A Comparison of Two Electrical Probing Techniques

Electromagnetic soundings were made using two techniques in an area around the summit of Kilauea Volcano, Hawaii. The target for the soundings was a conductive zone at a depth of about one kilometer, associated with geothermal heating. In one sounding method, an electromagnetic field was generated by passing current steps through a grounded-wire transmitter, while in the other, an electromagnetic field was generated by passing current steps through an ungrounded loop. The field was detected at a receiving site by measuring the vertical component of magnetic induction using an ungrounded loop. Both methods accurately reflected the presence of conductive rocks at depth. However, the use of a grounded-wire source had a strong advantage in that the field decayed with distance more slowly than that from the loop source, and a larger area could be surveyed using a single grounded-wire source.