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USGS2010

Introduction to the JEEG Agricultural Geophysics Special Issue

Near-surface geophysical methods have become increasingly important tools in applied agricultural practices and studies. The great advantage of geophysical methods is their potential rapidity, low cost, and spatial continuity when compared to more traditional methods of assessing agricultural land, such as sample collection and laboratory analysis. Agricultural geophysics investigations commonly focus on obtaining information within the soil profile, which generally does not extend much beyond 2 meters beneath the ground surface. Although the depth of interest oftentimes is rather shallow, the area covered by an agricultural geophysics survey can vary widely in scale, from experimental plots (10 s to 100 s of square meters), to farm fields (10 s to 100 s of hectares), up to the size of watersheds (10 s to 100 s of square kilometers). To date, three predominant methods—resistivity, electromagnetic induction (EMI), and ground-penetrating radar (GPR)—have been used to obtain surface-based geophysical measurements within agricultural settings. However, a recent conference on agricultural geophysics (Bouyoucos Conference on Agricultural Geophysics, September 8–10, 2009, Albuquerque, New Mexico; www.ag-geophysics.org) illustrated that other geophysical methods are being applied or developed. These include airborne electromagnetic induction, magnetometry, seismic, and self-potential methods. Agricultural geophysical studies are also being linked to ground water studies that utilize deeper penetrating geophysical methods than normally used.

Journal of Environmental & Engineering Geophysics↗

USGS1996

Use of ground-penetrating radar and continuous seismic-reflection profiling on surface-water bodies in environmental and engineering studies

Ground‐penetrating radar (GPR) and continuous seismic‐reflection profiling (CSP) on shallow rivers, lakes, and ponds are efficient and economical ways of obtaining subsurface hydrologic and geologic information for environmental and engineering studies. These methods are similar in that they produce continuous subsurface profiles, are easy to use in some applications, and the records can occasionally be straightforward to interpret. They are dissimilar in that GPR cannot penetrate electrically conductive water or subsurface sediments, and CSP usually cannot operate in water less than 5 feet (ft.) deep. GPR records collected on a lake in New Hampshire have been interpreted to estimate the depth to bedrock and to evaluate the grain‐size characteristics of the underlying stratified drift at the lakeshore boundary. In a pond in Massachusetts, CSP and GPR were used to determine depth to bedrock and the grain‐size characteristics of the subbottom materials in part of the pond. Water‐column multiple reflections, depth and conductivity of water and subsurface materials, and diffractions degraded the quality of the GPR records. CSP records collected in the Connecticut River near Hartford, Connecticut were used to estimate the depth of till and bedrock interfaces and to evaluate grain‐size characteristics of subsurface materials. Interpreted CSP records also can indicate bedding planes within consolidated rock units. Water‐column multiple reflections and very shallow water degraded the quality of the CSP records. GPR and CSP methods have been used to delineate infilled scour holes near bridge piers. Scour holes that were filled with up to 8 ft. of loose sand were mapped during engineering scour studies near a bridge in Connecticut. Because GPR and CSP operate on different physical principles, the two geophysical methods complement each other. Depending on the required depth of penetration and the degree of resolution needed, one or both of these methods can be used to acquire accurate and reliable subsurface hydrologic and geologic information critical to environmental and engineering studies.

