GROUND PENETRATING RADAR AND ELECTRICAL RESISTIVITY SURVEY NEAR BREAKDOWN CAVE, CHRISTIAN COUNTY, MISSOURI

Breakdown Cave, on the banks of the James River in Christian county, near the Greene County line, is managed by the Springfield Plateau Grotto as a research and educational laboratory. In recent years, during a prolonged drought, a cave passage normally water-filled had water levels low enough to allow for exploration of additional passageway. The new exploration revealed a domed “room” with significant flowstone and water flow. This room was estimated to be approximately 30 -35 feet high, which suggests its top arch might be close to the land surface above the cave. However, the original explorers did not have surveying equipment with them during the initial visit and the water filling the access passage since that initial visit has prevented any accurate surveying of the room's depth below ground surface. Uncertainty over the actual thickness of competent rock beneath the local soils has become a concern for builders and developers after the recent (August 2006) collapse of a 72+-foot deep sinkhole under a house in the nearby city of Nixa. In order to determine where the cave exists and how close to the surface it is, ground penetrating radar and (GPR) and direct-current electrical resistivity survey were conducted. An 100 MHz GPR antenna was used to collect 14 one hundred meter long profiles over the suspected cave. In addition, to confirm the results of the 100 MHz survey, a 250 MHz GPR antenna was used to collect 4 profiles and 2 electrical resistivity profiles using the Schlumberger array were also collected. The processed GPR data does not reveal any significant reflections/diffractions below 4 meters which is the approximate thickness of the soil column. Two regions: one near the middle of the profiles and another near the eastern end indicated reflections that may be due to a thickening of the soil layer or due to narrow cavities. The electrical resistivity profiles additionally indicated higher electrical resistivity values in these regions which suggest that either there are narrow cavities or that the GPR reflections may be due to variations within the limestone. There was no electrical evidence of the top of the cave being within 3 meters of the surface in the region we surveyed.