Detection of isolated target such as underground tunnel located deeply may be possible by the cross-borehole or the single borehole measurements. In situ conductivities(σ) and permittivities(ε) of the underground geometry may be obtained by three borehole measurements; one for the transmitting(source) antenna and the others for the receiving antenna picking up the scattered fields at two different distances. By taking the division of these two fields, one may obtain the permittivity and the conductivity from the ratio of the amplitudes and the difference of the phases.
Multi-frequency averaging of these permittivities and conductivities smooths out not only the dispersive characteristics but also the rapid fluctuations across the planar discontinuities such as cracks and faults. It is shown by the two dimensional simulations that cracks and faults may be identified by the back projection of the multi-frequency averaged σ and ε. The isolated targets such as the air tunnel, however, produces highly fluctuating interference fringes of σ and ε at the target, even with the multi-frequency averaging. After the identifying of the isolated target from the interference fringing area and by defining the region of reconstruction, one may reconstruct the isolated target by using the iterative inversion method. The two-dimensional numerical simulation shows that the successful reconstruction of the air tunnel is possible.
It is shown here that the tomogram of σ and ε in the cross section covered by two boreholes is successfully obtained from the in situ data measured by Ra-Geovis. One may identify the fluctuating interference-fringe area from this three-dimensional measured data. The reconstruction of this isolated air tunnel requires larger computer capability and its reconstruction is proceeded. The tomogram obtained from this three-dimensional measurement gives acceptable distribution of σ and ε of the background medium.
