High-Resolution 3-D and 4-D Imaging of the Near Surface I: Cutting-Edge Electric and Electromagnetic Methods
Presiding: M Grasmueck, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami; C Bank, Colorado College
NS33A-01 INVITED 13:30h
Imaging 4-D hydrogeologic processes with geophysics: an example using crosswell electrical measurements to characterize a tracer plume
Geophysical methods provide an inexpensive way to collect spatially exhaustive data about hydrogeologic, mechanical or geochemical parameters. In the presence of heterogeneity over multiple scales of these parameters at most field sites, geophysical data can contribute greatly to our understanding about the subsurface by providing important data we would otherwise lack without extensive, and often expensive, direct sampling. Recent work has highlighted the use of time-lapse geophysical data to help characterize hydrogeologic processes. We investigate the potential for making quantitative assessments of sodium-chloride tracer transport using 4-D crosswell electrical resistivity tomography (ERT) in a sand and gravel aquifer at the Massachusetts Military Reservation on Cape Cod. Given information about the relation between electrical conductivity and tracer concentration, we can estimate spatial moments from the 3-D ERT inversions, which give us information about tracer mass, center of mass, and dispersivity through time. The accuracy of these integrated measurements of tracer plume behavior is dependent on spatially variable resolution. The ERT inversions display greater apparent dispersion than tracer plumes estimated by 3D advective-dispersive simulation. This behavior is attributed to reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and differential smoothing from tomographic inversion. The latter is a problem common to overparameterized inverse problems, which often occur when real-world budget limitations preclude extensive well-drilling or additional data collection. These results prompt future work on intelligent methods for reparameterizing the inverse problem and coupling additional disparate data sets.
NS33A-02 13:50h
Time-Lapse Monitoring of Salt Transport Using 3-D Electrical Resistivity Imaging
Remediation of salt contaminated soils is a priority for the Canadian upstream oil and gas industry due to historic pipeline breaks, flare pits and other releases of co-produced saline water and brine. Detailed mapping of the distribution of salt in contaminated soil and ground water is essential for planning a remediation program, evaluating remediation progress, and for conclusive risk-based contaminated site closure assessments. Time-lapse 3-D Electrical Resistivity Imaging (ERI) was used at a site with high concentrations of salt that was undergoing remediation. The site is underlain by glacial till with irregular thin sand units. Salt has penetrated in places 6 to 8 m below ground level (mbgl). The average water table was 3 to 4 mbgl and a tile drain system was installed at a depth of about 2 m. The installation is designed to remove saline waters and reduce the mass of salt in the soils. The first 3-D ERI surveys were conducted in July 2004 and repeated in November 2004. The late summer and early fall had unusually high precipitation and a significant mass of salt was removed by the tile drain system. Salt contaminated soils in the July electrical conductivity image are seen as elevated electrical conductivity (EC) anomalies as high as 400 mS/m. The thickness and the distribution of the high concentration areas are quite irregular highlighting the need for high resolution 3-D ERI to accurately characterize the distribution. The overall distribution EC in the November survey is similar to that of July. However, difference maps calculated by subtracting the EC of the July result from the November result shows an overall reduction in EC, up to 50 mS/m, from the surface to about 2 mbgl, the approximate depth of the tile drains. Below 2 m, EC increases by up to 50 mS/m and the depth to the bottom of the high EC zone has increased. The decrease in EC of the upper 2 m appears to reflect changes in site EC. However, imaging the bottom of a conductive anomaly can be problematic. Since the EC of the upper 2 m decreased, it may be that the bottom of the anomaly is better resolved in the November survey. Consequently, a question remains as to whether the increase in EC below 2 m reflects changing site conditions or a difference in image resolution. A series of numerical experiments is used to explore the reliability of the difference images below the 2 m.
NS33A-03 14:05h
Electrical Resistivity Imaging of Tidal Fluctuations in the Water Table at Inwood Hill Park, Manhattan
Inwood Hill Park is located on the northern tip of Manhattan and has been extensively modified over the years by human activities. In its current form, it has a backbone of exposed or lightly covered bedrock along the Hudson River, adjacent to a flat area with two tidal inlets along the northern shore of Manhattan. The tidal motions in the inlets are expected to drive corresponding fluctuations in the water table along the borders of the inlets. In the Fall of 2002, a group of students from the Department of Earth and Atmospheric Sciences at the City College of New York studied these fluctuations. Electrical resistivity cross sections were obtained with a Syscal Kid Switch 24 resistivity meter during the course of a tidal cycle at three locations surrounding the westernmost inlet in the park. No change was seen over a tidal cycle at Site 1, possibly due to the effect of concrete erosion barriers which were located between the land and the water surrounding this site. Measurements at Site 2 revealed a small, regular change in the water table elevation of approximately 5 cm over the course of a tidal cycle. This site is inferred to rest on alluvial sediments deposited by a small creek. The cross sections taken at different times during a tidal cycle at Site 3 were the most interesting. They show a very heterogeneous subsurface, with water spurting between blocks of high resistivity materials during the rising portion of the cycle. A small sinkhole was observed on the surface of the ground directly above an obvious plume of water in the cross section. Park personnel confirmed that this sinkhole, like others scattered around this site, is natural and not due to recent construction activity. They also indicated that debris from the construction of the New York City subways may have been dumped in the area in the past. Our conclusion is that the tidal fluctuations at Site 3 are being channeled by solid blocks in the construction debris, and that the sinkholes currently present result from removal of sediments from below, as a result of the tidal fluctuations.
