Douglas J. LaBrecque

ELECTRICAL RESISTIVITY TOMOGRAPHY OF VADOSE WATER MOVEMENT

Cross borehole electrical resistivity tomography (ERT) was used to image the resistivity distribution before and during two infiltration experiments. In both cases water was introduced into the vadose zone, and the change in resistivity associated with the plume of wetted soil was imaged as a function of time. The primary purpose of this work was to study the capabilities and limitations of ERT to image underground structure and ground water movement in the vadose zone. A secondary goal was to learn specifics of unsaturated flow in a complex geologic setting. Tomographs of electrical resistivity taken before infiltration image coarser, well-drained soils (sands and gravels) as more resistive zones, whereas finer grained soils (silts and clays), which hold more water by capillarity, are imaged as more conductive. Images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology. In the ERT images we see the effects of capillary barriers and infer differences between capillary-driven flow through fine sediments and gravity-driven flow through very permeable sediments. Images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs. ERT can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements.

The effects of noise on Occam's inversion of resistivity tomography data

An Occams inversion algorithm for crosshole resistivity data that uses a finite-element method forward solution is discussed. For the inverse algorithm, the earth is discretized into a series of parameter blocks, each containing one or more elements. The Occams inversion finds the smoothest 2-D model for which the Chi-squared statistic equals an a priori value. Synthetic model data are used to show the effects of noise and noise estimates on the resulting 2-D resistivity images. Resolution of the images decreases with increasing noise. The reconstructions are underdetermined so that at low noise levels the images converge to an asymptotic image, not the true geoelectrical section. If the estimated standard deviation is too low, the algorithm cannot achieve an adequate data fit, the resulting image becomes rough, and irregular artifacts start to appear. When the estimated standard deviation is larger than the correct value, the resolution decreases substantially (the image is too smooth). The same effects are demonstrated for field data from a site near Livermore, California. However, when the correct noise values are known, the Occams results are independent of the discretization used. A case history of monitoring at an enhanced oil recovery site is used to illustrate problems in comparing successive images over time from a site where the noise level changes. In this case, changes in image resolution can be misinterpreted as actual geoelectrical changes. One solution to this problem is to perform smoothest, but non-Occams, inversion on later data sets using parameters found from the background data set.

Monitoring an underground steam injection process using electrical resistance tomography

We used electrical resistance tomography (ERT) to map the subsurface distribution of a steam flood as a function of time as part of a prototype environmental restoration process performed by the Dynamic Underground Stripping Project. We evaluated the capability of ERT to monitor changes in the soil resistivity during the steam injection process using a dipole-dipole measurement technique to measure the bulk electrical resistivity distribution in the soil mass. The injected steam caused changes in the soils resistivity because the steam displaced some of the native pore water, increased the pore water and soil temperatures and changed the ionic content of the pore water. We could detect the effects of steam invasion by mapping changes in the soil resistivity as a function of space and time. The ERT tomographs are compared with induction well logs, formation temperature logs and lithologic logs. These comparisons suggest that the ERT tomographs mapped the formation regions invaded by the steam flood. The data also suggest that steam invasion was limited in vertical extent to a gravel horizon at depth of approximately 43 m. The tomographs show that with time, the steam invasion zone extended laterally to all areas monitored by the ERT technique.

Difference Inversion of ERT Data: a Fast Inversion Method for 3-D In Situ Monitoring

A three-dimensional (3-D) Occam’s inversion algorithm for electrical resistivity tomography is modified to allow for inversion on the differences between the background and subsequent data sets. The algorithm is optimized for in situ monitoring applications. The resistivity obtained by the inversion of background data serves as the a priori model in the difference inversion. There are several advantages to this method. First, convergence is fast since the inverse routine needs only to find small perturbations about a good initial guess. Second, systematic errors such as those due to errors in field configuration and discretization errors in the forward modeling algorithm tend to cancel. The result is that we can fit the difference data far more closely than the individual potentials. Better data fits often equate to better resolution with fewer inversion artifacts. The difference inversion technique was applied to monitoring in-situ steam remediation in Portsmouth, Ohio and monitoring of flow in fluid fra...

