Reflection Traveltimes Research Articles

Acoustic studies of submarine gas hydrates on the Cascadia Margin.

Submarine gas hydrates were detected over large areas in the Cascadia margin off the west coast of British Columbia by the presence of bottom simulating reflectors in seismic surveys. Initial estimates of gas quantity based on these data and results from an ocean drilling program leg were highly optimistic, and subsequently many different experiments were carried out to quantify the estimates. This paper presents results from an air gun survey with ocean bottom seismometers deployed around a hydrated cold vent to determine the hydrate concentration in the vicinity of the vent. Reflection travel time data from the survey were used in a linearized inversion to determine the velocity structure within the hydrate stability zone. Reflection amplitude data were analyzed to generate maps of the reflectivity at the sea floor and at the base of the stability zone. The travel time inversion revealed that the sediment velocities were very low, indicating low concentration of hydrate (less than 2% of sediment volume) in the stability zone. The sea floor reflection amplitude data provided a map showing an elliptical extent of the sea floor vent. Inversion of amplitude data from the BSR indicated a small decrease in velocity, less than 7%.

Layer-stripping full waveform inversion with damped seismic reflection data

Full waveform inversion (FWI) directly minimizes errors between synthetic and observed data. For the surface acquisition geometry, reflections generated from deep reflectors are sensitive to overburden structure, so it is reasonable to update the macro velocity model in a top-to-bottom manner. For models dominated by horizontally layered structures, combination of offset/time weighting and constant update depth control (CUDC) is sufficient for layer-stripping FWI. CUDC requires ray tracing to determine reflection traveltimes at a constant depth. As model complexity increases, the multi-path effects will have to be considered. We developed a new layer-stripping FWI method utilizing damped seismic reflection data, which does not need CUDC and ray tracing. Numerical examples show that effective update depth (EUD) can be controlled by damping constants even in complex regions and the inversion result is more accurate than conventional methods.

An Improved Technique for Static Correction in a High Resolution Shallow Seismic Reflection Data using Difference in Reflection Times.

Shallow Seismic reflection survey was carried out in Zaria, located in the basement complex of central northern Nigeria, with the aim of characterising the granitic batholith. However, the effect of near surface material, which could make reflection events appear disjointed or deeper than they really are, could be enormous if not corrected for. This work sets out to correct for this events by making use of difference in reflection times estimated from initially known models of the subsurface. During the data acquisition the receivers where placed at 1 m interval, with a constant offset of 1 m. the common midpoint (CMP) method with a 12 fold coverage was employed. Previous refraction tomography model carried out in the area was used as a guide during the 2D velocity model generation, making use of the observed travel times. The observe time was used to generate the initial model, which was later corrected using the previous known subsurface model, thereby noting the difference in travel time induced by the near surface materials. The results obtained showed that the reflection events were more coherent and in their actual reflecting point on the statically corrected seismic sections, than the section without static correction because of the excess time from the near surface material with variable p wave velocity. This experiment has proved that difference in reflection travel times obtained from previous known model or borehole information in conjunction with the observed travel times serve as a better tool in applying static correction to seismic reflection data, than any previous known conventional methods that relies on a single technique. Key Words: Static correction, High resolution, Seismic reflection, Reflection times

Optimized nonhyperbolic moveout correction equation of long-offset seismic data for TI media with an arbitrary spatial orientation

The conventional long-offset nonhyperbolic moveout equation is derived for the transverse isotropic media with a vertical symmetric axis (VTI). It cannot be extended to the transverse isotropic media with an arbitrary spatial orientation of symmetry axis (ATI). In this paper, we optimize a modified long-offset nonhyperbolic moveout equation for ATI media based on the conventional nonhyperbolic moveout equation and the exact analytical solution of the quartic moveout coefficient (A4) and NMO velocity for ATI media that were derived in our previous work. Compared with the exact traveltimes of the ray-tracing algorithm for anisotropic media, this optimized equation can be used to calculate the traveltime varying with survey line azimuth in arbitrary strong or weak ATI media. It can replace the time-consuming, multi-offset, and multi-azimuth ray tracing method for forward modeling of longoffset reflection traveltimes in ATI media, which is useful to further anisotropic parameter inversion using long-offset nonhyperbolic moveout.

Unconstrained direct Dix-type inversion in a layered VTI medium

In standard Dix equations, developed for a stack of homogeneous horizontal layers with VTI symmetry, the traveltime parameters estimated from PP and PS reflections are used to compute the interval traveltime parameters. These interval traveltime parameters can be converted into the model parameters for each layer. A drawback of the standard inversion technique is its instability in estimated model parameters which require involving constraints in the inversion. I propose a method for the unconstrained direct Dix-type inversion of PP and PS reflection traveltimes into the model parameters. The method is based on the average of parameter estimates computed for each horizontal slowness or propagation angle.

