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At least 19 recordsLinked to original sources
USGS2004

Acoustic and optical borehole-wall imaging for fractured-rock aquifer studies

Imaging with acoustic and optical televiewers results in continuous and oriented 360° views of the borehole wall from which the character, relation, and orientation of lithologic and structural planar features can be defined for studies of fractured-rock aquifers. Fractures are more clearly defined under a wider range of conditions on acoustic images than on optical images including dark-colored rocks, cloudy borehole water, and coated borehole walls. However, optical images allow for the direct viewing of the character of and relation between lithology, fractures, foliation, and bedding. The most powerful approach is the combined application of acoustic and optical imaging with integrated interpretation. Imaging of the borehole wall provides information useful for the collection and interpretation of flowmeterand other geophysical logs, core samples, and hydraulic and water-quality data from packer testing and monitoring.

Journal of Applied Geophysics↗
USGS2004

Application of ground-penetrating radar, digital optical borehole images, and cores for characterization of porosity hydraulic conductivity and paleokarst in the Biscayne aquifer, southeastern Florida, USA

This paper presents examples of ground-penetrating radar (GPR) data from two study sites in southeastern Florida where karstic Pleistocene platform carbonates that comprise the unconfined Biscayne aquifer were imaged. Important features shown on resultant GPR profiles include: (1) upward and lateral qualitative interpretative distribution of porosity and hydraulic conductivity; (2) paleotopographic relief on karstic subaerial exposure surfaces; and (3) vertical stacking of chronostratigraphic high-frequency cycles (HFCs). These characteristics were verified by comparison to rock properties observed and measured in core samples, and identified in digital optical borehole images. Results demonstrate that an empirical relation exists between measured whole-core porosity and hydraulic conductivity, observed porosity on digital optical borehole images, formation conductivity, and GPR reflection amplitudes-as porosity and hydraulic conductivity determined from core and borehole images increases, formation conductivity increases, and GPR reflection amplitude decreases. This relation allows for qualitative interpretation of the vertical and lateral distribution of porosity and hydraulic conductivity within HFCs. Two subtidal HFCs in the uppermost Biscayne aquifer have significantly unique populations of whole-core porosity values and vertical hydraulic conductivity values. Porosity measurements from one cycle has a median value about two to three times greater than the values from the other HFC, and median values of vertical hydraulic-conductivity about three orders of magnitude higher than the other HFC. The HFC with the higher porosity and hydraulic conductivity values is shown as a discrete package of relatively low-amplitude reflections, whereas the HFC characterized by lower porosity and hydraulic-conductivity measurements is expressed by higher amplitude reflections. Porosity and hydraulic-conductivity values measured from whole-core samples, and vuggy porosity identified on digital borehole images from shallowing-upward, peritidal HFCs show that the highest porosity occurs at the base of the cycles, moderate porosity at the middle of the cycles, and lowest porosity occurs at the top of cycles. Hydraulic conductivity is also highest at the base of the peritidal cycles and lowest in the middle to upper parts of cycles. This change in porosity and hydraulic conductivity from bottom to top is visible as an upward variation in reflection amplitude on GPR profiles-lowest amplitudes at the base and highest at the cycle tops. This study demonstrates that GPR can be used to show the qualitative distribution of porosity and hydraulic conductivity within a cycle-stratigraphic framework composed of carbonate HFCs. The distribution of porosity and hydraulic conductivity within HFCs is related to depositional textures. The upward and lateral patterns of the rock facies within the HFCs can be translated to geophysical-log properties and radar facies configurations that could aid in interpretation and prediction of ground-water flow through a carbonate aquifer. ?? 2003 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2012

Geophysical investigations of geology and structure at the Martis Creek Dam, Truckee, California

