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At least 91 records · Page 5Linked to original sources
USGS2011

Geophysical bed sediment characterization of the Androscoggin River from the former Chlor-Alkali Facility Superfund Site, Berlin, New Hampshire, to the state border with Maine, August 2009

The former Chlor-Alkali Facility in Berlin, New Hampshire, was listed on the U.S. Environmental Protection Agency National Priorities List in 2005 as a Superfund site. The Chlor-Alkali Facility lies on the east bank of the Androscoggin River. Elemental mercury currently discharges from that bank into the Androscoggin River. The nature, extent, and the speciation of mercury and the production of methyl mercury contamination in the adjacent Androscoggin River is the subject of continuing investigations. The U.S. Geological Survey, in cooperation with Region I of the U.S. Environmental Protection Agency, used geophysical methods to determine the distribution, thickness, and physical properties of sediments in the Androscoggin River channel at a small area of an upstream reference reach and downstream from the site to the New Hampshire–Maine State border. Separate reaches of the Androscoggin River in the study area were surveyed with surface geophysical methods including ground-penetrating radar and step-frequency electromagnetics. Results were processed to assess sediment characteristics including grain size, electrical conductivity, and pore-water specific conductance. Specific conductance measured during surface- and pore-water sampling was used to help interpret the results of the geophysical surveys. The electrical resistivity of sediment samples was measured in the laboratory with intact pore water for comparison with survey results. In some instances, anthropogenic features and land uses, such as roads and power lines affected the detection of riverbed properties using geophysical methods; when this occurred, the data were removed. Through combining results, detailed riverbed sediment characterizations were made. Results from ground-penetrating radar surveys were used to image and measure the depth to the riverbed, depth to buried riverbeds, riverbed thickness and to interpret material-type variations in terms of relative grain size. Fifty two percent of the riverbed in the study area was covered with gravel and finer sediments. The electrically resistive river water and sediment in this study area were conducive to the penetration of the ground-penetrating radar and step-frequency electromagnetic signals and allowed for effective sediment characterization by geophysical methods. The reach between the former Chlor-Alkali Facility and the Riverside Dam, had small areas of fine sediment (estimated 11 percent of riverbed area), found on the upstream left bank and the downstream right bank, with an electromagnetic conductivity (31.4 millisiemens per meter (mS/m) maximum) that was higher than the upstream reference reach. The greatest electromagnetic conductivity (195 mS/m), pore-water specific conductance (324 mS/m) and lab measured sediment conductivity of (76.8 mS/m, measured with a direct-current resistivity test box) in the study were measured approximately 1 mile (mi) downstream of the site from a sandbar on the left bank. Reaches adjacent to and within 2 mi downstream from the site had elevated electromagnetic conductivity despite having lower estimated percentages of riverbed area covered in sediment (11, 25, and 61 percent, respectively) than the reference reach (97). Typically finer grained sediment with similar mineralogy will be more conductive. The Shelburne Reservoir is approximately 8 mi downstream from the site had the second greatest pore-water specific conductance measured, 45.8 mS/m. Many of the locations with the largest step-frequency electromagnetic values have not been sampled for pore water and sediment.

New Hampshire↗
USGS2013

Geophysical-log and hydraulic-test analyses of groundwater-production wells at the Hannahville Indian Community, Menominee County, Michigan