Evolution from fore‐arc oceanic crust to island arc crust: A seismic study along the Izu‐Bonin fore arc

Petrological studies have suggested that oceanic crust is formed during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in the fore‐arc region. We conducted an active‐source seismic survey in the fore‐arc region of the Izu‐Bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction. We used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction‐stack migration of picked reflection travel times to derive reflectivity images. These data show a remarkably thin crust (<10 km thick) along the northern half of the Bonin ridge and abrupt crustal thickening (to ∼20 km) toward the south of the ridge. Comparison of velocity–depth profiles of the thin fore‐arc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. Boninitic magmatism is evident in the area of thin crust and tholeiitic–calcalkaline andesitic volcanism in the area of thick crust. High‐precision dating of the volcanic rocks showed that the thin fore‐arc oceanic crust was created soon after initiation of subduction (48–45 Ma) and that the nonoceanic thick crust was created with tholeiitic–calcalkaline andesitic magmatism after the boninitic magmatism was ceased. Our seismological image strongly supports the view that fore‐arc oceanic crust was formed by fore‐arc spreading during the initial stage of subduction along the Izu‐Bonin intraoceanic arc.

Downward and upward continuation of 2-D seismic data to eliminate ocean bottom topography’s effect

In order to eliminate the effect of ocean bottom topography on seismic wave field, we transformed curved (x, z) coordinate system grids into rectangular (ξ, η) coordinate system grids and derived a 2-D scalar acoustic wave equation in the ξ, η domain. The seismic wave field collected at the sea surface was downward continued to the ocean bottom by the inverse finite difference method with the water velocity and then was reversely continued to the ocean surface by the finite difference method using the layer velocity from just below the ocean bottom in the (ξ, η) domain. Simulation calculations and practical application show that this method can not only remove the reflection travel time distortion but also correct the dynamic parameter changes caused by the ocean bottom topography. The inverted velocity after wave field continuation is much more accurate than before continuation and the image section was greatly improved compared to the original wave field.

Generalized nonhyperbolic moveout approximation

Reflection moveout approximations are commonly used for velocity analysis, stacking, and time migration. A novel functional form for approximating the moveout of reflection traveltimes at large offsets is introduced. In comparison with the classic hyperbolic approximation, which uses only two parameters (zero-offset time and moveout velocity), this form involves five parameters that can be determined, in a known medium, from zero-offset computations and from tracing one nonzero-offset ray. It is called a generalized approximation because it reduces to some known three-parameter forms with a particular choice of coefficients. By testing the accuracy of the proposed approximation with analytical and numerical examples, the new approximation is shown to bring an improvement in accuracy of several orders of magnitude compared to known analytical approximations, which makes it as good as exact for many practical purposes.

3D 3-C full-wavefield elastic inversion for estimating anisotropic parameters: A feasibility study with synthetic data

Traveltime-based inversions cannot solve for all of the anisotropy parameters for orthorhombic media. Vertical velocities cannot be recovered simultaneously with the dimensionless anisotropy parameters. Also, the density cannot be solved because it does not affect the normal moveout of P and S reflections. These limitations can be overcome using full-wavefield inversion for anisotropy parameters for orthorhombic media and for transversely isotropic media with vertical and horizontal symmetry axes. Tsvankin’s parameters and the orientation of the local (anisotropic) coordinates are inverted from three-component, wide-azimuth data sets containing P reflected and PS converted waves. The inversions are performed in two steps. The first step uses only reflections from the top of an anisotropic layer, whichdoes not constrain the trade-offs between the vertical velocities, the anisotropies, and density, as shown by parameter correlation analysis. The results from the first step are refined by using them as the starting model for the second step, which fits reflections from the top and bottom of the layer. The properties of the target layer influence the amplitudes of top and bottom reflections as well as the traveltime of the bottom reflections; when all these data are used, the inversion is highly overdetermined and all model parameters are estimated accurately. When Gaussian noise is added, the inversion results are very similar to those for the noise-free data because only the coherent signal is fitted. The residual at convergence for the noisy data corresponds to the noise level. Concurrent inversion of data from multiple sources increases the azimuthal illumination of a target.

