A procedure for collecting electromagnetic data using multiple transmitters and receivers capable of deep and focussed exploration

Abstract

Many ground controlled-source electromagnetic (EM) systems have been deployed, and under ideal conditions these systems are capable of detecting large conductors to depths of approximately 800 m; however, more common detection limits are less than 400 m. Although these systems have been used with great success, they may experience two weaknesses when exploring for deeper conductors: poor coupling with the target and small signal-to-noise ratios (S/Ns), both of which decrease the quality and interpretability of the data. We evaluated a novel time-domain EM procedure that addresses these weaknesses. The coupling weakness was addressed through multiple transmitter locations and multiple receiver locations, and the S/N was increased by spatial stacking of measurements (from the various transmitter-receiver combinations). A field test of this procedure was undertaken. Reciprocity data indicated that the noise levels of the vertical component data we acquired were about −0.004μV/Am2. Spatial stacking of the data can reduce the noise levels by a factor of seven. This means that a small conductor previously only visible to 150 m could be seen to 275 m and a conductor visible to 300 m could be seen to 575 m. One challenge of the new procedure was the time required to collect all the transmitter-receiver combinations — this time can be reduced using the principle of reciprocity and not repeating approximately reciprocal measurements. Another challenge was to visualize and interpret the large volumes of data collected using the procedure — this has been partially addressed by creating equivalent-dipole depth sections. Synthetic and real equivalent-dipole depth sections appeared very similar and illustrated that these images of the subsurface could be interpreted. However, the features appeared too deep on the sections, so better visualization techniques could be developed.