The U.S. Geological Survey, in cooperation with the Hannahville Indian Community, evaluated the geohydrology of the bedrock formations and hydraulic properties of groundwater-production wells at the Hannahville Indian Community in Menominee County, Michigan. Geophysical logs were collected from five wells at two sites during September 2012. The logs were analyzed to characterize the lithostratigraphy, bedding and fractures, and hydraulic properties of the geologic formations and aquifers beneath the Hannahville Indian Community. The geophysical logs collected included natural gamma radiation, electromagnetic conductivity, wellbore image, caliper, ambient and stressed flowmeter, fluid resistivity, temperature, and wellbore deviation. The geophysical logs were analyzed with results from short-term hydraulic tests to estimate the transmissivity and water-level altitudes of flow zones penetrated by the wells. The geophysical log analysis indicated the wells penetrated four distinct lithostratigraphic units—shale and carbonate rock, upper carbonate rock, carbonate rock and glauconitic sandstone, and lower carbonate rock. Most of the fractures penetrated by the wellbores appeared to be related bedding partings. The lower carbonate rock unit contained solution features. Analysis of the geophysical logs and hydraulic tests indicated that each of the five wells penetrated from one to four flow zones. The Casino 5 well penetrated a flow zone that was associated with solution features and had an estimated total transmissivity of 4,280 feet squared per day (ft 2 /d), the highest estimate for all the wells. The Casino 3 well penetrated four flow zones and had an estimated total transmissivity of 3,570 ft 2 /d. The flow zones penetrated in the lower carbonate rock unit by the Casino 3 and 5 wells were hydraulically connected. The Golf Shack well penetrated two flow zones and had an estimated total transmissivity of 40 ft 2 /d, the lowest estimate for all the wells. The Community 1 and Community 2 wells penetrated three and four flow zones, respectively, and had estimated total transmissivity values of 185 and 280 ft 2 /d, respectively.

Michigan↗
USGS1985

Evaluation of surface geophysical methods for collection of hydrogeologic data in the Nebraska Sand Hills region

The practicality of using surface geophysical methods for obtaining geohydrologic data in the Nebraska Sand Hills region was studied during the summer of 1984. Seismic refraction and electrical-resistivity equipment were used, because an evaluation of geohydrologic data indicated that results of surveys made with this equipment probably would yield the most useful data. The study area, which included parts of Garfield, Holt, and Wheeler Counties, was selected because it is geohydrologically representative of the eastern part of the Sand Hills region, and because sufficient geohydrologic data were available for use in evaluating the results of geophysical surveys. Geophysical methods were evaluated for their ability to consistently detect selected geohydrologic horizons. These horizons in descending order, are: the water table, the top of Quaternary silt beds, the top of Quaternary sand and gravel beds , the top of the Tertiary Ogallala Formation, and the top of the Cretaceous Pierre Shale. The top of the Pierre Shale generally is the base of the aquifer, which consists of all of the 500 to 700 ft of overlying deposits. Evaluations of the geophysical data indicate that seismic refraction surveys are best suited for determining the depth to the water table, but are not effective in studying beds below the water table. Vertical electrical soundings provided data on the depth to water table and the top of the silt beds. Available geohydrologic data, however, indicate that with some changes in data collection or interpretation techniques, it may be possible to obtain information on the top of the sand and gravel deposits, the top of the Ogallala Formation, and the top of the Pierre Shale with vertical electrical soundings. Use of either geophysical method could enhance the results of geohydrologic investigations in the Nebraska Sand Hills region. (Author 's abstract)

Water-Resources Investigations Report↗
USGS1995

Surface-geophysical techniques used to detect existing and infilled scour holes near bridge piers