A recent evaluation of Martis Creek Dam highlighted the potential for dam failure due to either seepage or an earthquake on nearby faults. In 1972, the U.S. Army Corps of Engineers constructed this earthen dam, located within the Truckee Basin to the north of Lake Tahoe, CA for water storage and flood control. Past attempts to raise the level of the Martis Creek Reservoir to its design level have been aborted due to seepage at locations downstream, along the west dam abutment, and at the base of the spillway. In response to these concerns, the U.S. Geological Survey has undertaken a comprehensive suite of geophysical investigations aimed at understanding the interplay between geologic structure, seepage patterns, and reservoir and groundwater levels. This paper concerns the geologic structure surrounding Martis Creek Dam and emphasizes the importance of a regional-scale understanding to the interpretation of engineering-scale geophysical data. Our studies reveal a thick package of sedimentary deposits interbedded with Plio-Pleistocene volcanic flows; both the deposits and the flows are covered by glacial outwash. Magnetic field data, seismic tomography models, and seismic reflections are used to determine the distribution and chronology of the volcanic flows. Previous estimates of depth to basement (or the thickness of the interbedded deposits) was 100 m. Magnetotelluric soundings suggest that electrically resistive bedrock may be up to 2500 m deep. Both the Polaris Fault, identified outside of the study area using airborne LiDAR, and the previously unnamed Martis Creek Fault, have been mapped through the dam area using ground and airborne geophysics. Finally, as determined by direct-current resistivity imaging, time-domain electromagnetic sounding, and seismic refraction, the paleotopography of the interface between the sedimentary deposits and the overlying glacial outwash plays a principal role both in controlling groundwater flow and in the distribution of the observed seepage.

California↗
USGS2002

A pitfall in shallow shear-wave refraction surveying

The shallow shear-wave refraction method works successfully in an area with a series of horizontal layers. However, complex near-surface geology may not fit into the assumption of a series of horizontal layers. That a plane SH-wave undergoes wave-type conversion along an interface in an area of nonhorizontal layers is theoretically inevitable. One real example shows that the shallow shear-wave refraction method provides velocities of a converted wave rather than an SH- wave. Moreover, it is impossible to identify the converted wave by refraction data itself. As most geophysical engineering firms have limited resources, an additional P-wave refraction survey is necessary to verify if velocities calculated from a shear-wave refraction survey are velocities of converted waves. The alternative at this time may be the surface wave method, which can provide reliable S-wave velocities, even in an area of velocity inversion (a higher velocity layer underlain by a lower velocity layer). ?? 2002 Elsevier Science B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2011

Challenges of using electrical resistivity method to locate karst conduits-A field case in the Inner Bluegrass Region, Kentucky

Conduits serve as major pathways for groundwater flow in karst aquifers. Locating them from the surface, however, is one of the most challenging tasks in karst research. Geophysical methods are often deployed to help locate voids by mapping variations of physical properties of the subsurface. Conduits can cause significant contrasts of some physical properties that can be detected; other subsurface features such as water-bearing fractures often yield similar contrasts, which are difficult to distinguish from the effects of the conduits. This study used electrical resistivity method to search for an unmapped karst conduit that recharges Royal Spring in the Inner Bluegrass karst region, Kentucky, USA. Three types of resistivity techniques (surface 2D survey, quasi-3D survey, and time-lapse survey) were used to map and characterize resistivity anomalies. Some of the major anomalies were selected as drilling targets to verify the existence of the conduits. Drilling near an anomaly identified by an electrical resistivity profile resulted in successful penetration of a major water-filled conduit. The drilling results also suggest that, in this study area, low resistivity anomalies in general are associated with water-bearing features. However, differences in the anomaly signals between the water-filled conduit and other water-bearing features such as water-filled fracture zones were undistinguishable. The electrical resistivity method is useful in conduit detection by providing potential drilling targets. Knowledge of geology and hydrogeology about the site and professional judgment also played important roles in locating the major conduit. ?? 2011 Elsevier B.V.

