Geophysical Study of Natural Bridge—A Comparison of Methods

Natural Bridge is a naturally occurring stone archway and historical site located in Rockbridge County, Virginia. It is the only natural stone archway in the country that supports a state road that carries everything from motorcycles to tractor-trailers. The arch is part of a state park, and as a tourist attraction hundreds of pedestrians walk under the arch each week. In 1993 a large slab of rock fell from the bottom of the arch and killed a tourist. Though rock bolting was undertaken to mitigate the risk of future rock falls, no thorough geologic study has ever been conducted. This study was undertaken to supplement a larger study conducted by the Radford University Geohazards Institute as part of a Virginia Department of Transportation initiative to gather geologic information on the arch. This initiative prohibited drilling as a means of obtaining the geologic information. Therefore, multiple non-destructive means were employed in the study that included geophysical, laser, optical, and acoustic methods. The authors were involved in the collection and analysis of the geophysical data on the arch that included resistivity imaging, seismic refraction, multi-channel analysis of surface waves (MASW), and ground penetrating radar (GPR). This paper will present the results of each method and how the various methods were integrated, as well as the advantages and disadvantages of each.