Fluid that fills boreholes in crosswell electrical resistivity
investigations provides the necessary electrical contact between the
electrodes and the rock formation but it is also the source of image
artifacts in standard inversions that do not account for the effects of
the boreholes. The image distortions can be severe for large resistivity
contrasts between the rock formation and borehole fluid and for large
borehole diameters. We have carried out 3D finite-element modeling using
an unstructured-grid approach to quantify the magnitude of borehole
effects for different resistivity contrasts, borehole diameters, and
electrode configurations. Relatively common resistivity contrasts of
100:1 and borehole diameters of 10 and 20 cm yielded, for a bipole
length of 5 m, apparent resistivity underestimates of approximately 12%
and 32% when using AB-MN configurations and apparent resistivity
overestimates of approximately 24% and 95% when using AM-BN
configurations. Effects are generally more severe at shorter bipole
spacings. We report the results obtained by either including or ignoring
the boreholes in inversions of 3D field data from a test site in
Switzerland, where approximately 10,000 crosswell resistivity-tomography
measurements were made across six acquisition planes among four
boreholes. Inversions of raw data that ignored the boreholes filled with
low-resistivity fluid paradoxically produced high-resistivity artifacts
around the boreholes. Including correction factors based on the modeling
results fora ID model with and without the boreholes did not markedly
improve the images. The only satisfactory approach was to use a 3D
inversion code that explicitly incorporated the boreholes in the actual
inversion. This new approach yielded an electrical resistivity image
that was devoid of artifacts around the boreholes and that correlated
well with coincident crosswell radar images.