Journal of Applied Geophysics↗
USGS1993

Using borehole geophysics and cross-borehole flow testing to define hydraulic connections between fracture zones in bedrock aquifers

Nearly a decade of intensive geophysical logging at fractured rock hydrology research sites indicates that geophysical logs can be used to identify and characterize fractures intersecting boreholes. However, borehole-to-borehole flow tests indicate that only a few of the apparently open fractures found to intersect boreholes conduct flow under test conditions. This paper presents a systematic approach to fracture characterization designed to define the distribution of fractures along boreholes, relate the measured fracture distribution to structure and lithology of the rock mass, and define the nature of fracture flow paths across borehole arrays. Conventional electrical resistivity, gamma, and caliper logs are used to define lithology and large-scale structure. Borehole wall image logs obtained with the borehole televiewer are used to give the depth, orientation, and relative size of fractures in situ. High-resolution flowmeter measurements are used to identify fractures conducting flow in the rock mass adjacent to the boreholes. Changes in the flow field over time are used to characterize the hydraulic properties of fracture intersections between boreholes. Application of this approach to an array of 13 boreholes at the Mirror Lake, New Hamsphire site demonstrates that the transient flow analysis can be used to distinguish between fractures communicating with each other between observation boreholes, and those that are hydraulically isolated from each other in the surrounding rock mass. The Mirror Lake results also demonstrate that the method is sensitive to the effects of boreholes on the hydraulic properties of the fractured-rock aquifer. Experiments conducted before and after the drilling of additional boreholes in the array and before and after installation of packers in existing boreholes demonstrate that the presence of new boreholes or the inflation of packers in existing boreholes has a large effect on the measured hydraulic properties of the rock mass surrounding the borehole array.

Journal of Applied Geophysics↗
USGS2021

Geoelectric survey of the Granite Gravel aquifer, Llano Uplift, Central Texas, to determine locations for water wells

An electrical geophysical survey was completed within a small area of the Llano Uplift of central Texas to determine locations to install two water wells in the Granite Gravel aquifer (GGA). Electrical resistivity tomography (ERT) was performed along two 188-m long profiles that intersected at the approximate center of a 100-m by 100-m self-potential (SP) map. The ERT survey was completed to map two-dimensional (2D) electric resistivity distributions in the GGA and the underlying Precambrian Town Mountain Granite (TMG) bedrock, whereas SP mapping was performed to delineate apparent streaming potential anomalies at the land surface that appeared correlated to the subsurface resistivity distributions, which exhibited strong lateral heterogeneity in the upper 35 m of the weathered layer that comprises the GGA. The depth to TMG bedrock, as shown by the resistivity distributions, varied substantially over relatively small profile distances and surface areas; however, the general electrical structure showed resistivity increasing with depth, beginning with a thin electrically conductive layer at the surface characterized by resistivity in the range of about 30–100 Ω-m, followed by a resistivity increase to 300–500 Ω-m at a depth that coincided with the water-table depth observed in the installed water wells. Resistivity of the TMG bedrock was generally greater than 500 Ω-m and exceeded 1000 Ω-m in some locations of the tomograms. Electrical structure beneath the survey area, as shown by the 2D resistivity distributions beneath the ERT profiles, delineated a relatively thick weathered section of GGA that spatially aligned with a conspicuous negative anomaly observed in the SP map after electrode-drift and terrain corrections were made. The combination of ERT and SP mapping guided selection of locations for two productive water wells within the small survey area despite substantial heterogeneity in the weathering profile of the GGA beneath the survey area.