Surface-geophysical techniques were used with a position-recording system to study riverbed scour near bridge piers. From May 1989 to May 1993, Fathometers, fixed- and swept-frequency continuous seismic- reflection profiling (CSP) systems, and a ground-penetrating radar (GPR) system were used with a laser-positioning system to measure the depth and extent of existing and infilled scour holes near bridge piers. Equipment was purchased commercially and modified when necessary to interface the components and (or) to improve their performance. Three 200-kHz black-and-white chart-recording Fathometers produced profiles of the riverbed that included existing scour holes and exposed pier footings. The Fathometers were used in conjunction with other geophysical techniques to help interpret the geophysical data. A 20-kHz color Fathometer delineated scour-hole geometry and, in some cases, the thickness of fill material in the hole. The signal provided subbottom information as deep as 10 ft in fine-grained materials and resolved layers of fill material as thin as 1 ft thick. Fixed-frequency and swept-frequency CSP systems were evaluated. The fixed-frequency system used a 3.5-, 7.0-, or 14-kHz signal. The 3.5-kHz signal penetrated up to 50 ft of fine-grained material and resolved layers as thin as 2.5-ft thick. The 14-kHz signal penetrated up to 20 ft of fine-grained material and resolved layers as thin as 1-ft thick. The swept-frequency systems used a signal that swept from 2- to 16-kHz. With this system, up to 50 ft of penetration was achieved, and fill material as thin as 1 ft was resolved. Scour-hole geometry, exposed pier footings, and fill thickness in scour holes were detected with both CSP systems. The GPR system used an 80-, 100-, or 300-megahertz signal. The technique produced records in water up to 15 ft deep that had a specific conductance less than 200 µS/cm. The 100-MHz signal penetrated up to 40 ft of resistive granular material and resolved layers as thin as 2-ft thick. Scour-hole geometry, the thickness of fill material in scour holes, and riverbed deposition were detected using this technique. Processing techniques were applied after data collection to assist with the interpretation of the data. Data were transferred from the color Fathometer, CSP, and GPR systems to a personal computer, and a commercially available software package designed to process GPR data was used to process the GPR and CSP data. Digital filtering, predictive- deconvolution, and migration algorithms were applied to some of the data. The processed data were displayed and printed as color amplitude or wiggle-trace plots. These processing methods eased and improved the interpretation of some of the data, but some interference from side echoes from bridge piers and multiple reflections remained in the data. The surface-geophysical techniques were applied at six bridge sites in Connecticut. Each site had different water depths, specific conductance, and riverbed materials. Existing and infilled scour holes, exposed pier footings, and riverbed deposition were detected by the surveys. The interpretations of the geophysical data were confirmed by comparing the data with lithologic and (or) probing data.

Connecticut↗
USGS2019

Geophysical assessment of a proposed landfill site in Fredericktown, Missouri

In June 2018, U.S. Geological Survey in cooperation with the U.S. Environmental Protection Agency (EPA) collected geophysical measurements to help evaluate the suitability of a proposed landfill site for disposing mine-waste materials in Fredericktown, MO. The geophysical investigation included electromagnetic induction, electrical resistivity tomography, horizontal-to-vertical spectral ratio passive seismic, and shear-wave refraction to characterize the unconsolidated sediment or regolith above the bedrock and the depth to crystalline bedrock. Water-based electromagnetic surveys and forward looking infrared imagery were used along the lake shoreline to identify potential groundwater-surface water interactions. Nested piezometers were installed at two locations along the shoreline to further characterize and validate the groundwater-surface water interactions identified with geophysical methods. Collectively, geophysical surveys were used to evaluate the overall suitability of the proposed site for use as a mine waste-soil and sediment repository and to evaluate this suite of geophysical methods for rapid collection and assessment of very shallow sediments.

Missouri↗
USGS2015

Potential application of radiogenic isotopes and geophysical methods to understand the hydrothermal dystem of the Upper Geyser Basin, Yellowstone National Park

