Streamer Data Research Articles

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.

PS imaging of ocean-bottom data

Opportunities and challenges exist for imaging seismic data acquired using conventional marine sources and receivers on the seafloor. Compared with conventional imaging of sea-surface streamer data, seabed acquisition and processing offer some opportunities to provide higher value of information. One of the opportunities is to use P-wave to S-wave converted reflection energy (PS imaging). Challenges include overcoming the effects of current seafloor receiver spacing, which can be large enough to negate the promised resolution gains and complicate the velocity model-building workflow. A synthetic data set illustrates possible imaging improvements that can result from seafloor acquisition as well as image degradation that can result when seafloor receivers are separated by typical current distances.

Research on improving conventional marine seismic streamer data imaging quality using OBS data from Northern South China Sea

It is very important to convert the seismic data from the time domain to the depth domain. Here we discuss the approaches of inverse modeling of travel times for determination of the P-wave velocity (Vp) using OBS data, and also using the inverted velocity to improve the conventional marine seismic streamer data imaging quality. The migration section of the single channel seismic data is used to define the model horizons and help to control their geometry. Wide angle hydrophone data of OBS are used to determine P-wave travel times. The picked travel times from various shots are inverted for P-wave interval velocities using RayInvr, which calculated theoretical travel times via ray tracing. Damped least squares optimization is performed to fine tune the fits between observed and calculated travel times. In the end, two conventional marine seismic streamer data's migration sections are compared and the result shows that the section using the inverted velocity of OBS data is much better than that using the velocity of conventional velocity analysis method to the streamer data.

Wave equation processing using finite-difference propagators, Part 2: Deghosting of marine hydrophone seismic data

We have developed a new and simple method for deghosting of conventional hydrophone streamer data towed at arbitrary variable depths. The method uses a time-space domain finite-difference (FD) solution to the wave equation with pressure field boundary conditions to predict and remove ghosts. Because it operates in the time domain, our method is unaffected by any number of notches in the frequency spectrum of the data and therefore will deghost “through notches.” Apart from the acquired hydrophone data, the method relies on the depth profile of the streamer recording the data beneath a sea surface with a known reflection coefficient as well as the propagation velocity in the water above the streamer. The method was applied to simple and more complex synthetic data, which illustrated its ability to deal with complex data and any acquisition geometry.

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.

Estimating shallow water sound power levels and mitigation radii for theR/VMarcusG.Langsethusing an 8 km long MCS streamer

For seismic surveys in shallow-water environments, the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species. This is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins. One potential solution to this problem is to measure received levels using the ship's multichannel seismic (MCS) streamer, which could allow for the dynamic real-time determination of sound levels and mitigation radii while a survey is underway. We analyze R/V Langseth streamer data collected on the shelf and slope near the Washington coast during the Cascadia Open-Access Seismic Transects (COAST) and Ridge2Trench projects to measure received levels up to a distance of approximately 8 km from the sound source array. We establish methods to filter, clean, and process streamer data to accurately determine received power levels and confidently establish mitigation radii. We show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features, but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued. The establishment of mitigation radii based on local conditions may help to maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies.

Migration of primaries and multiples using an imaging condition for amplitude-normalized separated wavefields

Migration of primary and multiple reflections leads to enhanced subsurface illumination and to increased image resolution. A joint migration approach using the complete wavefield requires properly imaged primaries and multiples of all orders. In recent works, primaries and multiples have therefore been imaged separately, using the upgoing and downgoing pressure wavefields obtained by decomposing dual-sensor streamer data. The matches between the corresponding depth images are still not found to be sufficiently accurate, so new and more appropriate imaging conditions need to be found. We reviewed the classical imaging approach used in one-way wave-equation migration, with the aim of extending it for simultaneous migration of primaries and all orders of multiples. Based on Rayleigh’s reciprocity theorem and well-known theoretical developments, we derived a new imaging condition described in terms of the upgoing pressure wavefield and a filtered version of the downgoing vertical-velocity wavefield. To evaluate the efficiency of this new imaging approach, primaries and multiples were separately migrated using synthetic and real data examples. These results were compared to those obtained using a conventional imaging condition. We found that the use of the new imaging condition led to a better match between the depth images and spectra of the migrated primaries and migrated multiples.

