RIASSUNTO
ABSTRACT
High-resolution geophysical techniques are now capable of routine assessment of sea-floor geology relevant to oil and gas exploration and production in the Gulf of Mexico to water depths of 7700 ft. Survey methods pioneered in deepwater areas off the Atlantic east coast have been significantly improved. Present practice uses satellite navigation for ship positioning and bottom-mounted telemetering transponder arrays for accurate positioning of deeply towed sensors. Deeply towed subbottom profiler and side-scan sonar systems provide very high resolution data for near-surface sediments and seafloor morphology. Digital recording provides capability for real-time image processing and enhancement. Bathymetric mapping uses surface-towed narrow-beam fathometers calibrated for water column velocity and bottom slope. Medium-penetration seismic data are displayed through a control module to reduce vertical exaggeration and improve resolution. The new techniques allow comprehensive engineering geologic evaluations of deepwater prospects in a cost- and time-effective manner.
INTRODUCTION
A capability has been developed to conduct very accurate high-resolution geophysical surveys to water depths of 7700 ft in the Gulf of Mexico. The surveys define geologic constraints for exploration drilling, determine sea-floor and near-surface geologic conditions relevant to various deepwater production engineering options, and provide data for correlation with geotechnical properties for foundation design.
Survey strategy and techniques are specifically designed to acquire sets of highly detailed acoustic data for prospects at various localities over the continental slope. This region exhibits great diversity in sea-floor relief, sediment properties, and geologic processes. For example, in inter-diapir areas bottom relief may reach 1000 ft, with slope gradients exceeding 20°. Near-surface sediments include sequences of parallel-bedded clays and silts resulting from Pleistocene slope progradation and deepwater sedimentation, but there are large variations in local sequence geometries and sediment consolidation properties. Geologic processes that influence local site characteristics include faulting, gas and fluid expulsion, clathrate formation, diapiric uplift and faulting, mass-movement phenomena, and sea-floor erosion.
A systematic program for assessment of deepwater geology for engineering purposes was first employed in 1981-84 off the Atlantic east coast. 2 The methods include deep-tow 100-kHz side-scan sonar and 3.5-7.0 kHz subbottom profiling. Recent technical improvements, which are the subject of this paper, have resulted in both substantially enhanced data quality and efficient, comprehensive deepwater site mapping and geologic assessment.
SURVEY STRATEGY
The total suite of acoustic data for each prospect area is generated by two separate survey missions, each with its own objectives and tools. The first-phase effort focuses upon acquisition of precision bathymetric profiles using a 24-kHz narrow-beam echo-sounder (Edo-Western 4077). Simultaneously, both sparker (8.4 kjoules) and minisparker (300 joules) profiles are collected to provide a complete overview of the subsurface stratigraphy and structure of the area, typically to depths of 1500-2000 ft and 300-600 ft, respectively, below the sea floor. Sediment sequence geometry and associated features such as faults, diapirs, subsurface gas, buried channels, and mass-movement features are identified from these data. Survey grids comprise 980-ft line spacings (primary) with 2950 oft tie lines. The grid orientation is predetermined using available regional data to give optimum coverage of local g