Geophysical tomographs made of oil shale at the mine of Occidental Oil Shale Inc., near DeBeque, Colorado, were used to map the in situ electrical attenuation of oil shale between 1 and 40 MHz in a plane 27 m by 12 m. Measured attenuation ranged from 0.10 to 0.53 Np/m. At 20 MHz the relative permittivity of oil shale is approximately 9.