Lateral Variations In Seismic Velocities Research Articles

Seismic Survey in Lesser Himalayan Thrust Belt, Western Nepal

Two hundred km of 2D seismic survey was carried out at the Lesser Himalayan Thrust Belts in Dailekh district, western Nepal. The main motivation is to elucidate the geologic relationship between the known oil and gas seeps, subsurface structure, and stratigraphy in the area. This is a challenging task which is from its extreme structural and geological complexity such as thrust faulting, tight folding, steep dip layers, and strong lateral variations in seismic velocity. Seismic data were acquired with SERCEL 428XL system and processed by GEOEAST computer software. In order to increase signal-to-noise ratio (SNR), suppress interference, and search for optimum acquisition parameters, a series of comparative tests on the different charge depth and size, group interval, CDP fold, geophone array, and single high-sensitivity geophone were conducted. We also tested 2S3L (two lines shooting and three lines receiving) wide line profiling. The results indicate that single hole with charge depth of 12 m, 4-16 kg charge size (less charge size for the densely populated areas), single high-sensitivity geophone, and 1S2L wide line profiling with 132 folds are the optimum acquisition parameters. On the basis of comparative process experiment, data processing workflow consisting of data preparation, prestack denoising, amplitude compensation, deconvolution, tomography static correction, velocity analysis, residual static correction, CRS stack, poststack migration, prestack time migration (PSTM), and prestack depth migration (PSDM) was selected. Maybe affected by problem of conflicting dip in complex media, CRS stack section does not show satisfactory geological characteristics. PSTM profile has moderate signal-to-noise (S/N) ratio; the shallow, medium, and deep continuous reflections can be observed in section. More details of the geological structures can be observed in PSDM section, especially in medium and shallow layers (less than 3000 ms or 4000 m), but PSDM method is more expensive and highly time consuming than that of CRS stack and PSTM. So, the PSTM section can be reasonably used for geological interpretation. By reference to field mapping, thrust characteristics, and MT data, the final interpretation to the PSTM section identified the interfaces of 6 geological units (Paleoproterozoic Nabhisthan Fm., Paleoproterozoic Dubidanda Fm., Neogene to Late Cretaceous Surkhet group, Late Carboneferous to Early Cretaeous Gondwana group, Mesoproterozoic Upper Lakharpata group, and Lower Lakharpata group) and delineated Main Boundary Thrust (MBT), Ramgarh Thrust (RMT), Padukasthan Thrust (PT), and Dailekh Thrust (DT). The bottom of Surkhet group which is our top target zone is about 4250 meters deep.

Seismic Velocity Structure Beneath the Tofino Forearc Basin Using Full Waveform Inversion

Given the effects of steep dips and large lateral variations in seismic velocity beneath the Vancouver Island continental shelf, seismic processing and travel time inversion are inadequate to obtain a detailed velocity model of the subsurface. Therefore, seismic full waveform inversion is applied to multichannel seismic reflection data to obtain a high-resolution velocity model beneath the Tofino fore-arc basin under the continental shelf off Vancouver Island margin. Seismic velocities obtained in this study help in understanding the shallow shelf sediment structures, as well as the deeper structures associated with accreted terranes, such as Pacific Rim and Crescent terranes. Shallow high velocities, as large as ∼5 km/s, were modeled in the mid-shelf region at ∼1.5–2.0 km depth. These coincide with an anticlinal structure in the seismic data, and possibly indicate the shallowest occurrence of the volcanic Crescent terrane. In general, seismic velocities increase landward, indicating sediment over-consolidation related to the compressional regime associated with the ongoing subduction of the Juan de Fuca plate and the emplacement of Pacific Rim and Crescent accreted terranes. Seismic velocities show a sharp increase about 10 km west of Vancouver Island, possibly indicating an underlying transition to the Pacific Rim terrane. A prominent low velocity zone extending over 10 km is observed in the velocity model at 800–900 m below the seafloor. This possibly indicates the presence of a high porosity layer associated with lithology changes. Alternatively, this may indicate fluid over-pressure or over-pressured gas in this potential hydrocarbon environment.

Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

Prediction of lateral variations in reservoir properties throughout an interpreted seismic horizon using an artificial neural network

Successful use of an artificial neural network is shown to predict lateral variations of seismic velocity, density, thickness, and gamma rays associated with sand dune reservoirs identified on a previously interpreted seismic horizon. The work is presented in two main sections. Section one is a feasibility analysis based on synthetic data. A known geologic model is used, performed by pseudowells, in which lateral variations in seismic velocity, density, and gamma ray values are related to the dunes. The synthetic seismic model and the attributes derived are used as training input in the neural network. Section two is a real case example where the methodology is applied to a real seismic data set. Results indicate that using a set of data and attributes restricted to a time interval corresponding to a previously interpreted seismic horizon is more efficient than using a whole data cube, involving a very large volume of data.

Uppermost mantle velocity from Pn tomography in the Gulf of Aden

We determine the lateral variations in seismic velocity of the lithospheric mantle beneath the Gulf of Aden and its margins by inversion of Pn (upper mantle high-frequency compressional P wave) traveltimes. Data for this study were collected by several temporary seismic networks and from the global catalogue. A least-squares tomographic algorithm is used to solve for velocity variations in the mantle lithosphere. In order to separate shallow and deeper structures, we use separate inversions for shorter and longer ray path data. High Pn velocities (8.2–8.4 km/s) are observed in the uppermost mantle beneath Yemen that may be related to the presence of magmatic underplating of the volcanic margins of Aden and the Red Sea. Zones of low velocity (7.7 km/s) are present in the shallow upper mantle beneath Sana’a, Aden, Afar, and along the Gulf of Aden that are likely related to melt transport through the lithosphere feeding active volcanism. Deeper within the upper mantle, beneath the Oman margin, a low-velocity zone (7.8 km/s) suggests a deep zone of melt accumulation. Our results provide evidence that the asthenosphere undergoes channelized flow from the Afar hotspot toward the east along the Aden and Sheba Ridges.

Lithospheric expression of geological units in central and eastern North America from full waveform tomography

The EarthScope TA deployment has provided dense array coverage throughout the continental US and with it, the opportunity for high resolution 3D seismic velocity imaging of both lithosphere and asthenosphere in the continent. Building upon our previous long-period waveform tomographic modeling in North America, we present a higher resolution 3D isotropic and radially anisotropic shear wave velocity model of the North American lithospheric mantle, constructed tomographically using the spectral element method for wavefield computations and waveform data down to 40 s period. The new model exhibits pronounced spatial correlation between lateral variations in seismic velocity and anisotropy and major tectonic units as defined from surface geology. In the center of the continent, the North American craton exhibits uniformly thick lithosphere down to 200–250 km, while major tectonic sutures of Proterozoic age visible in the surface geology extend down to 100–150 km as relatively narrow zones of distinct radial anisotropy, with Vsv>Vsh. Notably, the upper mantle low velocity zone is present everywhere under the craton between 200 and 300 km depth. East of the continental rift margin, the lithosphere is broken up into a series of large, somewhat thinner (150 km) high velocity blocks, which extend laterally 200–300 km offshore into the Atlantic Ocean. Between the craton and these deep-rooted blocks, we find a prominent narrow band of low velocities that roughly follows the southern and eastern Laurentia rift margin and extends into New England. We suggest that the lithosphere along this band of low velocities may be thinned due to the combined effects of repeated rifting processes and northward extension of the hotspot related Bermuda low-velocity channel across the New England region. We propose that the deep rooted high velocity blocks east of the Laurentia margin represent the Proterozoic Gondwanian terranes of pan-African affinity, which were captured during the Rodinia formation but left behind after the opening of the Atlantic Ocean. Our results suggest that recurring episodes of tectonic events that are well exposed at the surface also leave persistent scars in the continental lithosphere mantle, marked by isotropic and radially anisotropic velocity anomalies that reach as deep as 100–150 km.

