How to Map the Seafloor When You Can’t See Bottom

August 17, 2009 · Filed Under 2009, Journey

Rachael Soraruf, NOAA’s Office of Coast Survey

For millennia, people have been navigating the seas and waterways for commerce and transportation. However, lack of knowledge of shoals and hazards on the seafloor resulted in the loss of human life, as well as damage to ships and cargo. To safely traverse waters, mariners began measuring the depths of navigable waters and developing nautical charts.

Until the early 1930s, depths were determined by lead-line sounding techniques. Lead-line sounding involves dropping a lead weight on a graduated line (rope) until it hits the seafloor, and then reading the depth from the line. While fairly accurate for measuring seafloor depths, especially in shallow waters, this technique is extremely labor intensive and results in gaps between measurements. These gaps in coverage of the bottom could conceal the discovery of dangerous obstructions on the seafloor.

During the early 20^th century, naval requirements contributed to the development of the single beam echo sounder, which became the primary means of depth measurement for charting by the 1940s. Although the single beam echo sounder was more efficient than lead-line sounding techniques, it still did not provide continuous coverage of the seafloor.

In the 1970s, the multibeam echo sounder was developed to cover large areas of the seafloor. Unlike lead-line and single beam echo sounders, multibeam echo sounders collect data in a series of adjacent swaths that provide complete seafloor coverage, eliminating the chance of missing an obstruction or dangerous feature. The technology rapidly improved through the 1980s and 1990s, and is now the standard instrument for surveying the seafloor.

However, the majority of depths found on nautical charts in United States coastal waters remain those captured by lead-line and single beam surveys pre-dating the mid-1980s. Why? Planning and executing hydrographic surveys is time and money-intensive, and there is a lot of seafloor to cover: including the Exclusive Economic Zone (EEZ), the U.S. is responsible for 3.4 million square nautical miles of seafloor. At the current rate of seafloor mapping, it will take over century of continuous surveying to completely chart. Still, it is the aim of NOAA’s Office of Coast Survey (OCS) to provide complete and accurate charts for navigation of all vessel traffic within U.S. coastal waters.

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