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GoNear-surface velocities could vary with azimuth, impacting seismic data processing and interpretation. In this study, we developed a methodology to investigate the variations of near-surface velocities with azimuth, using 3D turning-ray tomography. The input data are the first arrivals selected from pre-defined azimuth sectors in terms of shot-receiverpair directions. The output velocities from tomography correspond to the selected azimuth sectors. A near-surface tomography study based on seismic data from a shallow heavy-oil reservoir in Canada has suggested that the observed azimuthal traveltime variations are not necessarily related to azimuthal (HTI) anisotropy induced by the stress field or fractures. It could also be caused by the nearsurface heterogeneity or acquisition footprint. Near-surface complexity could masquerade as anisotropy. Potentially this can influence statics and prestack imaging.

acquisition footprint, anisotropy, azimuth, azimuth sector, azimuthal, azimuthal near-surface, azimuthal turning-ray, Masquerade, near-surface complexity, near-surface heterogeneity, predefined azimuth sector, Reservoir Characterization, reservoir description and dynamics, sector, seg las vegas, seismic processing and interpretation, slowness, tomography, topography, traveltime, Upstream Oil & Gas, variation

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)

A workflow combining (1) plane-wave migration (PWM) in tilted-coordinates and (2) tomography with an automatic picking scheme can reduce turnaround time while producing accurate velocity updates. PWM in tiltedcoordinates can image steeply dipping events which can help better constraining the velocity model and better defining the salt geometry.

approach, backprojection, backprojection point, building, estimation, inversion, iteration, model, plane-wave migration, point, PWM, Reservoir Characterization, reservoir description and dynamics, salt, seg las vegas, seismic processing and interpretation, semblance, tilted coordinate, tomography, Upstream Oil & Gas, volumebased picking

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)

**Summary**

Wide azimuth towed streamer surveys (WATS) have recently proven quite successful at improving illumination and attenuating multiple energy in difficult subsalt imaging environments. However, they present new challenges in terms of survey design, acquisition and processing methodology and operations. In particular, the sheer volume of data acquired requires pragmatic approaches to data processing. A 2007 deepwater Gulf of Mexico WATS survey of exploration scale (10,000 km2) illustrates these challenges and demonstrates the need for increased fold and cross-line offset.

acquisition, crystal, exploration-scale wide azimuth towed, gather, image, Imaging, radon, Reservoir Characterization, reservoir description and dynamics, seismic processing and interpretation, sequence, source, source line, source vessel, streamer, survey, tomography, towed streamer case study, Upstream Oil & Gas, vessel, WAT

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)

Checkshot survey and sonic measurement are used to pursue perfect seismic-well tie. The prime purpose of these in-situ surveys is to find a time-depth curve that matches best with velocity model.

It is widely believed that sonic wave always travels faster than seismic wave based on the dispersive effect (Wuenschel, 1965), so travel time at any depth measured by checkshot should be greater than sonic log integrated time. However, some observations show opposite results. Figure 1 displays a real well log survey. The right curve shows the result of subtracting checkshot travel time (left) by sonic integrated time (middle). The negative result means seismic wave travels faster than sonic wave. Such observation is supposed to be caused by heterogeneity effect of checkshot survey, such as a horizontal variant velocity formation. To resolve the horizontal variant velocity in such a narrow space, cell tomography method (Stork and Clayton, 1991) is applied to derive 2D velocity model. The dispersion effect is ignored in this study.

The ray tracing approach (Stork and Clayton, 1992) is used to model the checkshot survey to detect the influence of heterogeneity on the travel time. Figure 2(a) display a pseudo velocity model with the velocities increasing steadily with depth.

Checkshot measurements are modeled by shooting on the surface with offset as 50m and the downgoing direct wave. Figure 2(a) shows a synthetic velocity model and ray paths. As shown in 2(a) , ray paths always bend into curves instead of straight line because of depth variant velocities.

Ray paths in Figure 2(a) model the checkshot survey by ray tracing method. Time-depth curve calculated from integrated inverse vertical velocity (modeling the sonic measurement) is shown in black curve in Figure 2(b) and time-depth curve from ray tracing is shown in yellow curve in Figure 2(b). Two curves appear very small deviation that means horizontally homogeneous media does not cause much difference between checkshot and sonic measurement. Then lets go on to the real data. Figure 3(a) shows a real sonic log data; (b) is the layer velocity model expanded horizontally from the sonic data and the modeled ray path of checkshot survey through the formation; (c) blue curve is the time-depth curve from real checkshot data and yellow curve is modeled time-depth curve from the ray path in (b). As shown in the Figure 3(c), modeled travel time is larger than real measurement at deeper formation that demonstrates the presence of heterogeneity at deeper part. I will derive the 2D horizontally variant velocity from the checkshot survey based on the sonic measurement by means of tomography.

