information about historic period cemeteries. GPR can often determine grave attributes such as depth of burial, Ground-Penetrating Radar grave size, type of caskets and their orientation; Techniques to Discover numbers of graves in certain locations; and the spatial distribution of graves within certain areas and Map Historic Graves of a cemetery. This information can then be integrated with birth and death records, infor- mation found on headstones, or other historical ABSTRACT documents to provide a database on the lives and behaviors of the individuals buried there. Often Ground-penetrating radar is a geophysical technique that can be used to identify and map features commonly associated this information is not available by other means. with historic graves, including intact or partially collapsed Some Euroamerican cemetery characteristics coffi ns and vertical shafts. Data are collected by moving such as the depth, orientation, and spatial distri- radar antennas that transmit pulses of energy into the ground bution of grave shafts have changed over time. along parallel transects within grids, recording refl ections Often they refl ect the economic background, eth- of those pulses from signifi cant discontinuities within the ground. Visual analysis of radar refl ection profi les can be nicity, and religious, social, or aesthetic values used to identify both coffi ns and the vertical shaft features of both the dead and those doing the burying commonly associated with human burials. Spatial analysis (Farrell 1980). Although in some cases these of the reflection amplitudes within a grid consisting of characteristics are well documented (Crissman many profi les (when converted to depth using site-specifi c 1994; Sloan 1995) they have not generally been velocities) produces three-dimensional maps of these burial features. The identifi cation and mapping of graves can applied to the study of specifi c communities or identify remains for possible excavation and study, and the integrated with historic records, especially in results can also be used for statistical and spatial analysis older cemeteries where grave markers are moved when integrated with historical records. If identifi ed by these or missing. GPR has the potential to precisely methods, previously unidentifi ed graves can be preserved in map these graves and add an important data areas threatened by construction or erosion. layer to any historical study involving burials and burial practices. Introduction Lacking geophysical means, fi nding historic graves using traditional probing or excavation Locating, studying, and sometimes excavating methods has often been a “hit or miss” task historic period graves can produce a great deal for most archaeologists. Attempts to locate of information about the past not otherwise these subsurface features using visual analysis available from archival documents or other data of surface soils or vegetation changes are also sources. If the goal is to study skeletal remains fraught with problems. Head- and footstones for osteological or molecular studies, the fi rst that were once present in many historic cemeter- step must be identification of the graves of ies are often deteriorated, relocated, or missing. interest. Many historic cemeteries are poorly Written documentation about grave locations is maintained and often threatened by erosion, often incomplete, inaccurate, or absent. Fall- development, and agricultural operations, making ing trees can uproot underlying sediments as the identifi cation of graves important if they are well as human remains; animals can burrow to be preserved. Sometimes unmarked graves into graves; and the wood associated with need to be identified so that human remains coffi ns and surface markers quickly rots with may be removed if threatened by construction little or no trace. Often there is little to assist or even to make way for additional burials researchers in locating graves other than vague when cemeteries expand their boundaries or memories about where burials were located or fi ll in areas that appear to be vacant. poorly drawn sketch maps. Geophysical techniques such as ground-pen- Archaeologists have attempted to locate etrating radar (GPR) can be used to located graves by inserting probes in the ground in Historical Archaeology, 2006, 40(3):64–73. Permission to reprint required LAWRENCE B. CONYERS—Ground-Penetrating Radar Techniques to Discover and Map Historic Graves 65 an attempt to detect soil changes, voids, or in ground conditions that might be related to areas that might be less compacted (Killam the presence of graves. The differences in the 1990). Some have resorted to dowsing, with readings within the grid, when mapped spatially, little success (Barrett and Besterman 1968; can often be related to burial phenomena, such Reese 1985; Van Leusen 1998) or employed as the presence or absence of artifacts associated psychics (Goodman 1977), and a few have with human remains or geological changes that even attempted to use dogs, purported to have can be