Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-09-23
2001-08-07
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06272435
ABSTRACT:
BACKGROUND OF TBE INVENTION
Migration to zero offset (MZO), alternatively known as the combined procedures of normal moveout and dip moveout (NMO/DMO), is a technique by which reflection data from a finite source and receiver offset is mapped to corresponding zero-offset traces. The MZO processes that are currently used are inadequate for a number of reasons. First, an inherent problem with current MZO processes is that they assume the source and the receiver are located in the same horizontal plane, called the acquisition plane. This assumption is almost invariably incorrect. Actually, the introduction of a rather small vertical source and receiver offset can introduce serious error in current MZO impulse responses. Furthermore, in ocean bottom seismic (OBS), or even land surveys with a deep borehole, there is a large vertical offset which is inherent to the survey. In these situations, assuming that the source and the receiver are in the same acquisition plane is simply not feasible.
Alternatively, to deal with a large vertical offset, current MZO processes shift the source and receiver to a common plane, called the datum plane. This is often called datumming or redatumming. Such conventional techniques result in a large amount of error and are extremely unreliable. Current techniques make this unreliable shift because they are without an acceptable and accurate alternative.
Thus, with the increasing importance of vertical cable and ocean bottom seismic (OBS) technologies, vertical offsets between the sources and receivers deployed in field acquisitions are significant, and must be formally accounted for in the MZO processes used in seismic processing.
There is a long felt need for a method of generating zero-offset traces from offset data, which does not shift the source and receiver to a common datum plane, and which does not presume a source-receiver pair in an acquisition plane. The present work fulfills this long felt need.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, a method of generating a zero-offset trace, at a zero-offset location on a processing plane, for a time sampled recording is provided. The time sampled recording has a source and a receiver, and a source coordinate and a receiver coordinate associated therewith. The source and receiver coordinates have a non-zero vertical offset associated therewith. The time sampled recording also has an amplitude and a sampled time associated therewith. The method comprises reading the source and receiver coordinates and computing a source-receiver offset from the source and receiver coordinates. The method further comprises determining an ellipsoid dependant upon the source-receiver offset, the sampled time, and the velocity of the medium. The ellipsoid has the source as one focus and the receiver as the other focus. The method further comprises determining a set of normal points on the ellipsoid, wherein a line from the zero-offset trace location to the ellipsoid is normal to the tangent of the ellipsoid at each of the normal points. The method further comprises determining the distance between each of the normal points and the zero-offset trace location and dividing twice the distance by the velocity of the medium, wherein the zero-offset travel time for each normal point is determined. The method further comprises assigning an amplitude to each zero-offset travel time, wherein a zero-offset trace is generated.
In an even further embodiment of the present invention, a method of generating a zero-offset trace, at a zero-offset location, for a time sampled recording is provided. The time sampled recording has a source location and receiver location associated therewith. The method comprises determining a reflecting surface and determining a set of normal points on the reflecting surface. A line from the zero-offset trace location to the reflecting surface is normal to the tangent of the reflecting surface. The method further comprises determining the distance between the zero-offset trace location and a normal point and dividing twice the distance by the velocity of the medium, wherein the zero- offset travel time for each normal point is determined. The method further comprises assigning an amplitude to each zero-offset travel time, wherein a zero-offset trace is generated.
In still a further embodiment of the present invention, a system of generating a zero-offset trace, at a zero-offset location on a processing plane, for a time sampled recording is provided. The time sampled recording has a source and receiver, and a source coordinate and a receiver coordinate associated therewith. The time sampled recording also has an amplitude and a sampled time associated therewith. The system comprises a means for reading the source and receiver coordinates. The source and receiver coordinates have a non-zero vertical offset associated therewith. The system further comprises a means for computing a source-receiver offset from the source and receiver coordinates and a means for determining an ellipsoid dependant upon the source-receiver offset, the sampled time, and the velocity of the medium. The ellipsoid has the source as one focus and the receiver as the other focus. The system further comprises a means for determining a set of normal points on the ellipsoid, wherein a line from the zero-offset trace location to the ellipsoid is normal to the tangent of the ellipsoid at each of the normal points. The system further comprises a means for determining the distance between each of the normal points and the zero-offset trace location and a means for dividing twice the distance by the velocity of the medium, wherein the zero-offset travel time for each normal point is determined The system further comprises a means for assigning an amplitude to each zero-offset travel time, wherein a zero-offset trace is generated.
In still a further embodiment, a system of generating a zero-offset trace, at a zero-offset location, for a time sampled recording is provided. The time sampled recording has a source location and receiver location associated therewith. The system comprises a means for determining a reflecting surface and a means for determining a set of normal points on the reflecting surface. A line from the zero-offset trace location to the reflecting surface is normal to the tangent of the reflecting surface. The system further comprises a means for determining the distance between the zero-offset trace location and a normal point, and a means for dividing twice the distance by the velocity of the medium, wherein the zero-offset travel time for each normal point is determined. The system further comprises a means for assigning an amplitude to each zero-offset travel time, wherein a zero-offset trace is generated.
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Arnold & Associates
McElheny Jr. Donald E.
PGS Tensor Inc.
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