Structural joint inversion of time‐lapse crosshole ERT and GPR traveltime data

Time‐lapse geophysical monitoring and inversion are valuable tools in hydrogeology for monitoring changes in the subsurface due to natural and forced (tracer) dynamics. However, the resulting models may suffer from insufficient resolution, which leads to underestimated variability and poor mass recovery. Structural joint inversion using cross‐gradient constraints can provide higher‐resolution models compared with individual inversions and we present the first application to time‐lapse data. The results from a synthetic and field vadose zone water tracer injection experiment show that joint 3‐D time‐lapse inversion of crosshole electrical resistance tomography (ERT) and ground penetrating radar (GPR) traveltime data significantly improve the imaged characteristics of the point injected plume, such as lateral spreading and center of mass, as well as the overall consistency between models. The joint inversion method appears to work well for cases when one hydrological state variable (in this case moisture content) controls the time‐lapse response of both geophysical methods.