MAPPING FISSURES IN PAHRUMP, NEVADA USING GEOPHYSICAL METHODS

Pahrump, NV is experiencing rapid growth in population, which is leading to increasing groundwater withdrawal. These groundwater issues are similar to other urban areas of the desert southwest that typically leads to subsidence issues that are in need of assessment. A common feature that accompanies subsidence in the Pahrump Valley is earth fissuring. Fissures are surface expressions of deep fracture systems typically caused by groundwater withdrawl greater than aquifer recharge. Fissures form as a result of the collapse of the aquifer sediments at depth, which leads to flushing of fine material in the sub-surface. Generally fissures are not identified until they are exposed at the surface induced from flash flooding or over-watering. Pahrump has experienced significant differential subsidence within the Valley causing major structural damage. These fissures can be 10's of meters deep and 100's of meters long. These ground failures can be exacerbated by faults at depth, shallow bedrock, and/or differential compaction.

The purpose of our study was to locate these features prior to their exposure at the surface. Magnetics, ground penetrating radar (GPR), and seismic reflection data were acquired across several fissures to calibrate their subsurface expression and trend, especially where there was no surface expression. The magnetics correlated well with the GPR results showing the shallow sub-surface fissures clearly in both exposed and non-exposed areas. The GPR signal penetrated to about 1.5 m depth with a clear anomaly of the fissure. The fissure signal widened at depth and appears to show slumping, which is typical of these features. Initial seismic reflection results are consistent with the magnetics and GPR for the location of the fissures. Additional processing will provide a cross section that images the fissures at a greater depth than the GPR results. Fissures are an unmitigatable environmental concern. Our results may aid in fissure identification prior to surface expression as well as develop a geophysical mapping technique that is efficient and economical, which may provide additional data for the local jurisdiction in zoning of the land surrounding earth fissures.