Numerous geochemical and geophysical studies have been conducted at Yellowstone National Park to better understand the hydrogeologic processes supporting the thermal features of the Park. This report provides the first 87Sr/86Sr and 234U/238U data for thermal water from the Upper Geyser Basin (UGB) intended to evaluate whether heavy radiogenic isotopes might provide insight to sources of groundwater supply and how they interact over time and space. In addition, this report summarizes previous geophysical studies made at Yellowstone National Park and provides suggestions for applying non-invasive ground and airborne studies to better understand groundwater flow in the subsurface of the UGB. Multiple samples from Old Faithful, Aurum, Grand, Oblong, and Daisy geysers characterized previously for major-ion concentrations and isotopes of water (δ2H, δ18O, and 3H) were analyzed for Sr and U isotopes. Concentrations of dissolved Sr and U are low (4.3–128 ng g-1 Sr and 0.026–0.0008 ng g-1 U); consequently only 87Sr/86Sr data are reported for most samples. Values of 87Sr/86Sr for most geysers remained uniform between April and September 2007, but show large increases in all five geysers between late October 2007 and early April, 2008. By late summer of 2008, 87Sr/86Sr values returned to values similar to those observed a year earlier. Similar patterns are not present in major-ion data measured on the same samples. Furthermore, large geochemical differences documented between geysers are not observed in 87Sr/86Sr data, although smaller differences between sites may be present. Sr-isotope data are consistent with a stratified hydrologic system where water erupted in spring and summer of 2007 and summer of 2008 equilibrated with local intracaldera rhyolite flows at shallower depths. Water erupted between October 2007 and April 2008 includes greater amounts of groundwater that circulated deep enough to acquire a radiogenic 87Sr/86Sr, most likely from Archean basement rocks. Details of how the shallow and deep components interact and mechanisms causing these interactions remain unknown, but the data demonstrate the usefulness of obtaining Sr-isotope data from future sample campaigns. Geophysical methods that would be useful for characterization of the UGB subsurface properties and geothermal system include electromagnetic (EM), gravity, and ambient seismic. A suite of ground-based EM methods could be used in a synergistic combination together with airborne EM surveys to provide data for a range of spatial scales and resolutions. Existing thermal data for the shallow subsurface could be used to relate ground and airborne EM survey data to locations of geothermal fluids near the surface. Gravity surveys would be useful for mapping subsurface density anomalies and possibly monitoring changes in degree of saturation with groundwater. Ambient seismic surveys would be useful for estimating the thickness of unconsolidated deposits that contain the shallow groundwater system. A study that combines radiogenic isotope tracers with geophysical methods has the potential to better characterize the geothermal workings in the UGB. Insights gained could lead to a better understanding of the geothermal system and how Park infrastructure may cause perturbations. Measurements of radiogenic isotopes from multiple geysers and pools in localized areas within the UGB that are coupled with data from geophysical surveys would help refine conceptual models of mixing between deep- and shallow-derived subsurface fluids.

Wyoming↗
USGS2026

Revisiting the utility of regional-scale, high-quality geophysical data in mineral exploration - A case study featuring the Mammoth Magnetic Anomaly, Pinal County, Arizona

Regional aeromagnetic surveys passively measure the total magnetic intensity (TMI) and are a foundational tool used in mineral exploration (Airo, 2015). With the increased global demand and the number of critical mineral resources required for manufacturing high-tech devices, developing high-quality, regional-scale geophysical surveys could aid critical mineral exploration efforts and geologic mapping. In 2019, the U. S. Geological Survey launched the Earth Mapping Resources Initiative (Earth MRI) to modernize the geologic and geophysical mapping of regions that have the potential to contain critical mineral resources within the United States. In support of planning Earth MRI geophysical surveys, Drenth and Grauch (2019) defined five aeromagnetic data quality rankings (rank 1 through rank 5) applying them to the airborne geophysical survey inventory of the United States (Johnson et al., 2021). Rank 1 aeromagnetic surveys are of the highest quality, meeting modern standards and allowing best practices for qualitative and quantitative interpretation; whereas rank 5 aeromagnetic surveys are of the lowest quality, being useful only for qualitative interpretation of broad features. Through the Earth MRI effort, 48 high-quality, regional-scale rank 1 and 2 airborne magnetic and radiometric geophysical surveys have been planned, collected, or publicly release through May 2025 (U. S. Geological Survey, 2025). Here, a portion of a rank 1 Earth MRI aeromagnetic survey in southeast Arizona is presented and compared to a legacy rank 5 aeromagnetic survey over the Mammoth Magnetic Anomaly (MMA), demonstrating how modern, high-quality aeromagnetic data improves our view of crustal geology, aiding mineral exploration.

