Marine Streamer Data Research Articles

Pseudoelastic pure P-mode wave equation

We have developed a pseudoelastic wave equation describing pure pressure waves propagating in elastic media. The pure pressure-mode (P-mode) wave equation uses all of the elastic parameters (such as density and the P- and S-wave velocities). It produces the same amplitude variation with offset (AVO) effects as PP-reflections computed by the conventional elastic wave equation. Because the new wave equation is free of S-waves, it does not require finer grids for simulation. This leads to a significant computational speedup when the ratio of pressure to S-wave velocities is large. We test the performance of our method on a simple synthetic model with high-velocity contrasts. The amplitude admitted by the pseudoelastic pure P-mode wave equation is highly consistent with that associated with the conventional elastic wave equation over a large range of incidence angles. We further verify our method’s robustness and accuracy using a more complex and realistic 2D salt model from the SEG Advanced Modeling Program. The ideal AVO behavior and computational advantage make our wave equation a good candidate as a forward simulation engine for performing elastic full-waveform inversion, especially for marine streamer data sets.

Subbasalt Marchenko imaging with offshore Brazil field data

Subbasalt imaging for hydrocarbon exploration faces challenges with the presence of multiple scattering, attenuation, and mode conversion as seismic waves encounter highly heterogeneous and rugose basalt layers. A combination of modern seismic acquisition that can record densely sampled data and advanced imaging techniques make imaging through basalt feasible. Yet, the internal multiples, if not properly handled during seismic processing, can be mapped to reservoir layers by conventional imaging methods, misguiding geologic interpretation. Traditional internal multiple elimination methods suffer from the requirement of picking horizons of multiple generators and/or a top-down adaptive subtraction process. Marchenko imaging provides an alternative solution to directly remove the artifacts due to internal multiples, without the need of horizon picking or subtraction. In this paper, we evaluate a successful application of direct Marchenko imaging for subbasalt demultiple and imaging with an offshore Brazil field data set. The internal multiples in this example are generated from the seabed and basalt layers, causing severe artifacts in conventional seismic images. We have determined that these artifacts are largely suppressed with Marchenko imaging, and we have developed a general workflow for data preprocessing and regularization of marine streamer data sets. We find that horizontally propagating waves also can be reconstructed by the Marchenko method at far offsets.

Elastic Least-Squares Imaging in Tilted Transversely Isotropic Media for Multicomponent Land and Pressure Marine Data

Traditional elastic reverse-time migration (RTM) involves P-/S-wave separation for the source and receiver wavefields, followed by applying the zero-lag cross-correlation imaging condition to produce PP and PS images. In anisotropic media, P-/S-wave decomposition requires a higher memory and computational cost than that in isotropic media. In addition, finite acquisition apertures and band-limited source functions result in unsatisfactory resolutions and amplitudes. To mitigate these problems, we present an elastic least-squares imaging method for tilted transversely isotropic media and apply it to land multicomponent and marine pressure data. Unlike traditional RTM, we use the relative perturbations to the product of density and squared axial (compressional/shear) velocities as reflectivity models ($$\Delta \ln{C}_{33}$$ and $$\Delta \ln{C}_{55}$$), and estimate them by solving a linear inverse problem. Numerical experiments illustrate that subsurface reflectors can be well resolved in adjoint images for land multicomponent data, because of the presence of both P- and S-waves in seismograms. Least-squares migration helps to further improve spatial resolution and image amplitudes. Since there are no direct S-waves in marine streamer data, adjoint RTM images of $$\Delta \ln{C}_{55}$$ are mainly resolved with the converted S-waves and are not as good as those in $$\Delta \ln{C}_{33}$$ images. By approximating the Hessian inverse, least-squares migration allows us to take advantage of the weak converted P–S–P-waves and improve the $$\Delta \ln{C}_{55}$$ image quality. Numerical experiments for synthetic and field data demonstrate the feasibility and advantage of the proposed least-squares TTI RTM compared with wave-mode separation-based elastic RTM. In field data experiments, we observe that since there are no strong P–S–P converted waves in streamer pressure records from the marine survey, the reflectors in $$\Delta \ln{C}_{55}$$ image might be mainly imaged from P-waves due to the amplitude versus offset (AVO) effects.