Texas↗
USGS1994

Use of geoelectrical methods in groundwater pollution surveys in a coastal environment

The pollution of coastal aquifers by old landfills can contaminate valuable and scarce water resources in the freshwater lens utilized seasonably by overcrowded communities. The pollutants will ultimately flow into the sea where they may also cause a coastal water pollution problem. We have detected pollution in the freshwater lens from a sanitary landfill near Provincetown, Cape Cod, using the geoelectrical resistivity method. This survey included Schlumberger geoelectrical depth soundings and a horizontal geoelectrical profile using the Wenner configuration. The geoelectrical survey was conducted at a site along Highway 6 where it passes the coastal town of Provincetown and a sanitary landfill that has been in operation since 1954. The depth soundings suggest the characteristic decrease in resistivity vs. depth from the high resistivity of the unsaturated zone to the low resistivity of the saltwater saturated zone. The freshwater lens is clearly identified by the change in slope of the steeply dipping curve of resistivity versus electrode spacing. Interpretations made using a multilayer program, Geomate, resulted in layer resistivities between 460 and 95 ohm?m for the freshwater lens. A comparison with well water resistivities suggests that a layer resistivity of 230 ohm?m or lower is indicative of pollution in the freshwater lens. The results of the geoelectrical depth soundings were confirmed in the Wenner horizontal profile. Both measurements suggest that the pollutants do not spread evenly as one would expect for a homogeneous and isotropic medium. Instead, a preferred channel for the flow of the pollutants is observed along a path from the landfill toward the shoreline. The depth to the saltwater/freshwater interface or, more specifically, to the low resistivity-high resistivity interface appears to be shallow where the freshwater lens is polluted. This was confirmed by pore water well samples that were highly mineralized. The equilibrium postulated by the Ghyben-Herzberg relation appears to be disturbed in the area of aquifer pollution. This rise in the conductivity boundary is caused by the highly mineralized bottom of the contaminant plume that submerges into the saltwater saturated zone. In the area of high freshwater pollution the groundwater can be subdivided into three layers that show a decrease in resistivity with depth. The formation factor, F, defined as the ratio of bulk aquifer resistivity to pore water resistivity, shows unusually high values between 10 and 12. These high values are unexpected for an unconsolidated sand. Pollution residues are suspected to clog the pores and thus to increase the resistivity. It is possible that iron-oxidizing bacteria and the precipitation of dissolved iron or organic pollutants are the cause of the high values of F. If proven correct, these interesting possibilities could lead to future new applications of the geoelectrical resistivity method in contaminant hydroloy.

Journal of Applied Geophysics↗
USGS1994

Contributions to a shallow aquifer study by reprocessed seismic sections from petroleum exploration surveys, eastern Abu Dhabi, United Arab Emirates

The US Geological Survey, in cooperation with the National Drilling Company of Abu Dhabi, is conducting a 4-year study of the fresh and slightly saline groundwater resources of the eastern Abu Dhabi Emirate. Most of this water occurs in a shallow aquifer, generally less than 150 m deep, in the Al Ain area. A critical part of the Al Ain area coincides with a former petroleum concession area where about 2780 km of vibroseis data were collected along 94 seismic lines during 1981–1983. Field methods, acquistion parameters, and section processing were originally designed to enhance reflections expected at depths ranging from 5000 to 6000 m, and subsurface features directly associated with the shallow aquifer system were deleted from the original seismic sections. The original field tapes from the vibroseis survey were reprocessed in an attempt to extract shallow subsurface information (depths less than 550 m) for investigating the shallow aquifer. A unique sequence of reproccessing parameters was established after reviewing the results from many experimental tests. Many enhancements to the resolution of shallow seismic reflections resulted from: (1) application of a 20-Hz, low-cut filter; (2) recomputation of static corrections to a datum nearer the land surface; (3) intensive velocity analyses; and (4) near-trace muting analyses. The number, resolution, and lateral continuity of shallow reflections were greatly enhanced on the reprocessed sections, as was the delineation of shallow, major faults. Reflections on a synthetic seismogram, created from a borehole drilled to a depth of 786 m on seismic line IQS-11, matched precisely with shallow reflections on the reprocessed section. The 33 reprocessed sections were instrumental in preparing a map showing the major structural features that affect the shallow aquifer system. Analysis of the map provides a better understanding of the effect of these shallow features on the regional occurrence, movement, and quality of groundwater in the concession area. Results from this study demonstrate that original seismic field tapes collected for deep petroleum exploration can be reprocessed to explore for groundwater.