Combining controlled-source seismology and local earthquake tomography to derive a 3-D crustal model of the western Alpine region

SUMMARY We present a newly developed approach of combining controlled-source seismology (CSS) and local earthquake tomography (LET) data to obtain a new 3-D crustal model of the western Alpine region. Our approach combines either data by taking into account the strengths of the individual seismic methods. Our western Alpine 3-D model is primarily based on a welldefined Moho, constrained by CSS and LET data, and includes smooth lateral variations in seismic velocities mainly constrained by LET data, but locally also by CSS data. The consistent combination of results from the two different seismic methods is feasible due to LET Moho elements, as defined by characteristic P-wave velocities and their uncertainty estimates. These uncertainty estimates are based on values of the diagonal element of the resolution matrix, absolute P-wave velocities that are typical for crust and mantle and a specific velocity gradient across the Moho discontinuity. Finally, our definition of LET Moho elements and their uncertainties is validated by comparisons of highest quality Moho results from both methods coinciding in 353 localities. Our model clearly shows three Moho surfaces, being Europe, Adria and Liguria as well as major tectonic structures like suture zones and the high-velocity Ivrea body. In general, it is in a good agreement with previous studies. The biggest differences occur along plate boundaries, where the strong lateral velocity variations arebestresolvedbyLET.DuetothelargernumberofavailableMohoreflectorelementsamore accurate definition of plate boundaries at Moho level is possible and, therefore, new insights in deep lithosphere structures of the Alpine collision zone can be expected. Furthermore, our new 3-D crustal model directly includes a 3-D migrated image of the Ivrea body.

Non‐uniqueness with refraction inversion – the Mt Bulga shear zone

ABSTRACTThe tau‐p inversion algorithm is widely employed to generate starting models with many computer programs that implement refraction tomography. However, this algorithm can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide shear zone, which is the subject of this study.By contrast, the shear zone is successfully defined with the inversion algorithms of the generalized reciprocal method. The shear zone is confirmed with a 2D analysis of the head wave amplitudes, a spectral analysis of the refraction convolution section and with numerous closely spaced orthogonal seismic profiles recorded for a later 3D refraction investigation.Further improvements in resolution, which facilitate the recognition of additional zones with moderate reductions in seismic velocity, are achieved with a novel application of the Hilbert transform to the refractor velocity analysis algorithm. However, the improved resolution also requires the use of a lower average vertical seismic velocity, which accommodates a velocity reversal in the weathering. The lower seismic velocity is derived with the generalized reciprocal method, whereas most refraction tomography programs assume vertical velocity gradients as the default.Although all of the tomograms are consistent with the traveltime data, the resolution of each tomogram is comparable only with that of the starting model. Therefore, it is essential to employ inversion algorithms that can generate detailed starting models, where detailed lateral resolution is the objective.Non‐uniqueness can often be readily resolved with head wave amplitudes, attribute processing of the refraction convolution section and additional seismic traverses, prior to the acquisition of any borehole data. It is concluded that, unless specific measures are taken to address non‐uniqueness, the production of a single refraction tomogram that fits the traveltime data to sufficient accuracy does not necessarily demonstrate that the result is either correct, or even the most probable.

Exploiting lateral resolution of near-surface seismic refraction methods

The 1D τ-p inversion algorithm is widely employed to generate starting models with most computer programs that implement refraction tomography. However, this algorithm emphasizes the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases that are indicative of shear zones. This study presents a case that demonstrates the failure of the 1D τ-p inversion algorithm to define or even detect a major shear zone that is 50 m or ten stations wide. Furthermore, the majority of refraction tomography programs parameterize the seismic velocities within each layer with vertical velocity gradients. By contrast, the 2D generalized reciprocal method (GRM) inversion algorithms emphasize the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50-m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is corroborated by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, a 1D analysis of the head wave amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. While all seismic refraction operations should aim to provide as accurate depth estimates as is practical, the major conclusion reached in this study is that refraction inversion algorithms that emphasize the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D profiles in which the structural features can be recognized from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features that do not display any intrinsic variation or “signature” in seismic velocities.

