Dipping Events Research Articles

GPR, TDR, and geochemistry measurements above an active gas vent to study near-surface gas-migration pathways

The migration of deep gas to the atmosphere along faults and associated structures is important in many fields, from studying the natural contribution of atmospheric greenhouse gases leaking from geothermal areas to ensuring the safety of man-made natural gas and carbon dioxide [Formula: see text] geologic-storage sites. Near-surface geophysical and geochemical techniques were applied to a naturally occurring gas vent located along a deep terrestrial fault to better understand the structure and geophysical response of this gas-migration pathway. A number of ground-penetrating radar (GPR) profiles were first conducted across the vent. Spot samples were then measured along one of these profiles for in situ apparent permittivity (using time-domain reflectometry — TDR), complex permittivity on dried samples (using a capacitivecell), soil-gas composition, and clay and bulk mineralogy. Results show how the migrating gas induces secondary effects that modify the signature of the vent as seen in the GPR profiles. In particular, high flux rates across the vent core (i.e., the central portion of the vent) result in a total lack of vegetation, which in turn leads to high water content that is likely responsible, together with increased bulk conductivity, for GPR signal blanking. In the transition zone surrounding the core, a water-content minimum and other changes contribute to a deeper GPR signal penetration that highlights dipping events. Data show that the structure itself is slightly asymmetrical, which may indicate more fracturing to one side of the feature. This study shows that GPR surveys are capable of imaging secondary effects induced by gas migration in soils above a structural discontinuity, even if the structure itself cannot be imaged.

Effects of trench-zone scattering on receiver functions over a subduction zone: A 3-D finite-difference modelling study

As Morozov [Morozov, I. B. (2004). Crustal scattering and some artefacts in receiver function images. Bull. Seismol. Soc. Am., 94 (4), 1492–1499.] suggested, for a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the strong horizontal velocity heterogeneity beneath the trench zone itself produces subhorizontally-propagating waves that should be observed as coherent dipping events in receiver functions (RF). Due to similar RF delay times and moveouts, these events could be difficult to distinguish from backscattered P- and S-wave modes. To further verify this suggestion, we performed a full-waveform, 3-D visco-elastic finite-difference modelling of teleseismic wave propagation within a simplified model of a subduction zone. The synthetics show strong scattering from the area beneath the trench, dominated by the mantle and crustal P-waves propagating at 6.2–8.1 km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered converted modes, and also have similar amplitudes. Although their amplitude decay characters are different, the uncertainty in the knowledge of the velocity and density structure of the subduction zone could make distinguishing between these modes difficult. However, under minimal assumptions, recent observations of receiver function amplitudes decreasing away from the trench support the interpretation of (sub-) trench-zone scattering. Although still limited in its representation of crustal heterogeneity, 3-D modelling suggests that scattering from near-Moho crustal structures plays a key role in the formation of teleseismic wavefields. Recognition of scattered noise in teleseismic records could help to constrain major crustal structures, particularly those with strong horizontal velocity contrasts at near-Moho depths, such as crustal sutures, subduction fault zones, and mountain roots. Matching of the observed arrivals with wavefield synthetics could help constrain the locations and parameters of such structures and also help substantiate the interpretations.

Structure of Mesozoic oceanic crust in the vicinity of the Cape Verde Islands from seismic reflection profiles

