Oceanographic Tools: Sonar Single Beam
the Ocean Floor with Echo Sounding
Echo sounding is the key method scientists use to
map the seafloor today. The technique, first used by German scientists
in the early 20th century, uses sound waves bounced off the ocean bottom.
Echo sounders aboard ships have components called transducers that
both transmit and receive sound waves. Transducers send a cone of sound
down to the seafloor, which reflects back to the ship. Just like a flashlight
beam, the cone of sound will focus on a relatively small area in places
where the ocean is shallow, or spread out over the size of a football field
when water depths reach 3,000 meters. The returned echo is received by
the transducer, amplified electronically, and recorded on graphic recorders.
The time taken for the sound to travel through the ocean and back is then
used to calculate water depths. The faster the sound waves return, the
smaller the water depths and the higher the elevation of the seafloor.
Echo sounders repeatedly ping the seafloor as a ship
moves along the surface, producing a continuous line showing ocean depths directly
beneath the ship.
In the early days of ocean exploration until as recently as 20 years ago, marine
geologists wrote down individual readings from recorders, plotted them on navigation
charts showing their ships position, and then drew contour lines joining
points of equal depth. In this way, they produced bathymetry maps that
displayed the oceans changing water depths (and hence changes in seafloor
elevation). These charts were accurate only within about 20-50 m, but that was
good enough for scientists to discover the mid-ocean ridge system in the Atlantic
and Pacific Oceans.
Echo sounders use different frequencies of sound to find out different things
about the seafloor. Scientists typically use echo sounders that transmit sound
at 12 kiloHertz (kHz) to determine how far down the seafloor lies. However, they
use a lower frequency (3.5 kHz) sound, which penetrates the seafloor, if they
want to see accumulated layers of sediments below it.