The ground penetrating radar (GPR) method is based on the propagation of high frequency electromagnetic (EM) waves. The waves are sent into the ground and reflected back to the surface in response to the different dielectric properties of the material the waves travel through. Upon arrival to St. Rose Cemetery an initial visual investigation of the site was conducted to locate any surface expressions of burials (i.e. depressions in the ground). A grid (7 m by 9 m) was set up with transects every 0.5 meters (traces collected every 0.05 meters). The data presented here was collected using UWEC’s pulseEKKO 1000 GPR system and utilizing the 225 MHz antennae. The higher frequency GPR data allowed for a more detailed image of the shallow subsurface which is preferred for investigations of buried objects near the surface. The post-acquisition analysis involved processing and plotting the information using pulseEKKO software and interpreting the data sets with radar stratigraphic analysis.
As the antennae are moved across the surface, the EM waves respond to changes in the subsurface and bounce back to the receiving antennae. Since the waves propagate out in all directions the antennae can ‘see’ an anomaly before it passes directly over it, which is known as a diffraction.
When the antennae first ‘see’ an anomaly in the subsurface, it records the object as if it were directly beneath the antennae. Since the wave travel time is longer, the object is recorded at a greater depth. This creates a hyperbolic shape with the object being located at the apex of the hyperbola.
This is an image of the 'Baby Lot' and the grid that we shot on our third visit to St. Rose. Each blue line represents a transect that we shot with both antennae.