Automated seismic isochron analysis

Details Automated seismic isochron analysis Automated seismic isochron analysis

1999-11-04

2001-06-19

McElheny, Jr., Donald E. (Department: 2862)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

Reexamination Certificate

active

06249746

ABSTRACT:

This invention relates to seismic prospecting for oil and gas reservoirs, and more specifically it relates to processing seismic data in a high speed digital computer for automating a technique used for mapping isochron attributes of a subterranean layer.
BACKGROUND OF THE INVENTION
For many years seismic exploration for oil and gas has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected point on a terrain, or another energy source having capacity for delivering a series of impacts or mechanical vibrations to the earths surface. Offshore, air gun sources and hydrophone receivers are commonly used. The acoustic waves generated in the earth by these sources are transmitted back from strata boundaries and/or other discontinuities and reach the earth's surface at varying intervals of time, depending on the distance traversed and the characteristics of the subsurface traversed. On land these returning waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical analog signals, which are generally referred to as traces. In use on land an array of geophones is laid out along a grid covering an area of interest to form a group of spaced apart observation stations within a desired locality to enable construction of three dimensional (3D) views of reflector positions over wide areas. The source, which is offset a desired distance from the geophones, injects acoustic signals into the earth, and the detected signals at each geophone in the array are recorded for later processing using digital computers, where the analog data is generally quantized as digital sample points, e.g., one sample every two milliseconds, such that each sample point may be operated on individually. Accordingly, continuously recorded seismic field traces are reduced to vertical cross sections and/or horizontal map views which approximate subsurface structure. The geophone array is then moved along to a new position and the process is repeated to complete a seismic survey. A 3D seismic survey is data gathered at the surface and presented as a data volume representation of a portion of the subsurface.
After exploration of an area is completed, data relating to energy detected at a plurality of geophones will have been recorded, where the geophones are located at varying distances from the shotpoint. The data is then reorganized to collect traces from data transmitted at various shotpoints and recorded at various geophone locations, and the traces are grouped such that the reflections can be assumed to have been reflected from a particular point within the earth, e.g., a common midpoint. The individual records or “traces” are then corrected for the differing distance the seismic energy travels through the earth from the corresponding shotpoints, to the common midpoint, and upwardly to the various geophones. This step includes correction for the varying velocities through rock layers of different types and changes in the source and receiver depths. The correction for the varying spacing of shotpoint/geophone pairs is referred to as “normal move out.” After this is done the group of signals from the various midpoints are summed. Because the seismic signals are of a sinusoidal nature, the summation process serves to reduce noise in the seismic record, and thus increasing its signal-to-noise ratio. This process is referred to as the “stacking” of common midpoint data, and is well known to those skilled in the art. Accordingly, seismic field data undergoes the above-mentioned corrections, and may also undergo migration, which is an operation on uninterpreted data and involves rearranging of seismic information so that dipping horizons are plotted in their true location. Other more exotic known processing techniques may also be applied, which for example enhance display of faults and stratigraphic features or some attribute such as peak amplitude, instantaneous frequency or phase, polarity etc., before the continuously recorded traces are reduced to vertical or horizontal cross sections or horizontal map views.
In the course of seismic exploration, geoscientists often use isochron maps based on the vertical separation of seismic horizons. As used herein, an isochron is a line on a map which represents an equal time difference between two reflections events. Accordingly, an isochron indicates a time thickness between horizons which can be used to predict geological features such as sand thickness, basin outline, direction of sediment transport, etc. The vertical separation between horizons can range from several thousand feet to separation distances approaching zero feet at a pinchout. Construction of isochron maps, however, requires a preprocessing step for determination of upper and lower horizons of a target interval, which is a significant obstruction because the horizons are typically determined manually or interactively on a workstation. In either event a great cost in time and money is expended in determining the horizons.
Accordingly it is an object of this invention to reduce the manual effort expended in defining horizons in seismic data volumes.
A more specific object of this invention is to depict the azimuth of maximum thickness change of a subterranean layer.
Yet another object is to depict the amount of isochron thickening or thinning of a subterranean layer.
Another object of this invention is to provide isochron attribute data which can be displayed in many different ways.
Yet another object is to aid the geoscientists in analyzing geological features such as thickness of subterranean sand layers, which helps the geoscientists decide where to site oil drilling rigs.
SUMMARY OF THE INVENTION
According to the present invention, the foregoing and other objects and advantages are attained in a computer implemented method for automating a technical method used for mapping attributes of seismic isochron data, including the amount of isochron thickening or thinning, and the azimuth of maximum thickness change of a subterranean layer. The computer automated method comprises the following steps: defining a pair of planar surfaces corresponding to the upper and lower surface of a subterranean layer of interest within the 3D data volume in the computer memory; calculating the direction of maximum thickening between the planes, and writing this direction in a first 3D output data cube as an iso-azimuth value; calculating the magnitude of the isochron change per distance and writing this change per distance to a second 3D output data cube as an iso-change value; and displaying data recorded in the iso-azimuth and/or iso-change 3D data cubes.
In a preferred embodiment of this invention where horizons are automatically defined, a first step is determining if a planar surface can be associated with each digitized point in the data volume. This is done by examining each digitized point in the data volume to determine the probability that the data point resides on a planar subterranean surface. In this method, a trial plane is inserted in the data volume so as to include the point under test and to intersect a three-by-three array of nine traces with the digitized data point under test positioned at the center of the array. Then various azimuth and dip positions of the trial plane are examined using crosscorrelation techniques for comparing similarity among the nine traces that intersect the test plane, and a plane at a depth corresponding to the upper surface of the subterranean layer of interest is identified by selecting the position of the test plane having the highest overall similarity of traces in the three-by-three array. In a like manner a plane corresponding to the lower surface of the subterranean layer is identified.
In accordance with another aspect of this invention, the apparatus comprises a computer programmed to carry out the above

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