The fixed receiver electromagnetic (FREM) method for drill hole surveys

Abstract

The fixed receiver electromagnetic (FREM) method is the reverse of the fixed transmitter method. The receiver is fixed, the transmitter is moved, and measurements are plotted in profile or plan at the transmitter positions. In drill hole applications the method extends the way in which information can be gathered and provides improved definition of conductive mineralization. In order to predict and interpret anomaly patterns, it is convenient to invoke reciprocity and visualise the receiver as an electromagnetic source, and the transmitter as a signal measurer. Model and field results confirm that a finite conductor causes a dipolar type magnetic field anomaly, which readily gives information on the strike, dip and approximate location of the conductor. However, conductive overburden may also produce a significant down hole electromagnetic response, and a FREM anomaly that is approximately dipolar. It is therefore necessary to identify the overburden effects and perhaps attempt to remove them, before interpretation. In addition to rules of thumb, a dipole source, and a rectangular current filament source, have been used successfully for interpreting conductor geometry from FREM data. The FREM approach is novel; it complements conventional drill hole electromagnetic survey measurements. A further appreciation of its advantages as well as drawbacks will come from experience.