Optimal full-waveform inversion strategy for marine data in azimuthally rotated elastic orthorhombic media

The orthorhombic (ORT) anisotropic description of earth layers can allow the capture of much of the earth’s anisotropic complexity. The inversion for high-resolution azimuthal variation of anisotropy is important for reservoir characterization, among other applications. A high-resolution description of the azimuth of fractures can help us to predict flow preferences. To verify the feasibility of multiparameter full-waveform inversion (FWI) for marine data assuming azimuthally rotated elastic ORT media, we have analyzed the radiation patterns and gradient directions of ORT parameters to the reflection data. First, we express the gradient direction of the ORT parameters considering the azimuthal rotation of the symmetric planes. Then, to support our observations in the gradient direction, the radiation patterns of the partial derivative wavefields from each parameter perturbation are also derived under the rotated elastic ORT assumption. To find an optimal parameterization, we compare three different parameterizations: monoclinic, velocity-based, and hierarchical parameterizations. Then, we suggest an optimal multistage update strategy by analyzing the behavior of the rotation angle as a FWI target. To analyze the trade-off among parameters in different parameterizations, we calculate the gradient direction from a hockey-puck model, in which each parameter is perturbed at the different location on a horizontal layer. The trade-off analysis supports that the hierarchical parameterization provides us with more opportunities to build up subsurface models with less trade-off between parameters and less influence of the azimuthal rotation of ORT anisotropy. The feasibility of the proposed FWI strategy is examined using synthetic marine streamer data from a simple 3D reservoir model with a fractured layer.

Full‐waveform inversion with multisource frequency selection of marine streamer data

ABSTRACTThe theory and practice of multisource full‐waveform inversion of marine supergathers are described with a frequency‐selection strategy. The key enabling property of frequency selection is that it eliminates the crosstalk among sources, thus overcoming the aperture mismatch of marine multisource inversion. Tests on multisource full‐waveform inversion of synthetic marine data and also the Gulf of Mexico data show speedups of 4 × and 8 × , respectively, compared with conventional full‐waveform inversion.

Multi-source design and penta source case study from the NWS Australia

Marine towed streamer acquisition has been largely restricted to just a few geometries which are determined by the streamer separation and number of sources. Most acquisition uses two sources with 100m streamer separations, or sometimes 75m and 50m for higher resolution surveys. Greater flexibility can be gained by using more than two sources and this leads to a range of design options more commonly associated with land acquisition. The aim here is to allow greater tuning of the acquisition to meet efficiency, quality or time restraints of the survey so that so a better match is obtained between the actual and desired survey. The use of multiple sources is in part enabled by the commercialisation of interfering shot energy removal which has long been practiced in land seismic, but is much more challenging with marine streamer data due to the lack of azimuthal and offset variation of the source positions relative to the receivers. The ability to remove interfering shot energy means shotpoint intervals can be reduced, enabling multi-source designs by increasing inline fold, which is needed for successful processing of the data in domains such as 2D CMP and common-trace. With triple sources we show that the options are greatly increased to either improve efficiency or quality and sometimes both, but we can also consider 5 or penta sources with which we obtain very high density data — 6.25m cross-line cells — with acquisition efficiency. A 400km2 survey was acquired on behalf of Quadrant Energy using the 5 source method and in addition a smaller 50km2 using a conventional geometry. Direct comparisons can be made between the geometries and the simple fast-track processing shows the benefits of decreasing the cross-line sampling to fully realise the benefits of broadband data as high-frequencies are fully sampled in the cross-line domain.

An efficient first arrival picking procedure for marine streamer data

Picking first breaks is a tedious and time-consuming process. We suggest a procedure for an efficient manual first arrival picking of marine towed-streamer data. The procedure contains three methods: we reduce the total number of manual picks by skipping shots and interpolating picks of each shot gather, reusing previous picks, and sorting three-dimensional (3D) shot gathers using offset. Two-dimensional and 3D simulations demonstrate that we can accelerate the picking process 11–117 times compared to that of a complete picking method. Although the efficiency varies depending on the data, the suggested procedure can save considerable manpower while maintaining the accuracy of manual picking.