Cell tomography is kind of traveltime tomography (Stork and Clayton, 1991). By this means, the velocity field is parameterized as an effective continuum of desired accuracy. This parameterization places no inherent restrictions on the structure the velocity field can take on. Three common methods for representing a continuum are with two-dimensional (2-D) splines, wavenumbers, or a grid of cells. I here use cells model.

borehole imaging, cell, checkshot, checkshot survey, checkshot-sonic microscale tomography, deviation, formation evaluation, geophysics, model, production control, production monitoring, ray, ray path, raypath, Reservoir Characterization, reservoir description and dynamics, Reservoir Surveillance, seg las vegas, slowness, time-depth curve, tomography, traveltime, Upstream Oil & Gas, wellbore seismic

Seismic imaging in depth is limited by the accuracy of velocity model estimation. Slope tomography uses the slowness components and traveltimes of picked reflection or diffraction events for velocity model building. The unavoidable data incompleteness requires additional information to assure stability to inversion. One natural constraint for ray based tomography is a smooth velocity model. We propose a new, reflectionangle- based kind of smoothness constraint as regularization in slope tomography and compare its effects to three other, more conventional constraints. The effect of these constraints are evaluated through comparison of the inverted velocity models as well as the corresponding migrated images. We find the smoothness constraints to have a distinct effect on the velocity model but a weaker effect on the migrated data. In numerical tests on synthetic data, the new constraint leads to geologically more consistent models.

Slope tomography is one of the many methods that try to determine a macrovelocity model for time or depth imaging. It uses slowness vector components to improve and stabilize the traveltime inversion. Slope tomography was initially proposed by Billette and Lambaré (1998) as a robust tomographic method for estimating velocity macro models from seismic reflection data. They had recognized the potential efficiency of traveltime tomography (Bishop et al., 1985; Farra and Madariaga, 1988) but also the difficulties associated with a highly interpretative picking. The selected events have to be tracked over a large extent of the pre-stack data cube, which is quite difficult for noisy or complex data. The idea is to use locally coherent events characterized by their slopes in the pre-stack data volume. Such events can be interpreted as pairs of ray segments and provide independent information about the velocity model.

However, the data for slope tomography are incomplete (Bishop et al., 1985). This causes depth and velocity ambiguities that depend strongly on the size of the acquisition aperture (Bube et al., 2005). Therefore, stability and convergence can only be achieved if additional information is prescribed. This additional information contains desirable properties for the solution, reducing ambiguity (Menke, 1989). It can be shown that stability is obtained only if we try to determine a smooth model of the subsurface (Delprat-Jannaud and Lailly, 1992, 1993). Moreover, for ray based inversion, smoothness is a requirement, because rough models cause the forward problem to break down during linear iterations. The use of combined smoothness constraints enables an interpretation-oriented inversion while keeping solutions consistent with the data.

We investigate the effect of different kinds of smoothness constraints in slope tomography, prescribing lateral, vertical and isotropic smoothing constraints in different combinations. Moreover, we propose a structurally motivated smoothing constraint in the direction of a potential reflector. This regularization is based on information that is contained in the data, in contrast to standard regularizations that impose global smoothness constraints. We test the different regularizations on the Marmousoft data set (Billette et al., 2003).

Slope tomography differs from conventional reflection tomography by the data that are used for the inversion (Billette et al., 2003).

Billette, constraint, coverage, difference, direction, gradient, image, information, inversion, model, reflection, reflector, regularization, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, slope tomography, smoothness constraint, tomography, traveltime, Upstream Oil & Gas

Structural and stratigraphical improvement was obtained through a detailed re-processing of 2-D seismic data acquired in North Africa. A better definition of a fault system trend and its architecture allowed reducing the uncertainty to define new locations. The fault system in conjunction with shales is a seal that creates different compartments. An unconventionally seismic workflow entirely in shotreceiver domain was applied to obtain an accurate earth image in this area with irregular topography, complex nearsurface and complex subsurface. An important effort was done for appropriately solving problems related to nearsurface statics/solution, mixed-sources, noise reduction and multiples. True-amplitude processing was considered for future AVO analysis.