Multichannel seismic reflection profile data have been used to determine the internal structure of Mesozoic oceanic crust in the vicinity of the Cape Verde islands. The data show the oceanic crust to be characterized by both dipping and sub-horizontal reflectors. Several lines of evidence argue against the reflectors being scattering artifacts arising, for example, from rough basement topography. Instead, the reflectors are attributed to tectonic and magmatic processes associated with the accretion of oceanic crust at the Mid-Atlantic Ridge. The upper crust shows variable reflectivity due to both dipping and sub-horizontal events. We interpret the dipping reflectors, which have been identified on both ridge-normal and ridge-parallel profiles, as sub-surface expressions of normal faults that formed at or near the Mid-Atlantic Ridge. There is no evidence that the faults are caused by loading of the oceanic crust by either the Cape Verde islands or their associated topographic swell. Some faults, however, can be traced into the overlying sediments suggesting they may have been re-activated since their formation at the ridge. The origin of the sub-horizontal reflectors is not as clear. We believe them to be boundaries of different igneous lithologies, such as that between basalts and gabbros. The lower crust is highly reflective in some areas, whereas in others only a few dipping and sub-horizontal reflectors are observed. Some of the dipping reflectors can be traced into the upper crust, suggesting they are also normal faults. Others, however, appear to be confined to the lower crust. The sub-horizontal, discontinuous, reflectors about 2.0–2.5 seconds two-way travel time below the top of oceanic basement are attributed to the Moho.

Seismic imaging of steep dips with low fold data: Case studies for Broken Hill and the Lachlan Fold Belt

In hard rock regions, a large range of stacking velocities is required to correctly stack reflectors of different dips. Typically, horizontal reflectors stack at 6000 m/s, whereas reflectors with dips of 60° stack at 12,000 m/s. For high fold (vibrator) data, correct stack of conflicting dips can be achieved by dip moveout (DMO) correction. However, for lower fold (dynamite) data, the sparse offset distribution complicates application of DMO. An alternative technique involves producing stacks with different stacking velocities and stacking these stacks. This technique was applied to two seismic reflection data sets, low fold dynamite data from Broken Hill and high fold vibrator data from the Lachlan Fold Belt. The Lachlan data set was used as both full 60/120 fold and reduced 10/20 fold. Velocity analysis, both analytical and empirical, was carried out to determine the range of stacking velocities. Stacking velocity increases with dip angle (cos-1q), but the velocity range across which an event stacks coherently increases more rapidly (approximately cos-3q for velocities typical of hard rock). The most critical area for analysis is the first two seconds of data, due to greater sensitivity of NMO to stacking velocity. The optimum number of stacks is an important consideration, based on the number of stacks in which an event contributes coherently to the sum The Broken Hill stack data showed simultaneous imaging of horizontal and dipping events. For the Lachlan reduced fold data set, horizontal and moderate to steeply dipping events were stacked successfully, although not as well as the post-DMO stack of the full fold data. The technique has some problems at the shallowest levels, where the stack can be degraded due to time shifts of events in the individual stacks, but is a useful tool for low fold seismic data typical of older regional profiles.

F‐xyeigenimage noise suppression

F‐xy eigenimage filtering works by replacing constant‐frequency slices with the sum of their first few weighted eigenimages, and can be used to remove random noise from stacked 3D seismic volumes. It performs equally well on flat or dipping events, and has no effect on noiseless data when the number of dips is no more than the number of summed eigenimages. It is also independent of many x‐ and y‐consistent effects such as reordering, statics, and filtering. One consequence is that it tends to do a good job along the boundaries of the 3D grid. Execution time is comparable to f‐xy prediction filtering, but can be greatly reduced using approximations based on Lanczos bidiagonalization.

Prestack Kirchhoff migration and amplitude accuracy

Prestack Kirchhoff migration is a successful seismic imaging method, commonly used in exploration seismology. Amplitude analysis and AVO attributes are also a common part of exploration seismology; hence the amplitude integrity of prestack Kirchhoff migration is an important issue. Accurate amplitudes do not naturally fall out of Kirchhoff migration. Three corrections must be computed and applied to the migration operator to ensure accurate amplitudes. First, an anti-alias filter must be applied to ensure that residual operator noise is not created. The anti-alias filter can affect the amplitudes of both flat and dipping events. Next, a derivative filter is applied to ensure that the wavelet phase and frequency content are preserved after migration; this filter does not affect the migrated amplitudes. Finally, most of the amplitude corrections come from ``true-amplitude' weights that are applied to the migration operator. Accurate true-amplitude weights are costly to computeand may even not be worth the effort because of potential instability. In practice, various approximate weights are normally applied, resulting in migrated amplitudes that are usually more accurate than those obtained from conventional processing.

