Texas A&M University
Department of Oceanography

Measuring the Depths

An in-depth look at ocean instruments that use sound
to measure water depth and create seafloor images

by Will Sager

 Early sailors measured ocean depth with a weight on a rope.

 People have alway been curious about the "land beneath the sea," but until relatively recently that realm up to 10,000 meters (or 6.2 miles) below the surface was virtually unknown because of its impenetrable watery mantle.

Scientists make bathymetry (ocean depth) charts of the ocean basins for the same reason we send a space probe to map Mars: Surface structure and shape of the planet belies the formational processes and geologic events that shaped the seafloor. If not for mapping the seafloor, we might still be debating whether plate tectonics is fact or fantasy.

Weighty attempts

Early sailors needed to know ocean depths, particularly along coastlines and near harbors, so that their ships would not run aground. Ocean depths were measured with a weight on a line, and depth measurements were limited to shallow water by the lengths of line carried on board and the enthusiasm of the captain for making soundings. Not until the 1870s, during the famous Challenger Expedition, were deep soundings routinely made.

But the technique was the same: laboriously lowering a weight on a rope and trying to figure out when it hit bottom. This technique was arduous in deep water, and the results were seldom accurate for two reasons: 1) Strong currents might pull the rope and weight to the side, and 2) the length of rope needed to reach the deep seafloor was so heavy, it was difficult to tell when the weight hit the bottom.

Many people assume that it was the development of scuba and submersibles in this century that allowed the ocean's floor to be explored. However, scuba divers are typically limited to the upper two percent of ocean depths because of water pressure. Beneath the upper few hundred meters, all the sun's light is gone, and although submersibles have headlights, observers inside can only see a few tens of meters because particles in the water scatter the light. Groping around on the deep seafloor in a submersible is akin to walking around on a black, foggy night with nothing except a flashlight. One can only get a dim perception of the total picture that surrounds the sub.

Before high-tech instruments, sailors used a weight on a rope to measure water depth.
As you may expect, this method wasn't always accurate.

http://oceanography.tamu.edu/Quarterdeck/1998/3/bryant-2.html
Copyright 1998-1999, Department of Oceanography, Texas A&M University.
Updated November 24, 1998. (abdw)

Send Quarterdeck inquiries to quarterdeck@ocean.tamu.edu.

QD front page | What is Quarterdeck? | Archives & topical index | Subscribe! | Contact us | Publication info