THREE METHODS FOR IDENTIFYING UNMARKED BURIALS AT HISTORIC CEMETERIES USING GROUND PENETRATING RADAR AND ELECTRICAL RESISTIVITY

Three methods for identifying unmarked burials have been developed through the analysis of geophysical data from colonial, antebellum, and post-civil war era cemeteries, and African American burial grounds in the southeastern U.S. Wooden coffins, shrouds or remains in burials dating to the 19^th century and earlier have decayed and are not detectable using geophysics. When prospecting for these older burials the goal is to identify the contrast between the undisturbed soil profile and the backfill in the grave shaft.

Ground penetrating radar (GPR) is used to identify burials on radar profiles or on time/depth slices. Radar profiles are observed while collecting data in the field and later in the processed data. Shallow truncated reflections paired with a reflection at depth is evidence of an unmarked burial on radar profiles. Shallow reflections are produced by a contrast in soil density and moisture along the sides of the grave shaft. The reflection at depth is produced by the base of the grave shaft. These elongate, rectangular anomalies observed on multiple adjacent profiles are good evidence of an unmarked burial.

A second method is to identify areas of homogenous backfill on GPR depth slices. Layering and foliation in saprolite/residuum and pedogenic horizons in a soil profile produce medium to high amplitude reflections on the depth slices. In contrast, homogenous backfill produces reflection-free anomalies the size and shape of a burial on the depth slices.

The third method is an electrical resistivity (ER) survey. Surface water infiltrates into backfill in a grave shaft easier than into the surrounding undisturbed soil. This increase in water content results in lower resistivity in the grave shaft. ER surveys are very time consuming, but the data can be used to produce maps that even the layman can understand. ER should be used to ground truth GPR radar anomalies and limit the number of false positives.

Lastly, it is important to observe patterns in the arrangement of anomalies. An east-west orientation of the long axis of an anomaly (Christian burials) and the anomalies aligned in groups or rows from north to south helps verify the interpretations. Some soils are better suited for identifying burials in the field and on GPR profiles and other soils show the burials better on GPR depth slices.