Seismic and gravity modelling of crustal structure in the Central Graben, North Sea. Observations along MONA LISA profile 3

High quality wide-angle and deep normal-incidence seismic data sets recorded during the MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) experiment have allowed detailed crustal investigations in the area of the Central Graben and south-eastern North Sea to be conducted. An integrated velocity–density model based on a joint inversion of traveltimes and gravity data is presented for the crust and uppermost mantle along MONA LISA line 3, which crosses the Central Graben approximately perpendicular to its strike direction. This model shows pronounced crustal thinning and associated Moho uplift beneath the deepest part of the Central Graben. In the graben area, Moho is interpreted to be situated at 25km depth and the crystalline crust is about a factor of 2 thinner than in the surrounding crustal provinces, where Moho is interpreted to reach depths between 32 and 36km. Maximum thickness of the sedimentary column in the Central Graben is 10–11km. The mass deficit of the thick sediments is compensated by the Moho uplift. The modelled Moho coincides with the base of lower crustal reflectivity observed in the coincident normal-incidence seismic section. Good overall agreement between the integrated velocity–density model and the normal-incidence data is observed. The migrated deep seismic reflection section recorded to 26s traveltime contains deep west-ward and east-ward dipping events symmetric about the deepest part of the Central Graben. These dipping upper mantle reflections may be linked to extensional processes that formed the Central Graben. At least one of the upper mantle dipping reflectors may be a reactivated structure formed originally during Caledonian collision.

What is DMO coverage?

‘Coverage’ or ‘fold’ is defined as the multiplicity of common-midpoint (CMP) data. For CMP stacking the coverage is consistent with the number of traces sharing a common reflection point on flat subsurface reflectors. This relationship is not true for dipping reflectors. The deficiencies of CMP stacking with respect to imaging dipping events have long been overcome by the introduction of the dip-moveout (DMO) correction. However, the concept of coverage has not yet satisfactorily been updated to a ‘DMO coverage’ consistent with DMO stacking. A definition of constant-velocity DMO coverage will be proposed here. A subsurface reflector will be illuminated from a given source and receiver location if the time difference between the reflector zero-offset traveltime and the NMO- and DMO-corrected traveltime of the reflection event is less than half a dominant wavelength. Due to the fact that a subsurface reflector location is determined by its zero-offset traveltime, its strike and its dip, the DMO coverage also depends on these three parameters. For every surface location, the proposed DMO coverage consists of a 3D fold distribution over reflector strike, dip and zero-offset traveltime.

Structural evolution of the Northern Tyrrhenian Sea from new seismic data

We present the results of the deep penetrating LISA seismic survey supplemented by several industrial oil surveys. The LISA cruise shows the deep part of the crust and the Moho reflections are seen at 7.5–8 s TWTT on several seismic lines on the Tuscan (eastern) margin of the North Tyrrhenian Sea. The Moho depth has been previously evaluated by refraction data at about 22–25 km and this depth corresponds to the reflections observed on our seismic profiles. Two kinds of dipping reflectors are identified in the crust. The western dipping events are interpreted as traces of thrusts with an Apenninic vergence, the eastern dipping events are probably extensional detachments faults. These faults are seen deep in the crust and may be shear zone in the ductile crust. The extensional faults and the thrusts have been previously identified on the industrial lines but these lines are processed at a depth of 4 s TWTT and consequently the deep reflections are not imaged. The industrial lines and the correlations with the exploratory wells helps to extend the results of the LISA cruise in the entire region. The compilation of all the seismic data allow us to present the first depth to basement map of this region. The major feature of the study area is the Corsica basin that is more than 8.5 km deep. The deep part of this huge basin is probably filled by Oligocene–early Miocene deposited in an extensional regime. However this extension persisted during the Miocene. The steep faults that bounds the basin toward the west are imaged and we present a model in ramp, and flat system, that explains the particular geometry of the Corsica margin. The eastern flank of the Corsica basin which is bounded by the Pianosa ridge, is completely different. This region has been intruded by granitic plugs during the Messinian and the early Pliocene and these intrusions are coeval with horst and graben formation on the Tuscan margin. Before this extension, Pianosa ridge and Elba island were deformed by a compression related to the formation of the Apennines during the Miocene. Our observations on the North Tyrrhenian Sea are set in the tectonic framework of the Western Mediterranean. A first extension occurred during the Oligocene–early Miocene in the Proven e ̧ al basin, east of Corsica and Sardinia. The eastern basins are bounded by strike-slip faults and are located on the top of deformed tectonic prism. The compressional deformation is related to the middle Miocene Apennines mountain building. Then compression migrated to the east and the inner Apennines belt collapsed during the late Miocene and the early Pliocene. This late extension, associated with a high heat flow and a shallow Moho, is well illustrated by the LISA survey in the North Tyrrhenian Sea.