Estimation of interval anisotropy parameters using velocity-independent layer stripping

Moveout analysis of long-spread P-wave data is widely used to estimate the key time-processing parameter [Formula: see text] in layered transversely isotropic media with a vertical symmetry axis (VTI). Inversion for interval [Formula: see text] values, however, suffers from instability caused by the trade-off between the effective moveout parameters and by subsequent error amplification during Dix-type layer stripping. We propose an alternative approach to nonhyperbolic moveout inversion based on the velocity-independent layer-stripping (VILS) method of Dewangan and Tsvankin. Also, we develop the 3D version of VILS and apply it to interval parameter estimation in orthorhombic media using wide-azimuth, long-spread data. If the overburden is laterally homogeneous and has a horizontal symmetry plane, VILS produces the exact interval traveltime-offset function in the target layer without knowledgeof the velocity field. Hence, Dix-type differentiation of moveout parameters used in existing techniques is replaced by the much more stable layer stripping of reflection traveltimes. The interval traveltimes are then inverted for the moveout parameters using the single-layer nonhyperbolic moveout equation. The superior accuracy and stability of the algorithm are illustrated on ray-traced synthetic data for typical VTI and orthorhombic models. Even small correlated noise in reflection traveltimes causes substantial distortions in the interval [Formula: see text] values computed by conventional Dix-type differentiation. In contrast, the output of VILS is insensitive to mild correlated traveltime errors. The algorithm is also tested on wide-azimuth P-wave reflection data recorded above a fractured reservoir at Rulison field in Colorado. The interval moveout parameters estimated by VILS in the shale layer above the reservoir are more plausible and less influenced by noise than those obtained by the Dix-type method.

Three‐Dimensional Seismic Transmission Tomography of the Deep Earthquake Faults in Yinchuan Basin

AbstractIn order to prove the validity and application value of three‐dimensional seismic transmission technology in active fault detection, a seismic array was deployed in the middle and north of Yinchuan basin. The basement and upper crust structure under the seismic array were reconstructed based on first‐arrival refraction traveltimes and reflection traveltimes from Moho at critical distances by using finite difference method, time‐term method, and the inversion method without model blocks. The results show that the basement of the studied area is shallow in the east and west and deep in the centre and it is steeper in the west than in the east. The greatest depth of the basement occurs from Luhuatai to Xidatan, and its depth reaches 7 km there. The Luhuatai fault, Yinchuan‐Pingluo fault, and Huanghe fault all appear as NNE‐trending velocity difference strips and there is significant variation in the depth of sediment interface across the faults. The east‐dipping, high‐angle Luhuatai fault extends under the basement. The west‐dipping Yinchuan‐Pingluo fault is a buried fault penetrating the basement. Huanghe fault dips toward the west and penetrates the basement as well. These exploration results prove that the three‐dimensional seismic transmission technology, which combines first‐arrival refraction wave exploration with Moho reflection wave exploration, is applicable in the reconstruction of fine upper crustal image of the studied area. With this technology, we can not only reveal the distribution and shallow features of the major faults, but also their extension under the basement.

Indication to distinguish the burst region of coal gas from seismic data

The velocity of an over-burst coal seam is about 1/3 compared to a normal coal seam based on laboratory test results. This can be considered as a basis to confirm the area of coal and gas burst by seismic exploration technique. Similarly, the simulation result of the theoretical seismic model shows that there is obvious distinction between over-burst coal and normal coal based on the coal reflection’s travel-time, energy and frequency. The results from the actual seismic data acquired in the coal and gas over-burst cases is consistent with that of the laboratory and seismic modeling; that is, in the coal and gas burst region, seismic reflection travel time is delayed, seismic amplitude is weakened and seismic frequency is reduced. Therefore, it can be concluded that seismic exploration technique is promising for use in distinguishing coal and gas over-burst regions based on the variation of seismic reflection travel time, amplitude and frequency.

Combining borehole and surface seismic data for velocity field estimation through slope tomography

A joint traveltime and slope (or slowness) tomography method of marine surface reflections and walk-away transmitted arrivals is investigated. Based on our previous developments of 2-D isotropic slope tomography dedicated to reflection data, the transmitted arrivals are added into the inversion scheme. The results of the transmitted arrival tomography are compared to those of the joint (transmitted and reflected arrivals) tomography on synthetic data. Moreover, a quality control (QC) procedure is proposed. Finally, walk-away transmitted arrivals, surface reflected arrivals and joint tomography of both arrivals are applied on real data sets from Oseberg, North Sea. These data include 2-D surface and walk-away lines along the same direction. The resulting velocity fields are used for pre-stack depth migration, allowing us to conclude that joint inversion leads to significantly improved images both in terms of focusing and event location. A key point is that improvements are not restricted in the area covered by transmitted arrivals. As the number of available borehole receivers is small, this case study demonstrates the strong impact direct arrivals that may have on depth imaging.