Journal of Applied Geophysics↗
USGS2004

Application of deterministic deconvolution of ground-penetrating radar data in a study of carbonate strata

We successfully applied deterministic deconvolution to real ground-penetrating radar (GPR) data by using the source wavelet that was generated in and transmitted through air as the operator. The GPR data were collected with 400-MHz antennas on a bench adjacent to a cleanly exposed quarry face. The quarry site is characterized by horizontally bedded carbonate strata with shale partings. In order to provide groundtruth for this deconvolution approach, 23 conductive rods were drilled into the quarry face at key locations. The steel rods provided critical information for: (1) correlation between reflections on GPR data and geologic features exposed in the quarry face, (2) GPR resolution limits, (3) accuracy of velocities calculated from common midpoint data and (4) identifying any multiples. Comparing the results of deconvolved data with non-deconvolved data demonstrates the effectiveness of deterministic deconvolution in low dielectric-loss media for increased accuracy of velocity models (improved at least 10-15% in our study after deterministic deconvolution), increased vertical and horizontal resolution of specific geologic features and more accurate representation of geologic features as confirmed from detailed study of the adjacent quarry wall. ?? 2004 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2006

Quantitative estimation of minimum offset for multichannel surface-wave survey with actively exciting source

Multichannel analysis of surface waves is a developing method widely used in shallow subsurface investigations. The field procedures and related parameters are very important for successful applications. Among these parameters, the source-receiver offset range is seldom discussed in theory and normally determined by empirical or semi-quantitative methods in current practice. This paper discusses the problem from a theoretical perspective. A formula for quantitatively evaluating a layered homogenous elastic model was developed. The analytical results based on simple models and experimental data demonstrate that the formula is correct for surface wave surveys for near-surface applications. ?? 2005 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2007

Wave field features of shallow vertical discontinuity and their application in non-destructive detection

The geotechnical integrity of critical infrastructure can be seriously compromised by the presence of fractures or crevices. Non-destructive techniques to accurately detect fractures in critical infrastructure such as dams and highways could be of significant benefit to the geotechnical industry. This paper investigates the application of shallow seismic and georadar methods to the detection of a vertical discontinuity using numerical simulations. The objective is to address the kinematical analysis of a vertical discontinuity, determine the resulting wave field characteristics, and provide the basis for determining the existence of vertical discontinuities based on the recorded signals. Simulation results demonstrate that: (1) A reflection from a vertical discontinuity produces a hyperbolic feature on a seismic or georadar profile; (2) In order for a reflection from a vertical discontinuity to be produced, a reflecting horizon below the discontinuity must exist, the offset between source and receiver (x0) must be non-zero, on the same side of the vertical discontinuity; (3) The range of distances from the vertical discontinuity where a reflection event is observed is proportional to its length and to x0; (4) Should the vertical crevice (or fracture) pass through a reflecting horizon, dual hyperbolic features can be observed on the records, and this can be used as a determining factor that the vertical crevice passes through the interface; and (5) diffractions from the edges of the discontinuity can be recorded with relatively smaller amplitude than reflections and their ranges are not constrained by the length of discontinuity. If the length of discontinuity is short enough, diffractions are the dominant feature. Real-world examples show that the shallow seismic reflection method and the georadar method are capable of recording the hyperbolic feature, which can be interpreted as vertical discontinuity. Thus, these methods show some promise as effective non-destructive detection methods for locating vertical discontinuities (e.g., fractures or crevices) in infrastructure such as dams and highway pavement. ?? 2007 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2007

Joint inversion of high-frequency surface waves with fundamental and higher modes

