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Summary
Gravity meters are commonly used to map spatial changes in the earth’s gravity, achieving resolution of parts per billion on microgravity surveys. The application of gravity surveys to monitor change and recent improvements in gravity instrumentation will be summarized.
The use of gravity to track the progress of a waterflood in Prudhoe Bay, Alaska, provides a 4D Gravity case history. The surface gravity change caused by the injection of water into the gas cap is measured annually and compared to the expected change calculated from the reservoir model. 4D Gravity provides an effective “early warning system” for injection and sequestration projects.
A recent borehole gravity survey in Hanford, Washington, demonstrates the ability to measure bulk densities using a borehole gravimeter. The applications include geotechnical studies at waste disposal sites, bridges and structures, locating and monitoring thief zones in reservoirs, and grade control in iron mines. A new borehole gravity meter will be introduced during 2008 that can be used in smaller, inclined boreholes.
Earth tides and other long period movements of the earth can be recorded by suitably configured gravity meters. The ground shaking in Luxembourg before and after the January 13, 2007, earthquake in Japan was recorded on a long period seismometer, a superconducting gravity meter and a new portable gPhone gravity meter. The match between the seismometer data and the gPhone data during the earthquake is excellent. The relatively inexpensive portable gPhone gravity meter provides the means to record earth movements along active fault zones and other critical locations. gPhone data recorded in Denver, Colorado on August 15, 2007, indicates preshock and post-shock ground motion as well as the 8.0 earthquake in Peru.