Common Midpoint Stacking Research Articles

Sub-salt imaging in the Gulf of Suez

Abstract Sub-salt imaging on seismic data is a serious geophysical problem affecting exploration in the Gulf of Suez. Here, thick Miocene evaporite, shale and salt sequences with salt tectonics overlie rift structures involving pre-Miocene and Miocene oil-producing formations. This paper discusses the seismic imaging problems associated with salt swells in the Gulf of Suez and presents experimental work by GUPCO that has provided a better understanding and possible solution to the imaging problem. A realistic model for the Gulf was used to generate synthetic shot records using finite difference modelling. Processing of the simulated data in conjunction with actual field data led to improved understanding of the wave propagation behaviour in the subsurface, and to a realization of the validity of the common midpoint stacking technique and post-stack depth migration.

Reverse time migration of CMP-gathers an effective tool for the determination of interval velocities

One of the most important steps in the conventional processing of reflection seismic data is common midpoint (CMP) stacking. However, this step has considerable deficiencies. For instance the reflection or diffraction time curves used for normal moveout corrections must be hyperbolae. Furthermore, undesirable frequency changes by stretching are produced on account of the dependence of the normal moveout corrections on reflection times. Still other drawbacks of conventional CMP stacking could be listed.

EFFECTIVENESS OF WIDE MARINE SEISMIC SOURCE ARRAYS1

AbstractIn recent years, the use of wide source arrays in marine seismic surveys has been a topic of interest in the seismic industry. Although one motivation for wide arrays is to get more guns in a source array without increasing the in‐line array dimension, wide arrays can also provide the benefit of suppressing side‐scattered energy. Comparisons of common midpoint (CMP) stacks of data acquired offshore Washington and Alaska with wide and conventional‐width source arrays, however, show only small and sometimes inconsistent differences. These data were acquired in areas where side‐scattered energy is a problem. Comparisons of pre‐stack data, however, show substantial differences between the wide and conventional source array data.The disparity between the stacked and prestack data is explained by analysing the effective suppression of back‐scattered energy by CMP stacking. Energy reflected from scatterer positions broadside to a given CMP location has a lower stacking velocity than that of the primary reflection events. Thus, CMP stacking attenuates the side‐scattered energy. In both survey areas the action of CMP stacking was so powerful in suppressing the broadside energy that the additional action of the wide array was inconsequential in the final stacked sections. In other areas, where the scattering velocity is comparable to the primary stacking velocity, wide arrays could provide considerable advantage.Even though CMP stacked data from wide and conventional‐width arrays may appear similar, the reduced amount of side‐scattered energy in wide‐array prestack data may provide a benefit for data dependent processes such as predictive deconvolution and velocity analysis. However, wide arrays cannot be used indiscriminately because they can degrade cross‐dipping primary events. They should be considered primarily as a special tool for attacking severe source‐generated noise from back‐scattered waves in areas where the action of CMP stacking is insufficient.

Some Properties of Backscattered Seismic Returns

Seismic amplitudes occurring in typical marine records indicate that the source-generated wavefield commonly exceeds ambient noise for much, if not all, of the normal recording time. When this behaviour is observed in areas of poor specular reflections and with small source-receiver separation, a reasonable inference is that the wavefield is sustained by backscattered source energy. There is abundant evidence that sea-floor irregularities play a particularly important role in this. Even in good reflection areas, sea-floor scattering can influence the seismic section in subtle, often unsuspected ways. Out-of-plane scatterers generate artifacts in response to normal 2D migration. Velocity stratification allows multipath propagation of scattered energy, so that a single scatterer may evoke several responses at different times characterised by different velocities. Whereas common midpoint (CMP) stack selectively attenuates scattered noise propagating within some range of azimuth angles determined by the primary velocity function, an unfortunate side effect is to enhance backscattered noise over the complimentary range of angles. Moreover, the source-receiver offset distances usually employed for stacking must increase susceptibility to interference from converted modes.

