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Tucson, AZ, United States

Cokriging has been applied to estimate the distribution of moisture within a rock pile of low-grade gold ore, or heap. Along with the primary data set of gravimetric moisture content obtained from drilling, electrical resistivity was used to supplement 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 regression (LSR) modeling of colocated resistivity and moisture. In general, the wells from which moisture data were derived were separated by distances far greater than the horizontal correlation scale. The kriging results showed that regions generally undersampled by drilling reverted to the mean of the moisture data. The LSR technique, which provides a simpletransformation of resistivity to moisture, converted the low resis-tivity 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 considers the correlation scale and secondary data, produced the best 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 application likely could be improved through optimal well placement, whereby the resistivity results guide the drilling program through gross target characterization, and the moisture estimation could be updated iteratively. © 2010 Society of Exploration Geophysicists. Source

Loke M.H.,Geotomo Software | Chambers J.E.,Natural Environment Research Council | Rucker D.F.,HydroGEOPHYSICS Inc. | Kuras O.,Natural Environment Research Council | Wilkinson P.B.,Natural Environment Research Council
Journal of Applied Geophysics

There have been major improvements in instrumentation, field survey design and data inversion techniques for the geoelectrical method over the past 25. years. Multi-electrode and multi-channel systems have made it possible to conduct large 2-D, 3-D and even 4-D surveys efficiently to resolve complex geological structures that were not possible with traditional 1-D surveys. Continued developments in computer technology, as well as fast data inversion techniques and software, have made it possible to carry out the interpretation on commonly available microcomputers. Multi-dimensional geoelectrical surveys are now widely used in environmental, engineering, hydrological and mining applications. 3-D surveys play an increasingly important role in very complex areas where 2-D models suffer from artifacts due to off-line structures. Large areas on land and water can be surveyed rapidly with computerized dynamic towed resistivity acquisition systems. The use of existing metallic wells as long electrodes has improved the detection of targets in areas where they are masked by subsurface infrastructure. A number of PC controlled monitoring systems are also available to measure and detect temporal changes in the subsurface. There have been significant advancements in techniques to automatically generate optimized electrodes array configurations that have better resolution and depth of investigation than traditional arrays. Other areas of active development include the translation of electrical values into geological parameters such as clay and moisture content, new types of sensors, estimation of fluid or ground movement from time-lapse images and joint inversion techniques. In this paper, we investigate the recent developments in geoelectrical imaging and provide a brief look into the future of where the science may be heading. © 2013 Elsevier B.V. Source

Rucker D.F.,HydroGEOPHYSICS Inc.
Geophysical Journal International

The resolution of the long electrode electrical resistivity tomography method is investigated through the comparison of arrays. The investigations included a synthetic model study and a pilot-scale field experiment, in which data from the 2-pole and 4-pole arrays were used to reconstruct known targets through inverse modelling. The results confirmed that the 2-pole array maps conductive targets with low lateral resolution and no vertical resolution. The 4-pole array performs extremely well or extremely poorly depending on the specific subset of data used in the inversion modelling. The worst performance was observed from using a randomized 4-pole subset. In this case, the reconstructed target was offset from the known location. The best performance came from evaluating the comprehensive data set comprising all possible 4-pole combinations and choosing favourable subsets that minimized outliers in transfer resistance, geometric factor, data error and apparent resistivity. These favourable 4-pole subsets were capable of resolving both conductive and resistive targets with higher fidelity than the 2-pole array. Unfortunately, it may not be possible to acquire the comprehensive 4-pole data set, especially for a large number of electrodes. A viable alternative is to acquire the comprehensive 2-pole data set and calculate any desired 4-pole subset using superposition. In this way, the geophysicist will also have full advantage of signal strength and shorter measurement cycle that accompanies the 2-pole array. © 2012 The Author Geophysical Journal International © 2012 RAS. Source

Modeling unsaturated flow in porous media requires constitutive relations that describe the soil water retention and soil hydraulic conductivity as a function of either potential or water content. Often, the hydraulic parameters that describe these relations are directly measured on small soil cores, and many cores are needed to upscale to the entire heterogeneous flow field. An alternative to the forward upscaling method using small samples are inverse upscaling methods that incorporate soft data from geophysical measurements observed directly on the larger flow field. In this paper, we demonstrate that the hydraulic parameters can be obtained from cross borehole ground penetrating radar by measuring the first arrival travel time of electromagnetic waves (represented by raypaths) from stationary antennae during a constant flux infiltration experiment. The formulation and coupling of the hydrological and geophysical models rely on a constant velocity wetting front that causes critical refraction at the edge of the front as it passes by the antennae. During this critical refraction period, the slope of the first arrival data can be used to calculate (1) the wetting velocity and (2) the hydraulic conductivity of the wet (or saturated) soil. If the soil is undersaturated during infiltration, then an estimate of the saturated water content is needed before calculating the saturated hydraulic conductivity. The hydraulic conductivity value is then used in a nonlinear global optimization scheme to estimate the remaining two parameters of a Broadbridge and White soil. © 2010 Elsevier Ltd. Source

Rucker D.F.,HydroGEOPHYSICS Inc. | Noonan G.E.,HydroGEOPHYSICS Inc. | Greenwood W.J.,Pacific Northwest National Laboratory
Engineering Geology

Dredging and widening of the Panama Canal is currently being conducted to allow larger vessels to transit to and from the Americas, Asia, and Europe. Dredging efficiency relies heavily on knowledge of the types and volumes of sediments and rocks beneath the waterway to ensure the right equipment is used for their removal. To aid this process, a waterborne streaming electrical resistivity survey was conducted along the entire length of the canal to provide information on its geology. Within the confines of the canal, a total of 663 line-kilometers of electrical resistivity data were acquired using the dipole-dipole array. The support of the survey data for dredging activities was realized by calibrating and qualitatively correlating the resistivity data with information obtained from nearby logged boreholes and geological maps. The continuity of specific strata was determined in the resistivity sections by evaluating the continuity of similar ranges of resistivity values between boreholes. It was evident that differing geological units and successions can have similar ranges of resistivity values. For example, Quaternary sandy and gravelly alluvial fill from the former river channel of the Chagres River had similar resistivity ranges (generally from 40 to 250. Ω m) to those characteristic of late Miocene basalt dikes (from 100 to 400 Ω m), but for quite different reasons. Similarly, competent marine-based sedimentary rocks of the Caimito Formation were similar in resistivity values (ranging from 0.7 to 10 Ω m) to sandstone conglomerate of the Bohio Formation. Consequently, it would be difficult to use the resistivity data alone to extrapolate more complex geotechnical parameters, such as the hardness or strength of the substrate. A necessary component for such analyses requires detailed objective information regarding the specific context from which the geotechnical parameters were derived. If these data from cored boreholes and detailed geological surveys are taken into account, however, then waterborne streaming resistivity surveying can be a powerful tool. In this case, it provided inexpensive and highly resolved quantitative information on the potential volume of loose suctionable material along the Gamboa Sub-reach, which could enable large cost savings to be made on a major engineering project involving modification of one of the most important navigable waterways in the world. © 2010 Elsevier B.V. Source

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