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Why
do geophysicists use seismic surveying in the ocean?
Geophysical seismic surveying enables
geophysicists to "see" the structure of the Earth beneath the
seafloor. Instead of using light waves reflected off objects to perceive
things visually, they use sound waves to "see" deep into the
earth's crust. The reason is that sound waves can travel through seawater
to penetrate layers of rock and sediment beneath the seafloor, while light
waves can't. These waves are reflected or refracted (bent) as they pass
through the Earth and can be detected by devices placed on the seafloor
or towed by a ship, providing scientists with an image of the structure
and geology beneath the seafloor. The overarching goal of this work is
to document the history of the earth and the processes by which it evolves.
This knowledge is vital for understanding climate and sea-level change,
earthquakes and other natural hazards, and the formation and distribution
of natural resources.
How
many seismic surveys does UTIG conduct each year?
UTIG scientists are involved in about two to three marine seismic surveys
each year. In comparison, the oil and gas industry conducts over 100 seismic
surveys annually in the Gulf of Mexico alone, particularly off Louisiana
and Texas.
What
are the scientific objectives of UTIG seismic survey's of Hess Deep?
The Hess Deep is a steeply plunging rift adjacent to the East Pacific
Rise, one of the planet’s fastest-spreading mid-ocean ridges. Because
the Hess Deep slices through recently created crust, it provides a window
into the Earth’s crust-making processes. The purpose of the seismic
survey is to generate a “picture” of the structure of Earth’s
crust at Hess Deep from the seismic data, which scientists can compare
with the outcrop geology observed by NSF-sponsored submersible dives carried
out in 1999 and 2001.
How
will these seismic data be acquired?
The Hess Deep seismic survey will involve the R/V Maurice Ewing,
which uses an array of ten to twenty airguns as the energy source. A 6-km
towed streamer equipped with hydrophones receives the returning acoustic
signals. We will also deploy a suite of Ocean Bottom Seismometers (OBS)
on the seafloor to receive and record the returning acoustic signals.
The energy to the air gun array is air supplied by compressors on board
the ship.
What
is the radius of impact?How loud?
The sound field around the ship will be 180 dB out to one km
and 150-160 dB up to 160 km away.
Points:
1. The decibel system is on a logarithmic scale so, for example, 240 dB
is one billion times greater in volume than 140 dB. The noise level next
to a jet engine is around 120 dB.
2. But this is in air, not water!
3. Comparing sound in air to underwater levels is not an exact match--61.5
dB must be added from a sound level in water to yield an equivalent intensity
in air.
4. Sounds travel much farther and faster in water than in air.
Do
you have permission to carry out a seismic cruise at Hess Deep?
Hess Deep is located in international waters of the Eastern Tropical
Pacific. However, since the seismic ship that we will use is U.S. owned
and operated, we must comply with the provisions of the Marine Mammal
Protection Act (MMPA) enacted by Congress in 1972 (16 U.S.C. 1361-1407),
which is intended to protect marine mammals in U.S. waters and by any
person under U.S. jurisdiction on the high seas. Columbia University,
the operator of the Ewing, applied for and received a permit
through the National Marine Fisheries Service (NMFS) for the Hess Deep
cruise. This permit is for Incidental Harassment Authorization (IHA) for
mammals.
UT
Institute for Geophysics
4412 Spicewood Springs Rd. #600
Austin, TX 78759-8500 USA
Phone: 512-471-6156
Fax: 512-471-8844
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