Ground Penetrating Radar Imaging Prehistoric Animal Skeletons and Paleofeatures in Ash Beds at Ashfall Fossil Beds Historical State Park, Nebraska

Abstract

Ground penetrating radar (GPR) is a non-invasive imaging method of the shallow subsurface. However, prior to my study, the capabilities of GPR for imaging bones encased in fine volcanic ash deposits had not been determined. To evaluate those capabilities, I tested 500 MHz and 1 GHz frequency GPR on 11.86 ± 0.13-million-year-old bone assemblages buried in fine Konservat-Lagerstätte ash deposits at Ashfall Fossil Beds State Historical Park in northeastern Nebraska. The main objectives of this research are to test the ability of GPR to image ash beds, buried animal skeletons and ichnofossils, and map their presence across the site. For this investigation, I acquired 173 GPR lines in dense grids covering approximately 220 m2 of the unexcavated section of the Hubbard Barn at Ashfall.Fine volcanic ash is a low electrical conductivity environment that favors GPR imaging due to low attenuation of radar signals. GPR images ash layers as strong continuous reflections. Both the 500 MHz and 1 GHz GPR frequencies imaged through the entire thickness of ash deposits, detecting the interface with the underlying sandstone at depths of up to 1.5 m. The ash to sandstone interface is identified by a characteristic loss of signal strength in the underlying sandstone. Areas of interest containing animal skeletons are characterized by low amplitude, discontinuous reflectors encased within continuous high amplitude ash layers. Four distinctive GPR reflection characteristics identified within the ash beds correspond to the presence of animal skeletons or multiple skeletal remains as well as vertebrate and invertebrate ichnofossils (trace fossils). The four characteristic GPR signatures were mapped across the site. This study demonstrates that GPR is a suitable method for paleontological investigations in ash deposits. Also, the study proved successful in imaging the subsurface paleostratigraphy as well as identifying areas of interest that may contain the remains of prehistoric animal bone assemblages.