Journal of Environmental & Engineering Geophysics↗

USGS2011

Hydrogeophysical investigations at Hidden Dam, Raymond, California

Self-potential and direct current resistivity surveys are carried out at the Hidden Dam site in Raymond, California to assess present-day seepage patterns and better understand the hydrogeologic mechanisms that likely influence seepage. Numerical modeling is utilized in conjunction with the geophysical measurements to predict variably-saturated flow through typical two-dimensional dam cross-sections as a function of reservoir elevation. Several different flow scenarios are investigated based on the known hydrogeology, as well as information about typical subsurface structures gained from the resistivity survey. The flow models are also used to simulate the bulk electrical resistivity in the subsurface under varying saturation conditions, as well as the self-potential response using petrophysical relationships and electrokinetic coupling equations. The self-potential survey consists of 512 measurements on the downstream area of the dam, and corroborates known seepage areas on the northwest side of the dam. Two direct-current resistivity profiles, each approximately 2,500 ft (762 m) long, indicate a broad sediment channel under the northwest side of the dam, which may be a significant seepage pathway through the foundation. A focusing of seepage in low-topography areas downstream of the dam is confirmed from the numerical flow simulations, which is also consistent with past observations. Little evidence of seepage is identified from the self-potential data on the southeast side of the dam, also consistent with historical records, though one possible area of focused seepage is identified near the outlet works. Integration of the geophysical surveys, numerical modeling, and observation well data provides a framework for better understanding seepage at the site through a combined hydrogeophysical approach.

California↗

USGS1999

Integrating surface and borehole geophysics in ground water studies: An example using electromagnetic soundings in south Florida

Time domain surface electromagnetic soundings, borehole induction logs, and other borehole logging techniques are used to construct a realistic model for the shallow subsurface hydraulic properties of unconsolidated sediments in south Florida. Induction logs are used to calibrate surface induction soundings in units of pore water salinity by correlating water sample specific electrical conductivity with the electrical conductivity of the formation over the sampled interval for a two‐layered aquifer model. Geophysical logs are also used to show that a constant conductivity layer model is appropriate for the south Florida study. Several physically independent log measurements are used to quantify the dependence of formation electrical conductivity on such parameters as salinity, permeability, and clay mineral fraction. The combined interpretation of electromagnetic soundings and induction logs was verified by logging three validation boreholes, confirming quantitative estimates of formation conductivity and thickness in the upper model layer, and qualitative estimates of conductivity in the lower model layer.

Journal of Environmental & Engineering Geophysics↗

USGS2004

Object-based inversion of crosswell radar tomography data to monitor vegetable oil injection experiments

Crosswell radar methods can be used to dynamically image ground-water flow and mass transport associated with tracer tests, hydraulic tests, and natural physical processes, for improved characterization of preferential flow paths and complex aquifer heterogeneity. Unfortunately, because the raypath coverage of the interwell region is limited by the borehole geometry, the tomographic inverse problem is typically underdetermined, and tomograms may contain artifacts such as spurious blurring or streaking that confuse interpretation. We implement object-based inversion (using a constrained, non-linear, least-squares algorithm) to improve results from pixel-based inversion approaches that utilize regularization criteria, such as damping or smoothness. Our approach requires pre- and post-injection travel-time data. Parameterization of the image plane comprises a small number of objects rather than a large number of pixels, resulting in an overdetermined problem that reduces the need for prior information. The nature and geometry of the objects are based on hydrologic insight into aquifer characteristics, the nature of the experiment, and the planned use of the geophysical results. The object-based inversion is demonstrated using synthetic and crosswell radar field data acquired during vegetable-oil injection experiments at a site in Fridley, Minnesota. The region where oil has displaced ground water is discretized as a stack of rectangles of variable horizontal extents. The inversion provides the geometry of the affected region and an estimate of the radar slowness change for each rectangle. Applying petrophysical models to these results and porosity from neutron logs, we estimate the vegetable-oil emulsion saturation in various layers. Using synthetic- and field-data examples, object-based inversion is shown to be an effective strategy for inverting crosswell radar tomography data acquired to monitor the emplacement of vegetable-oil emulsions. A principal advantage of object-based inversion is that it yields images that hydrologists and engineers can easily interpret and use for model calibration.

