GEOELECTRIC AND MAGNETIC CHARACTERISTICS OF FRACTURE TRACES: A TOOL FOR GROUNDWATER EXPLORATION IN IGNEOUS ROCKS

Utah Valley University has a long-term program of groundwater development in the Trans-Mexican Volcanic Belt in the state of Guanajuato, Mexico, in cooperation with Choice Humanitarian. Thus far, we have drilled a successful 50-m well in basalt for the village of Tamaula and an unsuccessful 22-m well in rhyolite for the village of Garbanzo. Well sites have been chosen based on spring locations and on fracture traces, which are linear features as seen on aerial photos. The objective of this study is to measure and model the geoelectric and magnetic characteristics of fracture traces as a means of determining which fracture traces are most indicative of actual fractured rock. Thus far, studies have been carried out on 20 fracture traces in rhyolite in the Trans-Mexican Volcanic Belt, six fracture traces in rhyolite / diorite / monzonite of the La Sal Intrusives in the La Sal Mountains of Utah, and one fracture trace in rhyolite of the Soldiers Pass Formation west of Utah Lake. The Earth’s total magnetic field has been measured at about 1-m spacing along profiles perpendicular to fracture traces using the Geometrics G-856 Proton Precession Magnetometer. Magnetic susceptibilities have been measured in the field using the portable SM-20 Magnetic Susceptibility Meter and in the lab using the Bartington MS3 Magnetic Susceptibility Meter. Geoelectric soundings for resistivity, chargeability and self-potential have been carried out both on and off fracture traces using the Iris Instruments Syscal Junior Resistivity Meter with the Schlumberger array parallel to the fracture trace at spacings in the range AB/2 = 2.29-137 m. Geoelectric profiles have been carried out perpendicular to fracture traces (with the array parallel to the fracture trace) at spacings AB/2 = 15.2 m and 137 m. Geoelectric and magnetic modeling is being carried out using the 1X2D-GM Magnetic Interpretation Software and the 1X1D Sounding Inversion Software. Preliminary results indicate that some fracture traces are associated with both magnetic and electrical resistivity lows consistent with intermediate depth (~ 50 m) fractured rock, although not all fracture traces have these characteristics. Further results will be reported at the meeting.