Abstract

Geophysical investigations of the first 100 m below the seafloor in deep-marine environments are done to assess drilling hazards, to plan routes for pipelines and cables, and to study benthic organism communities. The tool of choice for these investigations is subbottom profiling, which uses acoustic signals in the range of 1 to 25 kHz to image near-bottom stratigraphy. An existing engineering-scale, direct-current resistivity (DCR) system for use in deep-marine, near-bottom environments has been adapted. It is potentially useful in settings in which the presence of free gas, gas hydrates, coarse sediment, cemented carbonate, or highly deformed sediment limits the effectiveness of the subbottom-profiling method. The adapted DCR system was used to survey Woolsey Mound, Mississippi Canyon Block 118 (MC118), Gulf of Mexico, to characterize the shallow gas-hydrate system. Three conventional modes of DCR data acquisition — continuous-resistivity profiling (CRP), static array, and time lapse with a fixed array — were evaluated on the deep seafloor. High-resistivity anomalies likely associated with high concentration of hydrate deposits were imaged with all three modes of acquisition.

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