Arizona↗
USGS2012

Analysis of geophysical well logs obtained in the State 2-14 borehole, Salton Sea geothermal area, California

A complete suite of conventional geophysical well logs was obtained in the upper part of a 3220-m- deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the Salton Sea. In situ temperatures greater than 300°C and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction, spontaneous potential, un-calibrated neutron, natural gamma, and temperature. Bottom-hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355°C at a depth of 3220 m. Geophysical logs obtained in the State 2–14 borehole indicate that neutron porosity, gamma-gamma, and deep-induction logs provide useful information on lithologic trends with depth. The natural gamma log contains almost continuous, high-frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium. Acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1.8 to 3.0 km/s, whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3.0 km/s shallower than 1000 m in depth to almost 5.0 km/s at depths greater than 2000 m. Analyses indicate that most log values lie between two lithologic end points: an electrically conductive claystone with moderate neutron porosity, but no effective porosity, and an electrically nonconductive, fully cemented siltstone that has small but finite porosity. A limited number of clean sandstones depart from this trend; geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14%, and saturation with brines having equivalent NaCl concentrations greater than 100,000 mg/L. Depth- averaged trends in neutron porosity and deep-induction logs, along with trends in acoustic velocity determined from acoustic waveform logs, demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m. Although caliper logs were not obtained deeper than 2000 m, resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity. The transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar.

Journal of Geophysical Research Solid Earth↗
USGS2014

Airborne geophysical surveys conducted in western Nebraska, 2010: contractor reports and data

This report contains three contractor reports and data files for an airborne electromagnetic survey flown from June 28 to July 7, 2010. The first report; “SkyTEM Survey: Nebraska, USA, Data” describes data aquisition and processing from a time-domain electromagnetic and magnetic survey performed by SkyTEM Canada, Inc. (the North American SkyTEM subsidiary), in western Nebraska, USA. Digital data for this report are given in Appendix 1. The airborne geophysical data from the SkyTEM survey subsequently were processed and inverted by Aarhus Geophysics ApS, Aarhus, Denmark, to produce resistivity depth sections along each flight line. The result of that processing is described in two reports presented in Appendix 2, “Processing and inversion of SkyTEM data from USGS Area UTM–13” and “Processing and inversion of SkyTEM data from USGS Area UTM–14.” Funding for these surveys was provided by the North Platte Natural Resources District, the South Platte Natural Resources District, and the Twin Platte Natural Resources District, in Scottsbluff, Sidney, and North Platte, Nebraska, respectively. Any additional information concerning the geophysical data may be obtained from the U.S. Geological Survey Crustal Geophysics and Geochemistry Science Center, Denver Colorado.

Nebraska↗
USGS1995

Surface electromagnetic geophysical exploration of the ground-water resources of Isla de Mona, Puerto Rico, a caribbean carbonate island

Two electromagnetic surface geophysical techniques were used to explore the ground-water resources of Isla de Mona, Puerto Rico—a 55-square-kilometer island located between Puerto Rico and Hispañiola, Isla de Mona is a tectonically uplifted carbonate plateau of Neogene age that has an average elevation of about 50 meters above mean sea level. This plateau is bounded by vertical cliffs except on the southwest where there is a narrow, 3-square-kilometer coastal plain. The coastal plain is composed of Quaternary carbonate deposits, and has a maximum elevation of less than 10 meters above mean sea level. No large-scale surface-water features are found on the plateau or on the coastal plain. To better understand the aquifer characteristics of the island, terrain conductivity and transient electromagnetic data were collected on the coastal plain and on the plateau. Computer programs were used to analyze quantitatively the electromagnetic data. Geoelectric models were produced to approximate the depth below land surface of the saline-freshwater interface underlying both the coastal plain and the plateau. Because the geophysical methods could not discern the water-table, it was assumed that it was near sea level. The thickness of the freshwater lens was estimated by subtracting the elevation of the land surface above mean sea level from the depth to the saline-freshwater interface as determined from the geophysical data. Results from the geophysical methods and water-level observations indicate that a freshwater lens with a maximum thickness of about 10 meters exists under the coastal plain. This lens thins towards the ocean, and it also thins away from the ocean toward the plateau. The model produced from the transient electromagnetic data indicates that the freshwater lens under the plateau has a maximum thickness of about 14 meters, which is a much thinner lens than previously estimated. A freshwater lens thickness of 14 meters is similar to direct measurements by divers in Cueva de Agua at Punta los Ingleses (located on the southeast coast), and observations by divers of freshwater seeps into the ocean at depths of 8 to 10 meters along the north coast cliffs after a major rainstorm. Ground-water flow in the coastal plain appears to be radial from the center of the freshwater mound. At the intersection between the coastal plain and the plateau, the flow is parallel to the coastline. The direction of flow on the rest of the plateau could not be determined accurately with the available data.

