Abstract:

Borehole-to-ground resistivity survey is a geophysical method by which an electric current is injected from the borehole, and is received on the ground with different kinds of power supplied. Usually, in the field through the boreholetoground resistivity, the current source is divided into point current source injected from the bare hole and the linear current source injected from the cased borehole. Based on the practical work in the field, this paper studies the following aspects and obtains the corresponding conclusions: (1) when the current source is point current, the results of  three forward models (the current source above, inside and under the anomalous body), show that the apparent resistivity response on the surface varies greatly; (2) when the current source is a vertical line source, the results of three forward models (current source in the upper part of the anomalous body, extending to the middle of the anomalous body, as well as throughout the anomalous body), show that the apparent resistivity response amplitude changes gradually on the surface; (3) when the current source is inclined line source, the tilt angle of the line source has great effect on the apparent resistivity response on the surface, and the larger of the angle of inclination of the line source is, the smaller magnitude of the surface response is. These conclusions provide a guidance for field work by setting parameters of the apparatus in order to improve the accuracy of their observations, and make a qualitative analysis for the preliminary judging the depth of the anomalous body, and lay a solid foundation for the three dimensional inversion of the borehole-to-ground resistivity as well.

Key words: borehole-to-ground resistivity, forward, point current, line source, the apparent resistivity response on the surface