An Ocean Bottom Absolute Gravity Meter
We have completed the development of new absolute gravity meter that can be operated on the seafloor. The instrument is operated from a ship with a long cable and is capable of operation to 5000 meter depth. The design goal is an accuracy of 10 μGal, which, when achieved in repeat measurements during a period of many years, will provide information on variations in seafloor height with a precision of 3 to 5 cm.
The components needed for the absolute measurement of gravity are contained within a 60-cm-diameter aluminum pressure case. These include a vacuum chamber containing the test mass, an interferometer for tracking of the falling test mass, a fringe-signal processing system for calculation of acceleration, and various control systems. The entire system is mounted on motorized legs for leveling. Aside from the capability to be operated remotely, the instrument differs from other absolute gravity meters in two key ways. The first is that the new interferometer is based on optical fibers. The second is that the fringe signal is digitized.
Three measurements in the Pacific Ocean have been completed. The design of a benchmark re-occupation system is currently underway.
KeywordsGravity Gradient Test Mass Absolute Gravity Main Cable Pressure Case
Unable to display preview. Download preview PDF.
- Beyer, L. A., R. E. vonHuene, T. H. McCulloh, and J. R. Lovett, Measuring gravity on the seafloor in deep water, J. Geophys. Res., 71 ,2091–2100, 1966.Google Scholar
- Fox, C. G., Evidence of active ground deformation on the mid-ocean ridge: Axial Seamount, Juna de Fuca Ridge, April-June 1988, J. Geophys. Res., 95 ,12,813–12,822, 1990.Google Scholar
- Hildebrand, J. A., M. A. Stevenson, P. T. C. Hammer, M. A. Zumberge, R. L. Parker, C. G. Fox, and P. Meis, A seafloor and sea-surface gravity survey of Axial Volcano, J. Geophys. Res., 95 ,12,751-12,763, 1990.Google Scholar
- Segawa, J., and H. Fujimoto, Observation of an ocean bottom station installed in the Sagami Bay and replacement of the acoustic transponder attached to it, JAMSTECTR Deepsea Research ,256, 1988.Google Scholar
- Spiess, F. N., J. A. Hildebrand, and D. E. Boegeman, New systems for seafloor studies, Eos, Trans. Am. Geophys. Un., 68 ,1335, 1987.Google Scholar