Journal of Environmental & Engineering Geophysics↗

USGS2018

New insights on scale-dependent surface-groundwater exchange from a floating self-potential Dipole

In south-central Texas the lower Guadalupe River has incised into the outcrop of the Carrizo-Wilcox aquifer. The river and the aquifer are hydraulically connected across the outcrop, although the connectivity is obscured at the surface by alluvium and surface-water and groundwater exchange dynamics are currently poorly understood. To investigate surface-water and groundwater exchange dynamics between the lower Guadalupe River and the Carrizo-Wilcox aquifer, a geophysical study was completed along a 14.86 km reach of the river by using water-borne gradient self-potential (SP) profiling and two-dimensional direct-current electric resistivity tomography. This paper explores the applicability of these water-borne geoelectric methods in delineating gaining and losing channel reaches, and demonstrates that geoelectric signals in the form of total electric field strength can be logged with an electric dipole and decomposed into component SP signals depicting regional and local groundwater flow patterns attributable to regional and localized hydraulic gradients. Localized SP anomalies of several tens of millivolts, indicative of hyporheic exchange flows, are observed and superimposed upon a 124 mV regional SP anomaly indicative of ambient groundwater exchange flows between the river and the aquifer. The observed SP signals are interpreted through two-dimensional finite-element modeling of streaming potentials attributable to ambient groundwater exchange and hyporheic exchange flow patterns. Variables of the channel environment such as temperature and concentration gradients, depth, and velocity are considered and subsequently eliminated as alternative sources of the SP signals that are presented.

Texas↗

USGS2006

Design and application of an electromagnetic vibrator seismic source

Vibrational seismic sources frequently provide a higher-frequency seismic wavelet (and therefore better resolution) than other sources, and can provide a superior signal-to-noise ratio in many settings. However, they are often prohibitively expensive for lower-budget shallow surveys. In order to address this problem, I designed and built a simple but effective vibrator source for about one thousand dollars. The "EMvibe" is an inexpensive electromagnetic vibrator that can be built with easy-to-machine parts and off-the-shelf electronics. It can repeatably produce pulse and frequency-sweep signals in the range of 5 to 650 Hz, and provides sufficient energy for recording at offsets up to 20 m. Analysis of frequency spectra show that the EMvibe provides a broader frequency range than the sledgehammer at offsets up to ??? 10 m in data collected at a site with soft sediments in the upper several meters. The EMvibe offers a high-resolution alternative to the sledgehammer for shallow surveys. It is well-suited to teaching applications, and to surveys requiring a precisely-repeatable source signature.

Journal of Environmental & Engineering Geophysics↗

USGS2013

Blind test of methods for obtaining 2-D near-surface seismic velocity models from first-arrival traveltimes

Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the data's frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply dipping low-velocity fault zone. The estimated models do not contain a subtle low-velocity zone and other fine-scale features, in accord with conventional wisdom. Together, the results support confidence in the reliability and robustness of modern refraction inversion and tomographic methods.

Journal of Environmental & Engineering Geophysics↗

USGS2020

Permafrost mapping with electrical resistivity tomography in two wetland systems north of the Tanana River, Interior Alaska

Surface-based 2D electrical resistivity tomography (ERT) surveys were used to characterize permafrost distribution at wetland sites on the alluvial plain north of the Tanana River, 20 km southwest of Fairbanks, Alaska, in June and September 2014. The sites were part of an ecologically-sensitive research area characterizing biogeochemical response of this region to warming and permafrost thaw, and the site contained landscape features characteristic of interior Alaska, including thermokarst bog, forested permafrost plateau, and a rich fen. The results show how vegetation reflects shallow (0–10 m depth) permafrost distribution. Additionally, we saw shallow (0–3 m depth) low resistivity areas in forested permafrost plateau potentially indicating the presence of increased unfrozen water content as a precursor to ground instability and thaw. Time-lapse study from June to September suggested a depth of seasonal influence extending several meters below the active layer, potentially as a result of changes in unfrozen water content. A comparison of several electrode geometries (dipole-dipole, extended dipole-dipole, Wenner-Schlumberger) showed that for depths of interest to our study (0–10 m) results were similar, but data acquisition time with dipole-dipole was the shortest, making it our preferred geometry. The results show the utility of ERT surveys to characterize permafrost distribution at these sites, and how vegetation reflects shallow permafrost distribution. These results are valuable information for ecologically sensitive areas where ground-truthing can cause excessive disturbance. ERT data can be used to characterize the exact subsurface geometry of permafrost such that over time an understanding of changing permafrost conditions can be made in great detail. Characterizing the depth of thaw and thermal influence from the surface in these areas also provides important information as an indication of the depth to which carbon storage and microbially-mediated carbon processing may be affected.

