Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-11-11
2001-09-18
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06292754
ABSTRACT:
TECHNICAL FIELD
The present invention concerns 2D and 3D converted-wave seismic surveys. It also pertains to 2D and 3D wide-azimuth seismic surveys.
BACKGROUND
A seismic survey represents an attempt to map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is moved to various positions across the surface of the earth above a geological structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single straight line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both methods interrogate some volume of the earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. The term “unstacked” seismic traces is used by those skilled in the art to describe seismic traces as they are collected in field recordings. This term also is applied to seismic traces during the processing sequence up to the point where traces are “stacked” or averaged together after first being corrected for timing differences. General background information pertaining 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of SEISMIC DATA PROCESSING by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, the disclosure of which is incorporated herein by reference. Chapter 1, pages 9 to 89, of Yilmaz contains general information relating to conventional 2D processing and that disclosure is also incorporated herein by reference.
A modern seismic trace is a digital recording (analog recordings were used in the past) of the energy reflecting back from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Thus, each digital sample in a seismic trace is associated with a travel time (in the case of reflected energy, a two-way travel time from the surface to the reflector and back to the surface again). Further, the surface location of each trace in a seismic survey is carefully recorded and remains associated with that trace during subsequent processing. This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal “constant time slices” may be extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a 2D plane of seismic data. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2D seismic section from within the 3D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation subsequently applied to seismic data collected over other exploration targets.
Speaking in broad generalities, seismic energy propagates through the earth in one of two forms: compressional or “P” waves and shear or “S” waves, either of which might be generated by a wide variety of seismic sources. “Converted waves” travel first as one type of wave and then the other, the conversion between wave-types happening at any seismic discontinuity. If the conversion, from an incident P-wave to a reflected S-wave, happens at the reflector, this reflection mode will be called a C-wave.
Anisotropic media are those in which the P and S velocities depend upon the direction of wave propagation and of polarization. In anisotropic media, each conversion will in general reflect both fast and slow shear waves, which modes may be termed fast and slow C-modes. Flat-lying polar anisotropic (“VTI”) layers give rise to only one C-mode (polarized in-line) reflection. For general information pertaining to anisotropy in the context of geophysical exploration, see Thomsen, “Weak elastic anisotropy”,
Geophysics
, v. 51, no. 10, 1986, pp. 1954-1966, the disclosure of which is incorporated herein by reference.
Shear waves have vector displacements in the plane at approximately right angles to the raypath or direction of propagation and travel with a velocity dependent on the shear rigidity of the medium. Thus, shear waves contain different information about the subsurface structure along the raypath than do P-waves. Shear waves do not propagate through fluids, as fluids lack the stiffness necessary to support their passage. General background information on shear waves can be found, for example, in Helbig's article “Shear-Waves—What They Are and How They Can Be Used,” in SHEAR-WAVE EXPLORATION, 1986, Danbom and Domenico, eds., pp. 19-36, the disclosure of which is incorporated herein by reference.
A shear wave propagating through an anisotropic medium generally splits into two phases with the fixed polarizations and fixed velocities that propagate in that particular direction. The medium establishes two orthogonal directions of polarization (for each direction of shear wave propagation), with each polarization potentially having a different shear wave velocity (i.e., “fast” and “slow” polarizations). These special directions may be related to the “principle coordinate axes” of the medium. Of course, in general the orientation of the principle coordinate axes is unknown (before the collection and processing of the seismic data), and must be determined from the data. Note that the orientation of the source does not have any effect upon the polarization axes, except that the source o
BP Corporation North America Inc.
Gabala James A.
McElheny Jr. Donald E.
Watt Terry L.
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