Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Patent
1998-06-19
2000-06-27
McElheny, Jr., Donald E.
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
G01V 1137
Patent
active
060817658
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of determining the far-field signature of at least one marine seismic source and to apparatus for use in such a method.
In seismic exploration in a marine environment it is known to generate seismic pulses or waves from at least one seismic source and to measure or record the wavefield. Marine seismic sources commonly comprise a plurality of air guns arranged in an array to produce a combined seismic source which has more desirable characteristics than the individual sources of the array. In use each air gun generates a high pressure air bubble by the sudden discharge of a quantity of high pressure compressed air into the water. The elasticity of the air couples with the inertial mass of the surrounding water to produce an oscillating system as the air expands and contracts in size until its energy is dissipated in the water and the bubble reaches its equilibrium volume. These bubble oscillations generate spherical sound waves which form the seismic signal.
An array of marine seismic sources transmits a sound wave through the body of water which is directional, i.e. the shape or signature of the transmitted wave varies with direction. This is as a result of the array having dimensions which are not small compared with the wavelengths of sound in the transmitted wave. This contrasts with the individual elements of the array which are normally very small compared with these wavelengths and thus behave like point sources, i.e. the wave transmitted by any individual element has spherical symmetry and has the same shape in all directions.
The signature of an array of seismic sources varies according to the distance from the array. In a given direction, the signature of a transmitted wave varies in the so-called "near field" as the distance from the array increases until at a sufficient distance from the array, in the so-called "far field", the shape of the wave results substantially constant but the amplitude decreases, generally inversely in proportion to the distance from the array. The "far field" of an array generally exists at distances greater than D.sup.2 /.lambda., where D is the dimension of the array and .lambda. is the wavelength.
In U.S. Pat. No. 4,476,553, there is disclosed the use of an array of near-field hydrophones or pressure sensors arranged to measure the seismic signal generated by an array of air gun seismic sources in a marine body of water. Each hydrophone is placed no closer than about 1 m to an associated air gun so that the pressure measured at each hydrophone is a linear superposition of the spherical waves from all the oscillating bubbles. However this known system is expensive to implement and has the attendant problems of positioning the hydrophones accurately relative to the air gun, of calibrating the hydrophones and of transmitting the signals from the underwater hydrophones to the surface of the body of the water.
A known seismic source air gun made by Bolt Technology Corporation is disclosed in U.S. Pat. No. 4,240,518 and incorporates a stainless steel pressure sensor, known as the BSS, which is mounted within the air gun to measure the pressure inside the gun, as shown in FIG. 1. Hereinafter, such a seismic source air gun will be referred to as a "Bolt air gun". When the air gun fires, the air pressure in the gun changes, the change is measured by the sensor, and the sensor signal is sent back to the towing vessel. With an array of air guns it is important to ensure that all the guns fire at the same time and the signals from the pressure sensors on the different guns are used not only to measure the firing instant, or "time-break", of each gun but are also used as input to the gun controller, which adjusts the timing of the firing commands to the individual guns.
An aim of the present invention is to enable the signature of at least one marine seismic source to be predicted at a location remote from the at least one seismic source by measuring the pressure close to, e.g. at distances of under 1 m from, the at least one marine seismic sour
REFERENCES:
patent: 4108272 (1978-08-01), Paitson et al.
patent: 4476553 (1984-10-01), Ziolkowski et al.
Ziolkowski, "Measurement of Air Gun Signatures Using Gun-Mounted Pressure Sensors", EAGE 57th Conf. and Tech. Exhibition, Glasgow, Scotland, May 29-Jun. 2, 1995.
McElheny Jr. Donald E.
Skillman Henry H.
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