Removal of Reflection Point Smearing and the Dip-Dependence of the Stacking Velocities

The philosophy behind dip-moveout (Hale, 1984) is a slightly different approach to the problem of dip filtering, previously attacked by a number of methods such as a prestack process known generally as DEVILISH, which stands for dipping event velocity inequalities licked (Judson et al., 1978) and prestack partial migration (Yilmaz and Claerbout, 1980). The importance of the problem of dip filtering applied to common mid-point gathers is stated in great detail by the authors mentioned above. On the one hand, the conventional normal-moveout and common-midpoint stacking process reinforces reflections having a particular slope in a given common mid-point gather; on the other hand it attenuates reflections having different slopes. Therefore this process behaves as a dip filter on a common mid-point gather and decreases lateral resolution of events on a seismic section. A synthetic model and real field data have been employed to examine the algorithm of dip-moveout. These examples have shown that the algorithm may be included in the sequence of seismic data processing whenever it is needed. However, in terms of computer time and economy dip-moveout is not a cheap process. In order to overcome this difficulty, in this study some computational expense has been reduced by neglecting evanescent waves and only working in the seismic frequency spectrum.

Seismic Lithologic Modelling of Amplitude-Versus-Offset Data

For small angles of incidence, q, the reflection amplitude of plane p-waves from a planar interface between two elastic media is nearly linear with sin2q. For an NMO-corrected common-midpoint (CMP) gather, the linear fit of amplitude versus sin2q at each time sample yields two new kinds of seismic trace. The trace constructed from the zero-offset intercept, the so-called 'p-wave stack', represents the response to changes in acoustic impedance at reflecting boundaries. The trace constructed from the slope, the 'gradient stack', represents the response to changes in s-wave velocity as well as p-wave velocity and bulk density. Under certain assumptions, the difference between the p-wave trace and the gradient trace reflects changes in s-wave velocity alone, and the sum reflects changes solely in Poisson's ratio. Here, we demonstrate how the method of seismic lithologic modelling, a parameter-estimation technique that refines thin-layer models of p-wave velocity and density by iterative matching of synthetic data to CMP stack sections, can be extended to include s-wave velocity in the model. In this extended approach, a new form of synthetic data trace is matched to the gradient stack (or to either the 's-wave stack' or 'Poisson's ratio stack') as well. As with any other approach that attempts to obtain s-wave velocity structure from the offset-dependence of amplitude, success in application to field data depends heavily on the extent to which data can be conditioned so that they satisfy the assumptions upon which amplitude-versus-offset analysis is based.

The relationship of pre-stack apparent velocity filtering to the symmetry of the CMP stack response

Wearing my processing geophysicist's hat, I, like others, often ask questions to which there appear to be no answers readily available. One such question that had re-occurred to me was 'what is the relationship between pre-stack apparent velocity filtering and the common midpoint (CMP) stack response?' The elements of any answer were roughly -potential reduction in spatial resolution; -reduction in coherent noise; -apparent velocity in common source or geophone space is twice that observed on the CMP stack. During the summer of 1983 Walt Lynn presented some material at Western Geophysical's London office (as part of a week long 'Wave Equation Update School' lecture course given by Jon F. C1aerbout) entitled 'Dip filtering for noise suppression'. Part of this graphical material demonstrated how apparent velocity filtering could be viewed in source-geophone space as removing ciearly definable parts of a pre-stack diffraction. Upon this basis the following text is an attempt to answer my question by doing three things: • Setting down the necessary equations. • Showing how certain symmetry conditions affect the CMP stack. • Computing synthetic examples. The text will consider a point scatterer in the subsurface, how it responds to the CMP method and what happens when pre-stack apparent velocity filtering is included. The most important conclusion will be that certain apparent velocity filter symmetry conditions are necessary if the CMP stack is to respond similarly to positive and negative dips. The analysis will commence by describing the prestack traveltime surface that is associated with the point scatterer, which is sometimes called Cheop's Pyramid. Equations for apparent velocity on the commonly used axes of Cheop's Pyramid will be derived which show how certain areas of the surface are rejected by apparent velocity discrimination. The idea of symmetry will be described and a set of rules will be presented in order to test whether the pre-stack apparent velocity filter will prejudice dip direction on the CMP stack. Some computed examples will be presented along with a brief description of the computational method used. Finally a potential problem with 'marine' three-dimensional (3D) surveys that can be caused by pre-stack apparent velocity filtering will be briefly discussed.

