Cross-borehole Electrical Resistivity Tomography Research Articles

Optimization of Array Configurations and Panel Combinations for the Detection and Imaging of Abandoned Mineshafts using 3D Cross-Hole Electrical Resistivity Tomography

Cross-borehole electrical resistivity tomography was used to detect and image a concealed air-filled mineshaft at a greenfield test site. The measurement configurations and panel combinations were selected using a two-stage optimization process. An optimal set of array configurations was selected for each cross-borehole panel on the basis of the model resolution matrix. Subsequently, various combinations of panels were tested with synthetic and field data to determine the effects of coverage and data density on the resulting tomographic image. In the field trials, complicating factors were introduced by the use of resistive cement linings in the boreholes. A resistive feature was detected between the boreholes using a single panel and a 2.5D inversion, but the image quality was too poor to identify this as a mineshaft. A much-improved image was obtained using eight boreholes and eight panels with a full 3D inversion. Only four of these panels intersected the shaft. Crucially, the other panels provided coverage of outlying regions of the model, enabling the inversion algorithm to distinguish between the resistive effects of the borehole linings and the mineshaft.

Flow and transport in the unsaturated Sherwood Sandstone: characterization using cross-borehole geophysical methods

Abstract Cross-borehole radar and resistivity measurements have been used to characterize changes in moisture content and solute concentration due to controlled injection of 1200 1 of a saline tracer in the unsaturated zone of the Sherwood Sandstone at a field site in Yorkshire, UK. Borehole radar transmission profiles show the vertical migration of the wetting front during the tracer test. Three-dimensional cross-borehole electrical resistivity tomography was deployed to monitor changes over time in resistivity, caused by the increase in moisture content and pore-water salinity due to the tracer. The results show clearly the development of the tracer plume as it migrates towards the water table at a depth of 10 m. The tomographic results reveal the impact of a hydraulically impeding layer between a depth of 8 and 9 m. Geophysical and geological logs acquired at the site support this conceptualization. By combining the resistivity tomograms with cross-borehole radar tomograms, changes in pore-water concentration over time have been estimated. Changes in moisture content inferred from the geophysical results were compared with those produced by a three-dimensional unsaturated flow model. Using a sandstone effective hydraulic conductivity of 0.4 m day −1 in the model produced moisture profiles over time that were comparable with those inferred from the geophysical data during the early stages of the tracer test. Differences between modelled and field results were attributed to the impact of hydraulically impeding layers of finer sediments within the profile.

Monitoring DNAPL Pumping Using Integrated Geophysical Techniques

The removal of DNAPL during pumping has been monitored using integrated in situ geophysical techniques. At Hill Air Force Base in Utah, a free-product DNAPL plume (consisting predominantly of TCE) is pooled in water-wet soil on a thick clay aquitard. Groundwater pumping at Operable Unit 2 (OU 2) began in 1994; to date, nearly 30,000 gallons of DNAPL have been recovered from the site. From September, 1994 through September, 1995, changes in the basin during DNAPL pumping were monitored using an integrated geophysical system. Fiber optic sensors and neutron logs verify the presence of DNAPL in the vicinity of three boreholes which form a cross section from the perimeter of the basin to its center. Cross borehole electrical resistance tomography (ERT) images the changes in formation electrical properties due to the removal of DNAPL, extending the understanding of DNAPL removal between the boreholes. During pumping, electrical resistivities decreased; we suggest that these decreases are directly caused by the reduction in DNAPL. During ground water pumping, water with relatively low resistivity replaces some of the DNAPL pockets as the highly insulating DNAPL is removed. The results suggest that, as DNAPL is pumped from a nearby well, product slowly drains along themore » top of an aquitard and into the pump well, where it collects.« less

Air sparging in a sandy aquifer (Florence, Oregon, U.S.A.): Actual and apparent radius of influence

Injection of air into the saturated zone of a contaminated aquifer, air sparging, is a currently popular remediation alternative to the conventional approach of groundwater extraction followed by surface treatment. The air flow behavior around a single air sparging well was investigated under natural field conditions in a relatively homogeneous aquifer consisting of well-sorted dune sand. The response of conventional monitoring data (including water table mounding, soil gas pressure, soil gas composition and tracer gas response) was compared with the results of cross-borehole electrical resistance tomography (ERT), which provides a sensitive indication of the pattern of air flow in the saturated zone. Compared with the region of air flow imaged by ERT, conventional monitoring data provided an ambiguous indication of the region of air flow in the saturated zone, possibly leading to an overestimation of the radius of influence by a factor of two to eight. Critical evaluation of the conventional monitoring data indicates that the shortcomings are a function of various factors which include the transient aspects of pressure and water table changes, and differences in the controls on air flow within the saturated and unsaturated zones.

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.