GEOPHYSICAL DETECTION AND IMAGING OF A LOST MID-PLEISTOCENE BONE CAVE BENEATH VALLEY FORGE NATIONAL HISTORICAL PARK, PA

This paper describes a geophysical survey to locate the lost Port Kennedy Bone Cave of Valley Forge Nat. Hist. Park. The cave contains mid-Pleistocene fossils that were collected by famous paleontologists of the late 1800s, and were new to science, with some still found nowhere else. When workers were driven out by flooding, the cave was covered with quarry waste, later industrial asbestos waste, and was lost for over 100 years.

Re-locating the cave was the mission of Brian Lambert (Nat. Park Svc.) in the 1990s. In 2002, he enlisted Edward Daeschler (Phila. Acad. of Nat. Sci.) and Matthew Lamanna (UPenn) to do historical research. This team determined which of the local quarries is the Bone Cave Quarry, but could only estimate where the fissure might lie.

Given asbestos quarry fill, direct exploration for the fissure would entail health and environmental risks. Therefore, Edward Doheny (UPenn) proposed non-intrusive geophysics. Based on cave and overburden characteristics, a team of Ed's students and colleagues chose microgravity and 3-D electrical imaging as the best available geophysical technologies. In Summer 2004, matching gravity and resistivity lows were detected and provided evidence of the lost cave location.

To provide positive identification as the actual Bone Cave, a seismic profile across the gravity/resistivity anomaly was recorded in Spring 2006. A seismic refraction profile, using iterative ray tracing that provides only layer boundaries (SIP) did not detect a cave-like feature. However, using the same dataset, a full 2-D inversion (SeisOpt Pro) provided a P-wave velocity image of a feature that remarkably resembles 1800s sketches of the cave. Seismic surface wave data were also recorded/inverted using the refraction microtremor (ReMi) method. Surface wave data are fundamentally different from refraction data, and ReMi inversion is done on S-wave velocity in the amplitude/wavenumber domain rather than on P-wave velocity in space/time domain for refraction. Nevertheless, the ReMi image also reveals a feature consistent with 1800s sketches.

These results are high-confidence confirmation that the lost Bone Cave has been found, and provide the only existing modern images of the cave. In addition, results indicate that SeisOpt and ReMi may be good tools for imaging limestone cavities in (e.g.) engineering- and hydro-geophysics.