Ocean Bottom Cable Research Articles

Three-dimensional elastic full waveform inversion in a marine environment using multicomponent ocean-bottom cables: a synthetic study

Three-dimensional elastic full waveform inversion in a marine environment using multicomponent ocean-bottom cables: a synthetic study

Blended-Source Ocean-Bottom Seismic Acquisition

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25226, “Blended-Source Ocean-Bottom Seismic Acquisition,” by C.D.T. Walker, FairfieldNodal; D.J. Monk, Apache Corporation; and D.B. Hays, FairfieldNodal, prepared for the 2014 Offshore Technology Conference, Houston, 5–8 May. The paper has not been peer reviewed. The offshore application of blended, or simultaneous-source, seismic acquisition has been comparatively limited. For ocean-bottom seismic (OBS) applications, survey durations can be almost halved with a modest increase in costs by firing more than one source into the receiver spread simultaneously. In this paper, the authors describe the acquisition of what is believed to be the world’s largest ocean-bottom survey—more than 2200 km2—by use of two blended sources and a very large receiver spread. Introduction Blended, or simultaneous, sources have been introduced recently in land seismic operations, with dramatic improvements in both crew productivity and data quality. Their application offshore, however, has been relatively limited. For towed-streamer surveys, the firing of more than one source at a time increases the fold of coverage and hence data quality but does not reduce acquisition time; for OBS, however, the use of two or more blended sources can have a dramatic impact on survey duration and hence cost. The challenge has been how to ensure not only that data quality is not lost when multiple sources are fired at once but also that technical downtime of the equipment on the seafloor is minimized. Traditionally, the performance of OBS systems has been hampered by the inherent technical downtime experienced by all contractors operating ocean-bottom systems; the terminations, connectors, power distribution, and data-telemetry components within a traditional ocean-bottom- cable (OBC) system are inherently prone to failure because of the intrinsic nature of the cable-deployment-and- recovery cycle, during which the cables are stressed and destressed every time they are laid onto or recovered from the seabed. It is akin to recovering the full streamer spread at every line change for towed-streamer operations. The desire to improve operational performance was the driving force behind the development of autonomous nodal systems, and the extreme reliability of these systems has allowed ever larger seafloor spreads to be operated, with multiple benefits.

Complex structural imaging of transition zones in Bohai Bay, China, by OBC technology

A series of ocean-bottom cable (OBC) surveys has been conducted in the Bohai Sea in China in recent years to overcome difficulties experienced with streamer surveys in shallow water, such as strong currents, missing near offsets, and obstacles. The main challenges in OBC data imaging include steeply dipping structures, serious multiples in the shallow-water environment, large lateral velocity variations, fault shadow effects, and low signal-to-noise ratio (S/N). To obtain optimal images, advanced processing technologies have been developed and applied to OBC data which involve effective PZ summation and shallow-water demultiple, a high-fidelity beam migration in the wide-azimuth domain, and accurate velocity-model building in 3D tilted-transverse-isotropy (TTI) media. The PZ summation and shallow-water demultiple methods aim to effectively eliminate shallow-water ghosts to achieve broadband seismic data. Furthermore, high-fidelity controlled-beam migration (CBM) and TTI velocity-model updates greatly enhance steep dip imaging, improve S/N, and reduce turnaround time. Through the combination of these technologies, OBC data processing provides high-quality images with well-defined steeply dipping structures to reduce exploration risk in the Bohai area.

Enhanced imaging with high-resolution full-waveform inversion and reverse time migration: A North Sea OBC case study

In a case study from the Tommeliten Alpha area of the Norwegian North Sea, imaging problems were caused by the presence of gas in the overburden. In particular, a large part of the reservoir is in a seismically obscured area (SOA) caused by the gas. Full-waveform inversion (FWI) and reverse time migration (RTM) dramatically improve the imaging from ocean-bottom cable (OBC) acquisition over the region. The FWI algorithm is pushed to 22 Hz to generate an extremely high-resolution velocity model, and RTM then becomes required to honor the complexity in the resultant velocity model. Consequently, migration is done with the FWI model to generate a high-frequency RTM image to 80 Hz. This image is approximately double the maximum frequency commonly used for RTM in the North Sea and matches that of equivalent Kirchhoff products, but with all the benefits in imaging that RTM brings, providing a subsequent impact on interpretation of the area.

Ultralong-offset data acquisition can complement full-waveform inversion and lead to improved subsalt imaging

Full-waveform inversion (FWI) has proved itself as a valuable tool for updating and improving the accuracy of subsurface velocity models. For optimum performance, particularly in complex geological environments such as subsalt targets in the Gulf of Mexico (GOM), FWI requires 3D seismic data containing low frequencies and long offsets. For onshore seismic surveys, it is relatively straightforward and cost-effective to record long-offset data; however, for offshore surveys it can incur the additional cost of ocean-bottom cable (OBC) acquisition or multivessel towed-streamer techniques. The recent introduction of dual-coil-shooting multivessel full-azimuth acquisition has provided a highly efficient method of delivering marine 3D seismic data sets with long offsets. Tests using synthetic data and a real GOM dual-coil acquisition data set with full-azimuth distribution and long (up to 14.3-km) offsets demonstrate significant potential for improving the accuracy of velocity models and the reliability of the resulting depth images. The results indicate that the long offsets and low frequencies provided by the dual-coil acquisition method are of particular benefit in helping FWI to enhance the shallow section of the model. Furthermore, lower frequencies and longer offsets mitigate the sensitivity of the inversion process to the initial velocity model by enabling FWI to update the low-wavenumber component of the velocity model.