Carbonates and Evaporites↗
USGS2014

Application of near-surface geophysics as part of a hydrologic study of a subsurface drip irrigation system along the Powder River floodplain near Arvada, Wyoming

Rapid development of coalbed natural gas (CBNG) production in the Powder River Basin (PRB) of Wyoming has occurred since 1997. National attention related to CBNG development has focused on produced water management, which is the single largest cost for on-shore domestic producers. Low-cost treatment technologies allow operators to reduce their disposal costs, provide treated water for beneficial use, and stimulate oil and gas production by small operators. Subsurface drip irrigation (SDI) systems are one potential treatment option that allows for increased CBNG production by providing a beneficial use for the produced water in farmland irrigation. Water management practices in the development of CBNG in Wyoming have been aided by integrated geophysical, geochemical, and hydrologic studies of both the disposal and utilization of water. The U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) and the U.S. Geological Survey (USGS) have utilized multi-frequency airborne, ground, and borehole electromagnetic (EM) and ground resistivity methods to characterize the near-surface hydrogeology in areas of produced water disposal. These surveys provide near-surface EM data that can be compared with results of previous surveys to monitor changes in soils and local hydrology over time as the produced water is discharged through SDI. The focus of this investigation is the Headgate Draw SDI site, situated adjacent to the Powder River near the confluence of a major tributary, Crazy Woman Creek, in Johnson County, Wyoming. The SDI system was installed during the summer of 2008 and began operation in October of 2008. Ground, borehole, and helicopter electromagnetic (HEM) conductivity surveys were conducted at the site prior to the installation of the SDI system. After the installation of the subsurface drip irrigation system, ground EM surveys have been performed quarterly (weather permitting). The geophysical surveys map the heterogeneity of the near-surface geology and hydrology of the study area. The geophysical data are consistent between surveys using different techniques and between surveys carried out at different times from 2007 through 2011. This paper summarizes geophysical results from the 4-year monitoring study of the SDI system.

Wyoming↗
USGS2021

Multi-geophysical parameter classification of the Montserrat geothermal system

Multi-geophysical parameter classification can help to reduce the uncertainties of interpretations that often rely on one geophysical technique. Integrating these varying datasets requires a more robust classification approach rather than traditional qualitative methods. In this study, we applied the Fuzzy c-means (FCM) method to quantitatively classify similarities in a high resolution seismic tomography, a magnetotellurics and gravity datasets obtained in Montserrat. To group similar datapoints, this application uses a Euclidean distance measure and a membership function. Assigned membership values indicate the degree to which a datapoint belongs to a specific class. The spatial distribution of the derived classes, each classified with distinct geophysical parameters, helped to provide new structural and petrological information of the Montserrat geothermal system. In comparison to previous models, our new cluster model highlights two major improvements. These include the resolution and assessment of the spatial extension and 3D geometry of previously undetected features within the Montserrat geothermal system and the constrain and characterization of earlier identified anomalies. We additionally utilized geological and petrological data obtained from three geothermal wells in the Montserrat geothermal system to help validate our classifications. Based on a semi-quantitative approach we assessed the reliability of the FCM technique in relation to the likely uncertainties of the different geophysical models.

