Streamer Data Research Articles

Processing the first full azimuth OBN survey in Australia: a step change in imaging quality

SummaryOcean Bottom Node (OBN) acquisition has been widely used in oil and gas industry to provide improved reservoir imaging when compared to conventional narrow azimuth (NAZ) towed streamer data....

Full-waveform inversion of short-offset, band-limited seismic data in the Alboran Basin (SE Iberia)

Abstract. We present a high-resolution P-wave velocity model of the sedimentary cover and the uppermost basement to ∼3 km depth obtained by full-waveform inversion of multichannel seismic data acquired with a 6 km long streamer in the Alboran Sea (SE Iberia). The inherent non-linearity of the method, especially for short-offset, band-limited seismic data as this one, is circumvented by applying a data processing or modelling sequence consisting of three steps: (1) data re-datuming by back-propagation of the recorded seismograms to the seafloor; (2) joint refraction and reflection travel-time tomography combining the original and the re-datumed shot gathers; and (3) full-waveform inversion of the original shot gathers using the model obtained by travel-time tomography as initial reference. The final velocity model shows a number of geological structures that cannot be identified in the travel-time tomography models or easily interpreted from seismic reflection images alone. A sharp strong velocity contrast accurately defines the geometry of the top of the basement. Several low-velocity zones that may correspond to the abrupt velocity change across steeply dipping normal faults are observed at the flanks of the basin. A 200–300 m thick, high-velocity layer embedded within lower-velocity sediment may correspond to evaporites deposited during the Messinian crisis. The results confirm that the combination of data re-datuming and joint refraction and reflection travel-time inversion provides reference models that are accurate enough to apply full-waveform inversion to relatively short offset streamer data in deep-water settings starting at a field-data standard low-frequency content of 6 Hz.

Seismic characterization of submarine gas-hydrate deposits in the Western Black Sea by acoustic full-waveform inversion of ocean-bottom seismic data

Evidence for gas-hydrate occurrence in the Western Black Sea is found from seismic measurements revealing bottom-simulating reflectors (BSRs) of varying distinctness. From an ocean-bottom seismic data set, low-resolution traveltime-tomography models of P-wave velocity [Formula: see text] are constructed. They serve as input for acoustic full-waveform inversion (FWI), which we apply to derive high-resolution parameter models aiding the interpretation of the seismic data for potential hydrate and gas deposits. Synthetic tests indicate the applicability of the FWI approach to robustly reconstruct [Formula: see text] models with a typical hydrate and gas signature. Models of S-wave velocity [Formula: see text] containing a hydrate signature can only be reconstructed when the parameter distribution of [Formula: see text] is already well-known. When we add noise to the modeled data to simulate field-data conditions, it prevents the reconstruction of [Formula: see text] completely, justifying the application of an acoustic approach. We invert for [Formula: see text] models from field data of two parallel profiles of 14 km length with a distance of 1 km. Results indicate a characteristic velocity trend for hydrate and gas occurrence at BSR depth in the first of the analyzed profiles. We find no indications for gas accumulations below the BSR on the second profile and only weak indications for hydrate. These differences in the [Formula: see text] signature are consistent with the reflectivity behavior of the migrated seismic streamer data of both profiles in which a zone of high-reflectivity amplitudes is coincident with the potential gas zone derived from the FWI result. Calculating saturation estimates for the potential hydrate and gas zones yields values of up to 30% and 1.2%, respectively.

Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

AbstractThe northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2‐D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2‐D P wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the prestack depth‐migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of slow‐slip events to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intrawedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest that they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from International Ocean Discovery Program drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3‐D using a recently acquired, longer‐offset, seismic data set.

Simultaneous prestack inversion of variable-depth streamer seismic data

Variable-depth streamer seismic data are characterized by low and high frequencies and can be used to obtain high-quality and resolution images of complex subsurface structures. Taking advantage of the frequency range in the variable-depth streamer data, we propose the simultaneous inversion of prestack data from variable-offset stack gathers to obtain the P-wave impedance, S-wave impedance, and density. Next, we validate the method by using model and actual variable-depth streamer data from the Huizhou block. The results suggest that the broadband data recorded by variable-depth streamers improve the signal-to-noise ratio and quality of the inversion results and outperform the constant-depth streamer data in delineating the underground stratigraphy.

