Increased accuracy through variable-baseline gradient measurements with superconducting gravimeters

Despite the availability of sub-microgal superconducting gravimeters for many years, precise measurements of gravity change are still limited by our ability to resolve the signal of interest. Errors in Earth tide, ocean loading, and atmospheric corrections can obscure signals of interest for applications in hydrology, reservoir management, CO2 sequestration, volcanology, and other studies. Errors are largest for instruments deployed in new locations for short durations, in which case loading-model parameters must be determined a priori to calculate gravity residuals. Without better models or improved methods to remove unwanted signals, more accurate gravimeters provide limited benefit. One way to remove unwanted signals is to make 'variable-baseline gravity gradient' measurements using two (or more) gravimeters. With two gravimeters located in close proximity, common-mode signals that affect both instruments equally, such as Earth tides, ocean loading, and atmospheric changes, are removed from the differenced signal - essentially, this creates an average, single-tensor gradient measurement over the baseline separating the gravimeters. In winter 2012, such a variable-baseline gradient measurement was made for the first time adjacent to an artificial recharge basin in Arizona, USA. One of two iGrav gravimeters was deployed inside a newly-developed field enclosure, demonstrating that such instruments can be deployed for short periods with little preparation. The difference between the two instruments could be resolved to better than 0.3 microgals per day. As compared to the single-instrument signal, the gradient measurement better resolved the timing of the onset of infiltration and the depth of the wetting front beneath the basin over time. Furthermore, by varying the position of the two gravimeters, the measurement can be 'focused' at a specific depth. Making variable-baseline gravity gradient measurements can provide improved accuracy and sensitivity using today's gravimeters.