In the early 1990's the PI's developed an Ocean Bottom Absolute Gravity Meter (OBAGM) with funding from the Oceanographic Instrumentation Development program (proposal #89-20058). This system was then deployed on the Juan de Fuca ridge in 1994, 1995 and 1996. The PI's emplaced several seafloor benchmarks across the ridge and made gravity measurements on three of them (MG&G proposal #94-16939). This program was the initiation of a long-term monitoring program that will attempt to define cm-scale vertical deformation of the ridge.
During deployment of the sensors, and in subsequent land tests, the PI's have discovered a systematic error on the order of 1 mGal. If the instrument is operated outside of its pressure case, the measurements agree well with other land-based absolute gravity meters, indicating that the error source is vibrational coupling between the mechanical and optical components of the sensor. Although the PI's were able to correct for this systematic error, they propose here to redesign the system to eliminate the error, enabling the system to detect height changes of a few centimeters.
