Underwater sonar acoustics

by Claire Calcagno

Scientists turn to acoustic instruments to “see” through water because water becomes opaque to light rays within short distances: in Doc’s words, there is always “bad weather” under water, the equivalent of perpetual fog in the air.

Acoustic waves provide the means to make observations and measurements, as well as to communicate, under water. By measuring the time it takes for a sound pulse to reach an observer, it is possible to measure the distance between that source and the observer. Distances can be similarly calculated by timing the return echo of a sound pulse emitted by the observer.

During his first expedition with Cousteau, Edgerton noticed incidentally that with his depth-measuring sonar ‘pinger,’ the bottom signal had character: that is, the sound penetrated through the surface sediments and reflected off layers below the bottom sea floor. In his words, “it showed us more than we needed to see” — a phenomenon that piqued his interest and provided inspiration for a new path in his ocean engineering designs.

For the next several decades Edgerton worked on improving his sonar instrument, or sub-bottom profiler, which he nick-named a ‘mud penetrator’, to probe the sea floor seeking to identify and record what lay beneath the sediments. The constant challenge he faced was to balance the competing desires for precision and range: higher acoustic frequency gives greater precision but lower distance range, and vice versa. Sound is also absorbed differentially according to the type of sediment being examined and the interference such as caused by gas bubbles. So Edgerton sought as many opportunities as possible to field-test his experimental instruments in the greatest variety of conditions to determine their working parameters and optimize their performance.

Doc’s reach extended to the deepest layer of the earth’s crust, located with a Doc-designed ‘boomer’ and sampled for the first time from the Puerto Rico Trench with scientists of the Woods Hole Oceanographic Institute in 1960. He also spent many summers working with archaeologists who had begun exploring the underwater realm in earnest in the 1960s.

Edgerton experimented by shifting the angle of the sonar beam sideways, realizing that this was a way to be able to detect objects proud of the sea bed. His former student and onetime colleague Martin Klein went on to develop the first commercially viable dual-channel side scan sonar, based on these beginnings, becoming a recognized leader in the field.

The sonar instrument is typically towed behind a vessel on an instrument platform, or “fish”. With side scan sonar a fan-shaped beam of sound is directed off both sides of the survey ship, and typically surveys swaths of a few hundred meters to either side. The reflected sound is then recorded visually onto a recorder on board the ship.

Once again, the challenge was to balance range and resolution: the need for high resolution for clarity of image, and exended range so that coverage of the sea floor was as effective as possible. Edgerton’s 1986 publication Sonar Images, which brings together dozens of examples of his own sonargraphs as well as those recorded by colleagues at a wide variety of locations over the decades, amply illustrates the progress made from the earliest efforts of recording to results such as achieved by Marty Klein, featured on the cover of the volume – approaching photographic clarity.