Use of high-frequency electromagnetic waves for mapping an in situ coal gasification burn front

High-frequency electromagnetic waves transmitted between boreholes can be used to map the position of an underground coal gasification (UCG) burn front, as well as other geophysical anomalies. The technique uses transmitting and receiving antennas lowered down boreholes on either side of the gasified region. Results from the use of this technique in a UCG experiment show high resolution and close agreement with data from thermocouples. The depth and height of the burn front were easily determined and subsidence of overburden was also evident in the measurements. Several variations on the method were tried. This technique promises several advantages over other down-hole instrumentation: lower cost, better spatial coverage, and the ability to give measurements both during and after passage of the burn front. The UCG experiment also showed several improvements that could be made in the technique.