Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1993-02-16
1996-07-02
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 73, 364421, G01V 136
Patent
active
055329772
DESCRIPTION:
BRIEF SUMMARY
In order to ascertain the geometrical structure of the sub-soil geological layers, it is usual, in petroleum exploration, to study the propagation in the sub-soil of acoustic waves emitted at the surface by emitters situated at points termed "shotpoints". The waves propagate in the sub-soil and are then reflected by the reflectors constituted by the various layer boundaries and are thus returned to the surface where they provoke ground vibrations as a function of time which are recorded on receivers situated at points termed "reception points". Each recording is associated with the position of a midpoint situated in the middle of the segment joining the shotpoint and the reception point which are associated with the recording. The acquisition device is such that several different recordings can be associated with the same midpoint and that the various midpoints, generally equidistant from one another, line up along a line termed the "seismic profile".
Conventionally, each of the recordings of the set of recordings associated with a given midpoint, termed "common midpoint gather" (CMP), is processed in order to correct on the one hand the effects of obliqueness of the path followed by the acoustic wave as a function of the distance between the shotpoint and the reception point, and on the other hand to refer the recording to a given time origin. The dynamic corrections intended to correct the effects of obliqueness of the acoustic paths depend on propagation velocities termed "stacking velocities" and theoretically make it possible to simulate the recording which would have been obtained if the shotpoint and the reception point had been situated at the midpoint. Stacking the recordings of each of the gathers after corrections makes it possible to obtain stack recordings termed stack traces, whose juxtaposition following the ascending or descending coordinates of the midpoints constitutes the "stack seismic section" which can be similar to a two-dimensional deformed image of the sub-soil--time along the vertical and a distance along the horizontal. After stacking, another processing termed "post-stacking migration" is generally applied, intended to replace the seismic events in their real position so that each trace represents the acoustic image in the vertical of the associated midpoint as a function of depth.
Faced with a conventional stack section, the geophysicist usually assumes a priori that the section actually represents a section with zero offsets, that is to say comprising recordings associated with a zero emission/reception distance (offset). However, this is strictly true only if the travel times associated with a given reflector are hyperbolic. In many cases, in particular in the presence of a complex structure or large lateral variations in velocities, this assumption is false and the post-stacking migration applied to such a section will give a false image of the sub-soil.
One of the aims of the present invention is to obtain a true section with zero offsets by virtue of which the geophysicist will be able to establish as exact as possible an image of the sub-soil.
Various means of improving the stack seismic section are known, according to the problem posed. In the case of sloped events which interfere, each being associated with a different stacking velocity, it is known for example that the application of a processing operation, commonly referred to as NMO (Normal Move Out), intended to correct the effects of non-hyperbolicity as a function of the offset, followed by a partial prestacking migration commonly referred to as DMO (Dip Move Out), intended to correct the effects of dip, and which are applied to the elementary recordings of each gather is efficacious. In the case of rapid lateral variations in velocities, the problem can be solved in part by virtue of the application of static corrections compensating for their effects (prestack layer replacement).
In both cases, the processing operations applied consist in revising the estimate made at the start of the stacking velocity. However,
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Elf Aquitaine Production
Moskowitz Nelson
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