Alaska↗

USGS2014

Mapping saltwater intrusion in the Biscayne Aquifer, Miami-Dade County, Florida using transient electromagnetic sounding

Saltwater intrusion in southern Florida poses a potential threat to the public drinking-water supply that is typically monitored using water samples and electromagnetic induction logs collected from a network of wells. Transient electromagnetic (TEM) soundings are a complementary addition to the monitoring program because of their ease of use, low cost, and ability to fill in data gaps between wells. TEM soundings have been used to map saltwater intrusion in the Biscayne aquifer over a large part of south Florida including eastern Miami-Dade County and the Everglades. These two areas are very different with one being urban and the other undeveloped. Each poses different conditions that affect data collection and data quality. In the developed areas, finding sites large enough to make soundings is difficult. The presence of underground pipes further restricts useable locations. Electromagnetic noise, which reduces data quality, is also an issue. In the Everglades, access to field sites is difficult and working in water-covered terrain is challenging. Nonetheless, TEM soundings are an effective tool for mapping saltwater intrusion. Direct estimates of water quality can be obtained from the inverted TEM data using a formation factor determined for the Biscayne aquifer. This formation factor is remarkably constant over Miami-Dade County owing to the uniformity of the aquifer and the absence of clay. Thirty-six TEM soundings were collected in the Model Land area of southeast Miami-Dade County to aid in calibration of a helicopter electromagnetic (HEM) survey. The soundings and HEM survey revealed an area of saltwater intrusion aligned with canals and drainage ditches along U.S. Highway 1 and the Card Sound Road. These canals and ditches likely reduced freshwater levels through unregulated drainage and provided pathways for seawater to flow at least 12.4 km inland.

Florida↗

USGS2015

Ground-truthing electrical resistivity methods in support of submarine groundwater discharge studies: Examples from Hawaii, Washington, and California

Submarine groundwater discharge (SGD) is an important conduit that links terrestrial and marine environments. SGD conveys both water and water-borne constituents into coastal waters, where these inflows may impact near-shore ecosystem health and sustainability. Multichannel electrical resistivity techniques have proven to be a powerful tool to examine scales and dynamics of SGD and SGD forcings. However, there are uncertainties both in data aquisition and data processing that must be addressed to maximize the effectiveness of this tool in estuarine or marine environments. These issues most often relate to discerning subtle nuances in the flow of electricity through variably saturated media that can also be highly conductive ( i.e., seawater). Three contrasting field sites were examined for this study to assess the effectiveness of electrical resistivity techniques in varying coastal settings by comparing resistivity data to direct salinity and resistivity observations, quantifying changes in lithology and beach geomorphology, and fine-tuning inversion protocols. The three study sites all have substantial (up to 85 cm day −1 ) submarine groundwater discharge rates, but the hydrologic, oceanographic, and geologic characteristics of the sites are all very different. At a site in Pelekane Bay on the Big Island of Hawaii, seasonal flooding introduces very high concentrations of fine to coarse sediment into the bay. Near-shore circulation is limited in Pelekane Bay, so this newly introduced sediment can become deposited in the bay where it accumulates over time. At a site in Hood Canal, a fjord within Puget Sound, Washington, SGD rates can be high because of the large tidal range, abundant recharge, and steep hydrologic gradients. At Younger Lagoon in northern California, the flow of groundwater towards the coast is much more parsimonious, but here marine processes, including recirculated seawater, are important in controlling the flow of material towards the coast. Rigorous ground-truthing at each field site showed that multi-channel electrcial resistivity techniques can reproduce the scales and dynamics of a seepage field when such data are correctly collected, and when the model inversions are tuned to field site characteristics. Such information can provide a unique perspective on the scales and dynamics of exchange processes within a coastal aquifer—information essential to scientists and resource managers alike.

