Dale F. Rucker

Electrical-resistivity characterization of an industrial site using long electrodes

An electrical-resistivity survey was completed at the T tank farm at the Hanford nuclear site in Washington State, U.S.A. The purpose of the survey was to define the lateral extent of waste plumes in the vadose zone in and around the tank farm. The T tank farm consists of single-shell tanks that historically have leaked and many liquid-waste-disposal facilities that provide a good target for resistivity mapping. Given that the site is highly industrialized with near-surface metallic infrastructure that potentially could mask any interpretable waste plume, it was necessary to use the many wells around the site as long electrodes. To accommodate the long electrodes and to simulate the effects of a linear conductor, the resistivity inversion code was modified to assign low-resistivity values to the well’s location. The forward model within the resistivity code was benchmarked for accuracy against an analytic solution, and the inverse model was tested for its ability to recreate images of a hypothetical targe...

Moisture estimation within a mine heap: An application of cokriging with assay data and electrical resistivity

Cokriging has been applied to estimate the distribution of moisturewithinarockpileoflow-gradegoldore,orheap.Along with the primary data set of gravimetric moisture content obtainedfromdrilling,electricalresistivitywasusedtosupplement the estimation procedure by supplying a secondary data set. The effectiveness of the cokriging method was determined by comparing the results to kriging the moisture data alone and through least-squares regressionLSRmodeling of colocated resistivity and moisture. In general, the wells from which moisture data werederivedwereseparatedbydistancesfargreaterthanthehorizontalcorrelationscale.Thekrigingresultsshowedthatregions generally undersampled by drilling reverted to the mean of the moisture data. The LSR technique, which provides a simple transformationofresistivitytomoisture,convertedthelowresistivity to highmoisture, and vice versa. The sparse well locations created a high degree of uncertainty in the transformed data set. Extreme resistivity values produced nonphysical moisture values, either negative for the linear model or values greater than one for the power model. The cokriging application, which considersthecorrelationscaleandsecondarydata,producedthebest results, as indicated through the cross validation. The mean and variance of the cokriged moisture were closer to the measured moisture, and the bias in the residuals was the lowest.The applicationlikelycouldbeimprovedthroughoptimalwellplacement, wherebytheresistivityresultsguidethedrillingprogramthrough gross target characterization, and the moisture estimation could beupdatediteratively.

Surface geophysical exploration: developing noninvasive tools to monitor past leaks around Hanford's tank farms.

A characterization program has been developed at Hanford to image past leaks in and around the underground storage tank facilities. The program is based on electrical resistivity, a geophysical technique that maps the distribution of electrical properties of the subsurface. The method was shown to be immediately successful in open areas devoid of underground metallic infrastructure, due to the large contrast in material properties between the highly saline waste and the dry sandy host environment. The results in these areas, confirmed by a limited number of boreholes, demonstrate a tendency for the lateral extent of the underground waste plume to remain within the approximate footprint of the disposal facility. In infrastructure-rich areas, such as tank farms, the conventional application of electrical resistivity using small point-source surface electrodes initially presented a challenge for the resistivity method. The method was then adapted to directly use the buried infrastructure, specifically the steel-cased wells that surround the tanks, as “long” electrodes for both transmission of electrical current and measurements of voltage. Overcoming the drawbacks of the long electrode method has been the focus of our work over the past 7xa0years. The drawbacks include low vertical resolution and limited lateral coverage. The lateral coverage issue has been improved by supplementing the long electrodes with surface electrodes in areas devoid of infrastructure. The vertical resolution has been increased by developing borehole electrode arrays that can fit within the small-diameter drive casing of a direct push rig. The evolution of the program has led to some exceptional advances in the application of geophysical methods, including logistical deployment of the technology in hazardous areas, development of parallel processing resistivity inversion algorithms, and adapting the processing tools to accommodate electrodes of all shapes and locations. The program is accompanied by a full set of quality assurance procedures that cover the layout of sensors, measurement strategies, and software enhancements while insuring the integrity of stored data. The data have been shown to be useful in identifying previously unknown contaminant sources and defining the footprint of precipitation recharge barriers to retard the movement of existing contamination.