Full-waveform inversion for salt: A coming of age

Full-waveform inversion (FWI), proposed by Lailly and Tarantola in the 1980s, is considered to be the most promising data-driven tool for automatically building velocity models. Many successful examples have been reported using FWI to update shallow sediments, gas pockets, and mud volcanoes. However, successful applications of FWI to update salt structures had almost only been seen on synthetic data until recent progress at the Atlantis Field in the Gulf of Mexico. We revisited some aspects of FWI algorithms to minimize cycle-skipping and amplitude discrepancy issues and derived an FWI algorithm that is able to build complex salt velocity models. We applied this algorithm to a variety of data sets, including wide-azimuth and full-azimuth (FAZ) streamer data as well as ocean-bottom-node data, with different geologic settings in order to demonstrate the effectiveness of the method for salt velocity updates and to examine some fundamentals of the salt problem. We observed that, in multiple cases, salt velocity models from this FWI algorithm produced subsalt images of superior quality. We demonstrate with one FAZ streamer data example in Keathley Canyon that we do not necessarily need very high frequency in FWI for subsalt imaging purposes. Based on this observation, we envision that sparse node for velocity acquisition may provide appropriate data to handle large and complex salt bodies with FWI. We believe the combination of advanced FWI algorithms and appropriate data acquisition will bring a step change to subsalt imaging.

Long‐Lasting Evolution of Layer 2A in the Western South Atlantic: Evidence for Low‐Temperature Hydrothermal Circulation in Old Oceanic Crust

AbstractPrevious seismic studies suggest that hydrothermal processes are active only within young oceanic crust (<10–16 Ma). However, differences between measured and predicted heat flow at the ocean floor indicate that hydrothermal fluids may be transporting heat advectively in crust up to ages of 65 Ma. We report on seismic velocities of 0–71 Ma slow to intermediate spreading rate upper crust in the western South Atlantic. Thirteen high‐resolution 2‐D velocity models were built using traveltime tomography on downward continued streamer data acquired during the Crustal Reflectivity Experiment Southern Transect. In the Crustal Reflectivity Experiment Southern Transect area, velocities at the top of seismic layer 2A increase rapidly from ~2.4 km/s at 0 Ma to ~4.2 km/s at 6 Ma and then undergo a more gradual increase to ~4.9 km/s at 58 Ma. These new results resolve the long‐standing debate about the duration of interaction between ocean crust and seawater, providing seismic evidence for hydrothermal circulation continuing to crustal ages predicted by heat flow studies. Seismic layer 2B does not show a systematic off‐axis velocity trend but has an average value of 5.15 km/s. We interpret this result to indicate that the hydrothermal system becomes too shallow to affect the physical properties of layer 2B shortly after crustal accretion. Upper crustal heterogeneity in ridge‐parallel profile orientation is more pronounced for crust accreted at slow spreading rates, compared to intermediate rates. This result is consistent with shorter magmatic segments at slower spreading rates, increasing the frequency of tectonic and magmatic accretion alternately occurring along the ridge.

AVO analysis of a shallow reverse polarity in Sydney Basin

A vast shallow reverse polarity which is extended across the PEP11 block in offshore Sydney basin and the lack of any wells in the area has wondered the interpreters about the mechanisms which has caused such a seismic signature in the area. This study aims to characterize this reverse polarity using the AVO analysis and pre stack inversion techniques in a pilot study. Baleen 2004 2-D towed streamer data are the only recent seismic data acquired in PEP11 block. Shallow depth water has left significant amount of multiple energy in shot records. The presence of a very high velocity sequence of layers which terminate at the shore line cause a strong ray bending letting small amount of the energy penetrate to deeper parts. The reverse polarity reflector is part of a sequence of progradational sediments which are accommodated at the top of the high velocity layer.

