Abelardo Ramirez

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.

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.

Complex resistivity tomography for environmental applications

Complex resistivity may provide valuable information about the structural and hydraulic nature of porous media and fluids contained within such media. The environmental value of such a property is obvious. To date most environmental applications of complex resistivity have focussed on relatively crude data analysis methods and restrictive electrode configurations. New tomographic methods are becoming available that will allow complex resistivity to be employed with arbitrary electrode arrangements. Laboratory trials of our extensions of electrical resistivity tomography to a complex form are reported. The inversion procedure is presented and demonstrated for a range of targets, with resistive and reactive characteristics. The approach is shown to provide useful magnitude and phase images, giving spectral information about the region of interest. Cole–Cole analysis of the inversion results reveals specific material relaxation characteristics. The usefulness of imaging complex resistivity is evident when compared with more conventional resistivity tomography, in particular when examined over a range of input frequencies.

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...

Electrical resistance tomography during in-situ trichloroethylene remediation at the Savannah River Site

Abstract Electrical resistance tomography was used to monitor in-situ remediation processes for removal of volatile organic compounds from subsurface water and soil at the Savannah River Site near Aiken, South Carolina. This work was designed to test the feasibility of injecting a weak mixture of methane in air as a metabolic carbon source for natural microbial populations which are capable of trichloroethylene degradation. Electrical resistance tomograms were constructed of the subsurface during the test to provide detailed images of the process. These images were made using an iterative reconstruction algorithm based on a finite element forward model and Newton-type least-squares minimization. Changes in the subsurface resistivity distribution were imaged by a pixel-by-pixel subtraction of images taken before and during the process. This differential tomography removed all static features of formation resistivity but clearly delineated dynamic features induced by remediation processes. The air-methane mixture was injected into the saturated zone and the intrained air migration paths were tomographically imaged by the increased resistivity of the path as air displaced formation water. We found the flow paths to be confined to a complex three-dimensional network of channels, some of which extended as far as 30 m from the injection well. These channels were not entirely stable over a period of months since new channels appeared to form with time. Also, the resistivity of the air injection paths increased with time. In another series of tests, resistivity images of water infiltration from the surface support similar conclusions about the preferential permeability paths in the vadose zone. In this case, the water infiltration front is confined to narrow channels which have a three-dimensional structure. Here, similar to air injection in the saturated zone, the water flow is controlled by local variations in formation permeability. However, temporal changes in these channels are minor, indicating that the permeable paths do not seem to be modified by continued infiltration.

Electrical resistance tomography

Electrical resistance tomography (ERT) is a method that calculates the subsurface distribution of electrical resistivity from a large number of resistance measurements made from electrodes. For in-situ applications, ERT uses electrodes on the ground surface or in boreholes. It is a relatively new imaging tool in geophysics. The basic concept was first described by Lytle and Dines as a marriage of traditional electrical probing (introduced by the Schlumberger brothers) and the new data inversion methods of tomography. Development of both the theory and practice of ERT was confined mostly to the late 1980s and the 1990s. Tomographic inversion added important new capabilities as it was more general, accurate, and rigorous at spatial imaging of geophysical electrical resistance data than earlier pseudosection or curve fitting methods.

ELECTRICAL IMAGING OF ENGINEERED HYDRAULIC BARRIERS

Electrical resistance tomography (ERT) was used to image the full-scale test emplacement of a thin-wall grout barrier installed by high-pressure jetting and a thick-wall polymer barrier installed by low-pressure permeation injection. Both case studies compared images of electrical resistivity before and after barrier installation. Barrier materials were imaged as anomalies which were more electrically conducting than the native sandy soils at the test sites. Although the spatial resolution of the ERT was insufficient to resolve flaws smaller than a reconstruction voxel (50 cm on a side), the images did show the spatial extent of the barrier materials and therefore the general shape of the structures. To verify barrier performance, ERT was also used to monitor a flood test of a thin-wall grout barrier. Electrical resistivity changes were imaged as a saltwater tracer moved through the barrier at locations which were later found to be defects in a wall or the joining of two walls.

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.

Liquid phase structure within an unsaturated fracture network beneath a surface infiltration event: Field experiment

[1] We conducted a simple field experiment to elucidate structure (i.e., geometry) of the liquid phase (water) resulting from ponded infiltration into a pervasive fracture network that dissected a nearly impermeable rock matrix. Over a 46 min period, dyed water was infiltrated from a surface pond while electrical resistance tomography (ERT) was employed to monitor the rapid invasion of the initially dry fracture network and subsequent drainage. We then excavated the rock mass to a depth of ∼5 m, mapping the fracture network and extent of dye staining over a series of horizontal pavements located directly beneath the pond. Near the infiltration surface, flow was dominated by viscous forces, and the fracture network was fully stained. With increasing depth, flow transitioned to unsaturated conditions, and the phase structure became complicated, exhibiting evidence of fragmentation, preferential flow, fingers, irregular wetting patterns, and varied behavior at fracture intersections. ERT images demonstrate that water spanned the instrumented network rapidly on ponding and also rapidly drained after ponding was terminated. Estimates suggest that our excavation captured from ∼15 to 1% or less of the rock volume interrogated by our infiltration slug, and thus the penetration depth from our short ponding event could have been quite large.

Geomechanical behavior of the reservoir and caprock system at the In Salah CO2 storage project

Significance In Salah is one of the largest carbon capture and storage projects to date and has played a central role in demonstrating the feasibility of onshore sequestration of CO2 in deep saline aquifers. The unique field experience at In Salah provides a valuable case study in managing commercial-scale CO2 injections. In particular, the current work highlights the importance of geomechanics and integrated monitoring in understanding field behavior and managing storage risk. Almost 4 million metric tons of CO2 were injected at the In Salah CO2 storage site between 2004 and 2011. Storage integrity at the site is provided by a 950-m-thick caprock that sits above the injection interval. This caprock consists of a number of low-permeability units that work together to limit vertical fluid migration. These are grouped into main caprock units, providing the primary seal, and lower caprock units, providing an additional buffer and some secondary storage capacity. Monitoring observations at the site indirectly suggest that pressure, and probably CO2, have migrated upward into the lower portion of the caprock. Although there are no indications that the overall storage integrity has been compromised, these observations raise interesting questions about the geomechanical behavior of the system. Several hypotheses have been put forward to explain the measured pressure, seismic, and surface deformation behavior. These include fault leakage, flow through preexisting fractures, and the possibility that injection pressures induced hydraulic fractures. This work evaluates these hypotheses in light of the available data. We suggest that the simplest and most likely explanation for the observations is that a portion of the lower caprock was hydrofractured, although interaction with preexisting fractures may have played a significant role. There are no indications, however, that the overall storage complex has been compromised, and several independent data sets demonstrate that CO2 is contained in the confinement zone.