Remote Sensing of an Underground Coal-Burn Cavity with a Wide-Band Induction System

A conducting cavity in an underground coal burn was remotely probed from the surface with a wide-band electromagnetic induction system. The cavity, located near Hanna, WY, was produced by underground gasification of a 30-ft-thick subbituminous-coal seam ranging in depth from 300 to 400 ft. The cavity filled with saline ground water after the burn. The wide-band loop-loop system employed pseudonoise and cross-correlation techniques to produce a transient-time response in the field. Additional computer processing produced normalized 3-dimensional signature maps in both the time and frequency domains. These horizontal profiling maps, corresponding to a 100-Hz-50-kHz passband, demonstrate that a significant anomaly is produced by the cavity as the system is moved across the site. Time-domain maps show nearly a 7-1 change in relative peak-to-peak values, whereas the frequency-domain magnitude response changes as much as 36-1. Results were corroborated using another single-frequency system. These anomalies demonstrate the feasibility of employing induction systems to remotely characterize underground coal-burn cavities filled with conducting fluid.