Texas A&M University
Department of Oceanography
Winter 1998 / Spring 1999 - Vol. 6, No. 3
Profiling the seafloor
DID YOU KNOW?
Seismic profiling is practiced in its most advanced form by oil companies who search offshore sediments for hydrocarbons.
When a recorder plots successive echo sounder
pings, it produces an acoustic picture of the sea bottom and shows the thickness
and geometry of the sediments. This technique is called "seismic profiling" and is practiced in
its most advanced form by oil companies who search offshore sediments for
hydrocarbons.
To explore deep within the sedimentary wedges surrounding
the continents, oil companies use explosive sound sources (air guns that
make a "pop" with compressed air) to generate low-frequency sound
waves, because lower frequency waves penetrate farther beneath the seafloor.
When a sound pulse echoes from the seafloor, typically
some fraction of its energy is transmitted into the sediments beneath the
sea bottom. These waves reflect off horizons between sedimentary layers,
where the physical properties change slightly, and the returning signals
tell the thickness and geometry of the sediments.
The returning sound waves are received by long arrays
of listening devices, called hydrophones, that stretch to kilometers in
length. In a process called "3D seismics," data are collected
in such a dense grid that computers can treat the reflections as a volume,
rather than a profile, so the analyst can look at the data from many angles
in three dimensions.
Three-dimensional seismic data are hideously expensive
to gather (about $1,000,000 per 30 km2) and are jealously guarded by the
companies that spent so much to acquire them, so such data have not had
a huge impact on mapping the sea bottom.
If oceanographers are not interested in deep imaging
of sediments, they can use two other acoustic techniques that allow more
rapid exploration of the seafloor at less expense: multibeam echo sounders
and side-scan sonar.
This portion of a seismic "profile" [201 K] of the seafloor on the outer edge of the Mississippi continental shelf was created by an echo sounder.
http://oceanography.tamu.edu/Quarterdeck/1998/3/sager-3.html
Copyright 1998-1999, Department
of Oceanography, Texas A&M University.
Updated November 24, 1998. (abdw)
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