The evolution of FWI and its perceived benefits

Despite the mathematics behind full waveform inversion (FWI) being published in the early 1980s, it was 30 years before the method could be efficiently implemented on the scale of conventional 3D marine seismic volumes. FWI has evolved from using only transmitted waves and being constrained because towed streamer data lacked the very long offsets and ultra-low frequencies necessary to derive stable velocity updates beyond shallow depths. FWI now uses the full seismic wavefield (both transmitted and scattered wavefields), recovers deep velocity updates for standard offsets and frequencies and increasingly uses a wider range of frequencies that contribute to seismic imaging. We use several case examples to consider the benefits and caveats for robust FWI application: for resolving near-surface features and reducing seismic imaging uncertainty in areas with complex overburden heterogeneities; for resolving near-surface features and improving volumetric estimates; for using an enlarged bandwidth to resolve small model features; for updating the velocity in high contrast regimes; and for the creation of survey-wide, high-resolution models to reduce imaging uncertainty, complement attribute analysis, estimate elastic properties and prospect derisking. Collectively, we demonstrate how to produce high-resolution velocity models when conventional methods cannot and how to generate earth models in an accelerated fashion to reduce project turnaround. We describe pragmatic limits to what maximum FWI frequencies are reasonable and suggest ways that may soon by-pass signal processing and obtain direct earth attributes.

Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis

ABSTRACTDuring the time taken for seismic data to be acquired, reservoir pressure may fluctuate as a consequence of field production and operational procedures and fluid fronts may move significantly. These variations prevent accurate quantitative measurement of the reservoir change using 4D seismic data. Modelling studies on the Norne field simulation model using acquisition data from ocean‐bottom seismometer and towed streamer systems indicate that the pre‐stack intra‐survey reservoir fluctuations are important and cannot be neglected. Similarly, the time‐lapse seismic image in the post‐stack domain does not represent a difference between two states of the reservoir at a unique base and monitor time, but is a mixed version of reality that depends on the sequence and timing of seismic shooting. The outcome is a lack of accuracy in the measurement of reservoir changes using the resulting processed and stacked 4D seismic data. Even for perfect spatial repeatability between surveys, a spatially variant noise floor is still anticipated to remain. For our particular North Sea acquisition data, we find that towed streamer data are more affected than the ocean‐bottom seismometer data. We think that this may be typical for towed streamers due to their restricted aperture compared to ocean‐bottom seismometer acquisitions, even for a favourable time sequence of shooting and spatial repeatability. Importantly, the pressure signals on the near and far offset stacks commonly used in quantitative 4D seismic inversion are found to be inconsistent due to the acquisition timestamp. Saturation changes at the boundaries of fluid fronts appear to show a similar inconsistency across sub‐stacks. We recommend that 4D data are shot in a consistent manner to optimize aerial time coverage, and that additionally, the timestamp of the acquisition should be used to optimize pre‐stack quantitative reservoir analysis.

Insight Into Frontal Seismogenic Zone in the Mentawai Locked Region From Seismic Full Waveform Inversion of Ultralong Offset Streamer Data

AbstractSumatra subduction zone is one of the most seismically active zones on Earth. After having produced three Mw > 8.4 earthquakes and several Mw > 7.5 earthquakes, including the Mw = 7.8 2010 tsunami earthquake, the northern Mentawai segment is still locked and is capable of generating a great earthquake, possibly a disastrous tsunami. We analyzed ultralong offset seismic reflection data from this locked zone to characterize the nature of the accretionary prism and the plate interface using a combination of traveltime tomography, full waveform inversion, and prestack depth migration. In order to enhance the refractions, we downward extrapolate the streamer data to the seafloor, allowing the refraction arrivals to be observed from near‐zero offset up to far offset, and use a traveltime tomography to determine the background velocity in the upper sediments. Starting from these velocities, we perform a multiscale elastic full waveform inversion to determine the detailed P wave velocity structure of the subsurface. Based on this velocity, we perform a prestack depth migration to obtain seismic image in the depth domain and compute the porosity of the sediments to determine fluid content along faults. Our results show a low‐velocity subduction channel with high porosity at the plate interface that connects the likely active frontal thrusts at the toe of accretionary wedge, suggesting that the frontal section of the prism is seismogenic. We have also observed a low‐velocity layer in the middle of wedge separating old sediments below from new sediments above, defining the roots of bivergent thrust faults up to the seafloor, which can be interpreted as a psudo‐décollement.