related to grave shafts. The human acute senses of smell, that are trained to sniff remains themselves cannot generally be detected out human remains (Killam 1990). since there is not enough contrast between them A more reliable method that has been used to and the surrounding material. locate and then map historic graves is the use GPR is one of the best methods to map of geophysical devices that can measure physi- graves because it is capable of measuring both cal and chemical changes in the ground. These physical and chemical changes in the ground in changes may be related to grave shafts, coffi ns, three dimensions; therefore, depth as well as the void spaces, and even the human remains them- spatial distribution of graves can be determined selves (Bevan 1991; Nobes 1999; Davenport (Bevan 1991; Davis et al. 2000). This can be 2001). The most common of these are mag- accomplished because radar pulses are trans- netic gradiometry, electrical resistivity, GPR, and mitted from a surface antenna and refl ected off electromagnetic conductivity. Magnetic methods buried discontinuities. The returning pulses are use passive devices that measure small changes measured in elapsed travel time. When time is in the Earth’s magnetic fi eld that are infl uenced converted to distance (using measurable veloci- by changes in soils and buried materials below ties common to each site), depth in the ground the surface. These changes can result from the can be readily determined. In addition, radar presence or absence of metal in coffi ns or even energy is readily refl ected from any disconti- minute differences in soil and sediment types nuity in the ground, including soil compaction that exist between grave shafts and undisturbed changes, mineralogical differences, sediment size adjacent materials. The other three most com- distinctions, void spaces, and the type and con- monly used geophysical methods use tools that centration of associated artifacts. Amplitudes of transmit energy into the ground and then mea- the refl ected waves can also be precisely mea- sure how that energy is affected by changes in sured, indicating differences in material proper- the ground related to the presence or absence ties within the ground, producing an additional of graves, grave goods, and soil changes. The measurement that is valuable in locating subtle resistivity method transmits an electrical current buried features. into the ground and measures the differences in GPR systems are compact and easily trans- voltage between the transmitting device and a ported to and from the fi eld. A typical system recording device some distance away. When consists of a radar control system and associ- mapped spatially, changes in these resistance ated computer, antennas, and a power source readings can be related to the presence or (Figure 1). Grids of data (up to 40 x 40 m) absence of graves. A similar method of energy can be collected in a day, depending on the transmittal is used in electromagnetic (EM) con- transect spacing and the number and complex- ductivity, where an EM field is induced into ity of surface obstructions. Refl ection data are the ground and measurements are taken, which easily transferred from the GPR system to a indicate how that fi eld is affected by the under- laptop computer for immediate analysis, with lying deposits. GPR is also an active method preliminary results often available just hours that transmits pulses of radar energy of differ- after collection. ing frequencies into the ground and measures properties of the refl ections derived from buried Grave Characteristics materials in the ground. All of these geophysical methods collect data Physical anthropologists have long concerned along a series of transects within a grid, which themselves with fi nding human remains, whether can be interpreted individually as two-dimensional intentionally buried or covered and preserved profi les or as a group to spatially map differences by natural means. A large body of literature 66 HISTORICAL ARCHAEOLOGY 40(3) and its associated artifacts, (3) the backfill used to fill in the vertical shaft, and (4) the surface layers of sediment or soil that have accumulated on that shaft after interment. Of these four features, the contact between the shaft and the surrounding material, coffi ns containing remains, and sometimes associated artifacts are what can be readily imaged using GPR. When human bodies, coffi ns, urns, or any other grave goods are placed in the ground, a vertical shaft is excavated through surface soils and underlying sediment or rock units, producing an aerially FIGURE 1. This Geophysical Survey System Inc. (GSSI) distinct and often recognizable feature that can Subsurface Interface Radar (SIR) system model 2000 with be seen in GPR reflection profiles. During antenna and carrying case. (Photo by author.) excavation of a grave, the natural substrate and surface soils are almost always placed on the ground nearby and then returned to the grave addresses the detection of human remains for shaft after interment.