Key Words: Ground Penetrating Radar, Clandestine Burials, Geophysical Applications in Anthropology, Historic Cemeteries introduction and purpose

As with other GPR surveys, it is important to conduct a GPR survey of a clandestine burial on a grid with transect intervals remaining under 1 meter (Bevan, 1991; Conyers, 2006). Some more recent research suggests that best practices fora clandestine burial survey should even stay below a half meter (Wardlaw, 2009). The grid should attempt to traverse the possible burial at aright angle in order to best visualize the anomaly (Bevan, 1991).
GPR in Historic US. Cemeteries and an Example from Florida
There area number of reasons that an historic cemetery may require a geophysical search written documentation about these sites maybe missing or incorrect, thus providing inaccurate information as to the site-limits; markers may have gone missing or have been moved encroachment from development threatens possible interments the location maybe lost due to the overgrowth of the environment human remains may need to be removed due to imminent construction projects (Conyers, b. Information on historic cemeteries also provides an invaluable insight into community practices at known points in history burial practices, orientation of graves, gravestone iconography, and associated grave goods all provide a rich depth of data to the investigation of community practices. GPR surveys thus provide an excellent additional source of information to research questions associated with historic cemeteries. The ability of GPR to detect a burial in an historic cemetery is largely dependent on the materials used in burial, the preservation of the body or coffin, and the condition of the soil
(Conyers, b. Differing interment practices are associated with different time periods in US. history. Early settlers often wrapped the dead in shrouds and buried them in wooden caskets, which were most commonly used into the latter half of the 19
th century. This may prove

difficult for GPR surveys to detect as caskets collapse and deterioration overtime, along with a settling of grave shaft soil, may not provide enough of an amplitude difference for the GPR unit to detect an anomaly. Metallic coffins became more widely available after mass production began just prior to the Civil War and, if they are relatively intact, will produce high amplitude signatures during a GPR survey. Currently, many burials in cemeteries are encased within a liner or cement vault that is designed to preserve the ground stability. These provide large, distinct targets for radar energy during a survey (Doolittle and Bellatoni, 2010). There are several physical features that can be potentially imaged using GPR (Table 5), but each has to do with the detection of differences in the subsurface media as detected by differing patterns of amplitude as discussed previously.
Table 5. Features that have the potential to be imaged by GPR in an historical cemetery survey. Adapted from (Conyers, b. Natural soil substrate below and surrounding grave shaft. Buried coffin or body and associated artifacts.
3.
Backfill used to fill in vertical shaft. Surface layers of sediment that has accumulated over shaft after interment. The following example provides a good overview of GPR field methods for the detection of clandestine burials as well as provides an understanding for how this type of research maybe applied. Wardlaw (2009) in his research investigated an historic cemetery with many marked graves which he was able to use as controls for the detection of unmarked graves. This research has applicability to the forensic search for clandestine burials (Schultz, 2007; Schultz and
Dupras, 2008; Schultz, 2012) as well as providing anew dimension of data to archaeological investigations (Bevan, 1991; Nobes, 1999; Conyers, 2006; Sutton et al., 2013). As part of his thesis research at the University of Central Florida, Dennis Wardlaw
(2009) conducted a survey of Greenwood Cemetery in Orlando, Florida to determine the ability

of GPR surveys to detect burials at different points of interment. This granted an opportunity to test the utility of GPR at an historic cemetery site that provided a wide range of burial dates that may also be tested against current burials, as the cemetery is still in operation. The Greenwood cemetery was incorporated in 1880 and purchased by the city of Orlando in 1892. A local ordinance then required the removal of smaller cemeteries interspersed throughout the Orlando area with the remains reinterred at Greenwood, which became the only authorized burial site within city limits from this point forward. Greenwood is large, 68.7 acres, and due to its size and age has a wide variety of interment types from different historical periods represented (Wardlaw, 2009). The survey was conducted on 29 out of 36 sections at Greenwood utilizing a 500 MHz antenna. Transects crossed graves at a perpendicular angle at 50 cm intervals. As anomalies associated with burials were encountered they were marked and noted for depth. This research also compared the detection of anomalies with T-bar probe ground-truthing. The comparison of these methods showed that utilizing the T-bar probe proved to bean effective method of confirmation for GPR data collection in the field. This GPR survey depicted the difficulty in detecting earlier graves by solely using GPR, though the use of the T-bar probe in conjunction with a GPR survey was shown to be worthwhile. Earlier graves likely proved harder to detect due to deterioration of the casket and body, lack or loss of casket hardware that may have provided a significant amplitude signature, and the settling of soil in the grave shaft overtime. As GPR detects the difference in the electrical or chemical properties of soil, compaction of the soil and weathering overtime may create a situation where there is little detectable difference in the substrate. By the s,

however (Figure 7), the GPR unit readily detects both homogenized soils owing to the digging of a grave shaft as well as the coffin or burial vault (Wardlaw, 2009). This survey provided a worthwhile initial investigation of the relationship between time and GPR detection capabilities. Further research in this vein may provide a refinement of burial detection abilities, especially for burials dating before the middle part of the last century. This, in turn, may prove worthwhile for historic cemetery location in other locations that have a relatively high pH level in their soils as does Florida. Figure 7. Bar graph showing total graves comparison from Greenwood cemetery survey. Note the efficacy of GPR as tested against the probe (Wardlaw, 2009).

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