Capacitive Resistivity Imaging with Towed Arrays

Abstract

The capacitive resistivity (CR) technique is a generalization of the conventional DC resistivity method that facilitates measurements of electrical resistivity on engineered surfaces and highly resistive ground. The CR methodology allows the use of towed sensor arrays, thus enabling the rapid collection of high-resolution resistivity data. Under quasi-static conditions CR data are equivalent to galvanic DC measurements so that CR datasets can be interpreted with conventional DC inversion algorithms. In this study, we demonstrate that the methodology and fundamental parameters of the CR technique facilitate spatial sampling at the centimeter scale for towing speeds of the order of 2.5 km/h. We argue that the dipole-dipole array is the most suitable geometry for dynamic CR measurements and present example data acquired with a prototype instrument using plate-wire combinations arranged in an equatorial geometry. The information content of raw CR data is found to be dominant over towing-induced noise and a direct comparison with DC profile data shows good agreement between both techniques. Based on these findings, we show that tomographic imaging is possible using datasets acquired with moving arrays. Closer investigation of the practical aspects of towed-array capacitive resistivity imaging (CRI) highlights the similarities with galvanic multi-electrode surveys, but the different geometric constraints and sampling regime of CRI give rise to advantages (high lateral resolution) as well as disadvantages (limitations in vertical resolution). We conclude our study with recommendations for practical CRI survey procedures and present a field example where we have successfully imaged a subsurface target in 3D. Towed-array CRI is found to provide equivalent (and in some ways superior) information about the shallow subsurface when compared to DC ERT; it therefore widens the scope of electrical 4 imaging surveys to environments where the conventional methodology would be impractical.

Detecting a leachate plume in an aeolian sand landscape using a DUALEM‐421 induction probe to measure electrical conductivity followed by inversion modelling

“…The reason for this is attributed to the highly conductive nature of the landfill. In such circumstances a Geometrics OhmMapper could be used because such an instrument is not as sensitive to near-surface conductive anomalies (Kuras et al, 2007). In terms of identifying the source of the leachate plume, the ability to rapidly deploy a DUALEM-421, its ease of use and its ability to generate large amounts of data on topsoil, subsurface and subsoil r a quickly will allow us in future studies to explore its potential to develop a quasi 3D model (Monteiro Santos et al, 2010b).…”

“…The reason for this is attributed to the highly conductive nature of the landfill. In such circumstances a Geometrics OhmMapper could be used because such an instrument is not as sensitive to near‐surface conductive anomalies (Kuras et al. , 2007).…”

Parallel computation of optimized arrays for 2-D electrical imaging surveys

“…2-D resistivity surveys are widely carried out and even 3-D surveys are becoming more common in areas of very complex geology (Dahlin 1996;Auken et al 2006;Chambers et al 2006). The field applications range from agriculture (Samouëlian et al 2003), groundwater exploration (Seaton & Burbey 2000), engineering site investigation (Kuras et al 2007), environmental assessment , mineral exploration (Bingham et al 2006) to even hydrocarbon mapping (Bauman 2005).…”

Using integrated near‐surface geophysical surveys to aid mapping and interpretation of geology in an alluvial landscape within a 3D soil‐geology framework

“…A further 935 m of 2D ERT profile data from a concurrent project (Dabas andFavard 2006, 2007). Electrical mapping using DC resistivity methods (e.g., Panissod et al 1998;Kuras et al 2007) has advantages over inductive methodologies due to smaller uncertainties in sensor calibration and greater control over the depth of investigation (Dabas and Tabbagh 2003). In this context, resistivity measures the ability of the soil to conduct an electrical current, units being in Ohm-meters (Ωm).…”