Geophysical Surveys near the San Pedro River, Arizona

Abstract

During the Spring semester, 2001, the University of Arizona Geophysics Field Camp (GEN/GEOS 416/516) conducted geophysical surveys near the San Pedro River, Arizona. This site is being investigated by the U.S. Geological Survey as part of a basinwide study of water resources. Our geophysical investigation was designed to provide information that will assist in this hydrogeological study. The surveys were conducted near Boquillas Ranch (approximately 2 km south of Fairbank, AZ) and approximately 2 km north of Hereford, AZ. Dipole-dipole and Schlumberger DC resistivity surveys were conducted at both the Boquillas Ranch and Hereford sites. Depth to water table was approximately 10-20 meters at the Boquillas Ranch site and 0-to-1 0 meters at the Hereford site. The electrical resistivities at depth at the Hereford site were unusually low for Arizona basin-fill sediments (less than 10 ohm-m). Transient electromagnetic (TEM) soundings were also recorded at both the Boquillas Ranch and Hereford sites. Electrical resistivity models were determined that were very similar to the DC resistivity models. In general, the TEM soundings were able to provide similar information to the DC resistivity soundings with greater data acquisition rates and with a smaller crew. Electromagnetic induction (EM31 and EM34) surveys were run at the Hereford site. Qualitatively, an electrical resistivity structure was found that was similar to the DC resistivity survey and the TEM survey results (i.e. the resistivity decreased with increasing depth of investigation).These data are not well suited, however, to quantitative modeling. Seismic, magnetic, and gravity surveys were run at the Boquillas Ranch site. These surveys provided useful background information on the geologic structure at this site. They did not provide direct information on the hydrogeology at the site.