RSS 1.0Laboratory for Advanced Subsurface Imaging (LASI)
Recent Submissions
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Geophysical Surveys in the Harshaw Creek Area, Patagonia Mountains, ArizonaOver the weekends of February 15-16 and 22-23, 2020 geophysical data were collected by students from the University of Arizona in cooperation with the US Geologic Survey. The survey was conducted in the Lower Harshaw Creek area of Patagonia, Arizona. The objective of this survey was to define features of the setting for surface water and groundwater across three ranch properties that had varying levels of potable water. Local mineralization in the area led to high concentrations of dissolved solids and low pH in certain wells. DC Resistivity and Transient Electromagnetic were the primary methods used in the survey, with the GEM-2 Frequency Domain Electromagnetic system used in a smaller portion of the survey. These methods allowed for subsurface mapping of the study area with respect to local resistivities. The results of these surveys determined that the likely cause of the varying potability was a shallow rock body containing a high percentage of sulfide minerals. The local geology and nearby area’s mineral occurrences suggest the observed anomalies can best be explained as a buried skarn or epithermal base metal vein-type mineralization. This deposit lies within a shear zone, which caused an uplift of the body under certain sections of the survey area. This zone with the sulfide rich body is the leading theory for the occurrence of non-potable groundwater in the survey area.
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Geophysical Surveys in the Blue Nose Mine Area, Patagonia Mountains, ArizonaThe US Geological Survey and the US Forest Service requested an investigation of the sediment and groundwater properties of the Blue Nose Mine in the Patagonia Mountains of southern Arizona to help with future remediation efforts due to historical mining of the area. Information on the depth of the existing tailings piles was also requested to aid in determining the best removal strategy. The surveys carried out in this investigation include direct current (DC) resistivity, transient electromagnetics (TEM), EM-31 and EM-38, total-field magnetism, and petrophysical laboratory analysis. The 20m TEM loop and DC resistivity data revealed low-resistivity regions, less than 10 Ohm-m, surrounded by several-thousand Ohm-m resistive features, indicating a large conductive zone around the existing mine workings. The 10 m TEM loops, as well as the EM-31, and EM-38 data, collected on the tailings piles indicate a conductive layer reaching 8 m in depth from the surface. These help to delineate the conductive tailings material from the surrounding hillside. The total-field magnetic survey was useful in determining several linear features and magnetic trends, including the Harshaw Creek Fault on the eastern edge of the study site. The petrophysical analysis aimed to help refine subsurface interpretations, but due to the small sample size only provides a limited correlation to the DC resistivity and TEM data. Correlations between magnetic and resistive datasets reveal three zones with differing electromagnetic properties, one of which may correlate with the Blue Nose ore deposit.
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Geophysical Surveys in the Lead Queen Mine Area, Patagonia Mountains, ArizonaIn late 2015, after abnormally heavy monsoon rains in the Coronado National Forest south of Patagonia Arizona, an orange discharge was observed in the creek bed below the entrance to the abandoned Lead Queen Mine. This caused the U.S. Forest Service and United States Geological Survey (USGS) to mobilize to determine the cause and investigate actions to prevent future occurrences. Our studies included Transient Electromagnetics (NanoTEM), DC resistivity, EM-31 and Ground Penetrating Radar (GPR) surveys around the Lead Queen Mine for the United States Geological Survey. Data collected around Lead Queen Mine show a large highly conductive layer within the immediate area of the mine. Data from the NanoTEM and DC resistivity surveys show an anomaly with resistivities less than 6 ohm-m at depths varying from 20 to 60 meters. This anomaly is present in all TEM transects and the DC resistivity line. The DC resistivity line also shows a high-resistivity area near where the adit has been predicted to be. Further west, near another adit, a low-resistivity layer does not appear until beyond 60 meters depth. Data collected using the EM-31 shows no change in resistivity or conductivity along the stream-bed near Lead Queen Mine. GPR data did not effectively show a response to the adit, even with extensive manipulation using the RADAN software.
