Geophysical Surveys near Sierra Vista, Arizona

Abstract

Five different geophysical methods were used near Sierra Vista, Arizona in order to determine the presence or absence of impermeable silt-clay layers in reference to a proposed water-retention system. Geophysical investigations were conducted at Woodcutters 3 and Basin Floor 1 (both approximately one kilometer long north-south and east-west transects), and School Basin (10 meter by 50 meter area where only TEM data were collected). Magnetic, VLF, seismic, EM 31 and 34, and TEM survey data were collected on February 28th and 29th and March 20th and 21st of 2004. Magnetic and VLF surveys conducted at the Woodcutters 3 and Basin Floor 1 sites had relatively flat profiles that indicated only a few single-station anomalies with little difference between the two sites. Seismic survey data produced velocities in two-layer earth models that were similar between the two sites in the upper 5 meters, again indicating little difference between the sites. The EM 31 and 34 surveys, with depths of investigation in the 3-6 meter range, measured higher ground conductivity values in the upper 10 meters at the Woodcutters 3 site. This agreed with shallow borehole data from the Woodcutters 3 and Basin Floor 1 sites. The higher ground conductivity is indicative of higher water content, which may be due to the increased percentage of clay. For the TEM surveys, conducted at all three sites and with depths of investigations in the 0-50 meter range, it was found that a low resistivity layer at 30-50 meter depth resides at the Basin Floor 1 site. This layer is deeper than the available borehole data. At both sites the borehole data do not show definitive clay layers, making it difficult to correlate the high conductivity values with clay content percentages.