Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1988-04-13
1989-08-29
Tarcza, Thomas H.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 20, G01V 138
Patent
active
048624225
DESCRIPTION:
BRIEF SUMMARY
The object of the invention is a method for determining the geometry of a multisource seismic wave emission system.
A seismic wave emission system is used for generating seismic waves in a mass of water, which propagate toward the bottom and are reflected or diffracted by the roughnesses of the seabed. The waves sent back to the surface are picked up by sets of hydrophones distributed along one or more seismic cables towed by a ship. The seismic signals they produce are transmitted to a seismic laboratory in order to obtain recordings representative of the seabed configuration.
The seismic sources used are, for example, air guns, water guns, or other sources which are triggered simultaneously or sequentially. The purpose of these combined triggerings is to increase the acoustic power dissipated, to improve the shape of the resulting emitted pulse or signature, or to obtain a more directional emission diagram. The effect obtained by sequential triggering depends very precisely on the configuration of the sources used and the time intervals separating the emissions.
It is thus important to know the geometry of the emission system, i.e. the relative disposition of all the seismic sources with respect to each other.
The emission system comprises, for example, several subsets each composed of a line of several seismic sources disposed at intervals chosen along a towed "umbilical" which generally has towing cables, pipes for supplying gaseous fluids and/or liquids, electric power cables, signal transmission lines, etc.
The various subsets are towed parallel to each other and deflection means such as paravanes are used to keep them spaced laterally away from each other. Various depth control systems comprising for example platforms suspended on floats or platforms fitted with adjustableorientation ailerons and floats, allowing the sources to be submerged to selected depths are provided. Such deflection and depth control systems are described for example in French Patents 2,397,974, 2,240,478, and 2,523,542.
It is easy to measure the longitudinal distance separating the various sources along a given umbilical. However, it is far more difficult to determine exactly the actual distance of each source from the trajectory of the ship as it sails as well as its depth at the time when, the cruising speed having stabilized and the emission-reception system having assumed a stable disposition in the water, the seismic prospecting operation can commence. However, the condition of the sea, the currents, and the towing conditions may change, thus changing the actual disposition of the emission-reception array from one series of emissions to another. As a result, the signature and/or the directional diagram of the array are different for an identical triggering chronology.
The recordings obtained from these emissions are made more legible, in classical fashion, by correlation between the signals picked up by the reception device and the overall "signature" of the system. To determine the latter at all points in space, it is necessary to combine the individual signatures of each of the sources in a particular fashion, taking into account their relative disposition with respect to each other. A first method, described in published French Patent Application No. 2,582,107, consists of establishing a prior catalog containing the signatures of each of the sources of the emission system triggered separately, for a large number of values of parameters involved in the operation of the sources, particularly the immersion depth, and, in operation, taking into account the actual values of the parameters measured at the times of the "shots," combining the corresponding signatures taken from the catalog, possibly corrected, such as to reconstitute by synthesis the overall signature to infinity in all directions in space.
The overall signature of the emission system can also be obtained by measuring the signature of each of the sources of the emission system in the vicinity of the latter and combining them after correcting each one for oper
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Insitut Francais du Petrole
Lobo Ian J.
Tarcza Thomas H.
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