Geophysical Surveys near Fort Huachuca, Arizona

Abstract

The United States Geological Survey (USGS) has been studying the water resources in the vicinity of Fort Huachuca, Arizona. As a part of this study, they contracted an Airborne Electromagnetic (AEM) survey of the region, which was flown in 1997. During the spring semester of 2002, the University of Arizona Geophysics Field Camp class conducted Transient Electromagnetic (TEM) surveys at five locations near AEM flight lines in order to provide an independent test of the resistivity structure. We used 100 X 100 m transmitting loops. A central induction loop array was employed where the transient decay voltage after transmitter turn off was recorded in the center of the transmitter loop. The TEM data were inverted using a smooth modeling program from Zonge Engineering. The resulting apparent resistivity cross sections, in general, showed a resisitivity structure that was similar to the AEM cross sections down to the depth of the investigation of the TEM survey (approximately 200 m). The surface layers (zero to 50 meters) showed high resistivity (30 to 300 ohm-m) and deeper layers showed lower resistivities (10- 30 ohm meters). We also recorded low-induction number (LIN) EM surveys over a sink hole feature in this area. There was a pronounced conductivity high coincident with the sinkhole. Background conductivity readings were typically 10-15 mS/m and over the sinkhole feature the conductivity readings were typically 20-25 mS/m.