Abstract The first test of the Bell Aerospace gravity Gradiometry Survey System (GSS) for geologic applications was conducted in April 1994 in collaboration with the U.S. Navy. The GSS is a recently declassified gravity sensing system that contains the world's only moving-base gravity gradiometer. The system measures both gravitational acceleration and gravity gradients, yielding six measurements that define the local gravity field and its gradients in three dimensions a technologic advance in measuring gravity analogous to the advance from 2-D to 3-D seismic profiling through the towing of multiple rather than single hydrophone arrays. The gravity gradiometry test survey was conducted over a buried salt structure southsoutheast of New Orleans in water depths of ˜1500 m. The quality of the survey data is excellent. In declassified grids of the data at 2-km wavelengths, gravity gradients are resolved to 0.5 and gravity to 0.07 mGal. Simple models are used to illustrate the power of this data in subsurface structure definition. The potential utility of gravity gradiometry in oil and gas exploration then is demonstrated through application of the survey data in improving a geologic model of a part of the survey area derived from 3-D seismic data.

Abstract We have begun the integration of rock physical properties, production data, reservoir modeling, and 4-D seismic monitoring from multiple generations of 3-D surveys to track changes of seismic attributes with pool drainage. Here we present the 4-D seismic monitoring technologies in order to (1) predict reservoir characteristics from seismic data, (2) locate bypassed pay, and (3) isolate drilling strategies that will maximize additional recovery for future fields. The test study is from the Eugene Island 330 Field of the offshore Gulf of Mexico. These results will have general application to other fields in the Gulf of Mexico, Nigeria, the North Sea, the Caspian Sea, and Indonesia—those with multiple generations of 3-D seismic coverage and seismically illuminated hydrocarbons.

Abstract The Okinawa Trough is a marginal sea that opened when the Ryukyu Arc was rifted away from the Asian mainland. Geophysical surveys in the southwestern portion of the Okinawa Trough have delineated two grabens formed in the basement and overlying 2.5 seconds of sediment. One graben extends along the trend of the basin from 124°E to 125°E longitude, to the east and west of which the well-developed graben structure is replaced by a more dispersed pattern of normal faults. About 40 km north of this broadening of the fault pattern, a second graben is found which continues to the northeast where it, too, is replaced by basin-wide normal faulting. The sediments in the basin can be divided into two formations. Widespread deformation by folding and faulting is found in the lower unit, whereas the unfolded upper unit is primarily deformed by the faulting associated with the graben. A tentative scheme for the evolution of the graben would have the lower unit deposited in the basin while extension was active, with the upper unit deposited after the main phase of spreading had ceased and as a result, subject only to limited deformation. Faulting in the grabens is presently active although extension is occurring at a very slow rate. These faults have offsets which in many cases increase with depth to at least the bottom of the upper unit. The dip of the bedding beneath the graben also increases with depth, indicating that incipient spreading has been occurring since the beginning of the deposition of the upper unit (at rates on the order of a few mm/yr). An east-west trending ridge between the two grabens was dredge-sampled and found to be composed of biotite-rich quartz diorite and metamorphosed pillow basalts. It is thought to be a sliver of the Ryukyu Arc stranded in the basin during the rifting event which formed the Okinawa Trough. In spite of the extremely slow spreading rate in the Okinawa Trough, two new heat flow measurements of 126 and 443 mW/sq m, in addition to previously reported values, show the heat flow to be high in the basin.

Abstract A compilation of heaf flow versus age of oceanic crust about all the midocean ridges demonstrates that low heat-flow anomalies associated with the crest and flanks appear to occur only on slow-spreading ridges. Such crestal anomalies are probably caused by large-scale hydrothermal circulation in fractures throughout the riewly formed oceanic crust, which results in rapid cooling of the crestal zone and hydration of the crust. Away from the crest, sediments seal the fractures and circulation stops. The crust then begins to heat until it reaches equilibrium with the conductively cooling mantle, after which it cools with the rest of the lithospheric plate If, however, the crust heats to above the equilibrium stability temperature for its hydrous assemblage, dehydration occufe, absorbing enough, heat to produce the observed low heat flow on the flanks of these slow spreading ridges.