Electromagnetic modeling of reflections inside an in situ gasified coal seam

As an aid in the design of an electromagnetic system for the detection, monitoring, and mapping of the burn front of an underground (in situ) coal gasification process, an analytical model has been developed that will predict the electromagnetic field reflected from the gasified portion of the coal seam. The coal and gasified section have been modeled electrically as lossy media, surrounded above and below by relatively high conducting overburden and underburden. Vertical and horizontal electric and magnetic dipoles have been investigated, utilizing a geometrical optics formulation which also takes into account the physical roughness of the coal‐overburden (underburden) and coal‐gasified section boundaries. Indications are that a swept frequency monitoring system is capable of locating the burn front of the in situ gasification process, with the vertical electric dipole being the most attractive element.