Acoustics – Geophysical or subsurface exploration – Seismic wave generation
Reexamination Certificate
1998-10-16
2001-05-15
Dang, Khanh (Department: 2837)
Acoustics
Geophysical or subsurface exploration
Seismic wave generation
C181S120000, C367S141000
Reexamination Certificate
active
06230840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to marine vibrators used in marine seismic systems. In particular, the invention is related to low frequency acoustic sources used in marine seismic surveys.
2. Description of Related Art
The prior art discloses a variety of marine vibrators (or underwater acoustic projectors) used in marine seismic surveys, including but not limited to U.S. Pat. Nos. 3,329,930, 3,384,868, 3,392,369, 3,394,775, 3,482,646, 3, 676,840, 4,853,905, and 4,885,726. Prior art vibrator devices have a variety of shapes ranging from cylindrical to bell-shaped to spherical. Operationally, these devices are sufficiently heavy so that they do not float and sink of their own accord.
The prior art discloses various underwater acoustic projectors (e.g. as in U.S. Pat. Nos. 3,875,552, 5,247,894 and 5,600,087) used in submarine warfare. These devices are streamlined, generally neutrally buoyant, have control systems which facilitate changing depth, course and speed, and may be remotely controlled or autonomous. They are small in size, operate in a frequency regime much higher than that useful to seismic exploration, and produce relatively low power acoustic signals.
In the marine seismic exploration industry, typical air gun projectors produce high powered acoustic signals (near 200 dB/Hz re &mgr;Pa) in a low frequency band (5 Hz to 120 Hz). Existing prior art marine vibrators do not achieve these power levels in the low end of the seismic spectrum. Several existing prior art units may be used in concert to achieve the desired acoustic power in the seismic frequency band; but simultaneous use of multiple marine vibrator units complicates deployment, towing, synchronization, and operation.
If the acoustic projector is small compared to the wavelength of the signal being produced, the simple source equation (e.g. discussed in
Theoretical Acoustics
by Morse and Ingard, 1968) provides a model for predicting the performance of acoustic projectors. The simple source can be modeled as a sphere having the same surface area as the projector being modeled. It relates the acoustic pressure produced by a projector to the area, frequency and displacement of the projector surface. To produce a high-level acoustic pressure at low frequencies, a projector must have a large surface area which is displaced a relatively large distance.
pressure
,
p
=
π
ρ
f
2
Ad
r
&rgr;=density
f=frequency
A=projector area
d=area displacement
r=observation distance
Subsequently, the force required to move the large surface area a large distance is also large:
driving force, F=4 &pgr;
2
&rgr;f
2
Aad
where a=radius of equivalent simple source sphere
In order for a single submersed marine vibrator to produce high-pressure, low-frequency acoustic signals, the surface area of the projector needs to be very large and the actuator which drives the project is relatively powerful.
Many prior art marine vibrators used in seismic survey operations minimize the enclosed volume of the device which results in strong negative buoyancy. The hydrodynamic drag coefficient of such prior art vibrators is relatively high.
There is a need for an effective and efficient marine vibrator useful in the seismic survey industry that is easily moved through water. Moreover, there is a need for a marine vibrator or an underwater acoustic projector operating in the seismic frequency band that has high power output (>190 dB re &mgr;pa) in the range of frequencies between 5 and 120 Hz, is lightweight to minimize difficulty in deployment and retrieval, is easily towed or pushed through the water, is capable of being steered to change depth and/or lateral position, and is remotely or autonomously controlled.
SUMMARY OF THE PRESENT INVENTION
The present invention, in certain embodiments, discloses a marine vibrator with a hollow hull formed of one or more shells defining an inner space therein, one or more control members on the hull for controlling the position of the marine vibrator in water, and vibrator apparatus within the hollow hull connected to the shell.
In other embodiments, the present invention discloses a marine vibrator with a hollow hull formed of one or more shells having an interior surface and defining an inner space therein and vibrator apparatus within the hollow hull connected directly to the shell.
In certain embodiments, the present invention provides a marine vibrator with a hollow hull formed of one or more shells that define an inner space therein and have a streamlined shape for reducing hydrodynamic drag.
In other embodiments, the present invention provides an autonomous marine vibrator with a hollow hull formed of one or more shells that define an inner space therein, propulsion apparatus for propelling the marine vibrator through water, power apparatus within the hull for providing power to the propulsion apparatus, and vibrator apparatus within the hollow hull connected to the shell.
The present invention, in certain embodiments, discloses a submersible marine vibrator with a hull made of one or two outer shells. In one aspect there are two shells that differ in shape, e.g. two shells with different curvatures or a curved shell and a relatively flat shell. In another aspect there are two matching outer shells, each shell having a relatively flat curved shape, in one aspect defining a spherical segment of one base. In multi-shell embodiments, the edges of shells may be joined by a flexible seal or flexible connector. In embodiments using either two shells or a single shell, one or more stress relief recesses, slots, or slot portions may be used around all or a portion of a shell. In certain aspects for streamlining to reduce hydrodynamic drag, the surfaces of the shell or shells are defined by a radius which is significantly greater than the diameter of a base of the shells.
REFERENCES:
patent: 4885726 (1989-12-01), Myers
patent: 6002648 (1999-12-01), Ambs
Dang Khanh
Western Atlas International Inc.
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