Compensating for source and receiver ghost effects in full waveform inversion and reverse time migration for marine streamer data

In conventional marine seismic surveys, due to the time-delayed reflections from the sea surface on both source and receiver sides, ghosts present in recorded seismograms and lead to both phase and spectrum distortions (especially near certain frequency notches). To achieve a high-quality broad-band image/velocity model with conventional reverse time migration (RTM)/full waveform inversion (FWI) that adopts a synthetic zero-phase source wavelet and absorbing surface condition during wavefield modelling, marine seismic data have to be pre-processed to remove ghost effects. However, seismic deghosting is not a trivial task. Instead of employing an external deghosting process, we propose a strategy to compensate for ghost effects during FWI and RTM, which consists of two parts: first, to address phase distortions due to ghost effects by means of obtaining an accurate source wavelet estimation and adopting an appropriate surface boundary condition in both forward and backward wave propagation to appropriately generate ghosts; secondly, to build a compensation operator in the adjoint state computation to mitigate spectrum distortions caused by dominant ghost effects. To demonstrate the success and robustness of the proposed strategy, we present both synthetic experiments and field examples, which suggest that this strategy can lead to successful applications of FWI/RTM directly on marine seismic data without an extra deghosting process.

Wavefield tomography based on local image correlations

The estimation of a velocity model from seismic data is a crucial step for obtaining a high‐quality image of the subsurface. Velocity estimation is usually formulated as an optimization problem where an objective function measures the mismatch between synthetic and recorded wavefields and its gradient is used to update the model. The objective function can be defined in the data‐space (as in full‐waveform inversion) or in the image space (as in migration velocity analysis). In general, the latter leads to smooth objective functions, which are monomodal in a wider basin about the global minimum compared to the objective functions defined in the data‐space. Nonetheless, migration velocity analysis requires construction of common‐image gathers at fixed spatial locations and subsampling of the image in order to assess the consistency between the trial velocity model and the observed data. We present an objective function that extracts the velocity error information directly in the image domain without analysing the information in common‐image gathers. In order to include the full complexity of the wavefield in the velocity estimation algorithm, we consider a two‐way (as opposed to one‐way) wave operator, we do not linearize the imaging operator with respect to the model parameters (as in linearized wave‐equation migration velocity analysis) and compute the gradient of the objective function using the adjoint‐state method. We illustrate our methodology with a few synthetic examples and test it on a real 2D marine streamer data set.

An algorithm adapting encoded simultaneous-source full-waveform inversion to marine-streamer acquisition data

In recent years, the encoded simultaneous-source full-waveform inversion (ESSFWI) algorithm, which can reduce the computational cost of the individual-source full-waveform inversion (FWI), has been studied by many researchers. Although the ESSFWI technique can be successfully applied to fixed-spread acquisition data, the application of this technique to marine-streamer data is difficult because of the acquisition geometry. The difference in the acquisition geometry between the observed and modeled data produces missing trace parts in a stacked seismogram and unwanted events in the residual seismogram. To avoid these unwanted residuals, the correlation-based ESSFWI technique has been suggested by several researchers. However, the correlation-based approach unpredictably modifies the residual seismograms, which may result in distortion of the inverted velocity model. We developed an alternative approach to properly cope with the acquisition geometry problem. Our algorithm modified the offset-limited observed data to full-offset data by combining the original observed data with the modeled data. This modification led to an undistorted residual seismogram for the simultaneous multiple shot data. However, if we applied this modification process at every iteration of the inversion, the computational cost increased. To accelerate our ESSFWI algorithm, we proposed a strategy that modified the observed data at the iterations corresponding to the square numbers. Through the numerical examples, we discovered that our ESSFWI strategy better described the velocity structure of Marmousi-2 model than the correlation-based approach. Based on the comparison of computational speed-up for several strategies, we also confirmed that our ESSFWI algorithm using square numbers was a reasonable strategy. An example using real data found that our algorithm is slightly sensitive to noise, but we could minimize this sensitivity to noise by using a reasonable strategy to reduce the problem of missing traces. As a result, we could perform an effective ESSFWI even for marine-streamer acquisitions.