A geostatistically based inverse model for electrical resistivity surveys and its applications to vadose zone hydrology

[1] A sequential, geostatistical inverse approach was developed for electrical resistivity tomography (ERT). Unlike most ERT inverse approaches, this new approach allows inclusion of our prior knowledge of general geological structures of an area and point electrical resistivity measurements to constrain the estimate of the electrical resistivity field. This approach also permits sequential inclusion of different data sets, mimicking the ERT data collection scheme commonly employed in the field survey. Furthermore, using the conditional variance concept, the inverse model quantifies uncertainty of the estimate caused by spatial variability and measurement errors. Using this approach, numerical experiments were conducted to demonstrate the effects of bedding orientation on ERT surveys and to show both the usefulness and uncertainty associated with the inverse approach for delineating the electrical resistivity distribution using down-hole ERT arrays. A statistical analysis was subsequently undertaken to explore the effects of spatial variability of the electrical resistivity-moisture relation on the interpretation of the change in water content in the vadose zone, using the change in electrical resistivity. Core samples were collected from a field site to investigate the spatial variability of the electrical resistivity-moisture relation. Numerical experiments were subsequently conducted to illustrate how the spatially varying relations affect the level of uncertainty in the interpretation of change of moisture content based on the estimated change in electrical resistivity. Other possible complications are also discussed. INDEX TERMS: 0903 Exploration Geophysics: Computational methods, potential fields; 1869 Hydrology: Stochastic processes; 1875 Hydrology: Unsaturated zone; 1866 Hydrology: Soil moisture; 3260 Mathematical Geophysics: Inverse theory; KEYWORDS: geostatistical inverse model, electrical resistivity tomography, vadose zone, resistivitymoisture relation, spatial variability, sequential/successive linear estimator

Detection of Leaks in Underground Storage Tanks Using Electrical Resistance Methods

Two field experiments were performed to evaluate the performance of electrical resistance tomography (ERT) as a leak detection method under metal underground storage tanks (UST). This document provides a summary of field experiment results performed under a 15 m diameter steel tank mockup located at the Hanford Reservation, Washington, and of supporting numerical simulations. Two different leak events were created. About 3800 liters of saline solution were first released along a portion of the tanks edge and another 1900 liters were later released near the tanks center. The release rate averaged about 26 liters/hour for the leak on the tanks side and about 3.0 liters/hour for the center leak. Two‐ and three‐dimensional tomographs were calculated using data collected before, during and after each spill. The tomographs mapped the spatial and temporal evolution of resistivity changes caused by the leak; as the solution penetrated the soil, readily detectable resistivity decreases were observed and used to...

Time-lapse ERT monitoring of an injection/withdrawal experiment in a shallow unconfined aquifer

Toquantifyperformanceof3Dtime-lapseelectricalresistivity tomography ERT, a sequential injection/withdrawal experiment was designed for monitoring the pump-and-capture remediation of a conductive solute in an unconfined alluvial aquifer. Prior information is incorporated into the inversion procedure via regularization with respect to a reference model according to threeprotocols:1independentregularizationinvolvingasingle reference model, 2 background regularization involving a referencemodelobtainedviainversionofpreinjectiondata,and3 time-lapseregularizationinvolvinganevolvingreferencemodel obtained via inversion of data from previous experimental stages.Emplacementandsequentialwithdrawalofthesoluteisclearly imaged for all protocols. Time-lapse regularization results in greater amounts of model structure, while providing significantcomputationalsavings.ERT-estimatedelectricalconductivity is used to predict solute concentration and solute mass in the aquifer. At any experimental stage, we are able to estimate total solute mass in the aquifer with a maximum accuracy of 60%‐85% depending on regularization protocol and survey geometry.Wealsoestimatethewithdrawnsolutemassforeveryexperimentalstagethechangeinmassbetweenexperimentalstages. Withdrawn mass estimates are more reliable than total mass estimates and do not exhibit systematic underprediction or dependence on regularization protocol. Withdrawn mass estimates areaccurateforchangesinmassbelow2‐4 kgofpotassiumbromide KBr for horizontal and vertical dipole-dipole surveys, respectively. Estimating the withdrawn solute mass does not require background subtraction and, thus, does not require backgrounddata.