Velocity-less imaging of linearly inhomogeneous media in 3D

We show that in a medium with constant velocity-gradient it is possible to find both the two velocity parameters, and the locations of the reflector for each source-receiver pair from the reflection travel times. This algorithm is a basis for a velocity-less migration; it allows for using the obtained velocity parameters and reflection position for migrating the data. The presented method relies on obtaining the derivatives of the traveltimes with respect to the locations of the sources and receivers in the two horizontal directions.

Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography

Axial morphology along the Southeast Indian Ridge (SEIR) systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100°–116°E, west of the Australian‐Antarctic Discordance (AAD). Basalt geochemistry has a consistent Indian–mid‐ocean ridge basalt (MORB) type isotopic signature, so changes in axial topography are attributed to variations in both mantle temperature and melt supply. Wide‐angle seismic refraction lines were shot to four ocean bottom hydrophones within SEIR segments P1, P2, S1, and T, where each segment is characterized by a different morphology. We constructed 2‐D crustal velocity models by jointly inverting hand‐picked P wave refraction (Pg) and Moho reflection (PmP) traveltime data using a top‐down, minimum‐structure methodology. The results show a 1.5 km eastward decrease in crustal thickness across the study area, with segment averages ranging from 6.1 km at P1 to 4.6 km at T. Melt generation models require a ∼30°C decrease in mantle temperature toward the AAD to account for the crustal thickness trend. Significant changes in axial morphology accompany small‐scale variations in crustal thickness, consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature, melt supply, and cooling from hydrothermal circulation. Layer 3 thins by 3.0 km as layer 2 thickens by 1.4 km between segments P1 and T, reflecting the eastward decrease in melt supply and increase in melt lens depth. The trade‐off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material.

Estimates of the transition zone temperature in a mechanically mixed upper mantle

Partial melting of mantle peridotite generates a physically and chemically layered oceanic lithosphere that is cycled back into the mantle in subduction zones. Stirring times of the mantle are too long to allow for complete re-homogenization of subducted basalt and harzburgite, given the low chemical diffusivity of the solid mantle. This suggests that the Earth's mantle is a mechanical mixture of basaltic and harzburgitic components. Using a recently developed thermodynamic formulism we determine the phase equilibria and the seismic properties of a mantle comprised of a mechanical mixture of basalt and harzburgite (MM) and a homogeneous mantle (EA) with identical pyrolitic bulk chemistry. We use the theoretical shear velocity profiles as a new thermometer of the mantle below the magma-genetic zone by modeling the difference ΔT 410-660 between traveltimes of shear wave reflections off the 410-km and 660-km with the potential temperature T P. ΔT 410-660 are measured from waveform stacks. They indicate that, over 1000+ km wave lengths, the temperature varies by about 200 K. Lowest and highest temperatures are resolved for the western Pacific subduction zones and the central Pacific, respectively. This variation is similar for the EA and MM and is in excellent agreement with estimates of transition zone thickness and shear velocity variations. The median value of T P for the EA is 1720 K. It is about 1625 K for the MM, a value that is in better agreement with the Normal-MORB values of 1610 ± 40 K inferred from olivine-liquid equilibria given that our sampling region encompasses the Western Pacific subduction zones and the oldest parts of the Pacific Plate. We argue therefore that a mechanical mixed mantle, with generally higher velocities and steeper velocities gradients, represents a better physical reference model than a model based on a fully equilibrated assemblage.

Racial differences in central blood pressure and vascular function in young men

Young African-American men have altered macrovascular and microvascular function. In this cross-sectional study, we tested the hypothesis that vascular dysfunction in young African-American men would contribute to greater central blood pressure (BP) compared with young white men. Fifty-five young (23 yr), healthy men (25 African-American and 30 white) underwent measures of vascular structure and function, including carotid artery intima-media thickness (IMT) and carotid artery beta-stiffness via ultrasonography, aortic pulse wave velocity, aortic augmentation index (AIx), and wave reflection travel time (Tr) via radial artery tonometery and a generalized transfer function, and microvascular vasodilatory capacity of forearm resistance arteries with strain-gauge plethysmography. African-American men had similar brachial systolic BP (SBP) but greater aortic SBP (P<0.05) and carotid SBP (P<0.05). African-American men also had greater carotid IMT, greater carotid beta-stiffness, greater aortic stiffness and AIx, reduced aortic Tr and reduced peak hyperemic, and total hyperemic forearm blood flow compared with white men (P<0.05). In conclusion, young African-American men have greater central BP, despite comparable brachial BP, compared with young white men. Diffuse macrovascular and microvascular dysfunction manifesting as carotid hypertrophy, increased stiffness of central elastic arteries, heightened resistance artery constriction/blunted resistance artery dilation, and greater arterial wave reflection are present at a young age in apparently healthy African-American men, and conventional brachial BP measurement does not reflect this vascular burden.

