A geophysical study of the Patagonia-Sonoita Nature Conservancy property was carried out to determine the depth to bedrock and the geometry of the underlying basin in the area. The survey was part of the ongoing U.S. Geological Survey studies of the region. CSAMT, TEM, DC Resistivity, Seismic, Gravity, and Magnetic surveys were performed. Many of the surveys were not able to map depth to bedrock because the depth of penetration of these surveys was less than the large depth to bedrock, which we encountered in this area. The best estimate of depth to bedrock in the center of the valley is of the order of one kilometer, as identified by the CSAMT surveys. The CSAMT survey found a significant low-resistivity anomaly in the vicinity of the Nature Conservancy Visitor Center. This low-resistivity anomaly could be due to clay-rich rocks or possibly mineralized rocks. Magnetic and TEM anomalies also occurred in the same area as the CSAMT low-resistivity anomaly. Seismic surveys showed a loosely consolidated surface layer above more consolidated alluvium. The model from the · Gravity survey shows an extensive alluvial basin overlying rhyolite with high and low-angle faults.

During the Spring semester, 2000, the University of Arizona Geophysics Field Camp (GEN/GEOS 416/516 class) conducted geophysical surveys in the vicinity of the United Musical Instruments facility near Nogales, Arizona. This site is a super-fund site, due to the presence of contaminants, including TCE, in the ground water. The contaminants are presumed to have come from cleaning and electroplating solutions, which had been dumped into a small pond on the UMI property. The U.S. Geological Survey provided the funding for our study. The objective was to determine subsurface structural information that would help interpret possible future movement of the contaminant plume. Transient electromagnetic (TEM) data were most useful for interpreting the subsurface geohydrology. Water table was found at a depth of approximately 30 meters, north of the UMI building. A particularly interesting feature in the TEM data was a high-resistivity anomaly and an associated low-resistivity anomaly 10 to 1 00 meters northeast of the UMI building. We interpret the high-resistivity anomaly as possibly being due to an impermeable levee that was associated with a buried stream channel and the low-resisitivity anomaly as possibly being due to the pooling of conductive contaminated fluids against the impermeable levee. DC resistivity surveys were dominated by cultural coupling, which was pervasive m the survey area. Magnetic readings were used to help locate potential cultural interference. Gravity surveys indicated a low-density anomaly, which may be due to a buried stream channel. This feature could be related to the features mapped with the TEM surveys. The seismic survey indicated progressively more compacted and cemented alluvium, overlying the Nogales formation. The seismic data did not provide any direct information about the geohydrology of the area

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.

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.

Four different geophysical methods were used near Chino Valley, Arizona in order to map a suspected andesite intrusive, identified as plug 15, which had originally been found using aeromagnetic data already acquired over the area. Magnetic, transient electromagnetic, seismic, and gravity measurements were collected between 3/1/2003-3/2/2003 and 3/22/2003-3/23/2003. The surveys were located near the center of section 35, township 17 North, Range 2 West, just north of Chino Valley, AZ. The magnetic and TEM surveys provided the best indication of the location and depth of the plug. The north-south spatial extent of the plug was estimated to be approximately 600 meters. The depth to the top of the plug was found from the TEM survey to be approximately 300 meters at the center of the survey. The seismic survey did not reach deep enough to find the andesite anomaly and the gravity survey did not appear to be affected by the plug. Magnetic, TEM, and seismic surveys were also performed at another site located approximately 1.25 km northeast of plug 15. The seismic survey did not reach deep enough and the magnetic survey was too short to provide a depth interpretation. The single TEM sounding measured a very high resistivity (approximately 900 ohm-m) at this site.

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.