공간적인 알리아싱을 포함한 탄성파 트레이스의 내삽을 위한 요소파 기반의 Matching Pursuit 기법

탄성파 탐사자료의 획득 시, 기계적인 결함이나 지형적인 접근성 등으로 인하여 자료를 부분적으로 획득하지 못하거나, 해상 탐사의 경우 스트리머의 간격으로 인하여 공간적인 알리아싱(aliasing)이 발생할 수 있다. 이러한 불규칙하고 공간적인 알리아싱이 존재하는 자료는 추후 탄성파 자료처리과정에서 문제를 일으키게 된다. 만약, 정확하고 효율적인 내삽(interpolation) 기법을 보유하고 있다면, 이러한 문제를 해결할 수 있으며, 나아가 탐사를 수행할 때 송신원 또는 수신기의 수를 줄일 수 있으므로 탐사의 시간 및 비용이 줄어들게 된다. 여러 내삽 방법들 중 Matching Pursuit 기법을 이용한 내삽 방법은 고르기(sampling)의 형태와 상관없이 적용이 가능하다는 장점을 가지고 있다. 그러나 정현파 함수(sinusoidal function)을 기저함수로 이용하는 경우, 공간적인 알리아싱이 존재하는 탄성파자료에 적용시 정확한 내삽이 불가능하다는 한계를 지니고 있다. 따라서 본 연구에서는 공간적인 알리아싱의 문제를 해결하기 위하여, 정현파 함수가 아닌 요소파(wavelet)를 기저함수로 이용하는 요소파 기반의 Matching Pursuit 기법을 개발하였다. 또한, L-2 norm을 이용한 기존의 방법과 달리, 내적을 이용하여 내삽의 수행속도를 향상시켰다. Due to mechanical failure or geographical accessibility, the seismic data can be partially missed. In addition, it can be coarsely sampled such as crossline of the marine streamer data. This seismic data that irregular sampled and spatial aliased may cause problems during seismic data processing. Accurate and efficient interpolation method can solve this problem. Futhermore, interpolation can save the acquisition cost and time by reducing the number of shots and receivers. Among various interpolation methods, the Matching Pursuit method can be applied to any sampling type which is regular or irregular. However, in case of using sinusoidal basis function, this method has a limitation in spatial aliasing. Therefore, in this study, we have developed wavelet based Matching Pursuit method that uses wavelet instead of sinusoidal function for the improvement of dealiasing performance. In addition, we have improved interpolation speed by using inner product instead of L-2 norm.

Multisource waveform inversion of marine streamer data using normalized wavefield

Multisource full-waveform inversion based on the [Formula: see text]- and [Formula: see text]-norm objective functions cannot be applied to marine streamer data because it does not take into account the unmatched acquisition geometries between the observed and modeled data. To apply multisource full-waveform inversion to marine streamer data, we construct the [Formula: see text]- and [Formula: see text]-norm objective functions using the normalized wavefield. The new residual seismograms obtained from the [Formula: see text]- and [Formula: see text]-norms using the normalized wavefield mitigate the problem of unmatched acquisition geometries, which enables multisource full-waveform inversion to work with marine streamer data. In the new approaches using the normalized wavefield, we used the back-propagation algorithm based on the adjoint-state technique to efficiently calculate the gradients of the objective functions. Numerical examples showed that multisource full-waveform inversion using the normalized wavefield yields much better convergence for marine streamer data than conventional approaches.

Inversion of refractions and reflections by full-waveform inversion for marine streamer data: Classification of problem types and solution methods

We classify full-waveform inversion (FWI) projects according to the types of events used to invert marine streamer data. In general, an adequate model update requires the seamless introduction of wavenumbers that are missing in the starting model. It has been our experience that a seamless update may require extraction of the lowest possible wavenumbers from the data, as well as the application of preprocessing procedures and scattering kernels that are optimal for each type of arrival. In a shallow-water environment, and with sufficient offsets, refracted arrivals and diving waves can provide an adequate inversion for shallow velocity structures. We demonstrate this principle using the linearized (Born) scattering kernel for field data from the North Sea.