Full waveform inversion of combined towed streamer and limited OBS seismic data: a theoretical study

In marine seismic oil exploration, full waveform inversion (FWI) of towed-streamer data is used to reconstruct velocity models. However, the FWI of towed-streamer data easily converges to a local minimum solution due to the lack of low-frequency content. In this paper, we propose a new FWI technique using towed-streamer data, its integrated data sets and limited OBS data. Both integrated towed-streamer seismic data and OBS data have low-frequency components. Therefore, at early iterations in the new FWI technique, the OBS data combined with the integrated towed-streamer data sets reconstruct an appropriate background model. And the towed-streamer seismic data play a major role in later iterations to improve the resolution of the model. The new FWI technique is tested on numerical examples. The results show that when starting models are not accurate enough, the models inverted using the new FWI technique are superior to those inverted using conventional FWI.

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.

Evolution of deghosting process for single‐sensor streamer data from 2D to 3D

ABSTRACTIn marine acquisition, reflections of sound energy from the water–air interface result in ghosts in the seismic data, both in the source side and the receiver side. Ghosts limit the bandwidth of the useful signal and blur the final image. The process to separate the ghost and primary signals, called the deghosting process, can fill the ghost notch, broaden the frequency band, and help achieve high‐resolution images. Low‐signal‐to‐noise ratio near the notch frequencies and 3D effects are two challenges that the deghosting process has to face. In this paper, starting from an introduction to the deghosting process, we present and compare two strategies to solve the latter. The first is an adaptive mechanism that adjusts the deghosting operator to compensate for 3D effects or errors in source/receiver depth measurement. This method does not include explicitly the crossline slowness component and is not affected by the sparse sampling in the same direction. The second method is an inversion‐type approach that does include the crossline slowness component in the algorithm and handles the 3D effects explicitly. Both synthetic and field data examples in wide azimuth acquisition settings are shown to compare the two strategies. Both methods provide satisfactory results.

Sparsity‐enhanced wavelet deconvolution

ABSTRACTWe propose a three‐step bandwidth enhancing wavelet deconvolution process, combining linear inverse filtering and non‐linear reflectivity construction based on a sparseness assumption. The first step is conventional Wiener deconvolution. The second step consists of further spectral whitening outside the spectral bandwidth of the residual wavelet after Wiener deconvolution, i.e., the wavelet resulting from application of the Wiener deconvolution filter to the original wavelet, which usually is not a perfect spike due to band limitations of the original wavelet. We specifically propose a zero‐phase filtered sparse‐spike deconvolution as the second step to recover the reflectivity dominantly outside of the bandwidth of the residual wavelet after Wiener deconvolution. The filter applied to the sparse‐spike deconvolution result is proportional to the deviation of the amplitude spectrum of the residual wavelet from unity, i.e., it is of higher amplitude; the closer the amplitude spectrum of the residual wavelet is to zero, but of very low amplitude, the closer it is to unity. The third step consists of summation of the data from the two first steps, basically adding gradually the contribution from the sparse‐spike deconvolution result at those frequencies at which the residual wavelet after Wiener deconvolution has small amplitudes. We propose to call this technique “sparsity‐enhanced wavelet deconvolution”. We demonstrate the technique on real data with the deconvolution of the (normal‐incidence) source side sea‐surface ghost of marine towed streamer data. We also present the extension of the proposed technique to time‐varying wavelet deconvolution.

Three‐dimensional receiver deghosting of seismic streamer data using L1 inversion and redundant extended radon dictionary

ABSTRACTIn this paper, we propose a novel three‐dimensional receiver deghosting algorithm that is capable of deghosting both horizontal and slanted streamer data in a theoretically consistent manner. Our algorithm honours wave propagation phenomena in a true three‐dimensional sense and frames the three‐dimensional receiver deghosting problem as a Lasso problem. The ultimate goal is to minimise the mismatch between the actual measurements and the simulated wavefield with an L1 constraint applied in the extended Radon space to handle the underdetermined nature of this problem. We successfully demonstrate our algorithm on a modified three‐dimensional EAGE/SEG Overthrust model and a Red Sea marine dataset.

