A Hybrid Occam and Marquardt-Levenberg Inversion Approach to Time-lapse Transient Electromagnetic Monitoring of Water Infiltration at a Recharge Basin

Recharge flux is one of the most difficult components of the water balance to measure. However, quantitative estimates of this flux are critical for water resource planning in semi-arid and arid areas. Electrical and electromagnetic methods may be useful for monitoring recharge because of their dependence on the electrical conductivity of sediments and therefore on the volumetric water content of a porous medium. In this study, we examine transient electromagnetic (TEM) methods for monitoring water infiltration into an initially dry soil and subsequent drainage of the wetting front with time at a recharge basin near Tucson, Arizona. To produce geologically reasonable subsurface conductivity images from a real TEM dataset, we suggest a hybrid time-lapse inversion approach consisting of a smooth Occam inversion for the baseline day, followed by Marquardt-Levenberg inversions for the subsequent monitor days. Such an approach is uncommon in conventional electromagnetic inversions but in our case is useful in emphasizing subsurface changes during a monitoring experiment. Results suggest an impermeable clay layer at approximately 30m depth and a subsequent buildup of water in the subsurface to the west of the site.