Joint inversion of multimode surface waves for estimating the shear (S)-wave velocity has received much attention in recent years. In this paper, we first analyze sensitivity of phase velocities of multimodes of surface waves for a six-layer earth model, and then we invert surface-wave dispersion curves of the theoretical model and a real-world example. Sensitivity analysis shows that fundamental mode data are more sensitive to the S-wave velocities of shallow layers and are concentrated on a very narrow frequency band, while higher mode data are more sensitive to the parameters of relatively deeper layers and are distributed over a wider frequency band. These properties provide a foundation of using a multimode joint inversion to define S-wave velocities. Inversion results of both synthetic data and a real-world example demonstrate that joint inversion with the damped least-square method and the singular-value decomposition technique to invert high-frequency surface waves with fundamental and higher mode data simultaneously can effectively reduce the ambiguity and improve the accuracy of S-wave velocities. ?? 2007.

Journal of Applied Geophysics↗
USGS2008

Generation of a pseudo-2D shear-wave velocity section by inversion of a series of 1D dispersion curves

Multichannel Analysis of Surface Waves utilizes a multichannel recording system to estimate near-surface shear (S)-wave velocities from high-frequency Rayleigh waves. A pseudo-2D S-wave velocity (vS) section is constructed by aligning 1D models at the midpoint of each receiver spread and using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. The receiver spread length sets the theoretical lower limit and any vS structure with its lateral dimension smaller than this length will not be properly resolved in the final vS section. A source interval smaller than the spread length will not improve the horizontal resolution because spatial smearing has already been introduced by the receiver spread. In this paper, we first analyze the horizontal resolution of a pair of synthetic traces. Resolution analysis shows that (1) a pair of traces with a smaller receiver spacing achieves higher horizontal resolution of inverted S-wave velocities but results in a larger relative error; (2) the relative error of the phase velocity at a high frequency is smaller than at a low frequency; and (3) a relative error of the inverted S-wave velocity is affected by the signal-to-noise ratio of data. These results provide us with a guideline to balance the trade-off between receiver spacing (horizontal resolution) and accuracy of the inverted S-wave velocity. We then present a scheme to generate a pseudo-2D S-wave velocity section with high horizontal resolution using multichannel records by inverting high-frequency surface-wave dispersion curves calculated through cross-correlation combined with a phase-shift scanning method. This method chooses only a pair of consecutive traces within a shot gather to calculate a dispersion curve. We finally invert surface-wave dispersion curves of synthetic and real-world data. Inversion results of both synthetic and real-world data demonstrate that inverting high-frequency surface-wave dispersion curves - by a pair of traces through cross-correlation with phase-shift scanning method and with the damped least-square method and the singular-value decomposition technique - can feasibly achieve a reliable pseudo-2D S-wave velocity section with relatively high horizontal resolution. ?? 2008 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS2011

Inversion of multi-frequency electromagnetic induction data for 3D characterization of hydraulic conductivity

Electromagnetic induction (EMI) instruments provide rapid, noninvasive, and spatially dense data for characterization of soil and groundwater properties. Data from multi-frequency EMI tools can be inverted to provide quantitative electrical conductivity estimates as a function of depth. In this study, multi-frequency EMI data collected across an abandoned uranium mill site near Naturita, Colorado, USA, are inverted to produce vertical distribution of electrical conductivity ( EC ) across the site. The relation between measured apparent electrical conductivity ( EC a ) and hydraulic conductivity ( K ) is weak (correlation coefficient of 0.20), whereas the correlation between the depth dependent EC obtained from the inversions, and K is sufficiently strong to be used for hydrologic estimation (correlation coefficient of − 0.62). Depth-specific EC values were correlated with co-located K measurements to develop a site-specific ln( EC )–ln( K ) relation. This petrophysical relation was applied to produce a spatially detailed map of K across the study area. A synthetic example based on EC a values at the site was used to assess model resolution and correlation loss given variations in depth and/or measurement error. Results from synthetic modeling indicate that optimum correlation with K occurs at ~ 0.5 m followed by a gradual correlation loss of 90% at 2.3 m. These results are consistent with an analysis of depth of investigation (DOI) given the range of frequencies, transmitter–receiver separation, and measurement errors for the field data. DOIs were estimated at 2.0 ± 0.5 m depending on the soil conductivities. A 4-layer model, with varying thicknesses, was used to invert the EC a to maximize available information within the aquifer region for improved correlations with K . Results show improved correlation between K and the corresponding inverted EC at similar depths, underscoring the importance of inversion in using multi-frequency EMI data for hydrologic estimation.