Moveout inversion ofP-wave data for horizontal transverse isotropy

The transversely isotropic model with a horizontal symmetry axis (HTI media) has been extensively used in seismological studies of fractured reservoirs. In this paper, a parameter‐estimation technique originally developed by Grechka and Tsvankin for the more general orthorhombic media is applied to horizontal transverse isotropy. Our methodology is based on the inversion of azimuthally‐dependent P-wave normal‐moveout (NMO) velocities from horizontal and dipping reflectors. If the NMO velocity of a given reflection event is plotted in each azimuthal direction, it forms an ellipse determined by three combinations of medium parameters. The NMO ellipse from a horizontal reflector in HTI media can be inverted for the azimuth β of the symmetry axis, the vertical velocity [Formula: see text], and the Thomsen‐type anisotropic parameter δ(V). We describe a technique for obtaining the remaining (for P-waves) anisotropic parameter η(V)(or ε(V)) from the NMO ellipse corresponding to a dipping reflector of arbitrary azimuth. The interval parameters of vertically inhomogeneous HTI media are recovered using the generalized Dix equation that operates with NMO ellipses for horizontal and dipping events. High accuracy of our method is confirmed by inverting a synthetic multiazimuth P-wave data set generated by ray tracing for a layered HTI medium with depth‐varying orientation of the symmetry axis. Although estimation of η(V)can be carried out by the algorithm developed for orthorhombic media, for more stable results the HTI model has to be used from the outset of the inversion procedure. It should be emphasized that P-wave conventional‐spread moveout data provide enough information to distinguish between HTI and lower‐symmetry models. We show that if the medium has the orthorhombic symmetry and is sufficiently different from HTI, the best‐fit HTI model cannot match the NMO ellipses for both a horizontal and a dipping event. The anisotropic coefficients responsible for P-wave moveout can be combined to estimate the crack density and predict whether the cracks are fluid‐filled or dry. A unique feature of the HTI model that distinguishes it from both vertical transverse isotropy and orthorhombic media is that moveout inversion provides not just zero‐dip NMO velocities and anisotropic coefficients, but also the true vertical velocity. As a result, reflection P-wave data acquired over HTI formations can be used to build velocity models in depth and perform anisotropic depth processing.