ERT monitoring of environmental remediation processes

The use of electrical resistance tomography (ERT) to monitor new environmental remediation processes is addressed. An overview of the ERT method, including design of surveys and interpretation, is given. Proper design and lay-out of boreholes and electrodes are important for successful results. Data are collected using an automated collection system and interpreted using a nonlinear least squares inversion algorithm. Case histories are given for three remediation technologies: Joule (ohmic) heating, in which clay layers are heated electrically; air sparging, the injection of air below the water table; and electrokinetic treatment, which moves ions by applying an electric current. For Joule heating, a case history is given for an experiment near Savannah River, Georgia, USA. The target for Joule heating was a clay layer of variable thickness. During the early stages of heating, ERT images show increases in conductivity due to the increased temperatures. Later, the conductivities decreased as the system became dehydrated. For air sparging, a case history from Florence, Oregon, USA is described. Air was injected into a sandy aquifer at the site of a former service station. Successive images clearly show the changes in shape of the region of air saturation with time. The monitoring of an electrokinetic laboratory test on core samples is shown. The electrokinetic treatment creates a large change in the core resistivity, decreasing near the anode and increasing near the cathode. Although remediation efforts were successful both at Savannah River and at Florence, in neither case did experiments progress entirely as predicted. At Savannah River, the effects of heating and venting were not uniform and at Florence the radius of air flow was smaller than expected. Most sites are not as well characterized as these two sites. Improving remediation methods requires an understanding of the movements of heat, air, fluids and ions in the sub-surface which ERT can provide. The Florence site provides an excellent example of using information from ERT to improve a remediation system design. At Florence, the injection well used too long a sand pack in the injection zone which decreased the injection depth and thus the zone of influence of the system. Though in retrospect this is obvious, it would not have been noticed without ERT.

Electrical resistance tomography experiments at the Oregon Graduate Institute

Abstract Three controlled experiments were conducted at the Oregon Graduate Institute (OGI) with the purpose of evaluating electrical resistance tomography for imaging underground processes associated with in-situ site assessment and remediation. The OGI facilities are unique: a double-wall tank 10 m square and 5 m deep, filled with river bottom sediments and instrumented for geophysical and hydrological studies. At this facility, liquid contaminants could be released into the confined soil at a scale sufficiently large to represent real-world physical phenomena. In the first test, images of electrical resistivity were made before and during a controlled spill of gasoline into a sandy soil. The primary purpose was to determine if electrical resistivity images could detect the hydrocarbon in either the vadose or saturated zone. Definite changes in electrical resistivity were observed in both the vadose and saturated soils. The effects were an increase in resistivity of as much as 10% above pre-release values. A single resistive anomaly was imaged, directly below the release point, principally within the vadose zone but extending below the phreatic surface. The anomaly remained identifiable in tomograms taken two days after the release ended with clear indications of lateral spreading along the water table. The second test involved electrical resistance measurements before, during, and after air sparging in a saturated soil. The primary purpose was to determine if the electrical images could be used to detect and delineate the extent of the zone influenced by sparging. The images showed an increase of about 20% in resistivity over background values within the sparged zone and the extent of the imaged zone agreed with that inferred from other information. Electrical resistivity tomography measurements were made under a simulated oil storage tank in the third test. Comparison of images taken before and during separate releases of brine and water showed effects of changes induced by the water or brine. The simulated leak and its location were imaged as a conductive anomaly centered near the point of origin and were observed to spread with time during the release.

Assessment of measurement errors for galvanic-resistivity electrodes of different composition

This research provides an empirical study of electrodes used to measure galvanic resistivity. The central element of this work is an estimation of errors in resistivity measurements that arise because of the type of electrode material used. Fourteen types of electrodes were tested including metal electrodes, metal-salt-compound (nonpolarizing) electrodes, and one nonmetal electrode, under conditions that are typical of those encountered during geophysical surveys. Measurement errors for resistance and chargeability were estimated using the reciprocity of data from an array of electrodes such as might be used for electric-resistance tomography. The same error analysis was applied to data from a network of high-precision resistors to separate instrument errors from electrode errors. Significant differences were observed in errors produced by different electrode materials. We conclude that the choice of electrode is very important for resistivity or chargeability surveys. Iron, steel (including rebar), lead, and phosphor bronze produced the smallest errors in resistance and chargeability. Aluminum, magnesium, titanium, copper, and zinc produced the largest errors. Stainless steel (alloy 316), tin, and brass performed reasonably well, as did carbon, which was the only nonmetal tested.