Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1998-01-26
2000-04-11
Oda, Christine K.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 73, G01V 130
Patent
active
06049509&
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of processing seismic data.
Seismic data are collected using an array of seismic sources and seismic receivers. The data may be collected on land using, for example, explosive charges as sources and geophones as receivers; or the data may be collected at sea using, for example, airguns as the sources and hydrophones as the receivers.
After the raw seismic data have been acquired, the reflected signals (known as traces) received by each of the receivers as a result of actuation of a seismic energy source are processed to produce a sub-surface image. The processing includes the steps of transforming (or "migrating") the signals to their actual sub-surface location. The traces are then corrected to account for the separation, known as offset, between the source and the receiver.
FIG. 1 of the accompanying drawings schematically illustrates an idealised source and receiver arrangement arranged along a line. First, second and third sources 2, 4 and 6, respectively, co-operate with first, second and third receivers 8, 10 and 12, respectively. The sources and receivers are arranged about a common mid-point 15. For the sake of simplicity, the stratum or rock 20 beneath the sources and receivers will be assumed to be isotropic and to contain first and second horizontal partial reflectors 22 and 24, respectively. Seismic energy produced from the actuation of the first source 2 is reflected from the partial reflectors 22 and 24 and received by each of the receivers 8, 10 and 12. However, for the sake of simplicity only energy reflected from beneath the common midpoint 15 will be considered herein. Thus, in this example, we only consider energy received at the first receiver 8 as a result of actuation of the first source 2, energy received at the second receiver 10 as a result of actuation of the second source 4, and energy received at the third receiver 12 as a result of actuation of the third source 6. The "round trip" travel time of the energy from a source to a respective receiver increases with increasing distance between the source and the receiver. The round trip travel time is also a function of the depth of the reflectors 22 and 24. FIG. 2 of the accompanying drawings schematically illustrates the travel time for the situation shown in FIG. 1 as the offset increases. The offset axis of FIG. 2 is labelled 1, 2, and 3 to refer to the travel time between the first source and first receiver, the second source and second receiver, and the third source and third receiver, respectively. The round trip travel time with respect to offset for each of the reflectors defines a curve, in this simplified situation the curve can be accurately defined by: rock.
During the processing of the seismic survey data, the traces are assigned to their respective common midpoints such that the geology beneath the line of sources and receivers can be probed at a plurality of positions. A velocity analysis is then performed for each common midpoint, and indeed, for each reflector 22 and 24. This is achieved by specifying a range of hyperbolas, as defined in the above equation, related to a range of velocities and computing the mean reflection amplitude along all specified hyperbolas within that range. The seismic traces for a plurality of offsets are then converted, in accordance with the hyperbolas, to equivalent traces having zero offset and the traces are then summed. The mean amplitudes at zero offset are then examined to determine which hyperbola gives the best result.
Once an appropriate hyperbola has been selected, all the seismic data relating to the common midpoint for which the hyperbola has been determined are then corrected for normal moved and stacked so as to provide a stacked trace for that particular common midpoint. The stacked trace has an improved signal to noise ratio compared to the traces recorded at the receivers.
Although this technique is powerful and is used in the processing of seismic data, it is not without its problems. The strata or rocks beneath the receiver and s
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Sonneland Lars
Tennebo Per Ola
Yrke Oyvind
Geco A.S.
Jolly Anthony
Oda Christine K.
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