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.

An interdisciplinary survey consisting of four geophysical methods was conducted on the western edge of the Apache Generating Station’s property in Willcox, Arizona. The aim of the survey was to apply various methods for the detection of earth fissures and desiccation cracks. The geophysical methods used were static magnetic field measurements, frequency domain electromagnetics (FEM), ground penetrating radar (GPR), and seismic. Two grids were delineated and surveyed by each method. Grid 1 was set up at a site containing a fissure with visible surface expression over some parts of the grid, and Grid 2 was set up at a site with little visible surface expression of the fissure, but was suspected to contain a fissure in the subsurface. At another location, northwest of the Apache Generating Station, three lines were surveyed in an area of known desiccation cracks. All of the methods showed an anomaly associated with the fissure in Grid 1. Furthermore, at locations where the fissure is not visible in Grid 1, there were still strong anomalies in line with the suspected location of the fissure extending below the surface. Magnetic data from Grid 2 suggests that magnetics may not be a useful method in subsurface earth fissure detection at this site, where we believe that the fissure is only a very small crack with small aperture at depth. The electromagnetic results from Grid 2 show anomalies extending from lines 1 through 5 where there is only minimal surface expression in lines 1 and 2 and no surface expression in lines 3-5. No anomaly was seen in the northern end of Grid 2. It was found that GPR in Grid 2 did not display conclusive results in distinguishing subsurface earth fissure anomalies from other anomalies, such as roots. Seismic lines in Grid 2 show anomalies in the profiles that could indicate the presence of earth fissures; however a thin high velocity horizon may appear as a subsurface fissure, and this made interpretations more challenging. At the desiccation crack site, there is evidence of a shallow feature, which we interpret to be a dessication crack and not an earth fissure. A dirt road was present at the desiccation crack site, and it is possible that the road may have produced some of the observed anomalies due to rain-water channeling effects. The locations of fissures were confirmed by trenches excavated at the locations where anomalies were visible in the geophysical data, but where the fissures were not exposed at the surface.

In the spring semester of 2010, the University of Arizona GEOS/GEN 416 Field Studies in Geophysics class, funded by the USGS, collected data in the Upper Santa Cruz River Basin, located in southeastern Arizona, near the US-Mexico border. In this region, surface water is scarce, so the population is almost entirely dependent on ground water. To understand temporal and spatial variability of ground-water quantity and quality, it is necessary to understand the hydrogeology of the subsurface. Using time-domain electromagnetic measurements (TEM), combined with other geophysical data, it is possible to interpret characteristics of the subsurface that might otherwise go unnoticed using just well logs or where well logs are not available. The goal of this work is to develop an understanding of hydrologically significant spatial variations in litho-stratigraphic units in the basin. Using forward and inverse modeling of electromagnetic fields and comparisons with measured data collected by ground based TEM surveys, it is possible to estimate depth to bedrock and water table. Through the analysis of 9 different TEM loops varying in size from 75 to 500 meters, groundwater in the region was interpreted to range from ~20 meters to ~100 meters. Correlation of groundwater with proximity to the Santa Cruz River differs between Guevavi Basin and Highway 82 Basin. Water table depth decreases with proximity to the Santa Cruz in the Guevavi Basin, but increases with proximity in the Highway 82 Basin. Furthermore, none of the TEM loops positively identified any bedrock material, and in some areas the bedrock is determined to be greater than 850 meters depth.

Transient Electromagnetic (TEM), Controlled Source Audio Magnetotellurics (CSAMT), Gravity, and Magnetic data were collected in the Tucson Mountains during the Spring semester, 2011. The goal was to investigate the extent of a low-resistivity porous sedimentary layer and faults that may form potential traps located under the surface volcanic layers, as interpreted by Lipman 1993. The sedimentary layer under the volcanics has the potential to be used for either water resources or compressed air storage to store solar energy. The results from the TEM and CSAMT surveys broadly correlated with the thickness of the volcanic layer and throw of the faults interpreted by Lipman, 1993. The gravity modeling suggested the faults may have a larger throw than what was indicated by the other methods. Because of the fundamental uncertainty in the densities to use in the modeling, it was concluded that the gravity modeling may not give as accurate a prediction of the structure in this region. For further investigation of the deep porous sedimentary layer, we suggest that TEM and CSAMT are the most effective methods.

