Geometric optics and diffraction techniques for data simulation in cross-borehole tomography

The method of reconstructing a cross-sectional image of the earth between two boreholes can be used in areas from tunnel detection to mineral exploration. Of great importance to this underground reconstruction process is the ability to adequately model the forward (simulation) problem. The authors consider the simulation of continuous wave data using geometrical optics with first-order diffractions from impedance surface discontinuities. The ray optics method using diffraction from lossy wedges was shown to give good simulation results of the electromagnetic response of a high-conductivity square cylinder imbedded in a homogeneous earth. The results obtained for high-conductivity anomalies using ray optics compare favorably with those obtained using the volume current method (VCM). Additionally, the ray optics method sheds light on the effects of diffraction in cross-borehole tomography, thereby suggesting ways in which the inversion (reconstruction) process can be improved.