A comparison of reflection and refraction processing and interpretation methods applied to conventional refraction data from coastal Maine

Abstract In 1984 the U.S. Geological Society collected 850 km of refraction data in lines parallel and perpendicular to regional strike of the Appalachian orogen in Maine and southeastern Quebec. A 100-km-long strike-line in coastal Maine, west of Penobscot Bay, is here interpreted on the basis of both refracted and reflected phases. Techniques used in seismic reflection processing, including normal movement velocity corrections and common mid-point stacking, were applied to the refraction data in order to enhance the visibility of reflected phases. The geometries of the most laterally continuous reflections as defined by these techniques can be correlated with reflections seen on a crossing, near-vertical reflection profile. Conventional ray-tracing analysis of refracted and wide-angle reflected phases in the refraction data constrains the velocity structure to about 14 km depth, and provides evidence for high-veiocity material (>6.5 km/sec) between 4 and 7 km depth, and for lower velocities (

Seismic noise‐reduction techniques for use with vertical stacking: An empirical comparison

A systematic comparison of a wide range of noise‐reduction techniques applied to a single data set collected for this purpose was attempted. The study includes a comparison of the relative benefits of noise‐reduction schemes for very different noise levels. The comparisons use computer simulations of field techniques for noise reduction as applied to deep crustal, 48-fold VIBROSEIS® data. The noise‐reduction techniques involve amplitude scaling (diversity stacking and automatic gain control), noise editing (the use of fixed‐gain and self‐updating noise‐rejection systems with and without buffered memory for recursive editing), and reduced numbers of recording bits (mantissa‐only and sign‐bit‐only formats). The effectiveness of each noise‐reduction procedure is assessed by a study of its effects on noise levels seen on source‐point gathers and on both true‐amplitude and amplitude‐balanced common‐midpoint (CMP) stacks. For low levels of ambient noise, true‐amplitude CMP stacks can be substantially improved by the appropriate noise‐reduction techniques, but CMP stacks incorporating gain control before CMP stacking show only minor improvements. In contrast, when a high level of ambient noise is present, both true‐amplitude and gain‐controlled CMP stacks can be greatly improved by appropriate noise‐reduction processing before vertical stacking. Of procedures involving zeroing of noise bursts, self‐updating noise‐rejection systems were more effective than fixed‐gain noise‐rejection systems for all the conditions simulated here. A system incorporating recursive editing techniques is still more effective at low levels of ambient noise, but this system tends to edit recorded traces too severely at very high noise levels. Mantissa‐only and sign‐bit‐only recording give very similar results, and show an effect comparable to that given by self‐updating editing systems. Diversity stacking produced significant noise reduction in all conditions studied, and may be the most widely applicable and most generally useful of the noise‐reduction methods studied here. ®Trade and service mark of Conoco Inc.

3D Seismic Imaging Without Stack: a Case History from the Irish Sea

Conventional 3D marine seismic surveys usually employ high-fold Common Midpoint (CMP) stacking techniques, long streamers, and elaborate positioning systems to determine both the source and receiver array locations for every shot. It is inevitable that the reflection responses occurring in seismic profiles obtained through the CMP stack process are spatially averaged. Responses are further averaged over a range of reflection incidence angles in consequence of the various offset distances represented in the CMP gather. Whilst these effects may benefit the continuity of reflected events in a processed section, they detract from the potential resolution that could be achieved through subsequent 3D migration and obscure detail that might otherwise contribute to a more rigorous interpretation.

