GROUND-PENETRATING RADAR INVESTIGATION OF GOPHER-TORTOISE BURROWS: REFINING THE CHARACTERIZATION OF MODERN VERTEBRATE BURROWS AND ASSOCIATED COMMENSAL TRACES

Large-sized vertebrate burrows are challenging structures for neoichnologists to assess, often defying accurate description unless excavated or otherwise disturbed in ways that degrade their incipient qualities. Moreover, such invasive procedures may erase other valuable information, such as the presence, locations, and forms of burrows made by commensal animals living in these structures. To avoid these potential problems, we propose the use of ground-penetrating radar (GPR) for imaging large vertebrate burrows. In our study, we profiled modern, active and inactive burrows of adult and juvenile gopher tortoise (Gopherus polyphemus) on Saint Catherines Island, Georgia. The GPR unit used for this procedure was a MALA system with cart-mounted 500 MHz and 800 MHz shielded antennae. In practice, the burrows produced GPR reflection hyperbolae similar to those of utility pipes and roots. Primary burrow trends were established quickly by starting at the burrow mouth and using a loose zig-zag profile pattern to identify the burrow, then flagging it on the surface. Profile grids were established and conducted to ensure the burrow system was fully represented, thus providing a detailed database. GPR-Slice™ software was used to create two-dimensional and three-dimensional images of the burrow structures without disturbance to any animals occupying the burrows, likewise avoiding our inadvertently adding to taphonomic processes affecting burrows. Our methods resulted in resolving the overall geometry of gopher-tortoise burrows. Single and multiple intersecting burrows of various sizes, presumably made by commensal animals or juvenile gopher tortoises, were also definable from the GPR data. To further test our technique, we are building a burrow model with PVC piping to establish radar response to profile and burrow orientation. Additionally, a time series of GPR surveys could show how burrow structures may change over time with taphonomic processes and evolve into forms that would more realistically mimic analogous trace fossils.