Shallow subsurface morphotectonics of the NW Sumatra subduction system using an integrated seismic imaging technique

In subduction zones, shallow subsurface structures are the manifestation of the plate interactions at depth. However, significant water depths, rough bathymetry and presence of heavily deformed accretionary wedge materials hamper imaging of the near-surface features to a great extent using conventional imaging techniques. In this study, we show results using an integrated processing technique to a multichannel seismic data set acquired in 2006 from the northwestern offshore Sumatra. We start with first downward continuing the 12-km-long surface streamer data to the seafloor, followed by a high-resolution traveltime tomography of refracted phases to determine a detailed velocity–depth model of subsurface, which in turns, is used for pre-stack depth migration in order to delineate the shallow subsurface structures beneath the trench, subduction front and outer accretionary wedge. Our velocity–depth model and the depth migrated image depict variation of sediment properties across the front and structures of uppermost sedimentary sequence with an unprecedented high resolution providing the precise location of the frontal and conjugate thrusts, highly folded sedimentary sequences, which in turns describe their relationship with the top of the subducting plate and factors that control rupture propagation to the trench. Furthermore, we estimate the porosity distribution across the front, where we find a 12 and 18 per cent decrease in porosity beneath the deformation front and the inner accretionary plateau at 500 m below the seafloor, respectively, which we interpret to be due to the compaction. A significant decrease in porosity at the plate interface below 5–6-km thick sediments near the deformation front would increase the coupling, leading to the rupture propagation up to the trench, uplifting 4.5 km water and producing large tsunami.

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.

Dual sensor streamer technology used in Sleipner CO2 injection monitoring

ABSTRACTCO2 has been injected into the saline aquifer Utsira Fm at the Sleipner field since 1996. In order to monitor the movement of the CO2 in the sub‐surface, the seventh seismic monitor survey was acquired in 2010, with dual sensor streamers which enabled optimal towing depths compared to previous surveys. We here report both on the time‐lapse observations and on the improved resolution compared to the conventional streamer surveys. This study shows that the CO2 is still contained in the subsurface, with no indications of leakage. The time‐lapse repeatability of the dual sensor streamer data versus conventional data is sufficient for interpreting the time‐lapse effects of the CO2 at Sleipner, and the higher resolution of the 2010 survey has enabled a refinement of the interpretation of nine CO2 saturated layers with improved thickness estimates of the layers. In particular we have estimated the thickness of the uppermost CO2 layer based on an analysis of amplitude strength together with time‐separation of top and base of this layer and found the maximum thickness to be 11 m. This refined interpretation gives a good base line for future time‐lapse surveys at the Sleipner CO2 injection site.

Fidelity and repeatability of wave fields reconstructed from multicomponent streamer data

ABSTRACTWave field reconstruction – the estimation of a three‐dimensional (3D) wave field representing upgoing, downgoing or the combined total pressure at an arbitrary point within a marine streamer array – is enabled by simultaneous measurements of the crossline and vertical components of particle acceleration in addition to pressure in a multicomponent marine streamer. We examine a repeated sail line of North Sea data acquired by a prototype multicomponent towed‐streamer array for both wave field reconstruction fidelity (or accuracy) and reconstruction repeatability. Data from six cables, finely sampled in‐line but spaced at 75 m crossline, are reconstructed and placed on a rectangular data grid uniformly spaced at 6.25 m in‐line and crossline. Benchmarks are generated using recorded pressure data and compared with wave fields reconstructed from pressure alone, and from combinations of pressure, crossline acceleration and vertical acceleration. We find that reconstruction using pressure and both crossline and vertical acceleration has excellent fidelity, recapturing highly aliased diffractions that are lost by interpolation of pressure‐only data. We model wave field reconstruction error as a linear function of distance from the nearest physical sensor and find, for this data set with some mismatched shot positions, that the reconstructed wave field error sensitivity to sensor mispositioning is one‐third that of the recorded wave field sensitivity. Multicomponent reconstruction is also more repeatable, outperforming single‐component reconstruction in which wave field mismatch correlates with geometry mismatch. We find that adequate repeatability may mask poor reconstruction fidelity and that aliased reconstructions will repeat if the survey geometry repeats. Although the multicomponent 3D data have only 500 m in‐line aperture, limiting the attenuation of non‐repeating multiples, the level of repeatability achieved is extremely encouraging compared to full‐aperture, pressure‐only, time‐lapse data sets at an equivalent stage of processing.