California, Hawaii, Washington↗

USGS2016

Seasonal electrical resistivity surveys of a coastal bluff, Barter Island, North Slope Alaska

Select coastal regions of the North Slope of Alaska are experiencing high erosion rates that can be attributed in part to recent warming trends and associated increased storm intensity and frequency. The upper sediment column of the coastal North Slope of Alaska can be described as continuous permafrost underlying a thin (typically less than 1–2 m) active layer that responds variably to seasonal thaw cycles. Assessing the temporal and spatial variability of the active layer and underlying permafrost is essential to better constrain how heightened erosion may impact material fluxes to the atmosphere and the coastal ocean, and how enhanced thaw cycles may impact the stability of the coastal bluffs. In this study, multi-channel electrical resistivity tomography (ERT) was used to image shallow subsurface features of a coastal bluff west of Kaktovik, on Barter Island, northeast Alaska. A comparison of a suite of paired resistivity surveys conducted in early and late summer 2014 provided detailed information on how the active layer and permafrost are impacted during the short Arctic summer. Such results are useful in the development of coastal resilience models that tie together fluvial, terrestrial, climatic, geologic, and oceanographic forcings on shoreline stability.

Alaska↗

USGS2016

Where is the hot rock and where is the ground water— Using CSAMT to map beneath and around Mount St. Helens

We have observed several new features in recent controlled-source audio-frequency magnetotelluric (CSAMT) soundings on and around Mount St. Helens, Washington State, USA. We have identified the approximate location of a strong electrical conductor at the edges of and beneath the 2004–08 dome. We interpret this conductor to be hot brine at the hot-intrusive-cold-rock interface. This contact can be found within 50 meters of the receiver station on Spine 5, which extruded between April and July of 2005. We have also mapped separate regional and glacier-dome aquifers, which lie one atop the other, out to considerable distances from the volcano.

Washington↗

USGS2016

Application of ground penetrating radar for identification of washover deposits and other stratigraphic features: Assateague Island, MD

A combination of ground penetrating radar (GPR) data, core data, and aerial photographs were analyzed to better understand the evolution of two portions of Assateague Island, Maryland. The focus of the study was to investigate the applicability of using GPR data to image washover deposits in the stratigraphic record. High amplitude reflections observed in two shore-perpendicular GPR profiles were correlated to shallow (<1 m) lithologic contacts observed in sediment cores. At these contacts, deposits consisting primarily of quartz sand overlie sediments with organic matter that include degraded plant root or stem material. The underlying organic matter likely represents the vegetated portion of the barrier island that was buried by washover fans deposited during hurricanes Irene (2011) and Sandy (2012), as indicated in high-resolution aerial photographs. The GPR data were able to delineate the washover deposits from the underlying stratigraphic unit; however, the radar data did not resolve finer structures necessary to definitively differentiate washover facies from other sand-rich deposits ( e.g., flood-tide deltas and dunes). Other GPR profiles contain reflections that likely correlate to geomorphic features like tidal channels and vegetated zones observed in historical aerial imagery. Burial of these features by overwash fluxes were observed in the aerial imagery and thus the resulting radar sequence is largely interpreted as washover deposits. Deeper, channel-like features that have been infilled were also observed in shore-parallel profiles and these features coincide with scour channels observed in the 1966 aerial photography. Additional sedimentological data are required to determine what role overwash played in the in-filling of these features.

Journal of Environmental & Engineering Geophysics↗

USGS2017

Comparing measurement response and inverted results of electrical resistivity tomography instruments

In this investigation, we compare the results of electrical resistivity measurements made by six commercially available instruments on the same line of electrodes to determine if there are differences in the measured data or inverted results. These comparisons are important to determine whether measurements made between different instruments are consistent. We also degraded contact resistance on one quarter of the electrodes to study how each instrument responds to different electrical connection with the ground. We find that each instrument produced statistically similar apparent resistivity results, and that any conservative assessment of the final inverted resistivity models would result in a similar interpretation for each. We also note that inversions, as expected, are affected by measurement error weights. Increased measurement errors were most closely associated with degraded contact resistance in this set of experiments. In a separate test we recorded the full measured waveform for a single four-electrode array to show how poor electrode contact and instrument-specific recording settings can lead to systematic measurement errors. We find that it would be acceptable to use more than one instrument during an investigation with the expectation that the results would be comparable assuming contact resistance remained consistent.