Maximising the lateral resolution of near-surface seismic refraction methods

The tau-p inversion algorithm is widely employed to generate starting models with most computer programs, which implement refraction tomography. This algorithm emphasises the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases which are indicative of shear zones. This study demonstrates the failure of the tau-p inversion algorithm to detect or define a major shear zone which is 50 m or 10 stations wide. Furthermore, the majority of refraction tomography programs parameterise the seismic velocities within each layer with vertical velocity gradients.By contrast, the Generalized Reciprocal Method (GRM) inversion algorithms emphasise the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50 m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is confirmed by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, an analysis of the shot record amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers.The major conclusion reached in this study is that while all seismic refraction operations should aim to provide as accurate depth estimates as is practical, those which emphasise the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D traverses in which the structural features can be recognised from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features such as faults which do not display any intrinsic variation or ‘signature’ in seismic velocities.

Influence of phase transformations on lateral heterogeneity and dynamics in Earth's mantle

Using a self-consistent computation of phase equilibria and physical properties, we determine isobaric velocity–temperature and velocity–density scalings as a function of depth, focusing on the upper 800 km. The scalings contain an isomorphic part due to the influence of temperature on the physical properties of individual phases, and a metamorphic part due to variation of phase abundances and compositions with temperature. We show that the contribution from phase transformations is comparable in magnitude to that of temperature alone, and has important consequences for mantle structure. Both scalings are highly non-linear functions of temperature and depth even in the elastic limit due to the influence of phase transitions: near sharp phase transitions seismic velocities become much more sensitive to temperature. We expect the magnitude of lateral variations in seismic velocity to vary rapidly with depth. This result has important implications for the interpretation of smoothed tomographic models, particularly in the upper 1000 km, and possibly the bottom few hundred km, where phase transformations have a large influence on the structure. It will be important to include the metamorphic contribution of the scalings in geodynamical studies relating seismic structure to thermal structure or the gravity field. We find that the combined phase buoyancy of transitions near 520 km depth is equal in magnitude to that of the olivine to wadsleyite transition and should be included in future dynamical studies.

Seismic imaging of a hot upwelling beneath the British Isles: Comment: COMMENT

Arrowsmith et al. present a three-dimensional model for the crust and mantle beneath Britain, which resolves lateral variations in seismic velocity of ~±1.5%, corresponding to temperature anomalies of ~±100 °C. Despite the distance of up to ~2000 km from Iceland, they assert that these anomalies

The evolution of lithospheric mantle beneath the Kalahari Craton and its margins

The compositional structure and thermal state of the subcontinental lithospheric mantle (SCLM) beneath the Kalahari Craton and the surrounding mobile belts have been mapped in space and time using >3400 garnet xenocrysts from >50 kimberlites intruded over the period 520–80 Ma. The trace-element patterns of many garnets reflect the metasomatic refertilisation of originally highly depleted harzburgites and lherzolites, and much of the lateral and vertical heterogeneity observed in the SCLM within the craton is the product of such metasomatism. The most depleted, and possibly least modified, SCLM was sampled beneath the Limpopo Belt by early Paleozoic kimberlites; the SCLM beneath other parts of the craton may represent similar material modified by metasomatism during Phanerozoic time. In the SW part of the craton, the SCLM sampled by “Group 2” kimberlites (>110 Ma) is thicker, cooler and less metasomatised than that sampled by “Group 1” kimberlites (mostly ≤95 Ma) in the same area. Therefore, the extensively studied xenolith suite from the Group 1 kimberlites probably is not representative of primary Archean SCLM compositions. The relatively fertile SCLM beneath the mobile belts surrounding the craton is interpreted as largely Archean SCLM, metasomatised and mixed with younger material during Paleoproterozoic to Mesoproterozoic rifting and compression. This implies that at least some of the observed secular evolution in SCLM composition worldwide may reflect the reworking of Archean SCLM. There are strong correlations between mantle composition and the lateral variations in seismic velocity shown by detailed tomographic studies. Areas of relatively low Vp within the craton largely reflect the progressive refertilisation of the Archean root during episodes of intraplate magmatism, including the Bushveld (2 Ga) and Karroo (ca. 180 Ma) events; areas of high Vp map out the distribution of relatively less metasomatised Archean SCLM. The relatively low Vp of the SCLM beneath the mobile belts around the craton is consistent with its fertile composition. The seismic data may be used to map the lateral extent of different types of SCLM, taking into account the small lateral variations in the geotherm identified using the techniques described here.

