Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1997-08-04
1999-10-26
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 21, 367 56, 367 58, G01V 116, G01V 138
Patent
active
059739959
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of and apparatus for marine seismic surveying.
There are two distinct surveying operations that are generally performed. The first is a relatively deep exploration of the geology of an area. This is referred to hereinafter as a "deep" survey. Current techniques allow this survey to be performed as a 3D survey. Thus a single pass of a survey vessel or vessels working together surveys a corridor of the sea floor rather than a line as in 2D surveying. FIG. 1 of the accompanying drawings is a schematic illustration of the streamer arrangement of a conventional 3D survey. A seismic survey vessel 2 tows a plurality of "long" streamers 4 and seismic sources 6. The streamers, each of which is typically at least 2 Km long, carry hydrophones along their length. The hydrophones are arranged into groups and the length of each group is known as a "group interval". The sources 6 typically comprise two or three sub arrays, each comprising six to ten airguns. The sources typically produce a peak pressure of 30-100 bar at one metre with a 8 to 70 Hz frequency range. Each source is fired in sequence and a shot is taken every 18 to 25 metres of boat travel.
After the raw seismic data have been acquired, the reflected signals (known as traces) received by each group of hydrophones from each actuation of a seismic energy source are processed to produce a subsurface image. The processing includes the steps of transforming (or "migrating") the signals to their actual sub-surface location. The traces may be corrected to account for the separation (also known as offset) between the source and the hydrophone or hydrophones. A first correction accounts for the fact that the velocity of sound within the earth tends to increase with depth as the earth layers become more compacted. The correction is derived empirically from the data itself and is known as normal moveout correction. To successfully make this correction, data from a large range of offsets are required. A further correction is made to account for the inclination (or dip) of the reflecting surfaces or interfaces within the earth. The area being surveyed may be notionally divided into an array of cells (or bins). All the traces which have been assigned to a bin are then summed (stacked) to obtain a single trace for each bin. The stacked trace has an improved signal to noise ratio compared to the individual traces as the signal tends to add constructively whereas the noise is generally incoherent and does not add constructively. A more detailed description of the data processing of traces can be found in GB 2347751.
The arrangement of sources and hydrophones defines the maximum resolution available. The bin size can be defined arbitrarily but in practice is normally a multiple of the smallest definable feature. On this basis, the smallest bin size is: each streamer divided by the number of energy sources used.
Thus a four streamer arrangement cooperating with three sources having a streamer separation of 150 m and a group interval of 12.5 m gives a bin size of 6.25 m "in line" with the travel of the survey vessel and 25 m cross-line (transverse).
The in-line and cross-line resolutions are different because it is relatively easy and inexpensive to divide the streamer into many short groups, but it is expensive and difficult to deploy more streamers.
The second type of marine seismic survey is a relatively shallow exploration of the geology of an area. This is referred to hereinafter as a "shallow" survey. A shallow survey may comprise a high resolution survey, an example of which is commonly known as a site survey (all such high resolution surveys are hereinafter collectively referred to as site surveys). Here it is required to derive a lot of information about a relatively thin portion of the earth adjacent and including the sea floor. The site survey is used to assess the risk to equipment and personnel that may be involved in drilling into a given region of the earth. Hazards include pockets of gas and an unstable sea floor.
Conven
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Lindtjoern Olav
Walker Robin Charles
Batzer William B.
Geco A.S.
Moskowitz Nelson
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