Nongeometrically converted shear waves in marine streamer data

Under certain circumstances, marine streamer data contain nongeometrical shear body wave arrivals that can be used for imaging. These shear waves are generated via an evanescent compressional wave in the water and convert to propagating shear waves at the water bottom. They are called “nongeometrical” because the evanescent part in the water does not satisfy Snell’s law for real angles, but only for complex angles. The propagating shear waves then undergo reflection and refraction in the subsurface, and arrive at the receivers via an evanescent compressional wave. The required circumstances are that sources and receivers are near the water bottom, irrespective of the total water depth, and that the shear-wave velocity of the water bottom is smaller than the P-wave velocity in the water, most often the normal situation. This claim has been tested during a seismic experiment in the river Danube, south of Budapest, Hungary. To show that the shear-related arrivals are body rather than surface waves, a borehole was drilled and used for multicomponent recordings. The streamer data indeed show evidence of shear waves propagating as body waves, and the borehole data confirm that these arrivals are refracted shear waves. To illustrate the effect, finite-difference modeling has been performed and it confirmed the presence of such shear waves. The streamer data were subsequently processed to obtain a shear-wave refraction section; this was obtained by removing the Scholte wave arrival, separating the wavefield into different refracted arrivals, stacking and depth-converting each refracted arrival before adding the different depth sections together. The obtained section can be compared directly with the standard P-wave reflection section. The comparison shows that this approach can deliver refracted-shear-wave sections from streamer data in an efficient manner, because neither the source nor receivers need to be situated on the water bottom.

Multisource least‐squares migration of marine streamer and land data with frequency‐division encoding

ABSTRACTMultisource migration of phase‐encoded supergathers has shown great promise in reducing the computational cost of conventional migration. The accompanying crosstalk noise, in addition to the migration footprint, can be reduced by least‐squares inversion. But the application of this approach to marine streamer data is hampered by the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by the finite‐difference modelling method. This leads to a strong mismatch in the misfit function and results in strong artefacts (crosstalk) in the multisource least‐squares migration image. To eliminate this noise, we present a frequency‐division multiplexing (FDM) strategy with iterative least‐squares migration (ILSM) of supergathers. The key idea is, at each ILSM iteration, to assign a unique frequency band to each shot gather. In this case there is no overlap in the crosstalk spectrum of each migrated shot gather m(x, ωi), so the spectral crosstalk product m(x, ωi)m(x, ωj) =δi, j is zero, unless i=j. Our results in applying this method to 2D marine data for a SEG/EAGE salt model show better resolved images than standard migration computed at about 1/10th of the cost. Similar results are achieved after applying this method to synthetic data for a 3D SEG/EAGE salt model, except the acquisition geometry is similar to that of a marine OBS survey. Here, the speedup of this method over conventional migration is more than 10. We conclude that multisource migration for a marine geometry can be successfully achieved by a frequency‐division encoding strategy, as long as crosstalk‐prone sources are segregated in their spectral content. This is both the strength and the potential limitation of this method.

Application of multi‐source waveform inversion to marine streamer data using the global correlation norm

ABSTRACTConventional multi‐source waveform inversion using an objective function based on the least‐square misfit cannot be applied to marine streamer acquisition data because of inconsistent acquisition geometries between observed and modelled data. To apply the multi‐source waveform inversion to marine streamer data, we use the global correlation between observed and modelled data as an alternative objective function. The new residual seismogram derived from the global correlation norm attenuates modelled data not supported by the configuration of observed data and thus, can be applied to multi‐source waveform inversion of marine streamer data. We also show that the global correlation norm is theoretically the same as the least‐square norm of the normalized wavefield. To efficiently calculate the gradient, our method employs a back‐propagation algorithm similar to reverse‐time migration based on the adjoint‐state of the wave equation. In numerical examples, the multi‐source waveform inversion using the global correlation norm results in better inversion results for marine streamer acquisition data than the conventional approach.