Small‐scale convection in the D″ layer

Small‐scale convection has been suggested as a possible explanation for seismic heterogeneities in the D″ layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core‐mantle boundary suggest that small‐scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D″ discontinuity can represent both convective and chemical boundary and that the presence or absence of small‐scale convection can be responsible for the observed intermittent nature of the D″ discontinuity. Most lateral heterogeneities in the D″ layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core‐mantle boundary, and the viscosity of the D″ layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow‐velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow‐velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

Faulting and overburden and bedrock seismic velocities at Buchans and Gullbridge, Newfoundland, from seismic refraction measurements: applications to shallow geology and exploration

P-wave arrival times from seismic reflection surveys recorded at two mine sites in Newfoundland were used to estimate lateral variations in seismic velocity in uppermost bedrock. The abundant information on medium- to long-wavelength lateral variations in seismic velocity in bedrock is a by-product of refraction statics analyses, which has rarely been fully exploited to constrain shallow geological interpretation. Local minima in seismic velocity in bedrock are associated with faults, showing that subvertical features that do not produce observable reflections can also be mapped. Significant variations in seismic velocity also indicate variations in lithology or formation boundaries. Variations in overburden and weathering thicknesses are accurately defined at Gullbridge where the careful choice of shot–receiver configuration allows estimates of both lateral and vertical variations in the seismic velocities of glacial overburden. The information on overburden properties is of value in drift prospecting for minerals and in studies of Quaternary geology.

Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin

Abstract The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Errors in hypocenter location: Picking, model, and magnitude dependence

AbstractThe location procedures of seismic events are influenced by two major classes of errors, the error in picking individual seismic phases and modeling error due to the departure of the real Earth from the reference model used in the location. Both classes of error influence the estimate of location and it is difficult to separate them.The role of picking errors can be assessed by a nonlinear analysis using a Monte Carlo procedure. Arrivals times are perturbed with random numbers drawn from a normal distribution, and the event is relocated using these perturbed arrival times. By repeating the procedure many times, a cluster of locations is obtained, which can be used to investigate the effects of picking errors on the hypocenter. This analysis is insensitive to velocity-model errors as these are fixed for a given combination of stations and phases. Some care must be exercised when analysing multidimensional distributions in two-dimensional slices because of a projection effect.The modeling error due to the influence of lateral heterogeneity in the Earth is examined by comparing the locations of the same event using different combinations of phases and network geometries, which reinforces the need to use arrivals other than P for accurate depth resolution. The sensitivity of P arrivals to changes in depth are swamped by model errors, and inclusion of depth-sensitive phases such as pP is highly recommended. The effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only P arrivals are used. Consequently, the Monte Carlo analysis, which is primarily aimed at picking errors only, is most appropriate when multiple phases have been used to more accurately constrain the hypocenter, especially for the depth component.Altering the type of phase data used in the location plays a similar role in changing the network geometry, in that both are mechanisms that influence the nature of the constraint on the hypocenter. By relocating events with network geometries corresponding to the different magnitudes, it is found that the location of the event can be affected significantly by the magnitude, and when using robust statistics to describe earthquake residuals, the mislocation can occur in a systematic manner. The effect is marked in regions with significant lateral variations in seismic velocities. For example, low-magnitude events in the Flores Sea are found to be dragged toward Australia as a result of the fast paths to Australian stations relative to the iasp91 reference velocity model.

