Seafloor Direct Current Resistivity Techniques for Deep Marine, Near-Bottom Gas Hydrate Investigation

摘要

Geophysical investigations of the first 100 m below the seafloor in deep marine environments are done to assess drilling hazards, plan routes for pipelines and cables, and the study of benthic organism communities. The tool of choice for these investigations is sub-bottom profiling, which uses acoustic signals in the 1 to 25 kHz range to image near-bottom stratigraphy. In this study we adapt an existing, engineering-scale, direct-current resistivity (DCR) system for use in deep-marine, near-bottom environments. This new tool is potentially useful in settings in which the presence of free gas, gas hydrates, course sediment, cemented carbonate, or highly deformed sediment limit the effectiveness of the sub-bottom profiling method. We use the adapted DCR system 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, are evaluated on the deep seafloor. High-resistivity anomalies likely associated with high concentration of hydrate deposits are imaged with all three modes of acquisition.