Elastic full waveform inversion of multicomponent ocean-bottom cable seismic data: Application to Alba Field, U. K. North Sea

Elastic full waveform inversion of multichannel seismic data represents a data-driven form of analysis leading to direct quantification of the subsurface elastic parameters in the depth domain. Previous studies have focused on marine streamer data using acoustic or elastic inversion schemes for the inversion of P-wave data. In this paper, P- and S-wave velocities are inverted for using wide-angle multicomponent ocean-bottom cable (OBC) seismic data. Inversion is undertaken using a two-dimensional elastic algorithm operating in the time domain, which allows accurate modeling and inversion of the full elastic wavefield, including P- and mode-converted PS-waves and their respective amplitude variation with offset (AVO) responses. Results are presented from the application of this technique to an OBC seismic data set from the Alba Field, North Sea. After building an initial velocity model and extracting a seismic wavelet, the data are inverted instages. In the first stage, the intermediate wavelength P-wave velocity structure is recovered from the wide-angle data and then the short-scale detail from near-offset data using P-wave data on the [Formula: see text] (vertical geophone) component. In the second stage, intermediate wavelengths of S-wave velocity are inverted for, which exploits the information captured in the P-wave’s elastic AVO response. In the third stage, the earlier models are built on to invert mode-converted PS-wave events on the [Formula: see text] (horizontal geophone) component for S-wave velocity, targeting first shallow and then deeper structure. Inversion of [Formula: see text] alone has been able to delineate the Alba Field in P- and S-wave velocity, with the main field and outlier sands visible on the 2D results. Inversion of PS-wave data has demonstrated the potential of using converted waves to resolve shorter wavelength detail. Even at the low frequencies [Formula: see text] inverted here, improved spatial resolution was obtained by inverting S-wave data compared with P-wave data inversion results.

Fourier reconstruction of marine-streamer data in four spatial coordinates

Many methods exist for interpolation of seismic data in one and two spatial dimensions, but few can interpolate properly in three or four spatial dimensions. Marine multi-streamer data typically are sampled relatively well in the midpoint and absolute offset coordinates but not in the azimuth because the crossline shot coordinate is significantly under sampled. We approach the problem of interpolation of marine-streamer data in four spatial dimensions by splitting the problem into a 1D interpolation along the densely sampled streamers and a 3D Fourier reconstruction for the remaining spatial coordinates. In Fourier reconstruction, the Fourier coefficients that synthesize the nonuniformly sampled seismic data are estimated in a least-squares inversion. The method is computationally efficient, requires no subsurface information, and can handle uniform grids with missing data as well as nonuniform grids or random sampling. The output grid of the 1D interpolation in the first step is arbitrary. When the output grid has uniform inline midpoints spacing, the 3D Fourier reconstruction in the second step is performed in the crossline midpoint, absolute offset, and azimuth coordinates. When the first step outputs to uniform absolute offset, the 3D Fourier reconstruction handles the crossline/inline midpoint and the azimuth coordinates. In both cases, the main innovation is the inclusion of the azimuthal coordinate in the Fourier reconstruction. The azimuth multiplicity must be increased for the method to be successful, which means that overlap shooting is required. We have tested the algorithm on synthetic streamer data for which the proposed method outperforms an approach where the azimuthal coordinate is ignored. Potential applications are interpolation of marine streamer data to decrease the crossline source sampling for the benefit of 3D multiple prediction and regularization to reduce sampling-related differences in processing of time-lapse data.

Over/under towed-streamer acquisition: A method to extend seismic bandwidth to both higher and lower frequencies

Seismic resolution of towed marine-streamer data is affected by free-surface reflections. They strongly modulate the spectrum reducing energy at the so-called notch frequencies, fn = iv/2z {i = 0,1,…,∞}, in which v is the water velocity and z is the source or receiver depth. As a result there is a very strong loss of useful low-frequency energy and usually a similar loss at higher frequencies. The ghost effect occurs both at the source and at the receiver. At the source, the upgoing part of the source wavefield is reflected off the free surface with inverted polarity before its delayed journey into the Earth. At the receiver, the upgoing part of the receiver wavefield is reflected off the free surface with inverted polarity before it travels down to the receiver again. The over/under acquisition method allows separation of the up- and downgoing wavefields at the source (or receiver) using a vertical pair of sources (or receivers) to determine wave direction. In this article, we present the results of two ...

On “Delayed-shot 3D depth migration” ()

We present an alternative perspective on a key equation in the paper “Delayed-shot 3D depth migration” by Zhang et al. (2005). An alternative perspective indicates that a greater number of P-values are needed for a complete delayed-shot migration of typical marine-streamer data than those presented by Zhang et al. With the alternative equation, delayed-shot WEM migration is mathematically no more efficient than shot-WEM migration for typical marine streamer data.