Bathymetry of the Gulf of Mexico
Texas Sea Grant has supported the development of several bathymetric maps of the northwestern Gulf of Mexico that integrate diverse historic bathymetric data collected between 1930 and 2003 by the NOAA/ U. S. Coast Survey. These data include NOAA Coast Survey multi-beam bathymetric data collected over large areas of the Outer Continental Shelf in 1988-1993.
More recently (since 1998), multibeam bathymetric data have been collected over features of limited areal extent in the northwestern Gulf of Mexico by the U.S. Geological Survey (USGS), National Oceanic and Atmospheric Administration (NOAA), the Harte Research Institute for Gulf of Mexico Studies (HRI) at Texas A&M University-Corpus Christi, the Schmidt Oceanographic Institute, and the Center for Coastal and Ocean Mapping/Joint Hydrographic Center at the University of New Hampshire.
Led by Dr. Troy Holcombe of the Texas A&M University Department of Oceanography, the bathymetric maps show the shelf and slope of the northwestern Gulf of Mexico, Gulf coastal/shelf bathymetry combined seamlessly with land topography in the region (“bathytopo”), and multiple offshore/shelf banks.
Even though most of the earlier NOAA data were collected for purposes of navigation safety and nautical charting, deriving bathymetry from all these depth measurements yields other valuable results. Bathymetric maps have many applications, including ocean engineering and construction, the management of habitat of fish and other marine organisms, ocean research and exploration, and prediction models of storm surge and sea level rise, among others.
Who Uses Bathymetry?
by Dr. Troy Holcombe
Bathymetry has many practical applications.
OCEAN ENGINEERING AND CONSTRUCTION
Building and operating petroleum drilling platforms, production platforms and pipelines over large areas of the northwestern Gulf of Mexico is a major enterprise. Knowing the depth, slope of the bottom, and the inherent strength and stability of bottom and sub-bottom sediments, are factors in selecting locations and in the design, construction and installation of platforms and pipelines.
Good bathymetry is one of the starting points for planning and implementation of ocean engineering projects. Though numerically not the largest potential user community, use of bathymetry for ocean engineering and construction is one of the most valuable, from an economic, environmental and safety point of view. Decisions made and carried out by the ocean engineering community often mean the difference between success or failure, profit or loss, environmental health or disaster, and basic safety or lack thereof.
Closer to shore, another ocean engineering/construction community designs and builds docks, piers, jetties, bridges and navigation channels. For them it is essential to have accurate information about water depth and bottom configuration.
Use of NOAA charts for navigation safety may not be their best option, because navigation charts are typically constructed using only a small fraction of the information contained in NOAA’s archived bathymetric and hydrographic surveys. The best bathymetry made readily available is a valuable resource for ocean engineering projects conducted in the shore zone, especially in the planning stages of those projects.
HABITAT OF FISH AND OTHER MARINE ORGANISMS
For quantitatively estimating the available habitat for brown shrimp, for example, it is necessary to know the areas within bays and estuaries which are less than one meter in depth, and also, one needs to know the area of ocean habitat out to a depth of about 50 meters on the Continental Shelf.
It is also useful to have a good estimate of the volume of water in bays and estuaries, in order to make forecasts of water budget and salinities in these environments, and the effect that these will have on the carrying capacity of brown shrimp. Estimating shrimp habitat is only as good as the bathymetry upon which the calculations are based. Bathymetry is also a factor in quantitatively estimating the carrying capacity for any other organisms whose habitats are depth dependent – oysters, fish, and other organisms.
Making quantitative habitat estimates is the starting point for governance and regulation of the fishing industry. Bathymetry results in better estimates. On the opposite end of the spectrum, good bathymetry may be valuable to the fishermen themselves, who can maximize the success of their efforts if they know depths to the habitats of the fish they are pursuing.
OCEAN RESEARCH AND EXPLORATION
Collecting and analyzing sediment and rock samples from the sea floor (marine geology, geotechnology), sending an ROV down for photographing and observing a shipwreck (nautical archaeology), running biological trawls in order to make species counts (marine biology), collecting water samples and salinity/temperature measurements at depth in the water column (physical oceanography) – all of these field activities, which provide the information base for ocean studies, are enhanced by having good bathymetry at the location of the study.
Some field exploration activities would in fact be of limited value if conducted in the absence of good bathymetry. Bathymetric maps are the base maps for planning and carrying out oceanographic expeditions, whatever the purpose of the investigation, which might be basic or applied research, surveying/mapping or even treasure hunting. The potentially interested may include scientists in research laboratories or conducting petroleum exploration, regulators in government agencies, environmentalists or decisionmakers.
With bathymetry one can see the shape of the ocean floor, with its banks, valleys, mounds, domes, channels, ridges, hills, seamounts, slump scars, submerged deltas, fans, canyons and many other features. People of all ages are curious to see what the ocean floor looks like (and how the features were formed), particularly if the bathymetry is portrayed in color and is in 3-D or shaded relief. Among the most interested are the geoscientists, who, armed with knowledge of the geological processes that shape the earth’s surface, can expand the horizons of knowledge by coming up with first-order explanations of the geomorphology obtained through studying the bathymetry in conjunction with other geological information.
Reliable bathymetry is essential for safe operation of our submarines, and conversely for antisubmarine warfare. Who can put a value on the safety of a multibillion-dollar submarine with all hands aboard? Good bathymetry anywhere off the coast of the United States, or anywhere in the world ocean, has the potential to enhance naval operations.
LAW OF THE SEA
The limits of a nation’s exclusive economic zone can be extended beyond 200 miles if certain defined conditions exist that are a function of bathymetric depth and thickness of the terrigenous sediment wedge. If certain depth and sediment-thickness conditions exist, the extended EEZ is judged to have the potential for discovery of petroleum and/or minerals.
There is a small area in the middle of the deep Gulf of Mexico that is beyond the 200-mile limit of either Mexico or the United States. There have been negotiations between the two countries attempting to reach agreement on who owns the oil and gas, since the “doughnut hole,” as it is referred to, is petroliferous.
In support of these negotiations, a multibeam survey was conducted in order to have good bathymetry over the contested area. Bathymetry from this survey was one of the source data sets used in the preparation of our new bathymetric map of the northwestern Gulf of Mexico.
FIBER-OPTIC COMMUNICATIONS CABLES
Since 1988, there has been an intense and growing effort aimed at laying transoceanic fiber-optic cables linking continents and countries, and providing high-speed, high-volume “land-line” communications the world over. This activity is still ongoing. Cable-laying companies use bathymetry to plan routes for these cables, selecting those routes that the bathymetry show are least likely to be susceptible to geological events that will result in cable breaks. The northwestern Gulf of Mexico, surrounded by mainland, is apparently not presently a target area for installation of transoceanic submarine cables.
PREDICTION MODELS OF STORM SURGE AND SEA LEVEL RISE
Among the factors taken into consideration by storm surge prediction models in the coastal zone are the directional path of the storm, wind speed, width, speed of movement, storm duration, precipitation – and the configuration of coastal topography and offshore bathymetry. Good prediction models that are based on good bathymetry and coastal topography have the potential to provide the basis for orderly evacuations of low-lying coastal areas that are at risk.
A different prediction that can be made with good coastal topography and bathymetry (bathytopo) is what areas will be inundated by sea level rise of a given magnitude. Contingency plans for mitigating the effects of sea level rise can be improved if the areas that will be flooded can be established with greater accuracy, such as would be possible using the best existing bathytopo.