Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1996-09-20
2000-06-13
Oda, Christine K
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 56, 367 58, 367 66, 367 37, 367 73, G01V 128
Patent
active
060757523
DESCRIPTION:
BRIEF SUMMARY
In geophysical prospecting, specialists attempt to define the geometry, particularly of stratigraphic traps and/or structural traps. In the former case, the problem is one of improving the accuracy of the measurements of the time and curvature of the seismic reflectors. In the latter case, the problem is mainly one of defining a geological interval velocity field and a time stack to obtain a depth image of the subsurface to be explored.
To solve these problems of time and velocity measurement, a plurality of acquisition can be employed in a seismic reflection survey. A seismic reflection survey is a common method for obtaining a seismic image of the subsurface. In this method, using appropriate energy sources, called sources, acoustic waves are transmitted, travel in the subsurface to be explored, and are reflected on the different reflectors which it contains. The reflected waves are recorded, as a function of time, on adapted receivers disposed on the ground surface or in the water. Each recording or trace provided by a receiver is then assigned to the location of the point which is situated at the middle of the segment connecting the source to the receiver. This operation is referred to as sorting into a common midpoint (CMP) gather.
A seismic prospecting technique, which is now conventional, is multiple coverage. In this technique, the sources and receivers are disposed on the ground surface in such a way that a given midpoint gathers several recordings. The series of recordings associated with the same midpoint forms what is generally called a common midpoint gather of recordings or traces. The set of. gathers is associated with a series of different midpoints preferably located along the same processing line. To obtain these gathers, it is essential to spread the sources and receivers on the surface of the medium, in a predetermined arrangement.
Two types of spread predominate today. In the first `2D acquisition` type, the sources and receivers are aligned on one and same line. Thus all the recordings are associated with wave paths situated in the same plane, at least in the theoretical case of a subsurface structured in plane and parallel layers. This type of acquisition is routinely used, particularly for large-scale acquisitions, and to survey subsurfaces featuring a calm tectonics. In the second `3D acquisition` type, the sources and receivers are more or less uniformly distributed on a surface, in such a way that the recordings associated with the same CMP, called `bin` in this case because the same CMP is in fact attributed to midpoint positions belonging to elementary surfaces or bins, correspond to paths which are not all situated in the same vertical plane. The latter type of acquisition is more particularly used today when higher resolution is desired (for example, reservoir engineering) or in the case of complex tectonics.
These two types of acquisition and the associated conventional seismic processing make it possible, from the CMP gathers, to obtain a depth seismic image in the vertical plane passing through all these common midpoints: subsurface, in plane and parallel layers, dipping or not, the reflections associated with each of the subsurface reflectors, observed on a common midpoint gather, are theoretically aligned along time/distance curves which are theoretically hyperbolas centered on the vertical to the midpoint, said time/distance curves generally being defined by two parameters: the stacking velocity Vs and the zero-offset ray propagation time to; in the case of dipping plane and parallel layers, a normal moveout (NMO) correction is applicable to the measurement of the stacking velocity, either after stacking by a migration and a dip measurement, or before stacking by DMO, making the Dix equation applicable, of a monoaxial cylindrical series.
It may be recalled that the Dix equation, which applies to reflections in a subsurface formed of horizontal plane and parallel layers, and for low offsets between associated sources and receivers, expresses the interval velocity Vn in a
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Elf Aquitaine Production
Jolly Anthony
Oda Christine K
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