Controlled Source Audio-Magnetotellurics (CSAMT) surveys were conducted in 2012 in the Tucson Mountains as a continuation of the 2010 and 2011 Laboratory for Advanced Subsurface Imaging (LASI) field studies in this area. This geologic setting was chosen for its high-resistivity impermeable volcanic layer overlaying porous sedimentary layers. This type of structure has potential for water resources and as a reservoir for compressed air energy storage (CAES). The data from 2,500 meters of CSAMT survey lines generated 900 meter-deep resistivity versus elevation cross-sections and six plan maps of the depth and elevation to the buried conductive layer. Our results are generally in agreement with the geologic cross sections developed by Lipman (1993) and previous TEM data that confirmed the presence of a deep conductive layer beneath a resistive volcanic sequence.

To assist the United States Geological Survey with an on-going groundwater study around Old Yuma Mine in Tucson, Arizona, the University of Arizona GEN/GEOS 416/516 Field Studies in Geophysics class conducted geophysical surveys along two transects near the mine. Transect 1 was situated across the mine site; Transect 2 was located to the northeast in a nearby residential area. The methods used were gravity, magnetics, transient electromagnetics (TEM) and inductive electromagnetics (Geonics EM-31, and Geonics EM-34). The goal was to use these data to investigate the subsurface density, magnetic susceptibility, and electrical conductivity contrasts. A large gravity anomaly was observed on Transect 1 where it crosses both a mapped fault and the Old Yuma Mine; the anomaly is thought to represent a density contrast related to the Mine and fault. A smaller gravity anomaly was observed on Transect 2, corresponding in location along the profile to a large anomaly in the Transect 2 magnetics data. These anomalies are possibly related to the local lithology. A second magnetics anomaly was observed on Transect 1; this anomaly was also visible in the EM-31 and EM-34 data and could be due to the presence of a nearby wash. Other variations in the magnetics and EM-31/34 data consisted of narrow peaks associated with cultural interference, and the EM-31/34 data showed no large conductivity change in the shallow sediments. Interpretation of TEM data for Transect 1 was limited by the wide station spacing; much of the cross-section’s resistivity contrasts were interpolated over a large distance. The TEM cross-section for Transect 2 displayed higher resistivity on the northwest side and lower resistivity on the southeast side of the transect. Since the mapped fault, if projected northwards, would pass through the middle of Transect 2, it is thought that this resistivity contrast represents the location of the fault. It is hoped that the results of these surveys will be beneficial to the USGS in further work at Old Yuma Mine.

The Tucson International Airport Area (TIAA) Superfund site is an approximately ten square mile area in southeast Tucson, Pima County, Arizona in which several known contaminated water plumes have been identified, which are the result of improper disposal of industrial waste from multiple sources during the past sixty years. The most prominent of these contaminants are 1, 4-dioxane, hexavalent Chromium, and trichloroethylene (TCE), which exist in varying concentrations throughout the site. Groundwater contamination in Tucson was first identified in the 1950’s; however TIAA was not recognized as a Federal Superfund site until 1982. Since then, much work has been carried out in an attempt to fully understand and remediate the contamination in the area. This study focusses on four areas within the TIAA: Samsonite North, Aero Park Blvd South (EW line), Aero Park Blvd South (NS line), and EPA-03. Several geophysical techniques have been used to understand the subsurface structure in the area and to better understand the contamination plume and its movement. Using the Transient Electromagnetic (TEM) technique at three sites: Samsonite North, Aero Park Blvd South (NS line and EW line), and EPA-03, it was found that there was a low-resistivity region going through the EPA-03 site, two low-resistivity regions through Aero Park Blvd South (NS line), and three low-resistivity zones through Aero Park Blvd South (EW line). These channels were consistent with the overall orientation of water flow in the region which is toward the Northwest. These zones may indicate higher moisture content, and this may be caused by porous, water-filled channels passing through the sites. These zones could also indicate non-porous clay-rich regions, which would also be low resistivity. The effect of a pipeline on the measurements in the Samsonite North area made it difficult to reach any useful conclusions at this site.

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