Deep Ocean Measurements of Gravity
Mark Zumberge, PhD
Scripps Institute of Oceanography
University of California, San Diego
Gravity measurements have long played a unique role in the characterization of oil and gas reservoirs. Originally, the observed gravity field was used to help model the static distribution of density beneath the surface for imaging of geologic structure. More recently gravity observations have obtained a precision adequate to infer time-dependent density variation, facilitating models of reservoir response to production.
Making such measurements in the offshore environment presents some unusual challenges and some interesting opportunities. The past decade has seen important improvements in gravity observations as a means to monitor producing hydro-carbon reservoirs and sites of fluid injection, such as a large scale CO2 sequestration experiment. This increased use of gravity is due in part to improvements in instrumentation for observing gravity. Interpretation of gravity changes is always subject to ambiguity because of the non-uniqueness in potential fields. Yet seismic imaging and knowledge of well locations provide a good basis for constraining the locale of density variations that depend both on position and time. Surveys on or near the seafloor have the added benefit of being closer to the source region than a survey undertaken on the sea surface.
Our work has been focused mostly on static sea floor measurements using a remotely operated vehicle (ROV). Recently we have adapted the instrument for use in a moving vehicle. For the static work we developed an ROV-deployable gravity and pressure (for depth determination) observation package. It consists of an underwater housing containing a spring-mass Scintrex gravity sensor mounted on motorized gimbals, a Paroscientific quartz pressure gauge, and a computer to control the sensors and transmit data. For AUV (autonomous underwater vehicle) surveys, we are testing the idea that the motions of a commercially available underwater vehicle (a Bluefin 21) are gentle enough that we can use a land gravity meter for the underway survey and make corrections based on vehicle tilts and vertical accelerations, obviating the need for a gyro-stabilized platform. We mounted our gravity system in an appropriately shaped pressure housing, added some tilt sensors having suitable precision and dynamic range, and performed a series of dives with an AUV to depths of 935 m.
Dr. Mark Zumberge is a Research Geophysicist at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics Gravity Lab. His early research related to absolute gravity, for which he co-developed a meter. Other interests include marine gravity techniques, marine geodesy, infrasound, and geophysical instruments. Mark has received many awards, including most recently winning Best Paper for 2003 from the journal Geophysics. Dr. Zumberge received a B.Sc. with distinction in Physics from the University of Michigan in 1976 and a Ph.D. in Physics from the University of Colorado in 1981. He has been with the Institute of Geophysics and Planetary Physics since 1982.