Method of generating a fold distribution and of evaluating a sei

Details Method of generating a fold distribution and of evaluating a sei Method of generating a fold distribution and of evaluating a sei

1999-04-06

2000-10-10

McElheny, Jr., Donald E.

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

702 10, G01V 136

Patent

active

061310700

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a method of generating a fold distribution and to a method of evaluating a seismic survey. Such methods may be used to produce a Dip Moveout Dip Coverage Spectrum (DDCS) for evaluating the quality of seismic data obtained from a given survey geometry.
In seismic exploration, acoustic signals produced by a seismic source travel downwardly into the earth and are reflected back to a number of seismic receivers, such as geophones for use on land or hydrophones for marine seismic. The digitally recorded signals received by the receivers are normally referred to as traces and are processed in order to yield information about the nature of the earth below the area being investigated. For instance, these signals carry information indicating the structure of reflective layers such as boundaries between different types of rocks.
The procedure of converting recorded traces into a subsurface image is typically divided into several steps, each producing an intermediate result which may be useful. Ideally all the reflected signals are transformed (or "migrated") to their actual subsurface location, and are there combined, by summation, with all data corresponding to the same location. This procedure may in principle be performed in a single step, referred to as "prestack migration" by those skilled in the art. However, in order to facilitate parameter selection and reduce computational requirements, this procedure is usually subdivided into four steps.
Firstly, two corrections are made to eliminate the effects of source-receiver separation (or offset). The first is a velocity dependent correction known as normal moveout (NMO), which assumes reflections occur at horizontal interfaces. The other is a velocity independent correction known as dip moveout (DMO), which compensates for the mispositioning due to any inclination (or dip) of the reflecting interfaces. The theory of dip moveout is generally based on constant velocity and uniform receiver geometries, but it is sufficiently accurate for most cases where velocity varies. Applications of NMO and DMO produce traces which simulate the recording of a survey with the source and receiver at the same location (zero offset traces) and permit the summation (or stacking) of traces with the same or similar locations to produce the "stack". As well as reducing the number of traces for subsequent processing, this step improves the signal-to-noise ratio of the data. Finally, reflectors are moved to their correct positions by a zero offset migration of the stack.
In the case of 3D seismic data, in which the survey has been conducted with the sources and receivers arranged to cover an area of the surface and so obtain data from a 3-dimensional portion of the earth, the traces are collected into geometric cells (or bins) which make up a regular grid, either at the surface or some reference plane defined for processing purposes. The stack is partly obtained by summing traces which fall within the same cell, to generate a single trace for each grid location. Due to the DMO correction, traces which fall within neighbouring cells also contribute to the stack if the corresponding line from the source to the receiver group hits the output cell.
It is sometimes difficult to obtain a uniform distribution of sources and receivers due to obstructions, such as buildings or roads or, in the case of seismic marine exploration, due to cable drift or drilling and production platforms.
U.S. Pat. No. 5,450,370 discloses a process for assessing a proposed geometry of sources and receivers in order to avoid data shadow zones and over- or under-sampled data zones, which can be caused by a non-uniform receiver distribution, and to optimise the resulting image. The process comprises, for a given dipping reflector, the steps of generating for a fold distribution a range of source-receiver azimuths and a range of angles of dip and analysing the results to detect undesired shadow zones. In the event that such a zone is found, the receiver distribution is reconfigured and the

REFERENCES:
patent: 3746122 (1973-07-01), Davis
patent: 4933912 (1990-06-01), Gallagher
patent: 5450370 (1995-09-01), Beasley
Slawson, "DMO implications for 3-D Survey Design", SEG 1995, pp. 935-936.
Beasley, "Quality Assurance of Spatial Sampling for DMO", SEG 1993, pp. 544-547.

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