Journal of Applied Geophysics↗
USGS2011

Feasibility of waveform inversion of Rayleigh waves for shallow shear-wave velocity using a genetic algorithm

Conventional surface wave inversion for shallow shear (S)-wave velocity relies on the generation of dispersion curves of Rayleigh waves. This constrains the method to only laterally homogeneous (or very smooth laterally heterogeneous) earth models. Waveform inversion directly fits waveforms on seismograms, hence, does not have such a limitation. Waveforms of Rayleigh waves are highly related to S-wave velocities. By inverting the waveforms of Rayleigh waves on a near-surface seismogram, shallow S-wave velocities can be estimated for earth models with strong lateral heterogeneity. We employ genetic algorithm (GA) to perform waveform inversion of Rayleigh waves for S-wave velocities. The forward problem is solved by finite-difference modeling in the time domain. The model space is updated by generating offspring models using GA. Final solutions can be found through an iterative waveform-fitting scheme. Inversions based on synthetic records show that the S-wave velocities can be recovered successfully with errors no more than 10% for several typical near-surface earth models. For layered earth models, the proposed method can generate one-dimensional S-wave velocity profiles without the knowledge of initial models. For earth models containing lateral heterogeneity in which case conventional dispersion-curve-based inversion methods are challenging, it is feasible to produce high-resolution S-wave velocity sections by GA waveform inversion with appropriate priori information. The synthetic tests indicate that the GA waveform inversion of Rayleigh waves has the great potential for shallow S-wave velocity imaging with the existence of strong lateral heterogeneity.

Journal of Applied Geophysics↗
USGS2012

Integrated characterization of the geologic framework of a contaminated site in West Trenton, New Jersey

Fractured sedimentary bedrock and groundwater at the former Naval Air Warfare Center in West Trenton, New Jersey (United States of America) are contaminated with chlorinated solvents. Predicting contaminant migration or removing the contaminants requires an understanding of the geology. Consequently, the geologic framework near the site was characterized with four different methods having different spatial scales: geologic field mapping, analyses of bedrock drill core, analyses of soil and regolith, and S-wave refraction surveys. A fault zone is in the southeast corner of the site and separates two distinct sedimentary formations; the fault zone dips (steeply) southeasterly, strikes northeasterly, and extends at least 550 m along its strike direction. Drill core from the fault zone is extensively brecciated and includes evidence of tectonic contraction. Approximately 300 m east of this fault zone is another fault zone, which offsets the contact between the two sedimentary formations. The S-wave refraction surveys identified both fault zones beneath soil and regolith and thereby provided constraints on their lateral extent and location.

New Jersey↗
USGS2013

Integrating stations from the North America Gravity Database into a local GPS-based land gravity survey

The ability to augment local gravity surveys with additional gravity stations from easily accessible national databases can greatly increase the areal coverage and spatial resolution of a survey. It is, however, necessary to integrate such data seamlessly with the local survey. One challenge to overcome in integrating data from national databases is that these data are typically of unknown quality. This study presents a procedure for the evaluation and seamless integration of gravity data of unknown quality from a national database with data from a local Global Positioning System (GPS)-based survey. The starting components include the latitude, longitude, elevation and observed gravity at each station location. Interpolated surfaces of the complete Bouguer anomaly are used as a means of quality control and comparison. The result is an integrated dataset of varying quality with many stations having GPS accuracy and other reliable stations of unknown origin, yielding a wider coverage and greater spatial resolution than either survey alone.

Missouri↗