How Do You Map What You Cannot See? Measure the Echo
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A map of the shape of the seabed underlies navigation, fishing, cable laying and the study of how the ocean moves. Most of it has never been measured directly.
What the map shows
The chart represents the shape of the floor beneath water, using contours joining points of equal depth, colour bands between them and spot depths at particular positions, which together let a reader see the form of the ground as they would a land map. Depths are given relative to a defined reference level rather than the actual water surface, since the surface moves with the tide, and choosing that reference matters, because navigation charts use a low tidal level so that the figure shown is close to the least water a vessel will find.
How the measurements were made
The methods have changed completely three times:
- •A weighted line lowered until it touched, one point at a time
- •Single-beam echo sounding, timing a pulse to the bottom and back
- •Multibeam systems, fanning hundreds of beams across a swath
- •Airborne laser scanning, which works in clear shallow water
- •Satellite altimetry, inferring the floor from the shape of the sea surface
- •Ships logging depth continuously as they go about other work
The trick with the sea surface
The satellite method is indirect and surprising enough to be worth explaining. A large undersea mountain has enough mass to pull water towards it very slightly, so the sea surface bulges above it by a metre or so, and a trench produces a corresponding dip. Satellites measuring the height of the sea surface to a few centimetres can therefore detect the shape of the floor kilometres below, and that is how most of the world ocean has been mapped. The method covers everything and is coarse, resolving features of several kilometres across and missing anything smaller, which is why a satellite-derived chart is a guide to the general shape rather than a survey.
How a multibeam survey works
The instrument that produces modern detailed charts is worth describing because it explains both the quality and the cost. A hull-mounted array transmits a pulse and listens with many receivers, using the timing differences between them to work out the direction each returning echo came from, so one pulse yields depths across a fan several times the water depth in width. The vessel then runs parallel lines with the fans overlapping, in effect mowing the seabed. Accuracy depends on knowing the vessel's motion, its exact position and the speed of sound through the water column, which varies with temperature and salinity and has to be measured repeatedly during the survey.
How little has been measured properly
Estimates of how much of the seabed has been mapped by direct sounding at modern resolution have risen from under ten per cent a decade ago to roughly a quarter, which is the result of an international project coordinating existing data and new survey towards a complete map. The gap matters practically. Uncharted seamounts have been struck by submarines, with an American vessel badly damaged in 2005. Tsunami models depend on the shape of the floor to predict where waves will go. Cable and pipeline routes need detail no satellite provides. And the biology of the deep sea is largely a matter of features nobody has found yet.
The takeaway
Contours and spot depths relative to a low tidal reference show the shape of the seabed, measured historically by weighted line, then by timing sound pulses, and now by fanning hundreds of beams across a swath. Most of the world ocean is mapped instead from satellites detecting the metre-scale bulge that an undersea mountain's gravity raises in the sea surface, which is coarse. About a quarter has been surveyed directly.