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Proceedings of the IEEE

Issue 10 • Date Oct. 1984

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Displaying Results 1 - 25 of 26
  • [Front cover and table of contents]

    Page(s): c1
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  • Scanning the issue

    Page(s): 1235 - 1237
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  • The common depth point stack

    Page(s): 1238 - 1254
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    The Common Depth Point (CDP) Method is a seismic data acquisition and processing technique which transforms field recordings from seismic surveys into pseudo-cross-sectional images of the earth's geologic layering beneath the survey line. Such images are the primary geophysical analysis tools used by explorationists to pinpoint likely drilling locations for oil and gas reservoirs. The CDP Method has evolved from an elementary data enhancement concept in the analog seismic era of the 1950s to a highly sophisticated digital imaging and parameter estimation technique in the 1980s. This paper reviews the history of the method, and its underlying mathematical-physical basis in terms of simple computer simulations of reflection seismic experiments. The method is illustrated with actual examples from several geologic provinces, and the paper closes with a discussion of some limitations of the CDP Stack and possible future directions in seismic imagery. View full abstract»

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  • Seismic interpretation

    Page(s): 1255 - 1265
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    In seismic exploration, an acoustic energy source radiates elastic waves into the earth from the surface; receivers on the surface detect acoustic energy reflected from geological interfaces within the earth. The recorded data are processed in ways which ease interpretation. Seismic interpretation is the interface between the exact mathematics of seismic data processing and inexact geological reasoning. Oil and gas occur in sedimentary rocks, where source rocks are present and a porous and permeable reservoir is sealed by an impermeable cap rock to form a trap. The principal use of seismic exploration is to find potential oil and gas traps primarily by mapping geological structure. Integration of exploration well data with seismic data can reduce the ambiguity of the interpretation. Because seismic measurements are affected only by the elastic properties of the earth, geological interpretation beyond structural mapping is difficult. Furthermore, resolution is limited by the bandwidth of the recorded data, which in turn is limited by frequency-dependent attenuation of elastic waves by the earth. However, in some circumstances we can detect gas occurrences directly. Recent developments include the use of seismic data in unraveling the geological history of an area; the use of amplitudes and other attributes of seismic recording; the use of transverse waves; and three-dimensional seismic exploration. In the future, interpretation will become more automated and more integrated with processing. View full abstract»

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  • Seismic data gathering

    Page(s): 1266 - 1275
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    We present a brief review of those aspects of seismic data gathering which have a bearing on seismic data processing and interpretation. A summary of sources, detectors, and other instruments is given. Noises that interfere with seismic reflection data are described. The most troublesome noises are source-generated coherent noises and much of the data gathering effort is directed towards attenuating them. The main methods of attenuating coherent noise involve the geometry of the source and receiver layout and the use of source and receiver arrays. Incoherent noise can be attenuated by increasing the multiplicity of the data. Proper choice of field parameters requires a knowledge of the exploration objective, noise characteristics, and data processing requirements. Field parameter design is discussed on that basis. View full abstract»

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  • Statistical pulse compression

    Page(s): 1276 - 1289
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    The seismic method in petroleum exploration is an echo-location technique to detect interfaces between the subsurface sedimentary layers of the earth. The received seismic reflection record (field trace), in general, may be modeled as a linear time-varying (LTV) system. However, in order to make the problem tractable, we do not deal with the entire field trace as a single unit, but instead subdivide it into time gates. For any time gate on the trace, there is a corresponding vertical section of rock layers within the earth, such that the primary (direct) reflections from these layers all arrive within the gate. Each interface between layers is characterized by a local (or Fresnel) reflection coefficient, which physically must be less than unity in magnitude. Under the hypothesis that the vertical earth section has small reflection coefficients, then within the corresponding time gate the LTV model of the seismic field trace reduces to a linear time-invariant (LTI) system. This LTI system, known as the convolutional model of the seismic trace, says that the field trace is the convolution of a seismic wavelet with the reflection coefficient series. If, in addition, the reflection coefficient series is white, then all the spectral shape of the trace within the gate can be attributed to the seismic wavelet. Thus the inverse wavelet can be computed as the prediction error operator (for unit prediction distance) by the method of least squares. The convolution of this inverse wavelet with the field trace yields the desired reflection coefficients. This statistical pulse compression method, known as predictive deconvolution with unit prediction distance, is also called spike deconvolution. Alternatively, predictive deconvolution with greater prediction distance can be used, and it is known as gapped deconvolution. Other pulse compression methods used in seismic processing are signature deconvolution, wavelet processing, and minimum entropy deconvolution. View full abstract»