Constraining the anisotropy structure of the crust by joint inversion of seismic reflection travel times and wave polarizations

In the context of wide-angle seismic profiling, the determination of the physical properties of the Earth crust, such as the elastic layer depth and seismic velocity, is often performed by inversion of P- and/or S-phases propagation data supplying the geometry of the medium (reflector depths) or any other structural parameter (P- or S-wave velocity, density...). Moreover, the inversion for velocity structure and interfaces is commonly performed using only seismic reflection travel times and/or crustal phase amplitudes in isotropic media. But it is very important to utilize more available information to constrain the non-uniqueness of the solution. In this paper, we present a simultaneous inversion method of seismic reflection travel times and polarizations data of transient elastic waves in stratified media to reconstruct not only layer depth and vertical P-wave velocity but also the anisotropy feature of the crust based on the estimation of the Thomsen’s parameters. We carry out a checking with synthetic data, comparing the inversion results obtained by anisotropic travel-time inversion to the results derived by joint inversion of seismic reflection travel times and polarizations data. The comparison proves that the first procedure leads to biased anisotropic models, while the second one fits nearly the real model. This makes the joint inversion method feasible. Finally, we investigate the geometry, P-wave velocity structure and anisotropy of the crust beneath Southeastern China by applying the proposed inversion method to previously acquired wide-angle seismic data. In this case, the anisotropy signature provides clear evidence that the Jiangshan-Shaoxing fault is the natural boundary between the Yangtze and Cathaysia blocks.

Resolution of slowness and reflectors in crosswell tomography with transmission and reflection traveltimes

We sometimes encounter situations in seismic imaging in which knowing the position of key reflectors between wells would be very useful. In many crosswell data sets, both transmission and reflection traveltimes for selected reflectors can be picked. We investigated the possibility that transmission-plus-reflection crosswell traveltime tomography can determine the position of these reflectors with a high level of accuracy, thereby providing an independent way of verifying (and perhaps improving) the position of these reflectors obtained from crosswell reflection imaging. We studied the effect of combining reflection traveltimes for selected reflectors with transmission traveltimes on the resolution of the interwell slowness field and depth determination of selected reflectors. We found that theoretically, the position of reflectors is determined uniquely from transmission and reflection traveltimes in a linearized continuum formulation ofcrosswell tomography. We also computed diagonal elements of the resolu-tion matrix for two crosswell geometries based on field experiments conducted in a west Texas oil field to see what effect noise has on the accuracy of our determination of reflector depths. These computational results indicate that reflector positions are indeed very well determined for these geometries, with expected errors of [Formula: see text] of the well spacing when noise in traveltimes is [Formula: see text]. Because reflector-position parameters are so well determined, including reflection traveltimes does not degrade the resolution of the slowness field as a result of introducing additional reflector-depth parameters. Actually, the resolution of the slowness field, particularly near reflectors, improves by including reflection traveltimes, in spite of the fact that we must solve for these additional depth parameters. The improvement in slowness resolution should provide velocity models that can yield more accurate reflection images.

The Characteristic of Quartic Moveout Coefficient of P‐Wave in the TI Homogeneous Media with an Arbitrary Orientation Symmetric‐Axis

AbstractDeviations of any pure mode (non‐converted) wave reflection traveltimes from a hyperbola, called the nonhyperbolic or quartic moveout, need to be corrected properly in processing long‐spread anisotropic seismic data. Based on our exact analytic expression for the quartic moveout coefficient (A4) of pure mode (non‐converted) wave reflection on a horizontal interface in the TI media with an arbitrarily orientated symmetry axis (ATI) in space, we numerically study the characteristics of the P‐wave A4 with the CMP survey line azimuth variation. The forward modeling results show that the characteristics of the A4 are related not only to the anisotropy parameters of the TI, but also the spatial orientation of the TI. The variation characteristic of the A4 is determined by the anisotropy parameters and tilt angle of the TI symmetric‐axis, and the azimuth of the TI symmetric‐axis results in the azimuth symmetry of the A4 with the survey line variation. Our numerical results will benefit the explanation of the anisotropic seismic data and the parameter inversion.