Natural Geological Weighing Lysimeters: Calibration Tools for Satellite and Ground Surface Gravity Monitoring of Subsurface Water-Mass Change

摘要

Increased accuracy in measuring temporal variations in the Barth's gravity field allow in principle the use of gravity observations to deduce subsurface water-mass changes. This can be with respect to a small area, or as a larger spatial average of water mass change using gravity observations from low-altitude satellites, such as the forthcoming GRACE mission. At both scales, there is a need to validate gravity-based estimates against field recordings of actual subsurface water-mass variations. In practice, this could prove difficult because the spatial integral of all water-storage change components can be subject to considerable field measurement error An alternative approach to the validation process is proposed by which suitable geological formations are utilized as giant weighing devices to directly measure area-integrated water-mass changes. The existence of such "natural geological weighing lysimeters" is demonstrated using observations from a replicated experimental site in New Zealand. Sites of this type could be used to verify water-storage change estimates derived from sensitive ground surface gravity instrumentation. In addition, geological lysimeters could be used to make local checks on the accuracy of any estimated regional water-mass time series, which is proposed for satellite calibration. The land area "weighed" by a geological lysimeter increases with formation depth and it is speculated that recordings made at oil well depth may allow direct monitoring of subsurface water mass changes at the regional scale.