Generation of long wavelength heterogeneity in the mantle by the dynamic interaction between plates and convection

Lateral variations in seismic velocity (through its dependence on temperature) can easily be generated at the gravest harmonics, including degrees one and two, by the dynamic interaction between plates and convection. Models of thermal convection with a single non‐subducting plate have been formulated in a cylindrical geometry. Plates of width one to four times times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath. During rapid motion off of hot regions, a large‐scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings. Both insulation and plume‐plume collisions pump energy into the lower wavenumber harmonics.

Velocity‐depth ambiguity of reflection traveltimes

The conversion of time horizons to depth is fundamental to exploration geophysics. The interval velocity used in the conversion is often estimated from the stacking velocity, assuming that each layer’s interval velocity is homogeneous. However, even for one laterally inhomogeneous layer above a flat reflector the stacking velocity can swing violently about its average and conventional methods of velocity estimation fail. I show that violent swings in the stacking velocity are a symptom of a long‐wavelength ambiguity between the burial depth to an interface and interval velocity. Lateral variations in seismic velocity with a spatial wavelength of about 2.7 D, where D is the depth to the reflecting horizon, cannot be unambiguously resolved from traveltime measurements. The spatial wavelength of this ambiguous component varies from 2.57 D, for very small source‐receiver separations, to 2.86 D for source‐receiver separations equal to D. Spectral components of the stacking velocity at wavelengths shorter than this ambiguous value are amplified in size and reversed in polarity relative to the interval velocity. A practical inverse filter that corrects for these distortions produces an interval velocity that is almost totally lacking in low‐frequency components, giving a very distorted picture of the interval velocity. Since the wavelength of total ambiguity changes with offset, a complete description of the velocity and depth fields can, in theory, be extracted from a combination of multiple‐offset traveltime measurements. However, the wavelength of total ambiguity is such a weak function of source‐receiver separation that multiple offset processing, in practice, does little to resolve the ambiguity. In fact, the Rayleigh resolution limit implies that three or more offset measurements are more effective than two only if the seismic‐line length is at least 20 D. In a series of numerical experiments with the line set to 100 D and a spatial noise level of .01% in each channel I used a two‐channel Wiener filter to successfully extract the full‐band response for a simultaneous step change in velocity and in depth. The method fails for lines shorter than 20 D because of the transients that arise when the data are shorter than the filter. Stability was achieved by increasing the noise level to 1% in the design of the Wiener filter, but low spatial frequencies were lost and the estimated velocity‐depth model was distorted. If the results of this single flat‐layer analysis apply to practical situations, the velocity‐depth ambiguity may continue to plague exploration seismologists for some time to come.

The interpretation of expanding spread reflection profiles: examples from central and eastern Australia

The Australian Bureau of Mineral Resources has recorded expanding reflection spreads coincident with regional deep Seismic reflection profiles on land, both in sedimentary basins and in areas of exposed basement rocks. Arrival times of refracted, reflected and diffracted signals have been matched with synthetic times computed by ray tracing to give one- or two-dimensional velocity models for the regions immediately below the expanding spreads. Significant lateral variations in Seismic velocity in both sedimentary and basement rocks over distances of about 20 km are widespread, making it difficult to fit refraction, near-vertical incidence and moderate- to wide-angle reflection data with reasonably simple velocity models. One expanding spread recorded across the Nebine Ridge in southern Queensland shows P-to-SV and SV-to-P converted reflections from a bright spot in the coincident near-vertical incidence reflection data at a depth of about 17 km. Another expanding spread recorded in the Amadeus Basin has provided a detailed velocity model for the 9 km thick sedimentary section. The resolution of deep basement velocities is best below the Surat Basin, although the absence of persistent, continuous reflections and the presence of reflections from outside the vertical plane through sources and receivers limits the reliability of estimates of basement velocities.