Topography Of Seafloor

[Francoise Witsell]

Topographicmaps of the sea floor. Detailed depth contours provide the size, shape and distribution of underwater features. The map serves as a tool for performing scientific, engineering, marine geophysical and environmental studies, that are required in the development of energy and marine resources.

Detailed multipurpose maps of NOS bathymetry and US Geological Survey (USGS) land topography. Maps support the Coastal Zone Management and Energy Impact Programs and the offshore oil and gas program. They may also be used by land-use planners, conservationists, oceanographers, marine geologists, and those interested in the coastal zone and the Outer Continental Shelf's (OCS) physical environment. All 1:250,000 and 1:1000,000 maps are overprinted with the Minerals Management Service's OCS Protraction Diagram data.

Topographic maps of the sea floor, produced at a 1:100,000 scale that contain Loran-C rates, bottom sediment types and known bottom obstructions. This product is intended to aid fishermen and those needing seafloor features and potential fishing grounds.

Each consist of three sheets (a base bathymetric map, a magnetic map, and a gravity map), and where practicable a sediment overprint (NOS 1308N-17S). The bathymetric map, when combined with the other three maps, serves as a base for making geological-geophysical studies of the oceans bottom's crustal geophysical data for the Continental Shelf and slope. The SEAMAP SERIES at a scale of 1:1,000,000, covers geophysical data gathered in the deep-sea area, sometimes including the adjacent Continental Shelf and Slope.

Bathymetric maps that have been compiled, but are not published. NOAA provides blackline copies of compilation manuscripts for bathymetric maps that were left in the production process but are sufficiently developed to include accurate bathymetric data. There are no plans to have these maps published.

If you would like to contribute data to the next version of the global topography grid we would be pleased to use it. Any format is welcome, even raw multibeamformat. Please send e-mail to dsan...@ucsd.edu

Maps are a visual representation of objects in space. Maps help us navigate and make sense of the world. They also give order to complex environments by revealing spatial relationships and patterns. The location of anything is mappable using three axes: latitude (x), longitude (y), and elevation or depth (z).

Sonar technology was developed in the 1920s and significantly improved seafloor mapping. Sonar systems emit a sound and then record the time it takes to receive an echo off the seafloor. Seafloor depth is calculated using the time difference from when the sounds are made to when the echoes are heard.

Early single-beam sonar systems, called fathometers, were used to measure the seafloor depth beneath a ship. In the 1960s, multibeam sonar systems were developed to measure a larger area using many beams of sound in a fan-shaped pattern. Multibeam sonar can map much broader areas than single-beam sonar.

Contemporary seafloor mapping is often conducted using multibeam sonar systems on ships and other marine vessels. Multibeam systems produce high-resolution data, meaning that there are many data points per area. More data points means you can see more details. Multibeam bathymetric maps are suitable for many applications including safe navigation and studying benthic habitats. Benthic habitats are those closest to the seafloor, or the benthos. Unfortunately, it is remarkably difficult and expensive to send vessels to map the most remote parts of the ocean. Due to cost and logistics, slightly more than 23% of the seafloor has been mapped in high resolution as of 2022.

While researchers would rather have high-resolution data in all regions, modern bathymetric maps use a combination of high- and low-resolution data. Although low-resolution data are not the best, they are still better than nothing. Low-resolution data can still help us understand ocean environments. Much of this low-resolution data comes from satellite altimeters. Large underwater features, like seamounts larger than 1.5 kilometers (0.9 miles), have enough mass to affect the gravitational force in a given area. This change in gravity creates tiny bumps and dips on the sea surface. Satellite altimeters are sensitive enough to detect and measure these changes.

The Mariana Trench is the deepest trench in the world, located in the western Pacific Ocean near Guam and the Mariana Islands. The deepest part of the trench is named Challenger Deep after the British ship HMS Challenger, whose crew first sounded, or measured seafloor depth, in 1875. Scientists recently reevaluated its depths using multibeam data and updated the deepest known point to approximately 10,935 meters (6.8 miles).

In 2015, NOAA and partner scientists deployed a hydrophone within the Challenger Deep trough in the Mariana Trench near Micronesia. Hear more: Ambient Sound at Full Ocean Depth: Eavesdropping on the Challenger Deep.