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  • Vibrator signals

    Page(s): 1290 - 1301
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    The seismic reflection method accounts for over 90 percent of the geophysical expenditures for oil and gas exploration. Controlled sources of seismic energy emit signals into the earth which are reflected to the earth's surface by density-velocity contrasts associated with variations in subsurface rock lithology, stratigraphy, and structure. The reflected signals detected on the earth's surface are digitized and computer processed to produce seismograms used for geological interpretation to evaluate the potential for oil or gas reservoirs. Original reflection seismology methods developed nearly sixty years ago utilized explosives for seismic sources. However, the use of earth vibratory sources has increased in recent years. The compressional wave vibrator is discussed giving a description of the electrical-hydraulic method of controlling the applied base plate force. The vibrator efficiency for transduction of hydraulic energy into seismic energy is analyzed in terms of vibrator and earth parameters. Source signal design is achieved through selection of several operational parameters specifying the temporal and spectral properties of the vibrator sweep control. Limitations on vibrator system resolution by the signal detection and registration system are considered. Methods of signal enhancement are applied in the field during data acquisition and in computer data processing. Interpretation of vibrator seismograms is optimized by analyzing the parameters influencing system resolution, understanding their interaction, and then specifying the acquisition and processing parameters which will provide the necessary data quality. View full abstract»

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  • Migration of seismic data

    Page(s): 1302 - 1315
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    Reflection seismology seeks to determine the structure of the earth from seismic records obtained at the surface. The processing of these data by digital computers is aimed at rendering them more comprehensible geologically. Seismic migration is one of these processes. Its purpose is to "migrate" the recorded events to their correct spatial positions by backward projection or depropagation based on wave theoretical considerations. During the last 15 years several methods have appeared on the scene. The purpose of this paper is to provide an overview of the major advances in this field. Migration methods examined here fall in three major categories: 1) integral solutions, 2) depth extrapolation methods, and 3) time extrapolation methods. Within these categories, the pertinent equations and numerical techniques are discussed in some detail. The topic of migration before stacking is treated separately with an outline of two different approaches to this important problem. View full abstract»

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  • Refraction statics

    Page(s): 1316 - 1329
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    The determination of statics corrections is one of the fundamental problems in seismic data processing. Statics errors lead to loss of resolution and less than optimum interpretation of the seismic data set. The arrival times of refraction events form a data set sufficient to unravel the structure of the near-surface layers of the earth leading to accurate static corrections and enhanced interpretation. This paper introduces modeling methods using refraction arrivals and shows examples of the application to seismic data processing. View full abstract»

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  • Seismic velocity estimation

    Page(s): 1330 - 1339
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    The estimate of propagation velocities in the reflection or refraction seismic method is essential to the effective imaging of the subsurface. Wave propagation in a fully elastic medium gives rise to several propagation modes, among them are the longitudinal and transverse compressional waves (P-waves and S-waves), which are commonly used in the reflection and refraction seismic methods. With appropriately sorted data, the arrival time of a wave returning from a particular reflecting interface in the subsurface will vary as a function of source-receiver offset. This variation in arrival time with offset is called "moveout" and is controlled by the propagation velocity. The velocity of propagation enters the processing of reflection seismic data first as an essential parameter of a time coordinate transformation required before data of varying source-receiver offsets can be stacked to enhance signal-to-noise ratio. Velocities estimated in this manner are called "moveout velocities." Velocities also enter the processing sequence as a parameter of an imaging operation called "migration." Early efforts at velocity estimation were only accurate enough to provide parameters to process data, but high-quality data collected using present-day technology allow us to make accurate enough estimates of propagation velocity to infer subsurface geology. Complications arise, however, due to the effects of reflector structure and lateral velocity gradients. Current developments in seismic velocity estimation include measurement of shear wave (S-wave) velocities, use of wide-angle arrivals for more accurate P-wave velocity estimates, and methods requiring areal coverage (three-dimensional seismic). View full abstract»

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  • Signal and noise estimation from seismic reflection data using spectral coherence methods

