CROSS-BOREHOLE RESISTIVITY TOMOGRAPHY OF A PILOT-SCALE, IN-SITU VITRIFICATION TEST

Direct current (dc) cross-borehole resistivity measurements were used to monitor the melting and solidification processes of an in-situ vitr ification (ISV) experiment at Oak Ridge National Laboratory (ORNL) in Tennessee. Six boreholes, 6-m deep, were augured around the ISV site, and five electrodes implanted in each hole. Three sets of crosswell, p ole-pole resistivity data were collected: prior to the melt phase, imm ediately after power shut-off, and after the melt zone had solidified and returned to ambient temperature. These three sets of data were inv erted using a conjugate-gradient scheme to produce conductivity images of the melt phase and the vitrified end products. The images obtained depend quite strongly on the model weighting function applied to the inversion. With an optimum weighting function based on a priori spatia l constraints, the resistivity images delineate the melt zone and prov ide a reasonable indication of its geometry. The resistivity data supp ort, but do not require, the existence of the melt zone.