Out-of-Plane Effects in Crosshole Radio-Frequency Tomography
Radio-frequency tomography (RFT) normally is assumed to sense the geological structure in a plane between the transmitters and receivers, loosely termed the image plane. In practice, out-of-plane objects also affect the tomograms. We illustrate these effects on synthetic crosshole tomograms generated for a conductive sphere in a more resistive host. The sources and receivers are vertical magnetic dipoles.
The tomographic image of a highly conductive sphere located wholly or partially between the sources and the receivers is a conductive feature the shape of which is governed by the conductivity of the host. As the sphere moves around one of the boreholes at a fixed distance, its expression on the tomogram is transformed from a conductive feature to a resistive feature. This reversal also occurs as the conductive sphere moves radially away from the transmitter. The period of these reversals can be related to the change in path length from source to sphere to receiver. Thus, if only amplitudes are recorded, an out-of-plane conductor could be misinterpreted as a resistive object in the plane; the two cases could probably be differentiated if phase data were also recorded.
The influence of the sphere on the tomogram is also negligible at certain azimuths and radial separations determined by the host conductivity and source frequency. A good conductor close to a borehole could be missed by an RFT survey if it were located in such a null. The risk of this occurrence is greatly reduced if multifrequency data are collected. Also, the shape of the tomographic expression of the sphere is insensitive to azimuth and radial separation.