Multisensor streamer recording and its implications for time-lapse seismic and repeatability

Effective time-lapse, or 4D, processing seeks to extract the 4D signature with minimal background noise to help understand reservoir behavior in the production phase and to optimize producer and injector well placement. Nonrepeatability of source and sensor positions in the presence of overburden heterogeneity limits our ability to repeat seismic data and to deliver 4D signals with low background noise levels. To compensate for imperfect receiver repositioning, wavefield-regularization techniques can enable wavefield matching on a grid common to both data vintages. Often there is a trade-off between reducing the receiver-positioning mismatch and crossline interpolation error from wavefield spatial aliasing. Through the use of towed multisensor marine seismic streamers that record collocated pressure (P) and acceleration measurements (Y and Z) and the application of a matching-pursuit interpolation technique, a 3D dealiased, reconstructed, and deghosted pressure wavefield can be obtained on a densely sampled grid that permits wavefield matching to prior positions in a highly effective way. From analysis on field multisensor streamer data, implications for time-lapse seismic and repeatability are investigated.

Constraints on the shallow velocity structure of the Lucky Strike Volcano, Mid-Atlantic Ridge, from downward continued multichannel streamer data

The shallow velocity structure of the Lucky Strike segment of the Mid-Atlantic Ridge is investigated using seismic refraction and reflection techniques applied to downward continued multichannel streamer data. We present a three-dimensional velocity model beneath the Lucky Strike Volcano with unprecedented spatial resolutions of a few hundred meters. These new constraints reveal large lateral variations in P wave velocity structure beneath this feature. Throughout the study area, uppermost crustal velocities are significantly lower than those inferred from lower resolution ocean bottom seismometer studies, with the lowest values (1.8–2.2 km/s) found beneath the three central volcanic cones. Within the central volcano, distinct shallow units are mapped that likely represent a systematic process such as burial of older altered surfaces. We infer that the entire upper part of the central volcano is young relative to the underlying median valley floor and that there has been little increase in the layer 2A velocities since emplacement. Layer 2A thins significantly across the axial valley bounding faults likely as the result of footwall uplift. The upper crustal velocities increase with age, on average, at a rate of ~0.875 km/s/Myr, similar to previous measurements at fast-spreading ridges, suggesting hydrothermal sealing of small-scale porosity is progressing at normal to enhanced rates.

Simplify the variable-depth streamer data processing through pre- migration deghosting: a case study from NWS Australia Data

Variable-depth streamer acquisition, as one of the key marine broadband solutions, has shown great advantages in providing high resolution seismic imaging and better low frequency penetration than conventional data from examples around the world. By utilizing the notch diversity, the receiver ghost was fully removed through proprietary joint deconvolution method in the imaging domain which produces broadband imaging with bandwidth up to six octaves. However, this post- migration deghosting method requires the receiver ghost to be well preserved prior to final migration thus introduces complexity in key steps like multiple attenuation, velocity analysis and migration than the conventional processing flow.The objective of this work is to simplify the processing flow by applying bootstrap deghosting method right after shot domain de-noise. After this step, processing flow will be very similar to the conventional flow. This new flow has been tested on the variable-depth streamer data from NWS Australia. Compared to the post-migration deghosting flow, the new results not only provide similar benefit as to broader bandwidth and rich images, but also show improvement on multiple attenuation and primary preservation as well as seismic inversion.