Our understanding of the seafloor has come a long way from the early days of lead lines and tales of sea monsters. However, global efforts to map the seafloor are far from complete. In 2005, the USS San Francisco, a nuclear submarine, collided with an uncharted seamount. The current nautical charts in the area were not detailed enough for navigators to see the underwater mountain. Luckily, the submarine was able to safely surface and no one was severely injured. This incident highlighted the need for high-resolution multibeam bathymetry data across the entire ocean.

Mapping the entire seafloor by 2030 is an ambitious goal. This global effort is too big a task for one ship or one nation to tackle alone, so governments are collaborating to map the seafloor as efficiently as possible. Technology is being pushed to new limits. Organizations are inventing new vessels and leveraging the use of autonomous vehicles. Geographic information systems (GIS) are allowing people to visualize the seafloor like never before.

Today, many professions need geographers, cartographers, engineers, and data scientists. GIS is necessary in many aspects of our society, including public works projects, business decisions, disaster response, epidemiology, and so much more. Additionally, many industries require seafloor mapping experts for the implementation and evaluation of offshore wind farms, petroleum platforms, and submarine communication cables. NOAA Ocean Exploration hosts an Explorer-in-Training program that allows students and early career scientists to learn seafloor mapping and other aspects of ocean exploration. Learn more about the Explorer-in-Training program.

We examined the topography of the sea floor from the perspective of plate tectonics in Chapter 10, but here we are going to take another look at the important features from an oceanographic perspective. The topography of the northern Atlantic Ocean is shown in Figure 18.2. The important features are the extensive continental shelves less than 250 m deep (pink); the vast deep ocean plains between 4,000 and 6,000 m deep (light and dark blue); the mid-Atlantic ridge, in many areas shallower than 3,000 m; and the deep ocean trench north of Puerto Rico (8,600 m).

A topographic profile of the Pacific Ocean floor between Japan and British Columbia is shown in Figure 18.3. Be careful when interpreting this diagram (and others like it), because in order to show the various features clearly the vertical axis is exaggerated, in this case by about 200 times. The floor of the Pacific, like those of the other oceans, is actually very flat, even in areas with seamounts or deep trenches. The vast sediment-covered abyssal plains of the oceans are much flatter than any similar-sized areas on the continents.

The continental shelf and slope offshore from Nova Scotia is shown in Figure 18.4. In this passive-margin area (no subduction zone), the shelf is over 150 km wide. On the Pacific coast of Canada, the shelf is less than half as wide. Continental shelves are typically less than 200 m in depth; 200 m is also the limit of the photic zone, the maximum depth to which sufficient light penetrates to allow photosynthesis to take place. As a result of that photosynthesis, the photic zone is oxygenated, and therefore suitable for animal life. Approximately 90% of marine life is restricted to the photic zone. The photic zone is also known as the epipelagic zone. The mesopelagic zone extends from 200 m to 1,000 m; the bathypelagic zone from 1,000 m to 4,000 m; and abyssalpelagic zone is deeper than 4,000 m. (Pelagic refers to the open ocean, and thus excludes areas that are near to the shores or the ocean floor.)

Although the temperature of the ocean surface varies widely, from a few degrees either side of freezing in polar regions to over 25C in the tropics, in most parts of the ocean, the water temperature is around 10C at 1,000 m depth and about 4C from 2,000 m depth all the way to the bottom.

The deepest parts of the ocean are within the subduction trenches, and the deepest of these is the Marianas Trench in the southwestern Pacific (near Guam) at 11,000 m (Figure 18.5). There are other trenches in the southwestern Pacific that are over 10,000 m deep; the Japan Trench is over 9,000 m deep; and the Puerto Rico and Chile-Peru Trenches are over 8,000 m deep. Trenches that are relatively shallow tend to be that way because they have significant sediment infill. There is no recognizable trench along the subduction zone of the Juan de Fuca Plate because it has been filled with sediments from the Fraser and Columbia Rivers (or their ancient equivalents).

Bathymetric charts (not to be confused with hydrographic charts), are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines (called depth contours or isobaths) and selected depths (soundings), and typically also provide surface navigational information. Bathymetric maps (a more general term where navigational safety is not a concern) may also use a Digital Terrain Model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a global relief model. Paleobathymetry is the study of past underwater depths.

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