Broadband seismic : case study modeling and data processing

Seismic data with wide range of frequency is needed due to its close relation to resolution and the depth of the target. Low frequency provides deeper penetration for the imaging of deep target. In addition, the wider the frequency bandwidth, the sharper the wavelet. Sharp wavelet is responsible for high-resolution imaging and is very helpful to resolve thin bed. As a result, the demand for broadband seismic data is rising and it spurs the technology development of broadband seismic in oil and gas industry. An obstacle that is frequently found on marine seismic data is the existence of ghost that affects the frequency bandwidth contained on the seismic data. Ghost alters bandwidth to bandlimited. To reduce ghost effect and to acquire broadband seismic data, lots of attempts are used, both on the acquisition and on the processing of seismic data. One of the acquisition technique applied is the multi-level streamer, where some streamers are towed on some levels of depth. Multi-level streamer will yield data with varied ghost notch shown on frequency domain. If the ghost notches are not overlapping, the summation of multi-level streamer data will reduce the ghost effect. The result of the multi-level streamer data processing shows that reduction of ghost notch on frequency domain indeed takes place.

A parameterization study for elastic VTI full-waveform inversion of hydrophone components: Synthetic and North Sea field data examples

Choosing the right parameterization to describe a transversely isotropic medium with a vertical symmetry axis (VTI) allows us to match the scattering potential of these parameters to the available data in a way that avoids a potential tradeoff and focuses on the parameters to which the data are sensitive. For 2D elastic full-waveform inversion in VTI media of pressure components and for data with a reasonable range of offsets (as with those found in conventional streamer data acquisition systems), assuming that we have a kinematically accurate normal moveout velocity ([Formula: see text]) and anellipticity parameter [Formula: see text] (or horizontal velocity [Formula: see text]) obtained from tomographic methods, a parameterization in terms of horizontal velocity [Formula: see text], [Formula: see text], and [Formula: see text] is preferred to the more conventional parameterization in terms of [Formula: see text], [Formula: see text], and [Formula: see text]. In the [Formula: see text], [Formula: see text], and [Formula: see text] parameterization and for reasonable scattering angles (<[Formula: see text]), [Formula: see text] acts as a “garbage collector” and absorbs most of the amplitude discrepancies between the modeled and observed data, more so when density [Formula: see text] and S-wave velocity [Formula: see text] are not inverted for (a standard practice with streamer data). On the contrary, in the [Formula: see text], [Formula: see text], and [Formula: see text] parameterization, [Formula: see text] is mostly sensitive to large scattering angles, leaving [Formula: see text] exposed to strong leakages from [Formula: see text] mainly. These assertions will be demonstrated on the synthetic Marmousi II as well as a North Sea ocean bottom cable data set, in which inverting for the horizontal velocity rather than the vertical velocity yields more accurate models and migrated images.

Rayleigh-Marchenko redatuming for target-oriented, true-amplitude imaging

Marchenko redatuming is a revolutionary technique to estimate Green’s functions from virtual sources in the subsurface using only data measured at the earth’s surface, without having to place either sources or receivers in the subsurface. This goal is achieved by crafting special wavefields (so-called focusing functions) that can focus energy at a chosen point in the subsurface. Despite its great potential, strict requirements on the reflection response such as knowledge and accurate deconvolution of the source wavelet (including absolute scaling factor) and co-location of sources and receivers have so far challenged the application of Marchenko redatuming to real-world scenarios. I combine the coupled Marchenko equations with a one-way version of the Rayleigh integral representation to obtain a new redatuming scheme that handles internal as well as free-surface multiples using dual-sensor, band-limited seismic data (with an unknown source signature) from any acquisition system that presents arbitrarily located sources above a line of regularly sampled receivers—for example, ocean-bottom, source-over-spread streamer, and horizontal borehole seismic data. The redatuming scheme is validated using synthetic and field data, and the retrieved subsurface wavefields are used for improved structural imaging and taken as input for the computation of true-amplitude angle gathers, which can lead to more accurate amplitude-versus-angle interpretation and velocity analysis.