Super-deep reflection profiling: exploring the continental mantle lid

Reflections from the continental mantle lithosphere have now been reported from common mid-point (CMP) profiles at numerous sites around the world. These reflections include: (1) distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle; (2) kilometer scale, discontinuous, diffusely distributed, or isolated reflections at varying depths; (3) continuous, sub-horizontal reflections tens of kilometers in extent, often interpreted as the ‘base of the lithosphere’ in spite of a wide variation in depth and; (4) very deep reflections in the mantle lithosphere (> 125 km). The first category of mantle reflections is most commonly observed in proximity to crustal sutures, lending credence to their interpretation as fossil subduction zones. Isolated, discontinuous mantle reflections are by far the most common and, though observed in both ancient and modern collisional environments, are difficult to interpret due to both their limited extent and a corresponding lack of three-dimensional control. Sub-horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide. Of special note are the various reports of specific reflections at ∼80–100 km (22–24 s twt) in the North Sea, eastern Europe, and the Urals which suggest a more ubiquitous mantle horizon. The deepest events imaged thus far in a controlled source CMP survey are the ‘super-deep’ mantle reflections of the URSEIS project that have been identified at ∼135–165 km (35–43 s) and ∼225 km (56 s). Of the many reported mantle reflections, these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo-mechanical transition should be reflective at high frequencies (10–20 Hz).

Reflection moveout and parameter estimation for horizontal transverse isotropy

Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here, I present an exact equation for normal‐moveout (NMO) velocities from horizontal reflectors valid for pure modes in HTI media with any strength of anisotropy. The azimuthally dependent P‐wave NMO velocity, which can be obtained from 3-D surveys, is controlled by the principal direction of the anisotropy (crack orientation), the P‐wave vertical velocity, and an effective anisotropic parameter equivalent to Thomsen's coefficient δ. An important parameter of fracture systems that can be constrained by seismic data is the crack density, which is usually estimated through the shear‐wave splitting coefficient γ. The formalism developed here makes it possible to obtain the shear‐wave splitting parameter using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore, γ can be estimated just from the P‐wave NMO velocity in the special case of the vanishing parameter ε, corresponding to thin cracks and negligible equant porosity. Also, P‐wave moveout alone is sufficient to constrain γ if either dipping events are available or the velocity in the symmetry direction is known. Determination of the splitting parameter from P‐wave data requires, however, an estimate of the ratio of the P‐to‐S vertical velocities (either of the split shear waves can be used). Velocities and polarizations in the vertical symmetry plane of HTI media, that contains the symmetry axis, are described by the known equations for vertical transverse isotropy (VTI). Time‐related 2-D P‐wave processing (NMO, DMO, time migration) in this plane is governed by the same two parameters (the NMO velocity from a horizontal reflector and coefficient ε) as in media with a vertical symmetry axis. The analogy between vertical and horizontal transverse isotropy makes it possible to introduce Thomsen parameters of the “equivalent” VTI model, which not only control the azimuthally dependent NMO velocity, but also can be used to reconstruct phase velocity and carry out seismic processing in off‐symmetry planes.

Migration of the 3-D deep-seismic reflection survey at the KTB location, Oberpfalz, Germany

A near-vertical deep 3-D seismic survey covering an area of approximately 17.85 × 19.1 km has been carried out across the German continental deep borehole (KTB) site. This survey is the first of this kind to investigate complicated crystalline crust. Two major problems were encountered during the post-stack 3-D migration of these data. First, the survey region includes areas with strong lateral velocity gradients, which are difficult to treat with many migration algorithms. Velocities in the sedimentary rocks are ∼ 3000 m/s and in the crystalline rocks ∼ 5500 m/s. Secondly, the data are sparsely sampled (50 × 50 m binning), but include steeply dipping events with true dips up to 60°. To determine the optimum 3-D migration approach three different algorithms have been tested: summation (modified Kirchhoff), finite-difference and phase-shift. Results of the various tests suggest that one-pass 3-D migration algorithms produce noticeably superior results to the much faster two-pass methods. The combination of steep dips and sparsely sampled data was a problem for the finite-difference migration, whereas shallow dipping features were imaged well by this technique. Although the adopted phase-shift migration algorithm could not handle lateral velocity gradients, it produced excellent results for both shallow and steeply dipping structures. Both of these migration algorithms are relatively fast. By comparison, the summation migration algorithm produced overall good results for regions containing steep dips and strong lateral velocity variations, but it requires a lot of computing time.