VLF (Very Low Frequency), magnetic, TEM (Transient ElectroMagnetics), and Seismic were performed in Yuma, Arizona over two weekends in February and March of 2005. The targets of interest in Yuma included the depth to the shallow bedrock, the trace of the Algodones Fault, and the distribution and thickness of clay units. The VLF and magnetic surveys both proved not to be valuable for the interpretation of groundwater or bedrock depth. The Seismic surveys found a quartz monzonite horst structure at a depth of 8 meters and 250 in length and a graben at 8 meters depth and 48 meters in length. The TEM surveys were performed along Line 2 and Line 4 for Site One, Line 1 and Line 3 for Site Two, and SG1 line and SG2 line for Site Three. The TEM survey, located near the Seismic surveys, was in agreement with the findings of these Seismic surveys. At all sites, the TEM surveys were useful for determining depth to water table. At one site, the TEM survey mapped substantial near-surface clay layers.

Four geophysical surveys were conducted at the Nature Conservancy about 20 miles north of Benson, AZ, in the Upper San Pedro River Basin, in order to better understand the nature of the sub-surface features of the basin. The geophysical methods included TEM (Transient Electromagnetic), seismic, EM34 and magnetic surveys. The TEM, seismic and magnetic surveys were conducted perpendicular to the river basin while the EM34 lines followed the riverbed. The perpendicular surveys were divided into two regions, referred to as the South and North Lines. The TEM, seismic, and magnetic surveys revealed a consolidated bedrock structure at shallow depths (30-40 m) along the South Line. The feature has an east-west extension of approximately 500 meters, and is located just east of the San Pedro River. None of the perpendicular surveys were able to detect bedrock features along the North Line, implying that the depth to bedrock exceeds the maximum depth of this investigation (360 m). Both lines showed regions of high porosity, and, potentially, of saturated materials. These regions were more prevalent along the North Line, where numerous highly porous areas were detected at various depths (including one region beginning at a depth of 50 meters and extending at least to 360 m). The EM34 failed to detect any appreciable long-wavelength changes in conductivity along the riverbed, though localized point anomalies were found.

Transient Electromagnetic (TEM) surveys were conducted in the San Pedro Valley starting approximately 1 mile northeast of Benson, Arizona, and extending about 2 miles farther northeast. The survey used loop sizes of 20x20, 100x100, and 200x200 meters with the objectives of determining the depth, thickness, and lateral extent of clay deposits, and comparing ground surveys with a previously acquired airborne TEM survey. The data were processed with Zonge Engineering smooth inversion software as well as Interpex TEMIX layered-earth inversion software. The interpreted depth to near-surface clay deposits was less than 5 m on the west end near the San Pedro River, and increased to about 15 m, 1.3 km to the east. Farther east, clay deposits were only detected at depths of 100 m or more. A possible bottom to the clay was detected near 100 m depth at selected stations in the western half of the survey, which would correlate with wells in the vicinity, but it was not laterally continuous. Surveys at the remainder of the stations did not detect a lower limit to the clay deposit. The results of the airborne survey versus the ground elevation surveys show similar resistivity values.

Groundwater resources are essential to support the growing population of Benson as well as the agriculture, and wildlife throughout the Middle San Pedro Basin. A refined model of the hydrogeologic framework within the region is necessary to allow for the most efficient allocation of the area’s ground water resources by city planners and water managers in charge of future development. New data were collected by the University of Arizona’s Geophysics Field Camp to update and improve this representation. This survey utilizes Controlled Source Audio Magnetotellurics (CSAMT) to characterize previously unstudied locations in the San Pedro Basin. CSAMT data were processed and interpreted using software from Zonge Engineering and Interpex Ltd. Seven receiver stations along a five-kilometer Middle San Pedro transect were used to determine the resistivities of several basin-fill units. Results show some resistivity variability with respect to depth among the sampled regions. In the simplest representation of the data, four layers were modeled. Resistivities ranged from 15-30 ohm-m in the near-surface units. Deeper units showed resistivities of 5-10 ohm-m. These results were plotted to help identify conductive aquitard (clay) and potentially more resistive aquifer units. This clay unit could correspond to a thin unit of the St. David Formation, which has previously been identified as a confining unit between two separate basin aquifers. CSAMT data indicate that the local bedrock is deeper than 500 m.