GEOELECTRIC AND GEOMAGNETIC CHARACTERISTICS OF FRACTURE TRACES IN THE LA SAL MOUNTAINS, UTAH

Fracture traces, linear features as seen on aerial photos, can be associated with deep-seated faults or zones of fracturing or dissolution, and have been useful in choosing sites for water wells in carbonate rocks. However, their use in choosing well sites in volcanic rocks has been limited. The objective of this study is to draw connections among the appearance of fracture traces as seen on aerial photos and on the ground, the geophysical signature of fracture traces, and the productivity of water wells drilled on and off fracture traces in volcanic rocks. The motivation for this study is a desire to increase the success rate of Utah Valley University’s program of groundwater development in the Trans-Mexican Volcanic Belt. The objective was addressed by carrying out geoelectric and geomagnetic surveys of fracture traces in the rhyolite / diorite / monzonite of the La Sal Intrusives in the La Sal Mountains of Utah, which can be regarded as a geologic analogy to the Trans-Mexican Volcanic Belt. Surveys of the total magnetic field were carried out perpendicular to seven fracture traces using the Geometrics G-856 Proton Precession Magnetometer. Magnetic susceptibilities of exposed rocks were measured both in the field and in the lab in order to constrain magnetic models. Surveys of electrical resistivity, chargeability and spontaneous potential were carried out across three fracture traces using the Iris Instruments Syscal Junior Resistivity System with the Schlumberger electrode array. The geoelectric surveys included one sounding on each fracture trace and 2-3 soundings 500 m off each fracture trace with the electrode array parallel to the fracture trace and electrode spacings in the range AB/2 = 2.3-137 m. Geoelectric profiles were carried out perpendicular to fracture traces (with the array parallel to the fracture trace) at spacings AB/2 = 15.2 and 23.8 m. Preliminary magnetic models require subsurface bodies beneath each fracture trace with positive magnetic susceptibility anomaly χ = 0.05 SI units at depth 55 m with horizontal extent 160 m. Inversions of the resistivity soundings require subsurface electrical resistivities about an order of magnitude lower on compared to off a fracture trace in areas of groundwater discharge, but the reverse in areas of groundwater recharge. Further results will be reported at the meeting.