Broadband Processing of Conventional and Deep Tow Marine Streamer Seismic Data

We have developed WiBand, a de-ghosting and broadband processing method that can be applied to conventional streamer data acquired with single component streamers. We review two case studies of WiB...

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.

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.

Virtual Real Source: Source signature estimation using seismic interferometry

Knowledge of the seismic source signature is crucial in numerousproblems in exploration seismology, especially in full-waveform inversion. However, existing methods of source signature estimation like statistical methods and well-log-based methods suffer from several drawbacks arising from assumptions such as whiteness of the reflectivity series and the minimum-phase character of the wavelet. Also, estimation of the source signature using wave-theoretical methods requires the recording of the wavefield and its normal derivative or additional recordings above the receiver surface which are not always available. We introduce a method, called the Virtual Real Source, of extracting the source signature based on the theory of seismic interferometry, also known as the virtual source method. This method is independent of the assumptions and drawbacks of the above-mentioned methods. The only requirement for the method of Virtual Real Source is to have a virtual source location coincide with the physical shot position whose source signature is desired. The virtual source location does not necessarily have to be a zero-offset receiver because one can use interpolation for it. The source signature is extracted by deconvolving the real recording at a receiver from the virtual source recording. Through modeling examples, we show that Virtual Real Source produces accurate source signatures even for complicated subsurface structures and source signatures, and is robust in the presence of noise. Source signature of every shot in a survey can be extracted reliably as long as the source signatures have similar amplitude spectra. The phase spectrum of the source signature is always extracted accurately even if it varies randomly from one shot to another. The Virtual Real Source applied on a 2D streamer data set from the North Viking Graben in the North Sea extracts all the airgun signatures with the main pulse and the bubble oscillation.

Full-waveform inversion of variable-depth streamer data: An application to shallow channel modeling in the North Sea

Industry implementations of full-waveform inversion (FWI) are driven by the lower frequencies in the seismic data. This is in conflict with conventional acquisition scenarios where the free-surface ghost attenuates these desired low frequencies. In this article, we discuss the application of FWI to variable-depth streamer data and show that FWI adapts naturally to this acquisition geometry, hence benefitting from the improved low frequencies recorded in this configuration. We illustrate this with an example from the central North Sea, where detailed velocity features associated with the shallow channels in the near-surface geology are revealed by FWI. Migration with this updated velocity model improves the imaging through the near surface.

L1 pseudo-Vz estimation and deghosting of single-component marine towed-streamer data

We present a novel technique estimating the vertical component of particle motion from marine single-component pressure data. The particle motion data, bar an angle-dependent obliquity factor, is computed by convolution of the output from L1 deconvolution of the pressure ghost wavelet with the corresponding ghost wavelet of the particle motion. The estimated particle motion data is then used in a conventional 2D technique for receiver ghost attenuation by combination with the original pressure-wave data. The proposed new technique operates in the τ-[Formula: see text] domain of individual shot-streamer records and in overlapping windows along the intercept-time axis. In each window, the L1 deconvolution is achieved by an iteratively reweighted-norm least squares algorithm. We applied our technique to deep-tow streamer data of a 3D over/sparse-under marine survey, in which six streamers were towed at a shallow depth, with two additional streamers towed deeper. Over/sparse-under technology allows using seismic measurements from a shallow streamer to be complemented by a low-frequency limited measurement from a deep streamer to achieve an estimate of the up-going pressure wave recording. The low frequencies of the deep streamer are used to boost the low frequencies of the shallow streamer, which have been heavily attenuated by the shallow tow ghost response. Our technique achieves, on this particular data, set improvements in bandwidth of the single-component pressure data, while not fully reaching the quality of the optimally deghosted data from the over/sparse-under survey.