SOIL MOISTURE IMPACTS ON GROUND-PENETRATING RADAR INTERPRETATIONS OF BURIED GRAVE SHAFTS

Ground-Penetrating Radar (GPR) is a powerful geophysical tool that has been used to non-invasively locate grave shafts, but interpretations can be challenging in differing soil types and with spatially variable soil moisture conditions. GPR emits an electromagnetic wave into the soil which bounces off any subsurface features and returns to the receiver. The time it takes the wave to do this is converted into a depth based on soil properties. Particularly conductive or insulative materials can interfere with the path of the wave, dispersing or dampening the signal, which can influence the travel time of the electromagnetic waves and therefore interpretations of depth. 10x9 m and 10.5x9 m GPR transect and soil moisture grids were taken of the P4 Parking Lot Cemetery in Colonial Williamsburg using a 200 mHz antenna and processed as radargrams and depth slices in EKKO_Project 6. Previously exposed (but never excavated) grave shaft locations were overlaid onto the two depth slices with the largest difference in soil moisture content. The accuracy of each depth slice was determined by calculating the error of the distance between the centroid of the known grave shafts and centroid of the GPR-derived hotspots in ArcGIS. There were three methods used to create the polygon for determining the hotspot centroids: polygons based on the actual shape of a hotspot, polygons based on a circular approximation for a hotspot, and polygons based on the shapes of hotspot groupings. Different comparison methods could be more accurate in different environmental conditions, leading to a more accurate interpretation of the location of subsurface anomalies. This project explores the effect of soil moisture content on accuracy of depth slice hotspots and how to correct for distortion in radargrams due to soil water content. Improving understanding of GPR interpretations in variable environmental conditions may help refine future gravesite detection protocol with implications for using GPR to identify subsurface geological structures.