Cause and countermeasures for the symmetric effect in borehole-to-surface electrical resistivity tomography

Abstract Researches and applications of borehole electrical resistivity tomography (BHERT) were mostly about cross-hole ERT (CHERT). Literatures on single borehole-to-surface ERT are limited, although this type of measurement could be useful when only one borehole is available. The undesirable symmetric effect is an important issue in the borehole-to-surface ERT survey. It has been observed that not only the pure in-hole data but also some borehole-to-surface configurations would induce mirrored images in the inversion model. In this study, all the possible borehole-to-surface configurations were classified and investigated for their sensitivity patterns and imaging resolutions, and the root cause of the symmetric effect was revealed to be the condition where one current pole and one potential pole coexist in the borehole. The configurations without symmetric effect suffer from low resolution. The investigation finally led to seeking a balance between resolution enhancement and symmetric effect reduction. Two countermeasures to suppress the symmetrical mirroring were proposed. The combined data inversion approach is based on the capability of each array to resist symmetric effect, while the weighted model stacking technique (WMOST) is derived from the characteristics of inversion models. Both methods were shown to be effective in suppressing the symmetric effect while maintaining sufficient resolution.

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