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Tectonic Plates
Subduction Zone
(convergent boundary)
Layers of the Earth
• Crust
– Oceanic and continental
– 5km to 50km thick
• Lithosphere
– 50km to 100km thick
• Asthenosphere
Bathymetry
Sonar mapping uses sound waves to map out the contours or the shape of the
ocean bottom. Sonar is an acronym for Sound Navigation Ranging. Typically,
a pulse of sound is generated using a kind of underwater loudspeaker towed
behind a boat. The pulse of sound, or "ping," spreads out through the water
and is reflected back again by objects in the water, such as the ocean
bottom, a fish, or a submarine. Underwater microphones measure the
reflected sound. The time that it takes the echo to return is roughly
proportional to the distance to the reflecting object. Like an echo in the
air.
Measuring the distance to a single object is relatively easy because it
returns a simple echo. Making a map of the ocean floor is much more
difficult because sound reflects back from many directions, from hills and
valleys in the ocean floor. Complicated mathematics is needed to calculate
a map of the ocean bottom.
One of the most insonified aircraft in the world, this PB4Y rests on the bottom of
Lake Washington near Seattle, WA. Low current velocities and lack of a corrosive
environment has kept the basic shape of the plane intact, although structurally
unsound. Many sonar manufacturers use this target in advertising because of it's
image in sonar data. Note the shadow of the outboard right propeller. The aircraft is
resting on its left wing as indicated by the wing's shadow.
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