Jonathan E. Nyquist

Self-potential The ugly duckling of environmental geophysics

Self-potential (SP) is the method that everyone knows about but nobody seems to appreciate. Out of more than 850 papers published in the Symposium for the Application of Geophysics to Environmental and Engineering Problems (SAGEEP) between 1988 and 2001, 63 included SP as a key word, but most mentioned it only in passing.

Stream Bottom Resistivity Tomography to Map Ground Water Discharge

This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Omega m) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Omega m) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Omega m) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Omega m in the area of the seeps and greater than 50 Omega m change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.

Simultaneous inversion of airborne electromagnetic data for resistivity and magnetic permeability

Where the magnetic permeability of rock or soil exceeds that of free space, the effect on airborne electromagnetic systems is to produce a frequency‐independent shift in the in‐phase response of the system while altering the quadrature response only slightly. The magnitude of the in‐phase shift increases as (1) the relative magnetic permeability is increased, (2) the amount of magnetic material is increased, and (3) the airborne sensor gets nearer the earth’s surface. Over resistive, magnetic ground, the shift may be evinced by negative in‐phase measurements at low frequencies; but over more conductive ground, the same shift may go unnoticed because of the large positive in‐phase response. If the airborne sensor is flown at low levels, the magnitude of the shift may be large enough to affect automatic inversion routines that do not take this shift into account, producing inaccurate estimated resistivities, usually overestimates. However, layered‐earth inversion algorithms that incorporate magnetic permeab...

Delineating a road-salt plume in lakebed sediments using electrical resistivity, piezometers, and seepage meters at Mirror Lake, New Hampshire, U.S.A

Electrical-resistivity surveys, seepage meter measurements, and drive-point piezometers have been used to characterize chloride-enriched groundwater in lakebed sediments of Mirror Lake, New Hampshire, U.S.A. A combination of bottom-cable and floating-cable electrical-resistivity surveys identified a conductive zone (<100 ohm-m) overlying resistive bedrock (<1000 ohm-m) beneath the lake. Shallow pore-water samples from piezometers in lakebed sediments have chloride concentrations of 200–1800 μeq∕liter , and lake water has a chloride concentration of 104 μeq∕liter . The extent of the plume was estimated and mapped using resistivity and water-sample data. The plume ( 20×35 m wide and at least 3 m thick) extends nearly the full length and width of a small inlet, overlying the top of a basin formed by the bedrock. It would not have been possible to mapthe plume’s shape without the resistivity surveys because wells provided only limited coverage. Seepage meters were installed approximately 40 m from the mouth o...

Characterizing lakebed seepage and geologic heterogeneity using resistivity imaging and temperature measurements

The contribution of groundwater-surface water exchange to lake budgets is poorly understood and depends in part on lakebed heterogeneities. These heterogeneities are difficult to characterize using traditional point sampling methods. The goal of this project was to use electrical resistivity to identify potential zones of groundwater discharge and recharge, providing focus for point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially-formed lake in northeastern Pennsylvania. Two types of resistivity surveys were conducted. In a continuous resistivity profile, a multi-electrode cable was towed parallel to shore to look for spatial variability in resistivity around the lake. Two parallel to shore surveys were conducted, an inner and outer loop, to help characterize the lateral extent of sediment types. The results of these surveys suggested lithology changes both along the shoreline and with distance from shore. Follow-up resistivity data were collected using cables laid along the lake bottom perpendicular to the shoreline to look for finer scale zonation that affects seepage as a function of distance from shore. Follow-up seepage measurements showed that seepage rates are very low, which is consistent with the resistivity data from which we concluded that most of Lake Lacawac is lined with glacial clay and that the lake is essentially perched above the groundwater flow system, with just minor amounts of seepage in a few locations where fingers of sandy sediments extend a short distance from the shore. Discontinuities in these patches of transmissive sediments can result in reversals in the direction of seepage at nearby locations. We conclude that towed resistivity is useful as a rapid reconnaissance tool for mapping geologic heterogeneity. The results can be used to guide the more time-consuming but higher-resolution, lake bottom resistivity measurements, which in turn can guide the placement of seepage meters.

Use of Underwater Resistivity in the Assessment of Groundwater‐Surface Water Interaction within the Burd Run Watershed

Characterizing groundwater interaction with streams is essential for understanding contaminant transport. We are investigating the use of multielectrode resistivity to improve the detection of seepage points, and the mapping of gaining and losing stream reaches. Our field area is the Burd Run watershed, Shippensburg, PA. Burd Run is of interest as a geophysical test case because the stream is dilute where it flows from the metasedimentary ridge of South Mountain across a colluvium wedge. Fluid conductivity increases from 35 μS/cm to 440 μS/cm in three abrupt increments as the stream flows across the steeply dipping carbonate units of the Great Valley before discharging into Mill Spring Creek. The increases can be attributed to the introduction of carbonate groundwater, both from municipal discharge and from vertical seeps within the streambed. Continuous dipole-dipole surveys were conducted over a 107 m reach using a 28-electrode cable with a 1-m electrode spacing deployed on the streambed. The survey resolved the conductivity contrast of the streambed sediments over the carbonate bedrock and detected a vertical zone of higher electrical conductivity that correlates with observed streambed seeps.

