Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Patent
1991-08-19
1993-07-06
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
181104, 367 25, G01V 140
Patent
active
052260178
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to a system for acquiring borehole seismic data in two opposite directions.
2. Description of Related Art
In a seismic data acquisition borehole or in other boreholes, pressure waves generated from a source may enable information to be acquired relating to the surrounding geological strata.
Attempts have been made to study the geological strata surrounding a borehole by studying the reflection and the refraction of pressure waves generated in the borehole, with the reflection and refraction phenomena acting on the geological strata surrounding the borehole.
These techniques may be of major interest when the borehole is substantially parallel to the surrounding geological strata. In that case, the borehole can be used for studying the above-mentioned geological strata in two opposite directions contained in a longitudinal plane of symmetry of the borehole.
Unfortunately, the above-mentioned study methods encounter the difficulty of discriminating echoes or reflected pressure waves in the two opposite directions when the apparent velocities of these echoes are identical or very close together, as shown in FIGS. 1a and 1b for the echo paths shown by arrows when the angle .alpha. is zero or very close to zero. The apparent velocities of the two echoes coming from opposite directions are defined as being inversely proportional to the propagation times between the source that emits the pressure wave and a receiver in the borehole for a corresponding reflection at a point outside the borehole, the receiver being offset at a distance from the source.
When the apparent velocities of echoes relating to two reflecting points situated on either side of the borehole axis are identical, as shown in FIG. 1a, then it is no longer possible to discriminate between the corresponding echoes as received on the basis of their apparent velocities.
SUMMARY OF THE INVENTION
The object of the present invention is to remedy the above-mentioned drawback by implementing a method and apparatus for acquiring borehole seismic data in two opposite directions, in which ambiguity due to identical apparent velocities is removed.
Another object of the present invention is to implement a method and apparatus making it possible, when the borehole is parallel or quasi-parallel to the surrounding geological strata, to determine the different horizons in two opposite directions that are symmetrical about the longitudinal axis of the borehole.
The method of acquiring borehole seismic data from two opposite directions in a borehole includes an improvement comprising: emitting a pressure wave in said borehole; and, at a point in said borehole that is offset relative to the pressure wave emission point, detecting echoes reflected and refracted by the adjacent geological formations in such a manner as to discriminate between pressure waves from opposite directions of the borehole that have been reflected and refracted by said geological formations in a longitudinal plane of symmetry of said borehole. The discrimination is performed on the basis of detecting the sum and a function of the difference of the echo signals coming from said opposite directions.
The apparatus for acquiring borehole seismic data from two opposite directions in a borehole that is parallel or quasi-parallel to the surrounding geological layers includes an improvement comprising: emission means for emitting an omnidirectional and/or bidirectional pressure wave; and bidirectional detection means for detecting echoes reflected and refracted by the adjacent geological formations, the detection means being disposed at a location offset from the emission means, and serving to discriminate between pressure waves from opposite directions of the borehole that have been reflected and refracted by said geological formations in a longitudinal plane of symmetry of said borehole. The discrimination is performed on the basis of detecting the sum and a function of the difference of the echo si
REFERENCES:
patent: 3976969 (1976-08-01), Meuser
patent: 4173748 (1979-11-01), Lewandowski
patent: 4207619 (1980-06-01), Klaveness
patent: 4794573 (1988-12-01), Bell et al.
patent: 4813029 (1989-03-01), Erich, Jr. et al.
patent: 4833658 (1989-05-01), Staron
patent: 5066916 (1991-11-01), Rau
Compagnie Generale de Geophysique
Lobo Ian J.
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