Estimation of mass flux and aquifer properties using Global Positioning System and microgravity in the Tucson Basin, southern Arizona.

Abstract

This research evaluates the feasibility of combining Global Positioning System (GPS) measurements with high precision gravity observations over time as a tool to calculate the mass flux within an aquifer system. This study has three major components: long-period tidal gravity measurements, repeated gravity surveys, and GPS measurements. Long-period gravity measurements were used to determine the coefficients for the gravity data reduction algorithm. Lunar and Solar components of the tidal potential were determined from long-period records to be 1.12 and 1.16 respectively. The barometric correction was calculated to be 0.40 μGals/mbar at Eloy, Arizona, and 0.23 μGals/mbar at station WR52 in Tucson, Arizona. The gravimeter was found to be sensitive to temperature and to rate of change of temperature during long-period observations. In an ongoing project (since September, 1986) in cooperation with the Water Resources Division, U.S. Geological Survey, monthly gravity observations were made at two wells and a bedrock base station and near Eloy. By regressing gravity with static water level, a specific storage of 0.12 was determined. Gravity readings showed an annual periodicity with a 20 μGal amplitude and 25 μGals/year linear drift. Gravity observations were taken at 27 stations throughout the Tucson basin at 3 month intervals for 2 years. These data were tried to bedrock reference stations and gravity variations over time were integrated to determine change in subsurface mass with time (mass flux). Temporal gravity changes ranged from 0 to 25 μGals/month (equivalent to 0-60 cm of water) in various parts of the basin. Accuracy on the order of ±30 μGals was obtained and seasonal gravity changes observed. Regional gravity change resulting from regional recharge/discharge could not be observed using my survey design and equipment. The survey precision was not sufficient to yield reliable estimates of aquifer properties. Good GPS positioning vectors were obtained for 27 of the 100 surveys. Accuracy on the order of 1-5 ppm was obtained for most vectors. Using 1987 NGS GPS data as a baseline, my observations did not indicate differences above noise levels (2-10 cm vertical); thus subsidence was not yet detectable. This research has shown that gravity measurements are useful in monitoring ground-water conditions.