Geotomography in oil shale

Abstract

Geophysical tomographs were made of oil shale at the Occidental Oil Shale, Inc., mine near DeBeque, Colorado. These tomographs mapped the in situ radio frequency electromagnetic attenuation of oil shale between 1 and 40 MHz in a plane 27 by 12 m. Measured attenuation rates (inverse skin depth) ranged from 0.1 to 0.5 m −1 with the attenuation rate approximately proportional to the square root of frequency. At 20 MHz, the measured relative permittivity of oil shale is about 9. Geotomographs were made in another plane 16.8 by 12 m, before and again after the shale in this region was explosively rubbled. Measured electromagnetic attenuation rates at 20 MHz were generally larger in the rubble than in the undisturbed shale, suggesting that geotomography may be useful in mapping a characteristic of the rubbled zone such as void content or void size distribution. Model calculations of the rubble using a theory of dielectric mixtures predict a smaller attenuation rate in the rubble, implying that the measurements result from phenomena such as scattering or oil shale anisotropy, which are not accounted for in the model.