Time-lapse inversion of airborne electromagnetic data to recover the evolution of interfaces between freshwater and saltwater

ABSTRACT Airborne electromagnetic surveys are frequently collected to delineate saltwater from freshwater and, occasionally in the case of repeat surveys, to monitor the evolution of freshwater-saltwater interfaces. Recovering these hydrological interfaces is commonly performed by inverting the individual surveys independently, typically using a 1D kernel with the interface modeled as an explicit parameter in the case of a few-layer model, or interpreted if a smooth model has been inferred from the data. For repeat surveys, simultaneous inversion combining information from the different surveys — for model parameters that are not anticipated to change — is likely to result in fewer artifacts and overall more accurate models for the evolution of a freshwater-saltwater interface. We introduce a time-lapse inversion combining information from different surveys via either (1) temporal parsimony, where model parameters that are expected to be constant in time are set to be the same for different surveys, or (2) temporal correlation, wherein the temporal model change is traded off against the fit to the data and spatial coherency. Numerical experiments show that when systems with different sensitivities to an identified feature of interest are combined, the temporal parsimony leads to fewer artifacts and lower uncertainty estimates when compared with an independent inversion or the temporally correlated inversion. Simultaneous inversion of one frequency-domain RESOLVE and two time-domain SkyTEM surveys collected over the Bookpurnong floodplain in South Australia confirms that the time-lapse inversion concepts introduced here can combine the information from surveys collected at different times to improve recovery of subsurface changes. More importantly, the temporally parsimonious time-lapse inversion results in fewer artifacts compared with independent inversion and infers a more hydrologically plausible freshwater-saltwater interface.