Submarine canyons: Velocity replacement by wave‐equation datuming before stack

Submarine canyons incised into the continental slope interfere with the quality of common‐midpoint (CMP) stacked seismic data obtainable from reflectors beneath the sea floor. The interference problem is caused by rough topography in conjunction with the contrast between the acoustic velocity of sea water and the velocity of the exposed rock layers. Geophysicists have long recognized that part of the solution is to replace the traveltimes of raypaths through the water by their traveltimes through an identical thickness of rock. However, use of wave‐equation datuming to effect velocity replacement yields an additional correction for the change in raypath direction that occurs in crossing from rock to sea water; the wave‐equation datuming implementation of velocity replacement is more comprehensive and complete. The wave‐equation datuming method requires an accurate sea‐floor profile as part of the input, along with values of replacement velocity; it does not require knowledge of geology or velocities at depths much greater than the sea floor. Unstacked common‐source and common‐receiver records are processed to appear as if sources and receivers were moved to the water bottom; the velocity of water is replaced; and the sources and receivers are moved back to the sea surface through the replacement medium. The computational method is well‐suited to the irregular surfaces and laterally variable velocities inherent in the problem of submarine canyons. The advantage of this method is that the corrected seismic records accurately emulate the data that would actually be observed if the acoustic velocity of water could be changed physically. The normal‐moveout (NMO) velocity for optimum CMP stacking becomes the root mean square of the layer velocities, including the velocity substituted for that of water. The spurious lateral variation of stacking velocity in the original data is eliminated. Processing of the corrected data through velocity analysis, stacking, migration, and conversion to depth is therefore more reliable.

Shoot between the group for simulated multichannel processing flexibility

Where standard recording techniques are used it is not uncommon for seismic reflection data to contain undesirable, spatially aliased energy. For conventional recording layouts, the receiver station spacing controls the spatial sampling density so that higher wavenumber definition requires smaller spacing. Selection of the receiver station spacing with a limited number of recording channels is governed by the minimum and maximum offsets required to sample adequately both near-surface and deeper horizons. All too often the shallower reflections, which contain a larger proportion of the shorter wavelengths, are inadequately spatially sampled. But if a relatively small station spacing is chosen then there is insufficient differential moveout for the suppression of multiples by common mid-point stacking. However, if the number of channels is limited, it is still possible to increase the spatial bandwidth by simulating the recording of any integer multiple of the number of channels actually available. The method is appropriate for the acquisition of land data using surface sources because it involves the recording of more than one conventional record on each receiver spread from source positions spaced at equal intervals over one receiver station interval. The individual records for each receiver spread can be interleaved to simulate one field record on a multiple number of channels. This acquisition technique is not a new concept and has been utilised in several countries. However, the full processing potential has not yet been fully realised. In this paper the main recording and pre-stack processing concepts are demonstrated by a series of examples showing data in the time-distance and frequency wavenumber (f-k) domains. A real data set allows comparisons to be made between different processing sequences using this flexible recording method.

Migration velocity analysis by wave‐field extrapolation

Velocity information is essential to both common midpoint (CMP) stacking and migration. CMP stacking provides the basis for conventional velocity estimation techniques in that, for a number of trial velocities, the stack response of a CMP gather is computed and displayed in the form of a velocity table. An alternative approach to velocity estimation makes use of the basic ingredients of migration—downward extrapolation and imaging of seismic wave fields. The procedure involves migration of a CMP gather with a number of trial velocities and collection of the zero‐offset information, again in the form of a velocity table. Operating on a CMP gather, the migration‐based approach produces results similar to those of the conventional method. Analyses of synthetic CMP gathers using both methods show essentially equivalent treatments of seismic signal, and similar dependence of accuracy and resolving power on recording geometry. We have extended the migration‐based approach to include more than one CMP gather in each analysis. This extension allows proper treatment of dipping events and yields velocity information that is more appropriate for use in migration. By using the intermediate wave field at each step of downward extrapolation, we need only do a single constant‐velocity migration of the unstacked data followed by a simple mapping procedure in order to recover the velocity information.

Migration Velocity Analysis by Wave-Field Extrapolation

Velocity information is essential to both common midpoint (CMP) stacking and migration. CMP stacking provides the basis for conventional velocity estimation techniques in that, for a number of trial velocities, the stack response of a CMP gather is computed and displayed in the form of a velocity table. An alternative approach to velocity estimation makes use of the basic ingredients of migration–downward extrapolation and imaging of seismic wave fields. The procedure involves migration of a CMP gather with a number of trial velocities and collection of the zero-offset information, again in the form of a velocity table. Operating on a CMP gather, the migration-based approach produces results similar to those of the conventional method. Analyses of synthetic CMP gathers using both methods show essentially equivalent treatments of seismic signal, and similar dependence of accuracy and resolving power on recording geometry. We have extended the migration-based approach to include more than one CMP gather in each analysis. This extension allows proper treatment of dipping events and yields velocity information that is more appropriate for use in migration. By using the intermediate wave field at each step of downward extrapolation, we need only do a single constant-velocity migration of the unstacked data followed by a simple mapping procedure in order to recover the velocity information.

