Electromagnetic tomography: consistency of images and groundwater prospecting

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Electromagnetic tomography: consistency of images and groundwater prospecting

In crosswell observation of electromagnetic tomography, now there is no feasible way to calibrate the in-situ radiation constant, while the effect of induced wave-field always exists in observational data. Therefore the absorption coefficient obtained by a common-used image reconstruction method is only a relative value. The consistency of tomograms in linked profiles inevitably is affected seriously. An improved equation involving the effect of induced wave-field and an alternative tracing method to calculate the in-situ radiation constant is proposed in the present paper. Based on the results from a series of parameter calibration performed in water and the experiments with rock samples, the standard meter readings of borehole electromagnetic systems, types EW-1 and JWQ-3, are accurately defined here. Additionally, frequency reduction and boundary smoothing of linked profiles are discussed. In research on groundwater prospecting in Tanggongta, Inner-Mongolia, we used the above mentioned method and obtained the tomograms of linked profiles with good consistency. The absorption coefficient in the tomogram are not only reasonable and reliable, but also coincided with the result from rock parameter measurement. The depth, extension direction and water-bearing structure of two aquifers were well detected. This result correctly guides hydrologic drilling there. Contribution No. 96A0078, Institute of Geophysics, SSB, China. The present project is supported by National Science Foundation of China, contract No. 49374203.