From shape to process

The articles on seabed mapping in the summer edition of Geoscientist represent the culmination of decades-long endeavours to map the oceans. International efforts began in 1899 at the 7th International Geographic Congress, in Berlin, which set up a Commission headed by Prince Albert I of Monaco to create a series of seabed charts known as the General Bathymetric Chart of the Oceans (GEBCO). GEBCO published its initial suite of charts in 1905 – the first global perspective of submarine morphology that could be studied and interpreted by geologists. By 1959, geologists working at the Lamont Geological Laboratory (now the Lamont-Doherty Earth Observatory) were able to publish a comprehensive physiographic map of the North Atlantic seabed, and by 1977 the US Navy published in colour Bruce Heezen and Marie Tharp’s wonderful interpretive map of the world’s ocean floor, which graces the walls of many an oceanographic laboratory.

Early maps lacked detail and suffered from data gaps, being based on where ships travelled. One of my first tasks on joining the New Zealand Oceanographic Institute in 1965 was to map a one-million-km2 area of ocean floor including the Campbell Plateau, part of the largely drowned southern continent of Zealandia, and the adjacent Macquarie Ridge. We early submarine topographers had to use data from single downward-pointing echo-sounding beams on scarce track lines navigated by star sightings, which had limited accuracy, especially in bad weather. The key to interpretation was a comprehensive understanding of subsea morphology.

Today’s submarine mappers have two great advantages over their forbears: their vessels use satellite GPS to fix position, and they use multiple, outwardly fanning beams to map a wide swath of topography. Early swath bathymetry systems were military, and their data were initially classified. Now such systems are standard fittings on oceanographic ships.

In the UK, the National Institute of Oceanography’s (NIO) sonar system was GLORIA, developed in the 1970s, which provided a 25-km-wide swath picture of the seabed in deep water. During the 1990s, the US National Science Foundation used GLORIA to map the USA’s Exclusive Economic Zone. For close-ups of deep-sea topography, the NIO’s successor, the Institute of Oceanographic Sciences (IOS), developed a deep-towed equivalent, TOBI, that could provide high-resolution maps of areas like the Atlantic Mid-Ocean Ridge. With these instruments, the UK became a leader in deep-ocean mapping.

In 1995, the IOS’s deep-water mapping activities moved to Southampton, to what became the National Oceanography Centre (NOC). To negate having survey instruments tied to an expensive research vessel, we designed an autonomous underwater vehicle (AUTOSUB) that could do what TOBI did but without the tether. The NOC now carries out high-resolution deep-ocean swath mapping with AUTOSUB-5.

A remedy to the large data gaps came from the use of satellite radar altimetry to measure the height of the ocean surface, which is affected by the gravity (hence by both the topography and geology) of the deep seafloor. In 1994, Walter Smith and David Sandwell produced the first bathymetry of the seabed predicted from satellite gravity measurements in areas never covered by a ship, discovering multiple previously unknown features, especially the locations and extents of fracture zones across mid-ocean ridges, and the locations of numerous seamounts. In 1996-97, I worked with Walter Smith and others on the Scientific Committee on Oceanic Research Working Group on Improved Global Bathymetry, which in 2001 recommended a project to map the entire ocean floor in high resolution. This operation finally began in 2017, when, in collaboration with the Nippon Foundation, GEBCO developed the Seabed 2030 project to map the entire ocean floor at “the best possible resolution” by 2030.

Seabed mapping highlights the adage that, in geology, “shape is the first clue to process”. Knowledge of bathymetry is also essential for understanding and forecasting ocean circulation and climate change, tsunami propagation, marine hazards, the siting of submarine cables, and the sustainable use, management and protection of marine ecosystems and resources. Detailed offshore geological maps now being created by the BGS will help, among other things, to show where deep trawling by fishing vessels is damaging seabed ecosystems; where pollution has been created by the discharge of cutting piles by oil rigs; or where offshore wind farms can best be sited.

DR COLIN SUMMERHAYES

Scott Polar Research Institute, University of Cambridge, UK

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