A superconducting gravimeter for evaluation of groundwater changes in the field

Abstract

The Superconducting Gravimeter (SG) is an extremely sensitive instrument that measures relative changes in gravity. It is based on the movement of a superconducting sphere levitated in a magnetic field created by current in superconducting coils. It is capable of detecting gravity variations as small as 10⁻¹¹ ms⁻². Because early production SG's lost helium at a steady rate, a large capacity dewar was required for reasonable periods of uninterrupted operation. In the late 1990's, Sumitomo Heavy Industries (SHI) developed a compact refrigeration system able to achieve liquid helium temperatures near 4K. It eliminates helium loss and allows a much smaller dewar to provide long intervals of continuous operation. These technical advances led us to develop an SG configured as a transportable field instrument. The goals were: 1) to package the entire SG system in two containers; 2) to test transport feasibility while the sensor remained in a superconducting levitated state; 3) to verify operability in field conditions; and 4) to determine the value of a transportable SG in groundwater and aquifer studies. We integrated the SG with a full weather station (measuring barometric pressure, rainfall, soil moisture etc.) and a geodetic GPS receiver (measuring vertical movement and atmospheric water vapor). All components were contained within enclosures constructed from angle and sheet aluminum. Each has dimensions ~1.5 x 0.8 x 1 m, total mass ~250 kg (including equipment) and can be transported easily by a medium truck while the sensor remained in a superconducting levitated state. Temporal gravity variations measured by the SG include solid Earth tides, pole-tide, atmospheric pressure effect, ocean loading effect, and terrestrial water storage variations. Most of these can be well modeled, except the last term. We developed a standard procedure for SG data processing. The first field deployment of the SG system was for the study of the Edwards aquifer, a Karst aquifer system that provides water resources in Central Texas, and was operated by a monitoring well equipped with transducer to measure water level changes. The residual gravity changes measured by the SG were only sensitive to the water storage changes underground. With well level measurements, it can be used to estimate aquifer's specific yield.