Standard, Random and Optimum Array Conversions from Two-pole Resistance DataRucker and Glaser: Array Conversions from Two-pole Resistance Data

We present an array evaluation of standard and non-standard arrays over a hydrogeological target. We develop the arrays by linearly combining data from the pole-pole (or 2-pole) array. The first test shows that reconstructed resistances for the standard Schlumberger and dipole-dipole arrays are equivalent or superior to the measured arrays in terms of noise, especially at large geometric factors. The inverse models for the standard arrays also confirm what others have presented in terms of target resolvability, namely the dipole-dipole array has the highest resolution. In the second test, we reconstruct random electrode combinations from the 2-pole data segregated into inner, outer, and overlapping dipoles. The resistance data and inverse models from these randomized arrays show those with inner dipoles to be superior in terms of noise and resolution, and that overlapping dipoles can cause model instability and low resolution. Finally, we use the 2-pole data to create an optimized array that maximizes the model resolution matrix for a given electrode geometry. The optimized array produces the highest resolution and target detail. Thus, the tests demonstrate that high quality data and high model resolution can be achieved by acquiring field data from the pole-pole array.

A Modified Wenner Array for Efficient Use of Eight-Channel Resistivity Meters

AbstractThe Wenner array is a popular measurement strategy for acquiring geoelectrical data for one-dimensional soundings and two-dimensional profiles. An advantage of using this array is the high signal strength, as the receiving dipole expands in size proportional to the transmitting dipole. A major disadvantage of the Wenner array is the low efficiency in data acquisition, as this array can only be used in a single-channel acquisition mode. To make better use of the available open channels on multi-channel resistivity meters, we explore modifications to the Wenner array by adding additional receiver dipoles. Two of the modifications include receiver dipoles that are either larger or smaller than the Wenner dipole, but are symmetrical around the center of the transect. A third modification includes supplementing the Wenner dipole with smaller gradient dipoles that are not symmetrically arranged around the center. We found that this latter array, termed Alt_3 Wenner, performed better than other modified arrays in terms of resolution and noise. We also tested the Alt_3 Wenner array against an optimized array and found the resolution to be nearly as high, but with better noise management and channel usage efficiency. We recommend the Alt_3 Wenner array for supplementing the Wenner array if that is the array of choice.n

Waterborne Electrical Resistivity of the Hypersaline Mono Lake

A short, waterborne streamer resistivity survey was conducted on Mono Lake in California. The lake, located on the eastern side of the Sierra Nevada Mountains, contains a significant amount of dissolved solids due to being a terminal lake with no outlet. The survey was conducted to determine if the method could be used to define subbottom sediments and other geological features to answer questions related to recent-past volcanic and tectonic events. The survey used a 15-meter dipole-dipole array towed at approximately 2 mph behind a small flat bottomed boat. The survey was limited in scope to collecting approximately 6,000 m of data within the western cove, where substantial core sampling has taken place. Comparison of the methods indicates the towed array can obtain good quality data despite the lake9s conductivity being in excess of 84,000 uS/cm. The resistivity models reconstructed geological material upwards of 400 ohm-m to depths of 35 m, which likely represent hard rock below the lake. We conclude that the method could be used to map most of the shallow areas of the lake, where the water column is less than 20 m. The deepest areas of the lake, south of Paoha Island, would require streamer resistivity cables with an electrode spacing of at least 60 m.

PANAMA CANAL EXPANSION PROJECT: HOW MARINE ELECTRICAL RESISTIVITY WAS USED IN SUPPORT OF CANAL DREDGING

The Panama Canal is currently being dredged to deepen and widen the waterway for passage of larger vessels. Dredging efficiency relies heavily on knowledge of the rock type and hardness, as this will determine the dredging method and machinery selected. For example, loose sediment can be removed via suction dredging, whereas more complex drilling and blasting techniques are needed to remove harder rock types. To support this effort, a detailed marine electrical resistivity survey was completed within the Canal to produce a spatial distribution of electrical resistivity values along the canal floor to understand overall rock and sediment distribution. Resistivity data were acquired along a series of parallel, two-dimensional transects, nominally spaced 25 meters apart across the width of the navigable portion of the waterway. Alone, resistivity values could not determine rock type; it was found in this survey that differing geological units can show similar ranges of resistivity values. Although weathered and competent rock generally displays lower resistivity values and loose sediment exhibit higher resistivity, the porosity, water saturation and ionic constituents could potentially affect specific values in localized areas. This survey effort showed however, that when resistivity data are combined with known data from geological maps and Canal boreholes, rock types, strength properties, and geologic boundaries can be identified successfully in the resistivity data.