Seismic reflections from subvertical diabase dikes in an Archean terrane

A 20 km seismic reflection profile in the Superior province of northwestern Ontario imaged polydeformed Archean terrane, including the Manitouwadge greenstone belt and the adjacent Wawa-Quetico subprovince boundary. In addition to reflectivity from Archean lithostructural elements, we recognize discrete reflections that correlate with undeformed subvertical Proterozoic dikes. Moderately to steeply inclined linear events in the stacked section are related to Matachewan dikes that intersect the seismic profile at 50°–75°. The reflections project to the surface at points corresponding to dike-profile intersections and have apparent dips in the seismic section consistent with the intersection angles in map view. Subhorizontal, linear to curvilinear reflections originate from Marathon dikes that are subparallel to the seismic line. On the basis of forward modeling studies, the events occur at two-way traveltimes for horizontally propagating waves. We conclude that a number of dipping and subhorizontal events represent refracted waves propagated along horizontal ray paths near the surface and reflected from subvertical dikes. In many other seismic profiles in crystalline crust, reflections originating from subvertical dikes have likely been overlooked or misinterpreted, either because the presence of dikes was not recognized, or because their seismic response was not evaluated.

Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment

Multichannel seismic (MCS) images are often contaminated with in‐ and out‐of‐plane scattering from the sea floor. This problem is especially acute in the mid‐ocean ridge environment where sea‐floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea‐floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water‐borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea‐floor‐scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea‐floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common‐midpoint (CMP) domain after DMO. Moveover, our DMO‐based approach suppresses out‐of‐plane scattering, and therefore is not limited to removal of in‐plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO‐based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO‐based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth.

Characteristics of Romanian lithosphere from deep seismic reflection profiling

The tectonic units of Romania contain both Alpine orogenic regions and platforms. The investigations of crustal structure by near-vertical reflection seismic profiling are based on less than 100 lines unequally distributed and of limited lengths (5–30 km). The seismic features studied are: the crustal seismic reflectivity and transparency; dipping events and diffractions; and the Moho and subcrustal reflectivity. The major common features are: a reflective sedimentary cover of mostly Neogene layers; a weak signal from the sedimentary-basement boundary; a quasi-transparent upper crust; a mid-crust with short and subhorizontal events; a transparent or layered lower crust; and a Moho that is either poorly visible or evidenced by a zone with reflection bands and some subcrustal reflections. The Moesian platform, together with the adjacent Carpathian foredeep, show a mid-crust with short and subhorizontal events, a crust-mantle transition zone in the west, an unclear or prominent Moho and some subcrustal reflectivity. The northern part of the Carpathian foredeep shows strong seismic layering of the lower crust. The central part of the Transylvanian depression is characterized by short and subhorizontal events at mid-crustal level, a slightly reflective lower crust with diffractions and a weak Moho signal. The southern sector of the Pannonian depression is generally marked by a reflective lower crust. The tectonic significance of these seismic signatures is interpreted as follows: a brittle upper crust; a decoupling mid-crustal zone; a lower crust with brittle to ductile behaviour; a possible extensional or isostatic rebalanced Moho and laminated zones with reduced viscosities at the lower crust and upper mantle levels. A series of crustal models based on the reflection seismic sections are presented for several areas of Romania.