Dip‐moveout by Fourier transform

The conventional normal‐moveout (NMO) and common‐midpoint (CMP) stacking process enhances reflections having a particular moveout velocity, while attenuating events (such as multiple reflections) having different moveout velocities. Unfortunately, this process also acts as a dip filter applied to the CMP stack. In other words, NMO and stacking enhances reflections having a particular slope in the CMP stack, while attenuating reflections having different slopes. NMO and stacking, like any dip filter, degrades lateral resolution. Fortunately, this dip‐filtering action can be suppressed by applying, in addition to NMO, a prestack process known variously as DEVILISH, prestack partial migration, and dip‐moveout. As the latter term implies, this process is a dip‐dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same NMO velocity. Stated another way, NMO velocities estimated from dip‐moveout‐corrected seismograms are independent of the dips of subsurface reflectors. Dip‐moveout by Fourier transform is a method for performing dip‐moveout (DMO) correction in the frequency‐wavenumber domain. The implementation of this method, which resembles the implementation of a discrete Fourier transform, is quite different from and compares favorably with previously published finite‐difference DMO algorithms. DMO by Fourier transform, unlike DMO by finite‐differences, is accurate for all offsets and all dips, provided that velocity is constant. Because velocity is never constant, some accuracy is inevitably lost; but the application of DMO by Fourier transform to recorded seismograms demonstrates the ability of this process to enhance (1) the dip bandwidth of CMP stacks and (2) the accuracy of velocity estimates.

Coherent noise in marine seismic data

Despite significant advances in marine streamer design, seismic data are often plagued by coherent noise having approximately linear moveout across stacked sections. With an understanding of the characteristics that distinguish such noise from signal, we can decide which noise‐suppression techniques to use and at what stages to apply them in acquisition and processing. Three general mechanisms that might produce such noise patterns on stacked sections are examined: direct and trapped waves that propagate outward from the seismic source, cable motion caused by the tugging action of the boat and tail buoy, and scattered energy from irregularities in the water bottom and sub‐bottom. Depending upon the mechanism, entirely different noise patterns can be observed on shot profiles and common‐midpoint (CMP) gathers; these patterns can be diagnostic of the dominant mechanism in a given set of data. Field data from Canada and Alaska suggest that the dominant noise is from waves scattered within the shallow sub‐buttom. This type of noise, while not obvious on the shot records, is actually enhanced by CMP stacking. Moreover, this noise is not confined to marine data; it can be as strong as surface wave noise on stacked land seismic data as well. Of the many processing tools available, moveout filtering is best for suppressing the noise while preserving signal. Since the scattered noise does not exhibit a linear moveout pattern on CMP‐sorted gathers, moveout filtering must be applied either to traces within shot records and common‐receiver gathers or to stacked traces. Our data example demonstrates that although it is more costly, moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data‐dependent processing steps of predictive deconvolution and velocity analysis.

VELOCITY ESTIMATION AND DOWNWARD CONTINUATION BY WAVEFRONT SYNTHESIS

A “wave stack” is any stack over a common shot or geophone gather in which the moveout is independent of time. It synthesizes a particular wavefront by superposition of the many spherical wavefronts of raw data. Unlike the common midpoint stack, wave stacks retain the important property of being the sampling of a wave field and, as such, permit wave‐equation treatment of formerly difficult or impossible problems. Seismic sections of field data generated by wave stacks that synthesized slanted downgoing plane waves showed a similarity in appearance to the common midpoint stacks. In signal‐to‐noise ratio they lay between the single offset section and the midpoint stack. The angle selectivity of the slanted plane‐wave stacks permitted detection of a reflector that was not visible on either the midpoint stack or the raw gathers. Simple velocity estimation in slant frame coordinates differs only in detail from standard frame coordinates. Because of the wave field character of data that have been slant plane‐wave stacked, wave‐equation techniques can be used to generalize migration and velocity estimation to regions in which exist a strong lateral velocity inhomogeneity within the distance of a cable spread.