The burn-cavity of LLNL’s large block coal gasification experiment near Centralia, Washington was probed by using the High Frequency Electromagnetic (HFEM) technique. Since the burn area was small and the boreholes closely spaced, there was less signal attenuation than previous experiments. Thus, higher frequencies were used for better resolution. Also, sufficient data was taken to use image reconstruction algorithms to produce a tomography of the burn-cavity. Since the large block will be excavated, this experiment provides an excellent opportunity to compare the results of HFEM probing to the actual burn cavity.