Cone-based electrical resistivity tomography

Determining the 3D spatial distribution of subsurface propertiesisachallenging,butcritical,partofmanagingthecleanupof contaminated sites. We have developed a minimally invasive technology that can provide information about the 3D distribution of electrical conductivity. The technique, cone-based electrical resistivity tomography C-bert, integrates resistivity tomography with cone-penetration testing. Permanent current electrodes are emplaced in the subsurface and used to inject currentintothesubsurfaceregionofinterest.Theresultantpotential fields are measured using a surface reference electrode and an electrodemountedonaconepenetrometer.Thestandardsuiteof conepenetrationmeasurements,includinghigh-resolutionresistivitylogs,arealsoobtainedandareanintegralpartoftheC-bert method.C-bertdataareinvertedusinganinexactGauss-Newton algorithm to produce a 3D electrical conductivity map.Amajor challenge with the inversion is the large local perturbation around the measurement location, due to the highly conductive cone.As the cone is small with respect to the total model space, explicit modeling of the cone is cost prohibitive.We have developedarapidmethodforsolvingtheforwardmodelwhichusesiteratively determined boundary conditions IDBC. This allows us to generate a computationally feasible, preinversion correctionfortheconeperturbation.WeassessedC-bertbyperforming a field test to image the conductivity structure of the Kidd 2 site nearVancouver,BritishColumbia.Atotalofninepermanentcurrent electrodes were emplaced and five C-bert data sets were obtained, resulting in 6516 data points.These data were inverted to obtain a 3D conductivity image of the subsurface. Furthermore, we demonstrated, using a synthetic experiment, that C-bert can yield high quality electrical conductivity images in challenging field situations. We conclude that C-bert is a promising new imagingtechnique.