Geothermics↗
USGS2020

Post-1978 tumescence at Long Valley Caldera, California: A geophysical perspective

Long Valley Caldera has been restless since at least 1978. Prominent symptoms of this unrest include earthquake swarms and tumescence (inflation) centered on the resurgent dome. Over the years, interpretations of physical processes underlying this unrest have varied considerably. Results from a collection of geophysical studies infer the presence and/or active intrusion of magma in the crust. Geologic evidence, however, does not support recent magmatic activity in the caldera, leading to an interpretation that the caldera volcanic system is moribund, and the current unrest is a result of second boiling (aqueous fluids released during crystallization of the rhyolitic magma that produced the Bishop Tuff). Here, we examine the collective constraints provided by geophysical studies over the past four decades. Although the current geophysical evidence does not conclusively discriminate between unrest driven by recent crustal magmatic intrusion versus second boiling, it does provide evidence that a large volume of partial melt persists within the mid- and lower crust. This implies that the Long Valley Caldera system as a whole is long-lived, and the magma reservoir remains at least partially molten. In the shallow crust, the possibility of small pockets of magma remains. In aggregate, the data suggest commonality with other large, long-lived silicic caldera systems, such as Yellowstone and Campi Flegrei. Although the possibility of eruption within Long Valley Caldera remains unlikely, the geophysical evidence argues that Long Valley has an active magmatic system at depth and we must retain the possibility of eruptive hazards.

California↗
USGS1998

Implications of seismic reflection and potential field geophysical data on the structural framework of the Yucca Mountain-Crater Flat region, Nevada

Seismic reflection and gravity profiles collected across Yucca Mountain, Nevada, together with geologic data, provide evidence against proposed active detachment faults at shallow depth along the pre-Tertiary–Tertiary contact beneath this potential repository for high-level nuclear waste. The new geophysical data show that the inferred pre-Tertiary–Tertiary contact is offset by moderate- to high-angle faults beneath Crater Flat and Yucca Mountain, and thus this shallow surface cannot represent an active detachment surface. Deeper, low-angle detachment surface(s) within Proterozoic-Paleozoic bedrock cannot be ruled out by our geophysical data, but are inconsistent with other geologic and geophysical observations in this vicinity. Beneath Crater Flat, the base of the seismogenic crust at 12 km depth is close to the top of the reflective (ductile) lower crust at 14 to 15 km depth, where brittle fault motions in the upper crust may be converted to pure shear in the ductile lower crust. Thus, our preferred interpretation of these geophysical data is that moderate- to high-angle faults extend to 12–15-km depth beneath Yucca Mountain and Crater Flat, with only modest changes in dip.

Nevada↗
USGS2013

Geophysical constraints on Rio Grande rift structure in the central San Luis Basin, Colorado and New Mexico

Interpretation of gravity, aeromagnetic, and magnetotelluric (MT) data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, one of the northernmost major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin determined from a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone, and reach nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. In between, Santa Fe Group thickness is negligible under the San Luis Hills and estimated to reach ~1.1 km under the Costilla Plains (although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km is geophysically reasonable). From combined geophysical and geologic considerations, pre-rift, dominantly sedimentary rocks appear to increase in thickness from none in the Sanchez graben on the east to perhaps 800 m under the San Luis Hills on the west. The pre-rift rocks are most likely early Tertiary in age, but the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. Geophysical data provide new evidence that an isolated exposure of Proterozoic rocks on San Pedro Mesa is rooted in the Precambrian basement. This narrow, north-south–trending basement high has ~2 km of positive relief with respect to the base of the Sanchez graben, and separates the graben from the structural depression beneath the Costilla Plains. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (≥1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south–trending structure with expression in gravity and MT data occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Although it has been interpreted as a down-to-the-east normal fault or fault zone, our modeling suggests that it also is likely related to pre-rift tectonics. Aeromagnetic anomalies over much of the area are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3–3.7 Ma Servilleta Basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates are interpreted to indicate patterns of subsidence following eruption of the basalt, with maximum subsidence in the Sanchez graben.

