Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1993-06-30
1998-06-23
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 19, 367 50, 367 58, 181112, G01V 120, G01V 138
Patent
active
057712026
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for acquisition of seismic data from an area which is to be surveyed by using at least two seismic energy sources and at least two seismic cables, and and more particularly relates to the acquisition of seismic data from a marine area.
Seismic surveys are conducted according to the principle that a seismic signal is transmitted from a seismic energy source, the signal is reflected from geological strata and the reflection is recorded in 2 seismic streamer cable which is preferably placed at a distance from the transmitter. In the case of symmetrical reflection the reflection point will lie midway between the source and the cable and this point is called the CDP (Common Depth Point).
In the present case the terms seismic streamer cable (also named cable), seismic energy source (also named source) and seismic total array are meant to cover the following type of elements:
The seismic streamer cable is a cable comprising a defined plurality of hydrophones or geophones. These hydrophones or geophones are registrating pressure pulses which are transferred to a registration/receiver system. The length of the cable may vary from some hundred meters to 6000-7000 meters. The hydrophones or geophones are positioned in groups and each group consists of a row of hydrophones/geophones. An example in practicle use may be a cable with a length of 3000 meters with each group having a length of 12.5 meters which results in 240 groups.
The term seismic energy source as used in this application comprises all types of sources, arrays and guns. The term also covers an amount of single guns provided in a pattern in such a way that when they are fired they will act as one seismic energy source. A typically seismic energy source may consist of two subgun arrays, each subgun array consisting e.g. of 4-20 single guns positioned partly in a row and the two subgun arrays being positioned a certain distance from each other, both in the direction of length and width, or only in one of these directions.
By total seismic array is meant an array comprising the seismic equipment being towed behind the vessel(s).
In marine seismic surveys a survey is made of a particular area of the sea, seismic vessels towing a set of seismic energy sources and or a set of seismic streamer cables. The total towing equipment consisting of seismic energy sources and seismic streamer cables is called a total seismic array. In principle, from several seismic energy sources, pulses are transmitted which are reflected from the seabed, received by receivers of hydrophone groups in the streamer cable, transmitted to recording equipment designed for the purpose of storage or processing. The vessel surveys the area by sailing back and forward along lines until a complete picture has been obtained of the geophysical conditions in the area. The principle of a survey of this nature is illustrated in the attached FIG. 1. Surveys of land areas can be conducted according to the same principle, but the following description refers exclusively to marine surveys, by means of which the principle of the invention can most easily be demonstrated.
Previous attempts have been made to make marine seismic measurements more efficient. Thus in U.S. Pat. No. 4,868,793, a method is described where seismic recording elements or individual sources are used in each area (array)/point source as elements in adjacent sources, so that the geometrical point for the source can be varied sequentially. However, this is a relatively complicated method and does not provide a wide lateral coverage. In British Pat. No. UK 1,193,507 a vertical positioning of receivers is described, but the same problems arise here in the case of faulty equipment as in the recognized technique. In British Pat. No. UK 1,285,769 an acquisition technique is described where two or more boats are used. In this case, however, the operation is conducted with a separate boat for the seismic energy sources and a separate boat for cables and recording equipment. With
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Bale Richard
McMillan Ian
Geco A.S.
Moskowitz Nelson
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