Fourier finite‐difference migration

Many existing migration schemes cannot simultaneously handle the two most important problems of migration: imaging of steep dips and imaging in media with arbitrary velocity variations in all directions. For example, phase‐shift (ω, k) migration is accurate for nearly all dips but it is limited to very simple velocity functions. On the other hand, finite‐difference schemes based on one‐way wave equations consider arbitrary velocity functions but they attenuate steeply dipping events. We propose a new hybrid migration method, named “Fourier finite‐difference migration,” wherein the downward‐continuation operator is split into two downward‐continuation operators: one operator is a phase‐shift operator for a chosen constant background velocity, and the other operator is an optimized finite‐difference operator for the varying component of the velocity function. If there is no variation of velocity, then only a phase‐shift operator will be applied automatically. On the other hand, if there is a strong variation of velocity, then the phase‐shift component is suppressed and the optimized finite‐difference operator will be fully applied. The cascaded application of phase‐shift and finite‐difference operators shows a better maximum dip‐angle behavior than the split‐step Fourier migration operator. Depending on the macro velocity model, the Fourier finite‐difference migration even shows an improved performance compared to conventional finite‐difference migration with one downward‐continuation step. Finite‐difference migration with two downward‐continuation steps is required to reach the same migration performance, but this is achieved with about 20 percent higher computation costs. The new cascaded operator of the Fourier finite‐difference migration can be applied to arbitrary velocity functions and allows an accurate migration of steeply dipping reflectors in a complex macro velocity model. The dip limitation of the cascaded operator depends on the variation of the velocity field and, hence, is velocity‐adaptive.

(f, x) Migration — Some Side Issues

Conventional methods of treating boundaries in finite difference (f, x) migration, namely padding in both space and time, have the disadvantage of increasing the data volume. This increase, whilst acceptable for 2-D processing, becomes uneconomical in the 3-D arena. Dipping events move up-dip during the migration process. Dips close to the sides of the data that move outside the data range can be reflected back if the boundary conditions are poorly set. This results in misleading events appearing in the migrated data. The addition of extra zero trace padding allows these events to move out of the data and so avoids these artefacts. The alternative to padding with zero traces is to use absorbing side boundary conditions. Absorbing sides have been proposed by several authors. A method developed by Clayton and Engquist is reviewed in detail, and implementation issues are discussed. Although this method is exact for only one propagation angle, it is found to attenuate significantly the reflections of other dipping events. Time padding is used to avoid the wraparound that causes shallow data to reappear as noise at later times in the migrated result. Wraparound is inherent in (f, x) finite difference migration, because of the cyclic nature of the fast Fourier transform. The time padding increases the run time of both the fast Fourier transform and the migration. Clearly, it is desirable to find a technique to remove the wraparound and so avoid the need for costly time padding. Kjartansson showed a method for wraparound removal for the case when the sampling interval is equal to the migration time step or depth interval. In practice, however, a coarser time step is usually employed. A new generalised method has been developed for wraparound removal under these conditions of coarse time step. The technique is applied at each time step extrapolation. Although this obviously involves more computation, the method is more efficient than using extra time padding. Both these techniques, the absorbing side boundaries and the wraparound removal, can be used in 2-D and one-pass 3-D implementations. Without the need for extra padding, migration run times and hence costs are reduced. The methods are demonstrated on synthetic data. The results compare well with those obtained using conventional padding, with a considerable reduction in run time.

Seismic structure of oceanic crust in the western North Atlantic

We examine the seismic structure of Cretaceous‐aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide‐angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge‐normal and ridge‐parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge‐parallel extensional faults commonly mapped at mid‐ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave‐upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge‐normal profiles the majority of these events dip to the east, toward the paleo‐spreading center, whereas on ridge‐parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1‐ to 2‐km‐thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge‐normal and ridge‐parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere.

Propagating rift pseudofaults ‐ Zones of crustal underplating imaged by multichannel seismic reflection data

Two multichannel seismic reflection lines recorded over the northern Juan de Fuca plate off western Canada provide images of the crustal structure of three pseudofaults, the traces of propagating rifts. Anomalous reflectivity, including both subhorizontal and dipping events at times below the normal crust‐mantle transition, is prevalent in the region of the pseudofaults. Although the crustal thickness elsewhere on the lines varies by only about 10%, crust associated with the pseudofaults is as much as 30% thicker than average. This may be due to an increase in thickness of the interpreted oceanic crustal layer 3B or crustal underplating. The results indicate that magma supply at propagating rifts may at times be large, perhaps due to the effects of nearby hotspots or convective partial melting associated with the initiation of rifting.