The resulting velocity model was accomplished by a structurally consistent driven reflector method to shows important lateral changes. This approach includes nearsurface non-linear tomography, together with wavefield datuming providing a robust image of the reservoir.

case study, filter, migration, near-surface correction, noise, nonlinear, prestack, refraction residual static, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, shot, shot gather, stratigraphical enhancement, tomography, Upstream Oil & Gas, Vibroseis, Western Sahara Desert, Yilmaz

We apply the P-wave refraction-tomography method to seismic data collected with a landstreamer. Refractiontomography inversion solutions were determined using regularization parameters that provided the most realistic near-surface solutions that best matched the dipping layer structure of nearby outcrops. A reasonably well matched solution was obtained using an unusual set of optimal regularization parameters. In comparison, the use of conventional regularization parameters did not provide as realistic results. Thus, we consider that even if there is only qualitative a-priori information about a site (i.e., visual)-as in the case of the East Canyon Dam, Utah-it might be possible to minimize the refraction nonuniqueness by estimating the most appropriate regularization parameters.

algorithm, east canyon, east canyon dam, geophysics, inversion, Ivanov, landstreamer, layer, mapping, Miller, model, refraction, refraction tomography, regularization, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, solution, surface wave, tomography, Upstream Oil & Gas

Tomostatics using first breaks is a popular way to estimate near-surface statics due to the presence of low-velocity weathering zone. However, it is difficult to constrain the geometry of the base boundary of the weathering zone using first breaks, because the corresponding raypaths tend to be parallel with this boundary, traversing on top of the basement layer. We devise here a method of first-break deformable-layer tomostatics with constrains on the depth range of the base boundary of the weathering zone using reflections. Initial test of the method is conducted using a 2D field data set from western China. Comparison between the unconstrained and constrained tomographic models shows a similar geometry of the model layers, but the constrained portion of the base boundary of the weathering zone tends to have a sharper velocity contrast and laterally smoother than that of the unconstrained model. At many places greater than 10 ms difference exists in one-way vertical traveltimes over the weathering zone of the two models, meaning large difference in their static corrections.

base boundary, basement, boundary, constraint, correction, DLT model, geophysics, iteration, layer, model, reflection, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, shallow reflection, shot, solution, Thickness, tomography, unconstrained dlt, Upstream Oil & Gas

Full wavefield tomography has become well established in two dimensions, but its extension into 3D for realistically sized problems is computationally daunting. In this paper, we present one of the first studies to apply 3D wavefield tomography to field data, and demonstrate that the method can solve useful exploration problems that that are not tractable by other methods.

analysis, approach, approximate solution, channel, composite source, dimension, equation, frequency, high-resolution velocity model, lower panel, method, model, obtain, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, source, tomography, Upstream Oil & Gas, Wavefield Tomography

Zhou, Chaoguang (Petroleum Geo- Services) | Martínez, Jaime Ramos (Petroleum Geo- Services) | Lin, Sonny (Petroleum Geo- Services) | Jiao, Junru (Petroleum Geo- Services) | Dahl, Sverre Brandsberg (Petroleum Geo- Services)

Tomography has been widely employed for velocity model building. The typical work flow starts with the migration of an initial velocity model. This is followed by picking residual moveouts and then updating the velocity through tomography. The migration process provides common image gathers and a stack. At the tomography stage, a ray tracer is used to trace specular rays from the image points to the surface to set up the system of linear equations for the tomographic inversion by linking valid ray pairs to their corresponding residuals. For narrow azimuth (NAZ) surveys, searching for valid ray pairs is usually limited to a narrow azimuth band. Since neither the gathers nor the stack contain acquisition geometry information, the selected specular ray pairs may not reflect the true ray paths, resulting in inaccurate rays being used in the inversion process. In addition to the problem of "which rays to choose", we also have the problem of "how many rays to choose". These problems are even more difficult to handle with acquisition configurations other than NAZ, such as the wide azimuth towed-streamer (WATS) surveys, multi-azimuth (MAZ) surveys, and ocean bottom cable (OBC) surveys. To overcome these problems, we propose a tomography method that incorporates the acquisition geometry information and uses vector offsets to account for both offsets and azimuths. To address the ill-posed nature of the system of equations, we developed an anisotropic Laplacian regularization operator that allows different smoothing along different directions. We validate the method with tests on both synthetic and field data with a WATS geometry.

acquisition, acquisition geometry information, anisotropic regularization, building, direction, gather, geometry, information, migration, model, pair, ray, regularization, Reservoir Characterization, reservoir description and dynamics, residual moveout, seismic processing and interpretation, specular, survey, tomography, true geometry tomography, Upstream Oil & Gas

Thank you!