Colorado, New Mexico↗
USGS2014

Geophysical expression of a buried niobium and rare earth element deposit: the Elk Creek carbonatite, Nebraska, USA

The lower Paleozoic Elk Creek carbonatite is a 6–8-km-diameter intrusive complex buried under 200 m of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S. and a rare earth element (REE) deposit. The carbonatite is composed of several lithologies, the relations of which are poorly understood. Niobium mineralization is most enriched within a magnetite beforsite (MB) unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes Proterozoic country rocks. Efforts to explore the carbonatite have used geophysical data and drilling. A high-resolution airborne gravity gradient and magnetic survey was flown over the carbonatite in 2012. The carbonatite is associated with a roughly annular vertical gravity gradient high and a subdued central low and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. Geophysical, borehole, and physical property data are combined for an interpretation of these signatures. The carbonatite is denser than the country rocks, explaining the gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than those of the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are present in large volumes. MB is very dense (mean density 3200 kg/m 3 ) and strongly magnetized (median 0.073 magnetic susceptibility), producing a gravity gradient high and contributing to the aeromagnetic high. Barite beforsite has physical properties similar to most of the carbonatite volume, making it a poor geophysical target. Geophysical anomalies indicate the presence of dense and strongly magnetized rocks at depths below existing boreholes, either a large volume of MB or another unknown lithology.

Nebraska↗
USGS2000

Evaluation of geophysical logs, Phase II, November 1998 to May 1999, at Crossley Farms Superfund Site, Berks County, Pennsylvania

Between November 1998 and May 1999, geophysical logging was conducted in 29 boreholes at the Crossley Farms Superfund Site, Hereford Township, Berks County, Pa., to determine the fluidproducing zones, fluid-receiving zones, zones of vertical borehole flow, and casing depth. The wells range in depth from 96 to 500 feet below land surface. Gamma logs only were collected in three bedrock wells. The geophysical logging determined the placement of well screens and packers, which allow monitoring and sampling of water-bearing zones in the fractured bedrock so that the horizontal and vertical distribution of contaminated ground water migrating from known sources could be determined. Geophysical logging included collection of caliper, video, fluid-temperature, fluid-resistivity, single-point-resistance, natural-gamma, fluid-flow, and acoustic-televiewer logs. Caliper and video logs were used to locate fractures, joints, and weathered zones. Inflections on fluidtemperature and fluid-resistivity logs indicated possible water-bearing fractures, and flowmeter measurements verified these locations. Single-point-resistance and natural-gamma logs provided information on stratigraphy. After interpretation of geophysical, video logs, and drillers notes, 24 of the wells were reconstructed such that water levels can be monitored and water samples collected from discrete water-bearing fractures in each well.

Pennsylvania↗
USGS2003

Geophysical, stratigraphic, and flow-zone logs of selected test, monitor, and water-supply wells in Cayuga County, New York

Volatile-organic compounds have been detected in water sampled from more than 50 supply wells between the City of Auburn and Village of Union Springs in Cayuga County, New York, and the area was declared a Superfund site in 2002. In 2001-04, geophysical logs were collected from 37 test, monitor, and water-supply wells as a preliminary part of the investigation of volatile-organic compound contamination in the carbonate-bedrock aquifer system. The geophysical logs included gamma, induction, caliper, wellbore image, deviation, fluid resistivity and temperature, and flowmeter. The geophysical logs were analyzed along with core samples and outcrops of the bedrock to define the stratigraphic units and flow zones penetrated by the wells. This report describes the logging methods used in the study and presents the geophysical, stratigraphic, and flow-zone logs.

Open-File Report↗