Journal of Environmental & Engineering Geophysics↗

USGS2022

Evaluation of electrical and electromagnetic geophysical techniques to inspect earthen dam and levee structures in Arkansas

Within the state of Arkansas there is an increasing number of aging dams and levees that have little to no documentation concerning their construction or composition. Surface geophysical surveys offer a non-intrusive method for investigating these structures: To describe their lithologic makeup, to evaluate the materials that they were constructed upon, and to identify potential flow paths through them. Techniques such as electrical resistivity tomography, seismic refraction, and electromagnetic induction have all been used to image dams and levees and require additional information from geologic outcrops, geotechnical borings, or drill cores in order to make informed geologic interpretations of the geophysical models. These geologic models then allow the owners of these structures to make more informed decisions about their operation and maintenance. Between 2011 and 2018, the U.S. Geological Survey conducted geophysical and geotechnical investigations of three earthen structures within the state of Arkansas. Electrical and electromagnetic geophysical data were used to develop lithologic models of these structures and the underlying geology. Self-potential surveys were utilized to detect the movement of water through these structures indicating possible seepage pathways. Geotechnical methods such as electric and hydraulic direct-push well logs and cores acted as both a control on the geophysical interpretations and a confirmation of anomalies. This integrated approach detected the lack of an impermeable core within a levee, imaged a change in lithology of the bedrock forming the seal beneath a gravity dam, and identified a potential seepage feature within the core of an earthen dam. These results further support that this method of extending known lithologic features via surface and borehole geophysics is a useful approach for characterizing earthen water control structures.

Arkansas↗

USGS2021

Investigation of scale-dependent groundwater/surface-water exchange in rivers by gradient self-potential logging: Numerical modeling and field experiments

Exchanges of groundwater and surface-water are fundamental to a wide range of water-supply and water-quality management issues but challenging to map beyond the reach scale. Waterborne gradient self-potential (SP) measurements are directly sensitive to water flow through riverbed sediments and can be used to infer exchange locations, direction (gain versus loss), scale, and relative changes, but to date applications to river corridor hydrology are limited. Numerical modeling and field experiments were therefore performed herein, each emphasizing waterborne gradient SP logging for identifying and locating focused vertical groundwater discharge (surface-water gain) and recharge (surface-water loss) in a river. Two and three-dimensional numerical models were constructed to simulate the polarities, appearances, and peak amplitudes of streaming-potential and electric-field anomalies on a riverbed and in the surface-water that were attributable to steady-state vertical fluxes of groundwater through high-permeability conduits in the riverbed. Effects of varied hydraulic length-scale of exchange and surface-water depth were tested through numerical modeling. Modeling results aided in data acquisition and interpretation for three repeated field experiments performed along a 1.5–2.0 km reach of the Quashnet River in Cape Cod, Massachusetts, where focused, meter-scale groundwater discharges occur at discrete locations within otherwise ubiquitous and more diffuse groundwater upwelling conditions. Strong gradient SP anomalies were repeatedly measured in the Quashnet River at previously confirmed locations of focused groundwater discharge, showing the efficacy of waterborne gradient SP logging in identifying and characterizing groundwater/surface water exchange dynamics at multiple river network scales.

Massachusetts↗

USGS2011

Critical steps for the continuing advancement of hydrogeophysics

Special hydrogeophysics issues published by hydrology and geophysics journals, special sessions and workshops at conferences, and an increasing number of short courses demonstrate the growing interest in the use of geophysics for hydrologic investigations. The formation of the hydrogeophysics technical subcommittee of AGU's Hydrology section adds further evidence of the recognized significance of this growing interdisciplinary field. Given the clear value of nondestructive and nonintrusive imaging for subsurface investigations, we believe the advances in the adoption of existing geophysical methods, the development of novel methods, and the merging of geophysical and other data made in hydrogeophysics could be applied to a wide range of geological, environmental, and engineering applications.

Eos Science News↗