oa The effect of dipole position errors on E-field measurements in electrical geophysical surveys: is close enough good enough?

Anomalous conductive responses that could not be related to obvious geological sources were observed in field data from a CSAMT survey in the Cobar area, New South Wales. Repeat measurements over the anomalous stations were made, yielding a substantially different resistivity response. Approximate positioning of the E-field dipole stations due to the presence of thick scrub was believed to be one possible source of error contributing to the differing responses.

Subsequent test measurements in which the E-field dipole angle was varied by 10 degrees east and west of the correct direction demonstrated that such an orientation change resulted in resistivity variations of up to 50% from the true value. This is significantly larger than expected from simple theory, which suggests that the results from location errors up to 10 degrees from correct should only be a few percent. This large variation observed in the Cobar area is believed to be due to a strong geo-electrical anisotropy within the steeply dipping and highly cleaved meta-sedimentary rocks of the Cobar Basin.

It was concluded that relatively small errors in receiver orientation could have a large effect on the magnitude of the received signal, which can potentially generate 'false' anomalies in the derived resistivity pseudosections and inversions. It is possible that such location errors are commonly made when a standard GPS is used to locate stations on a grid. Based on the experience from the Cobar area, accurate station positioning is clearly a pre-requisite for reliable electrical surveys in any geological terrain.

This paper presents field examples demonstrating the potential errors, and the results of applying more-accurate positioning techniques. Recommended methods to gauge the magnitude of the potential errors in any survey area are proposed.