A two week cruise aboard the R/V GYRE focused on mapping surficial sedimentary processes and their connection to the subsurface geology. The study area was on the upper continental slope in the northwestern Gulf of Mexico; an area of active hydrocarbon exploration. Active salt movement, hydrocarbon movement up faults, the presence of gas hydrates associated with biogenic and thermogenic methane, and overpressured sand deposits all present hazards to oil exploration in this area. This study used sidescan sonar and high-resolution chirp subbottom profiling techniques to map the surficial and shallow subsurface expression of these processes in two areas roughly 15 by 25 km in size. High-resolution multi-channel seismic-reflection data were collected simultaneously with the sidescan imagery to provide a link to the deeper subsurface. Additional multi-channel and Huntec seismic data were collected along regional lines between the detailed study areas to tie these studies to other areas of known hydrates and to wells where shallow overpressured sands have been drilled.
The detailed studies attempted to map entire geological systems in this upper slope environment to provide a broader perspective than can be seen from studying a single lease block. One study area focused on two adjacent salt withdrawal basins to assess the processes that are shaping the flanks of the basins, the processes affecting sedimentation in the basin floors, and to compare the deeper structures and surficial processes between the basins. The second study area focused on three salt domes, the processes associated with them, and how they affect the shallow subsurface stratigraphy. Initial observations suggest that most of the surface and shallow subsurface geohazards are associated with the flanks of salt structures. Here active salt movement generates faults along which hydrocarbons can escape to the seafloor, and tectonic oversteepenning generates slope failures.
Lines 15 and 16 of Mosaic Area Two do not have acoustic ranging data available. The SEG-Y file headers contain ship position (UTM zone 15). The ASCII shotpoint navigation files contain geographic coordinates for sonar fish position obtained by comparing fish depth and layback distance from lines 17 and 18 where acoustic slant ranges were good. Because lines 15 and 16 are in relatively shallow water and fish depths did not exceed 300 meters, this method of estimating layback is reasonable. Based on the layback verses fish depth analysis of lines 15 and 16, the horizontal errors may be as much as 50 meters. Layback depends on other variables other than fish depth. Ship speed was not considered in the analysis.
Lines 17 and 18 of Mosaic Area Two were determined to have good acoustic slant range, thus the calculated layback was used to obtain sonar fish position. The ASCII shotpoint contain geographic coordinates for sonar fish position as well as ship position. The SEG-Y file headers contain ship position (UTM zone 15).
Lines 19-27 of Mosaic Area Two had unreliable slant range information due to the length of fish layback and deeper water. Laybacks were estimated for lines 19 and 20 based on the analysis of fish depth and layback in lines 17 and 18, but when fish depths exceeded 300 meters the error in this method increases. Instead another method of comparing water depth from the sonar fish (fish depth and altitude combined) to NOAA gridded Seabeam bathymetry. Offsets between the bathymetry and the water depth from the fish showed the apparent layback for each line. Using this method, the layback was estimated and the corrected navigation compared to the gridded bathymetry. This showed good correlation between water depths and sea floor features. Furthermore, the estimated layback was used to navigate the sidescan sonar data in the mosaicking process, and features on adjacent sidescan sonar lines matched well. The ASCII navigation files contain ship position as well as layback positions. The SEG-Y file headers contain ship position (UTM zone 15).
The processing was done by David Foster, and is summarized in Foster and others (2000)
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