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Near-Surface Geophysics Surveys at Empire Gulch in the Las Cienegas Conservation Area, SE ArizonaElectrical resistivity and induced polarization surveys were conducted in the Las Cienegas National Conservation Area for the Nature Conservancy and in accordance with the Bureau of Land Management. Data collected around Empire Ranch HQ shows a thin, high resistivity layer at the surface underlain by a lower resistivity layer, indicative of typical regional alluvium. H0 and H1 show resistivity of approximately 80 Ω-m down to 5 and 10 meters, underlain by 25 Ω-m and 20 Ω-m respectively. H2 and H3 shows resistivity of 60 Ω-m down to 5 and 15 meters, respectively, underlain by 20 Ω-m resistivity. Moving east, the data displays a distinct change. A thin, high-resistivity surface layer is underlain by an extremely low resistivity layer, typical of clays. H5, Z2 and H6 display surface resistivity between 20 Ω-m and 15 Ω-m, typical alluvium values. The massive underlying layer displays resistivity values of approximately 5 Ω-m. Continuing east, H7 shows a thick layer of high (80 Ω-m) resistivity surface material down to 25 meters, underlain again by a massive, low resistivity layer with a typical clay value of 5 Ω-m. H8 and Z3, collected in the Cieneguita cienegas complex, both display a 5 Ω-m surface layer down to 5 meters, indicative of clays. The surface layer is underlain by a 15 Ω-m, low-porosity layer, essentially devoid of clays, interpreted as Late Tertiary and Quaternary basin fill.
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Geophysical Surveys Near Tucson Electric Power Sundt Generating StationTucson Electric Power (TEP) is carrying out subsurface investigations in order to locate more groundwater for cooling at the Sundt power-generating plant. To assist with this investigation, the University of Arizona GEN/GEOS 416/516 Field Studies in Geophysics class conducted geophysics surveys in an area just south of Davis-Monthan Air Force Base and between UTM coordinates 508,555 to 511,753 East and 3,553,705 to 3,556,895 North. Four geophysics methods (Gravity, Magnetics, Transient Electromagnetics (TEM), and Passive Seismic) were employed to locate a postulated fault, which may be correlated with ground water flow. A broad regional magnetic anomaly was mapped, as the magnetic field decreases steadily from NE to SW. There are no significant magnetic field anomalies that could be related to a potential fault. The Gravity results show a regional gravitational gradient, steadily decreasing from NE to SW, with a large isolated anomaly apparent around 1500m from the base station at the NE corner of the survey area. But, this large anomaly is a localized, and does not appear on the adjacent parallel survey lines, therefore it is not related to a potential fault contact. The Passive Seismic survey detected a deep boundary at 100m to 160m in elevation, but the depths interpreted from the 11 stations are scattered and do not show a clear trend. The Transient Electromagnetic (TEM) data show a consistent difference in depth to a low-resistivity layer along the profile line. The four TEM stations north of Interstate 10 (I10) have an average elevation for the 10 Ohm-m contour line of 720 meters. The four TEM stations south of I10 have an average elevation for the 10 Ohm-m contour line of 670 meters. This offset may be related to the postulated fault.
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Geophysical Surveys Near Tucson International AirportThe 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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Geophysical Surveys near Old Yuma Mine, Tucson Mountains, ArizonaTo 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.
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Controlled Source Audio Magnetotelluric (CSAMT) Surveys in the Tucson MountainsControlled 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.
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GEOPHYSICAL INVESTIGATION OF THE TUCSON MOUNTAINSTransient 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.
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TRANSIENT ELECTROMAGNETIC (TEM) INVESTIGATION OF MICROBASIN MORPHOLOGY ALONG THE SANTA CRUZ RIVER, NOGALES, ARIZONAIn 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.
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GEOPHYSICAL INVESTIGATION OF SUBSIDENCE FISSURES NEAR WILLCOX, ARIZONAAn 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.
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CONTROLLED SOURCE AUDIO MAGNETO- TELLURIC (CSAMT) GEOPHYSICAL INVESTIGATION OF THE MIDDLE SAN PEDRO RIVER BASIN, SOUTHEASTERN ARIZONAGroundwater 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.
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TRANSIENT ELECTROMAGNETIC (TEM) INVESTIGATIONS INTHE UPPER SAN PEDRO RIVER BASIN, BENSON, ARIZONATransient 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.
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GEOPHYSICAL INVESTIGATIONS IN THE UPPER SAN PEDRO RIVER BASIN, BENSON, ARIZONAFour 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.
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Geophysical Investigations near Yuma, ArizonaVLF (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.
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Geophysical Surveys near Sierra Vista, ArizonaFive 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.
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Geophysical Surveys near Chino Valley, ArizonaFour 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.
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Geophysical Surveys near Fort Huachuca, ArizonaThe 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.
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Geophysical Surveys near the San Pedro River, ArizonaDuring 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.
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GEOPHYSICAL SURVEYS AT THE UNITED MUSICAL INSTRUMENTS, SUPER-FUND SITE, NOGALES, ARIZONADuring 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