Marine Resistivity As A Tool For Characterizing Zones Of Seepage At Lake Lacawac, Pa

The groundwater-surface water exchange zones of lakes and streams are dynamic and difficult to characterize. The spatial variability of seepage zones makes them difficult to locate using traditional point sampling methods. The goal of this project is to use marine resistivity to identify potential zones of groundwater discharge and recharge, providing focus for point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially formed lake in northeastern Pennsylvania. One target for these surveys was the resistivity contrast between groundwater and surface water. Another target was resistivity contrasts created by geologic heterogeneities that control groundwater discharge into the lake. Two types of surveys were conducted using a SuperSting resistivity system. In a continuous resistivity profile, a multi-electrode cable was towed parallel to shore to look for spatial variability in resistivity around the lake. A second resistivity array was laid on the lake bottom perpendicular to the shoreline to examine how resistivity varied with distance from shore. The results of these surveys suggested several lithology changes both along the shoreline and with distance from shore. Seepage meters were used to provide ground truth about interpreted areas of seepage.

Freeze Core Sampling to Validate Time‐Lapse Resistivity Monitoring of the Hyporheic Zone

A freeze core sampler was used to characterize hyporheic zone storage during a stream tracer test. The pore water from the frozen core showed tracer lingered in the hyporheic zone after the tracer had returned to background concentration in collocated well samples. These results confirmed evidence of lingering subsurface tracer seen in time-lapse electrical resistivity tomographs. The pore water exhibited brine exclusion (ion concentrations in ice lower than source water) in a sediment matrix, despite the fast freezing time. Although freeze core sampling provided qualitative evidence of lingering tracer, it proved difficult to quantify tracer concentration because the amount of brine exclusion during freezing could not be accurately determined. Nonetheless, the additional evidence for lingering tracer supports using time-lapse resistivity to detect regions of low fluid mobility within the hyporheic zone that can act as chemically reactive zones of importance in stream health.

Rayleigh-wave diffractions due to a void in the layered half space

Summary Void detection is challenging due to the complexity of near-surface materials and the limited resolution of geophysical methods. Although multichannel, high-frequency, surface-wave techniques can provide reliable shear (S)-wave velocities in different geological settings, they are not suitable for detecting voids directly based on anomalies of the S-wave velocity because of limitations on the resolution of S-wave velocity profiles inverted from surface-wave phase velocities. Xia et al. (2006a) derived a Rayleigh-wave diffraction traveltime equation due to a void in the homogeneous half space. Encouraging results of directly detecting a void from Rayleigh-wave diffractions were presented (Xia et al., 2006a). In this paper we used four twodimensional square voids in the layered half space to demonstrate the feasibility of detecting a void with Rayleigh-wave diffractions. Rayleigh-wave diffractions were recognizable for all these models after removing direct surface waves by F-K filtering. We evaluate the feasibility of applying the Rayleigh-wave diffraction traveltime equation to a void in the layered earth model. The phase velocity of diffracted Rayleigh waves is predominately determined by surrounding materials of a void. The modeling results demonstrate that the Rayleigh-wave diffraction traveltime equation due to a void in the homogeneous half space can be applied to the case of a void in the layered half space. In practice, only two diffraction times are necessary to define the depth to the top of a void and the average velocity of diffracted Rayleigh waves.

Tracking tracer breakthrough in the hyporheic zone using time-lapse DC resistivity, Crabby Creek, Pennsylvania

Characterization of the hyporheic zone is of critical importance for understanding stream ecology, contaminant transport, and groundwater-surface water interaction. A salt water tracer test was used to probe the hyporheic zone of a recently re-engineered portion of Crabby Creek, a stream located near Philadelphia, PA. The tracer solution was tracked through a 13.5 meter segment of the stream using both a network of 25 wells sampled every 5-15 minutes and time-lapse electrical resistivity tomographs collected every 11 minutes for six hours, with additional tomographs collected every 100 minutes for an additional 16 hours. The comparison of tracer monitoring methods is of keen interest because tracer tests are one of the few techniques available for characterizing this dynamic zone, and logistically it is far easier to collect resistivity tomographs than to install and monitor a dense network of wells. Our results show that resistivity monitoring captured the essential shape of the breakthrough curve and may indicate portions of the stream where the tracer lingered in the hyporheic zone. Timelapse resistivity measurements, however, represent time averages over the period required to collect a tomographic data set, and spatial averages over a volume larger than captured by a well sample. Smoothing by the resistivity data inversion algorithm further blurs the resulting tomograph; consequently resistivity monitoring underestimates the degree of fine-scale heterogeneity in the hyporheic zone.