The Gulf of Mexico has had an incredible history, leaving scientists to puzzle over a landscape peppered with channels, canyons, domes, pockmarks, faults, and mini-basins. Now, with the recent release of a giga-pixel (1.4 billion pixel) bathymetric map of the Gulf of Mexico seafloor scientists are able to interpret one of the largest coherent 3-D seismic images in the world.
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The project was spearheaded by the Bureau of Ocean Energy Management (BOEM), a governmental agency that oversees oil exploration and development offshore the United States. During the exploration phase oil and gas companies bid on 3 mile by 3 mile blocks for the right to explore for hydrocarbons. In doing so, the government requires companies to hand over their seismic surveys to BOEM.
Now, the BOEM has convinced 7 different companies who own the seismic survey licenses to publicly share the bathymetric data in an aggregated map. Those companies are: CGG Services (U.S.), Inc.; ExxonMobil Corporation; Petroleum Geo-Services (PGS); Seitel, Inc.; Spectrum USA; TGS-NOPEC Geophysical Company; and WesternGeco, LLC.
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The completely free downloadable map is available on the BOEM website.
How Was The 1.4 Billion Pixel Map Created?
Oil exploration companies shoot 3-D seismic surveys to image the subsurface in order to understand the regional geology, structure, and hydrocarbon potential. To do this a boat tows along underwater air guns that create bursts of sound waves that travel to the bottom of the ocean and into the mud and rock beneath the ocean. Behind the sound guns are also receivers or microphones that can pick up very faint pulses of sound that are bounced back from the ocean bottom.
As the sound travels down through the water and into the mud some of the sound bounces back when it goes through a change in material (i.e. from water to mud). For instance, when the sound travels through the water-to-mud interface, some sound bounces back to the receiver and some continues downward into the mud. Eventually, the sound will hit another type of material such as sand and some sound waves will bounce back again to the receiver.
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Through this process, scientists are able to measure the time it takes for many pulses of sound in a 3-D array to travel down and bounce back to the receiver. Merging this data, geophysicists are able to create an image of the subsurface in terms of time measured for the sound to travel down and bounce back. Geologists can then input the expected types of material the sound has travelled through to create a model that converts time to depth. Finally, you have a 3-D seismic depth map of the subsurface.
Salt Underneath The Gulf Of Mexico
The pockmarks and mini-basins in the Gulf of Mexico are due to a deformed thick layer of salt that sits hidden beneath the surface. This salt was formed around 200 million years ago when the Americas separated from Africa and pulled open the Gulf of Mexico. The gulf was a smaller basin not fully open to the ocean, allowing water to enter, evaporate, and leave behind thick deposits of salt.
Eventually, the Gulf of Mexico fully opened to the Atlantic Ocean and sediment began to pile on top of the salt. The Mississippi River transported sediment from large portions of the continental US and dumped that sediment on top of the underlying salt. The weight of the sediment deformed the underlying salt, pushing it up, squeezing it into faults, and creating domes of salt.
The salt is less dense and more malleable than the surrounding mud and rock and thus tends to flow and get squeezed into overlying layers. The tenant of salt tectonics is one of the reasons the Gulf of Mexico is such a prolific hydrocarbon basin. The salt acts to seal in hydrocarbons and prevent them from leaking to the surface, a fundamental ingredient in hydrocarbon exploration.
Old vs. New Seismic Maps
This is in fact the second version of this bathymetric map, the first being much lower in resolution. The new map has an incredible resolution of one pixel equaling 1,600 square feet, approximately the size of a house. The new BOEM map is about 16 times higher in resolution compared to the previous map, which had a resolution of 27,000 square feet equaling one pixel.
To create the new map the BOEM stitched together over 200 individual maps from exploration companies. In total, the maps cover 135,000 square miles of the Gulf of Mexico with datasets that span over 30 years.
The public release of this 1.4-billion-pixel map will help scientists from academia, environmental agencies, oil companies, and governmental agencies further understand this prolific region of our ocean.