Direct measurement of subsurface mass change using the variable baseline gravity gradient method

Abstract

Time-lapse gravity data provide a direct, nondestructive method to monitor mass changes at scales from centimeter to kilometer. But, the effectively infinite spatial sensitivity of gravity measurements can make it difficult to isolate the signal of interest. The variable baseline gravity gradient method, based on the difference of measurements between two gravimeters, is an alternative to the conventional approach of individually modeling all sources of mass and elevation changes. This approach can improve the signal-to-noise ratio for many applications by removing the contributions of Earth tides, loading, and other signals that have the same effect on both gravimeters. At the same time, this approach can focus the support volume within a relatively small user-defined region of the subsurface. The method is demonstrated using paired superconducting gravimeters to make for the first time a large-scale, noninvasive measurement of infiltration wetting front velocity and change in water content above the wetting front.

Geophysics From Terrestrial Time‐Variable Gravity Measurements

“…Ground‐based gravimetry can determine the change of gravity related to Earth rotation fluctuations, to celestial body and Earth attractions, to the mass variations in the direct vicinity of the instruments, and also to distant sources, and to displacement of the instrument itself due to ground deformation. Gravity measurements contribute to risk assessment and mitigation, by improving our understanding of past and present ice mass changes (Kazama et al, ; Lambert et al, ; Larson & van Dam, ; Mazzotti et al, ; Mémin et al, ; Omang & Kierulf, ; Ophaug et al, ; van Dam et al, ), subsidence of low‐lying areas (Van Camp et al, ; Zerbini et al, ), ground water resources (Creutzfeldt et al, ; Fores et al, ; Hector et al, ; Imanishi et al, ; Jacob et al, ; Kennedy et al, ; Lampitelli & Francis, ; Van Camp, de Viron, Pajot‐Métivier, et al, ; Van Camp et al, ), and earthquakes (Imanishi, ; Montagner et al, ; Van Camp et al, ). Concurrently, terrestrial gravity measurements play a key role in the new definition of the kilogram (Stock, ), and our understanding of environmental effects affecting the gravity measurements will be useful to assess the Newtonian noise affecting gravitational wave detectors (Coughlin et al, ; Harms & Venkateswara, ).…”

Monitoring and modeling infiltration–recharge dynamics of managed aquifer recharge with desalinated seawater

“…The results reported here are unique for several reasons. First, we monitored a month (January 2015) of continuous MAR with 2.45 × 10 6 m 3 of DSW (loading of about 23 m month −1 ), higher than in most other reported MAR at infiltration basins, comparable only to a few studies (Kennedy et al, 2014;Nadav et al, 2012;Racz et al, 2012). Second, we focus on the temporal pond-surface infiltration and groundwater recharge using field measurements and both simplified analytical methods as well as detailed numerical modeling.…”

Superconducting Gravimetry

“…The goal is to improve the knowledge of the total subsurface water flow and hydrologic parameters that cannot be identified from hydrology measurements alone. Furthermore, by changing the spacing and geometric configuration of the gravimeters, the region of sensitivity can be 'focused' to the area of hydrologic interest (Kennedy et al, 2014) . In Figure 34, the orange lines show residual gravity from iSG 004 operating next to recharge pond RB-207 and iSG 006 operating 500 m to the south next to pond RB-207.…”