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Resistivity sounding and Imaging (Tomography)

Introduction

Electrical properties are among the most useful geophysical parameters in characterising earth materials. Variations in electrical resistivity typically correlate with variations in water saturation, fluid conductivity, porosity, permeability, and the presence of waste material. Depending on the particular site, these variations may be used to locate contaminant plumes, saline intrusion, stratigraphic units, sinkholes, fractures, buried structures, and any other feature whose electrical properties contrast with the surrounding materials.

Winsev 6, interpretation of geoelectrical soundings for Windows (see here)

A resistivity survey carried out using a modern automated multi-channel system involves deploying up to eighty regularly spaced electrodes along a section of survey line and through a complex sequence of switching current and potential electrodes a pseudo cross-section of apparent resistivity is constructed. ‘Pseudo-section’ data acquired in the field are later processed to produce a modelled true resistivity cross-section of the subsurface, which typically facilitates reliable interpretation of subsurface geological and hydrological conditions.

Applications of Electrical Methods

EM and resistivity can be applied to a wide variety of problems encountered in environmental, groundwater, geotechnical, and archaeological work, including:

Location of landfills and bulk buried materials.
Delineation of contaminant plumes.
Depth of water table and aquifer identification and mapping.
Continuity of stratigraphic interfaces such as clay layers.
Mapping of faults and fractures.
Location of karst features.
Bedrock profiling (useful in seismically noisy areas).

Survey Procedures

Multi-electrode Resistivity Traversing (MRT) involves deploying an array of electrodes along a survey line connected via multi-core cables to a control unit. Resistivity data is recorded via complex combinations of current and potential electrodes to build up a pseudo cross-section of apparent resistivity beneath the survey line. Direct currents are applied directly into the ground through a pair of electrodes and a voltage difference measured across a second electrode pair provides the necessary information to calculate the apparent earth resistivity. The depth of investigation depends on the electrode separation and geometry, with greater electrode separations yielding bulk resistivity measurements to greater depths.

Resistivity meter multy electrode SYSCAL

Data Processing and Interpretation

Measured apparent resistivity data acquired in the field require processing via a complex inversion algorithm to generate an interpreted true resistivity section. Data acquired with multi-electrode cables can be plotted initially as a ‘pseudo-section’ image based on the current and potential electrode separations or “n” level and horizontal position of the centre of the active electrodes. This term ‘pseudo-section’ means that the measured or observed values are only apparent in terms of magnitude, location and depth. The true subsurface resistivity image must be derived from finite-difference forward modelling via software.

Data processing is based on an iterative routine involving determination of a two-dimensional (2D) simulated model of the subsurface, which is then compared to the observed data and revised. Convergence between theoretical and observed data is achieved by non-linear least squares optimisation. The procedure is smoothness constrained to improve stability in the iterative process, the degree of smoothness being determined by a user-specified damping factor. The extent to which the observed and calculated theoretical models agree is an indication of the validity of the true resistivity model (indicated by the final root-mean-squared RMS error).

The true resistivity models are presented as colour-scaled contour plots of changes in subsurface resistivity with depth. The 2D method of presenting resistivity data is limited when irregular or complex geological features are present where the simple cross-section may not be indicative of the true geometry.

Geoelectrical model

Different geological materials have characteristic resistivity values that enable the identification of boundaries between distinct lithologies on resistivity cross-sections. At some sites, however, there are overlaps between the ranges of possible resistivity values associated with sediments of variable composition, e.g., tills and fluvial sediments. The distinction of two geological units depends on percentage differences in the content of high resistivity coarse-grained material and low resistivity clays and water. Borehole, seismic or other geophysical surveys can provide useful data to assist with the interpretation of resistivity cross-sections where complex lithologies are encountered.

Summary

Resistivity surveys can provide detailed cross-sectional information about a site and a series of traverses carried out along parallel lines can enable the production of a 3D model. High quality data can be rapidly acquired and error checked in the field using modern multi-channel equipment such as our ABEM Lund ES464 80 channel system. Electrical resistivity techniques are extremely useful in a wide variety of situations, e.g., for environmental investigations, hazardous waste sites, ground water exploration, karst features, archaeology, etc.

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