Use Of 3-D Exploration Seismic Data For Geohazards Analysis

Abstract

Abstract The following case study compares high resolution geophysical survey data to migrated 3-D exploration data for the interpretation and mapping of marine geohazards over deep water lease areas of the Northern Gulf of Mexico. While the results are mixed, it is apparent that both types of tools are needed, in that, they complement each other to give a more comprehensive picture of the sea floor. Single channel analog and digital high resolution tools provide finer details of the near sea floor geology; whereas, 3-D seismic data provides valuable acoustic imagery of the sea floor and detailed subsurface information when interpreted on a computer workstation. The net conclusion of this study is that both types of survey data should be integrated to produce comprehensive Hazards, Site-Specific, and Shallow Seismic Stratigraphic Studies (for the detection of high-permeability sands) over deep water lease tracts; and that the use of one type of data without the other can produce significant gaps to a thorough interpretation of deepwater shallow hazards. A best practices approach to the conduct of deep water geohazards it is concluded, would be a stepwise program. Starting with migrated 3-D exploration data, to produce a first-pass geohazards interpretation following which the needed coverage and types of tools to be deployed for a high resolution survey can be assessed to provide comprehensive coverage of any hazards to drilling operations. Introduction Since inception of requirements for shallow hazards and cultural resource assessments in 1983 by the US. Department of the Interior over leases in the Gulf of Mexico, standard practice has been to shoot high resolution geophysical surveys to meet the minimum criteria set forth in Federal regulations. The two basic needs for the conduct of the ‘hazards’ sutveys are an assessment of cultural resources such as the presence of pipelines and other man-made features, and natural geologic features which could represent a danger to exploration drilling and the installation of production structures. Such "hazards" include active faults, high sea floor gradients, juxtaposed areas of hard and soft sediments, shallow gas accumulations, unstable sea floor areas, etc. When industry began operating in deeper waters of the continental slope, it became apparent that standard practices for the conduct of geohazatds surveys would need modification, as surface-towed magnetometers and side scan sonar systems could not provide useful coverage of the deep seafloor. Deep-tow side scan sonar systems, while adequate for the work, are very expensive and necessitate the deployment of sub-sea acoustic navigation systems to track the side scan fish towed on thousands of feet of cable. The widespread availability of migrated 3-D exploration seismic data has recently provided a potential tool for an assessment of shallow geohazards, which is evaluated as such herein. In addition, the discovery of Chemosynthetic communities of organisms basking in abyssal depths and feeding off seeping hydrocarbons added a new twist in that they represented References, nomenclature, and figures at end of paper.