    Page(s): 1340 - 1356
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    Spectral coherence analysis is unrivaled as a quantitative tool over a range of practical problems in seismic interpretation, data processing, quality assessment for data acquisition, and research. Its great virtue is its ability to supply the detailed error information necessary for a thorough interpretation of results. Ordinary coherence analysis is employed in line intersection analysis and the design of filters to cross-equalize differently acquired seismic sections in a given area; both ordinary and partial coherence methods are indispensable in matching synthetic seismograms and seismic data; and multiple coherences are used to estimate the coherent signal and incoherent noise content of seismic sections and gathers. The precise meaning of the signal and noise estimates output by coherent analysis has to be related to the particular technique employed and the type of data input to it. The principles and procedures for analyzing seismic data with these methods are reviewed and illustrated with practical examples. View full abstract»

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  • Multidimensional filtering of seismic data

    Page(s): 1357 - 1369
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    Multidimensional filtering has been applied during recording and processing of seismic reflection data since the earliest days of analog recording on paper records. As the state of the art has evolved to digital recording and processing, and acquisition has expanded to include dense spatial sampling over a large number of channels, more sophisticated multichannel filters have been developed. These include simple "mixes" (spatial convolution with small operators), two-dimensional Fourier transforms with appropriate limits in spatial and temporal frequencies, and more geometrically, as well as geophysically, meaningful Radon transform techniques. All multidimensional filtering limits the data in some fashion, be it temporal frequency bandwidth, spatial frequency bandwidth, limits in apparent horizontal phase velocity across a recording array (antenna), or limits in apparent wave-propagation velocity. These limits generally are defined to pass regions of high signal level and reject regions of high noise levels. As more recent techniques have emerged, such as Tau-p transforms (special cases of the Radon transform), filter limits may be described in terms of geophysical knowledge as well as signal characteristics. Thus additional information, derived from regional geophysical knowledge, may be added to the data processing sequence. Many new considerations and potential problems have arisen as new multidimensional filtering techniques have been developed, including spatial sampling, aliasing with different transforms, maintenance of dynamic range, and effects of multidimensional filtering at different points in the processing sequence. View full abstract»

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  • Multiple reflections as an additive noise limitation in seismic reflection work

    Page(s): 1370 - 1384
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    A principal problem in seismic exploration is the heterogeneous partially reflecting medium that makes up the transmission zone to the target reflectors of interest. Multiple reflection between the free surface and the shallower reflectors produces additive noise that interferes with our view of the bedding plane geometry and rock properties in the target zone. Residual multiple energy on the interpreter's record section display can easily be mistaken for signal. Because the magnitude of multiples is dependent on reflectivity products, and because subsurface reflectivities are normally small, the importance of multiples is highly variable. The complementary combination of predictive deconvolution and common depth point stack is routinely used for the reduction of multiple reflections. Predictive deconvolution achieves a temporal prediction and subtraction of that class of short-period multiples that is most predictable, and common depth point stack reduces that class of multiples distinguishable from primaries on the basis of local horizontal phase velocity in the x, t observational space. Each of these methods can be justified in terms of a simplified one-dimensional model of the earth. However, a major reason for their success is that they are employed with an empirical, statistical approach. As a result, these two processes are particularly robust and forgiving and respond imperfectly, but often constructively, to real-world deviations from the simplified models. Both processes are capable of distorting or destroying the desired signal, and their noise-reduction effectiveness is dependent on the manner of application. Quality control methods are imperfect, and there is an element of human artistry in seismic data processing. Predictive deconvolution employs a one-dimensional (t) filter designed on a purely statistical basis. Seismic data represent observations in (x1, x2, t), a four-dimensional space, and multiple reflections are deterministically predictable (in principle) from the shallower primary reflection data. Common depth point stack operates in (x1, t) as an array steered for the arrival of the roughly spherical target reflection wavefront. Multidimensional filter design aimed at coherent noise rejection, based on- the nonrandom noise structure in (x1, t) is often used in problem areas. More effective reduction of multiple reflections is one of many potentially useful improvements in the seismic method. View full abstract»

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  • Seismic modeling and inversion