NS33A-04 INVITED 14:20h
Near Well-bore Imaging Algorithm for the Multi-Array Triaxial Induction Logging Measurements
Induction tools have been the standard resistivity devices for borehole geophysics for more than fifty years. Conventional induction tools are built with coils that have their magnetic moments directed along the tool axis. The resulting sensitivity to the formation is in a direction perpendicular to the well-bore axis. When the beds are dipping, the response is more complicated. For over thirty years a different design for an induction array has appeared in the literature. The most recent design of the induction tool has three orthogonal transmitter coils, all located at the same position on the tool axis, and three orthogonal receiver coils similarly arranged. This device is referred to as a triaxial induction tool. Each transmitter couples to each of the orthogonal receivers, hence in an arbitrary formation this tool produces nine independent measurement data. Further in order to obtain a different depth of investigation from the well-bore a multiple triaxial receiver arrays are used (the so-called multi-array induction tool). One of the important applications of the data from the multi-array triaxial induction tool is to form an image around the well-bore. To carry out this quantitative inversion (imaging) of this data we propose to use the so-called Multiplicative Regularized Contrast Source Inversion algorithm. In this algorithm the resistivity contrast and the contrast sources (the product of the resistivity contrast and the electric field) are updated by an iterative minimization of an appropriate cost functional. The non-regularized cost functional consists of two terms. The first term represents the misfit in the data (data equation) and the second term represents the error of the contrast and the total field in satisfying the integral equation over the computational domain (object equation). In each iteration step, we first update the contrast sources using the Conjugate Gradient (CG) direction so as to minimize the cost functional and then we update the contrast by minimizing again the cost functional using the updated contrast sources. Since all the updating parameters in the inversion procedure are available in closed form, we do not carry out any inversion of an ill-posed operator. The latter significantly increases the robustness of the algorithm. To enhance the ability of the algorithm to handle noisy data an extra regularization term is included in the algorithm. As known in the literature the drawback of adding an extra regularization term to the cost functional is the presence of an artificial weighting parameter in the cost functional, which can only be determined through considerable numerical experimentation and a priori information about the desired unknown. Therefore, we take the regularization as a multiplicative constraint and as a consequence the weighting parameter is now completely prescribed by the error norm in the data and the object equation. Since the weighting parameter is related to the error norm of the data equation, this multiplicative procedure suppress automatically the effect of noise in the reconstruction results. Finally we remark that since in each iteration we do not solve any full forward problem, the computational complexity of the algorithm is equivalent to the complexity of solving two forward problems using the CG method. In the presentation some simulated inversion results will be shown to demonstrate the power of the multi-array triaxial induction data and its imaging algorithm.
NS33A-05 14:40h
Cone-based Electrical Resistivity Tomography
Determining the 3D spatial distribution of subsurface properties is a critical part of managing the clean-up of contaminated sites. Most standard hydrologic methods sample small regions immediately adjacent to wells or testing devices. This provides data which are not representative of the entire region of interest. Furthermore, at many contaminated sites invasive methods are not acceptable, due to the risks associated with contacting and spreading the contaminants. To address these issues, we have developed a minimally invasive technology that provides information about the 3D distribution of electrical conductivity. This new technique, cone-based electrical resistivity tomography (C-bert), involves placing several permanent current electrodes in the subsurface and using electrodes mounted on a cone penetrometer to measure the resultant potential field while advancing the cone into the subsurface. In addition to potential field measurements, we obtain the standard suite of cone-penetration measurements, including high resolution resistivity logs; these data can then be used to constrain the inversion of the potential field data. A major challenge of working with these data is that the cone penetrometer is highly conductive, and thus presents a large local perturbation around the measurement location. As the cone is very small (approximately 30mm in diameter) with respect to the total model space, explicitly modeling the cone is computationally demanding. We developed a method for solving the forward model that reduces computational time by an order of magnitude. This solution method, iteratively determined boundary conditions, makes it possible to correct for the cone effect before inversion of the data. Results from synthetic experiments suggest that the C-bert method of data acquisition can result in high quality electrical conductivity images of the subsurface. We tested the practicality of this technique by performing a field test of the C-bert system to image a salt water intrusion in a fresh water aquifer in Vancouver, British Columbia. A total of nine current electrodes were emplaced at the site and five C-bert profiles were obtained, resulting in approximately 6500 data points. We conclude, from this first field test, that this method is a promising new way to image the subsurface.