This 3-D seismic map of the bathymetry and amplitude of Atwater Valley 340 (AT 340) provides a visual representation of the subsea depth and strength of the response of sound energy bouncing off the seafloor. The "hard" returns (red, yellow, and dark green areas) indicated hardbottom habitats. Many of the hardbottom areas indicated on this map also have chemosynthetic communities. Click image for larger view and image credit.
Pre-dive Geophysical Analysis of Atwater Valley Block 340 (AT340)
June 9, 2007
Bill Shedd
Minerals Management Service (MMS)
27° 38.70 N
88° 21.99 W
Federal regulations stipulate that 3-D seismic data collected for the oil and gas industry be made available to MMS. Though these data were originally generated by the oil industry to explore for deep oil and gas targets, MMS geophysicists Bill Shedd and Jessie Hunt, and Harry Roberts of LSU recognized that these data might be useful in finding seafloor seeps as well.
MMS used these data to generate bathymetric maps that highlighted areas in the Gulf of Mexico, which showed distinct seafloor areas of high elevation (mounds) and low areas (depressions). The seismic data for many of these same areas showed anomalous variation in the amplitude, which is the strength of the response of sound energy bouncing off the seafloor. The “hard” areas (red, yellow, and dark green on seismic amplitude maps, such as the one above at AT 340) have been correlated to hardbottom areas. Chemosynthetic tubeworms, mussels, and clams are commonly found on and around these carbonate rock outcrops. There are also low amplitude spots on the maps. These represent “softer” and frequently gassy sediments. “Softer” spots tend to be seeps with episodic flow rates and a general lack of chemosynthetic communities. These “soft” spots are usually surrounded by scattered hardground and sometimes chemosynthetic communities.
This map provides a general overview of the seafloor topography in the Gulf of Mexico. Atwater Valley 340 (AT 340) is highlighted by a yellow arrow in the eastern portion of the Gulf. Click image for larger view and image credit.
Though, both the March 2006 reconnaissance cruise and May 2006 Alvin cruise confirmed our interpretation of both the bathymetric and amplitude data, our understanding was still limited by the coarse horizontal and vertical resolution of the data. Resolution refers to the sharpness and clarity of the data. Most 3-D data have horizontal resolution of approximately 15 m by 30 m and vertical resolutions of 5-10 m. Since many chemosynthetic habitats are below the resolution of the 3-D seismic data, we miss many important details on the scale of 1-2 m. Though seismic data are critical in helping us narrow down areas of interest, we need more detailed bathymetric maps to identify many important but subtle features.
Autonomous Underwater Vehicles (AUVs) are one of several tools that scientists are using to obtain finer resolution data about the seafloor. You’ll hear more about one of these in an upcoming log.