    Page(s): 1385 - 1393
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    The seismic method of petroleum exploration has always been an inverse technique. The explorationist has used data from seismic and other sources to derive an indirect description of a potential reservoir. Key to his success has been the use of forward-modeling methods to aid him in relating what he sees in the seismic data to what he expects to see, based on his hypothesized geologic model These forward-modeling methods have developed from simple ray-tracing procedures that determine travel time, to more sophisticated methods that model the total wavefield. These methods include extensions of ray techniques, along with propagator and finite-difference methods. The formation of subsurface images from the seismic data has traditionally involved the use of inversion principles. The operations of deconvolution and migration are inverse methods that aid in the production of enhanced images of the subsurface. In recent years, the development of more direct methods of inversion has been initiated, based on scattering theory. These methods seek to estimate the acoustic or elastic parameters of the earth for ever more general model assumptions. Good progress has already been made using the assumption that the processed seismic data emulate the response of a horizontally stratified earth to a plane-wave excitation. Research is continuing to generalize the assumptions to include point-source excitation and a general scattering model of the subsurface. View full abstract»

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  • Wavelet estimation

    Page(s): 1394 - 1402
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    An important problem in seismic exploration is the estimation of and correction for the seismic wavelet. A seismic signal may be modeled as a convolutional model with the wavelet as one component. The wavelet propagated by the seismic energy source is complicated by transmission and recording filters. Some filters in the system can be deterministically defined while others are more conjectural. The estimation of the wavelet is useful in two major ways. Borehole measurements are used to model the surface seismograms. The wavelet used in the model needs to match that of the seismogram to correlate the two measurements. Conversely, the estimated wavelet can be used to design inverse filters which make the seismogram approach the borehole measures. Some well-known methods for estimation of the wavelet are based on assumptions about the wavelet or the earth reflectivity. Examples of the methods indicate success on some data even though each makes different assumptions. The methods serve to point out basic problems in reliably estimating the wavelet from the seismogram. Basic problems include noise, band-limiting, nonstationarity, uncertain theoretical models, assumption failure, and widely diverse geological sequences of the earth. Quality control or evaluation of the performance of an estimation algorithm is a nontrivial problem. The estimation of the wavelet from a seismic recording remains an area of challenging research and importance in exploration for hydrocarbons. View full abstract»

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  • Phase estimation using the bispectrum

    Page(s): 1403 - 1411
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    The information which is of importance in the reflection seismic method resides in the reflectivity series. In order to extract this information about the subsurface, the blurring effect of the seismic wavelet must first be removed. Since this signature is generally unknown, various wavelet estimation schemes have been developed. The one most currently used in the seismic industry is based on the assumption that the seismic wavelet has the minimum-phase property. This restrictive assumption is often incorrect. The purpose of this paper is to explore the use of the bispectrum in order to obviate the minimum-phase requirement. Specifically, using synthetic examples, we develop and compare three different algorithms which determine the wavelet phase from the bispectrum of the reflection seismogram. An important aspect of the problem not treated before is the application of the bispectral technique to band-limited data. View full abstract»

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  • Resonant characteristics and end effects of a slot resonator in unilateral fin line

    Page(s): 1416 - 1418
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    The resonant characteristics and end effects of a rectangular-slot resonator in unilateral fin line are presented for centered as well as off-centered fin locations in the guide. Sample theoretical results on the resonant frequency are verified through experiment at the Ka-band. View full abstract»

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  • Surface-acoustic-wave (SAW) voltage sensor with improved sensitivity

    Page(s): 1418 - 1419
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    The sensitivity of a surface-acoustic-wave (SAW) voltage sensor can be increased by reducing the thickness of the substrate across which the voltage is applied. Voltage sensitivity of 95 Hz/V has been obtained in a 77-MHz SAW oscillator fabricated on a 71-µm-thick, 728° Y-X LiNbO3substrate. View full abstract»

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  • Number of simultaneous colors versus gray levels: A quantitative relationship

    Page(s): 1419 - 1421
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    Growing interest in color graphics and image processing stresses the need for a quantitative basis for requirements. A result obtained via fuzzy entropy measure for gray-tone images is extended to color. A heuristically derived relationship between the number of simultaneous colors versus number of gray levels for comparable information is presented. View full abstract»

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  • The DP professional's guide to writing effective technical communication

    Page(s): 1422 - 1423
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  • How to write computer manuals for users

    Page(s): 1422 - 1423
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  • Documentation development methodology

    Page(s): 1422 - 1423
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  • Writing handbook for computer professionals

    Page(s): 1422 - 1423
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  • Feedback

    Page(s): 1423
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  • Book alert

    Page(s): 1423
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