Optimal acquisition and synthetic electrode arrays

Abstract

Present day commercial and academic multi‐electrode systems are still used with concepts that stem from times where we went into the field with four electrodes. Primarily for environmental investigations, the 2D earth resistivity measurement on the surface is routinely used for its ability to map the electric resistivity distribution in the earth from which an estimate of the heterogeneity is obtained. The relation between the electrode configuration used and the obtainable resolution in the recovered subsurface resistivity distribution is not well understood. Only recently, this aspect has become a subject of research. We have carried out investigations both numerically and in the field, to use a complete pole‐pole data set as the basic measurement configuration and compute any four‐electrode array that could have been used in the field from the basic pole‐pole measurements. This numerical construction of all possible array types from the pole‐pole measurement is called synthetic electrode arrays construction and the resulting arrays are called synthetic electrode arrays. These can be Schlumberger arrays, Wenner arrays, dipole‐dipole or pole‐dipole arrays, or any non‐regular array that can be thought of. The total amount of data is not limited to the conventional arrays. The conventional arrays are not necessarily the ones containing the most subsurface information. We have looked at the information in the full coverage pole‐pole data set using a regular grid on the earth surface and also looked at different combinations pole‐pole arrays used together with synthetic three‐ and four‐electrode arrays based on these on inversion results for line‐data. For comparison, we have also measured with conventional arrays and compared results with synthetic arrays. These results are also compared to numerical studies where we have taken the finite measurement accuracy of the instrument into account.