Seismic on the edge – a 3D transition zone seismic survey from concept to final volume

The Speculant 3D Transition Zone (3DTZ) Seismic Survey was acquired by Origin Energy in the Otway Basin, approximately 30km east of Warrnambool, Victoria during November and December 2010. The objective of the survey was to fill a data gap between an existing Marine 3D Seismic Survey (3DMSS) and Land 3DSS, with the target located offshore in the shallow waters of the coastal zone. Although the survey covered a small surface area, the coastal location combined with the economic and environmental significance of the region dictated numerous exclusion zones. To achieve coverage of the target under these exclusion zones, the survey was designed to simultaneously use two independent recording systems; a cable free system on land and an Ocean Bottom Cable (OBC) system offshore, two different sources (GI air gun offshore and Vibroseis on land) and three different receiver types. The final survey design was an orthogonal geometry with a high density of marine shots and a high density of land receivers, with large holes due to access restrictions. The field data were a collection of six different source - receiver combinations, each with unique noise characteristics and statics, covering separate but overlapping CMP?s. Combining these disparate data sets was the key challenge of the processing. The lessons learnt during planning, acquisition and processing are shared in this paper along with examples of the effect that survey design, field conditions and equipment use and limitations can have on field records and processed data.

Onboard data quality control and processing for ocean-bottom nodes

When we use streamers or ocean-bottom cables, we have the data in real time. When we use autonomous ocean-bottom nodes, we have the data only when we retrieve them. With 100 days endurance, we may be shooting blind for three months without knowing if we are recording good data, or not. This may cause anxiety for some people.

Wide angle reflections in OBC seismic physical model experiment

Wide angle acquisition has been taken as a significant measure to obtain high quality seismic data and is getting greater attention. In this paper, we discuss ocean bottom cable (OBC) seismic wide angle reflections on the basis of a layered model experiment. Some experiment results don’t support theoretical conclusions. The main experimental conclusions are: 1. Wide angle reflection energies are stronger than non-wide-angle reflections (up to twice as strong) but there is a big difference between observations and theoretical calculations that suggest the wide angle reflection energies are 15 times the nonwide-angle reflection energy. The reflection energy increases gradually rather than sharply as the theoretical calculations suggest. 2. The reflection events remain hyperbolic when the offset increases. 3. Wide angle reflection dominant frequency is about 20–30% less than nonwide-angle reflections and decreases as the offset increases. The non-wide-angle reflection dominant frequency shows no obvious variation for small offsets. 4. There is no wave shape mutation or polarity reversal near the critical angle. 5. The reflection event group features are the same for both cases of incidence angle greater and less than the critical angle. 6. Direct arrivals, multiples, and water bottom refractions influence the wide angle reflections of the sea floor.

Introduction to this special section: Marine and seabed technology

About 70% of the Earth and much of its hydrocarbons are covered by water, but the ocean does not prevent us from using geophysical methods offshore. The ocean helps. Compared to land seismic surveys, marine is like looking under the lamp. Surface-towed seismic streamers are now the primary method for offshore geophysics, but before we had streamers, the only way to acquire geophysical data offshore was to put receivers on the seabed. Ocean-bottom cables and nodes started almost 80 years ago. Ewing, Wortzel, and Vine deployed autonomous ocean-bottom nodes they developed at Lehigh University. In independent efforts at that same time, Bullard, Gaskell, and Flavill used geophones on an ocean-bottom cable they developed at Cambridge University. The sources were dynamite. I do not know if any of these pioneering OBC and OBN surveys made it to the fledgling SEG. The young seismic industry was too busy with the challenges and opportunities of land seismic surveys to spare attention and talent offshore. The oil industry was focused onshore. The early ocean-bottom seismometers could not record a large amount of data and there was little interest in what they did record. Their usage was limited to academia, which has been using ocean-bottom nodes continuously since the 1930s.

Improvements in development strategy for the Stag Oil Field, North West Shelf, Australia—implications for production success and drilling risk minimisation

Located on the North West Shelf of Western Australia, the Stag Oil field has proven to be a prolific reservoir, having produced more than 55 million barrels (MMbbls) of oil since 1998. This has not been without its challenges, however; with premature water breakthrough from injection wells occuring in several wells, potentially stranding large volumes of oil in the ground. Using the multicomponent processing and joint amplitude-versus-offset (AVO) inversion of an ocean bottom cable (OBC) seismic survey acquired in late 2007, new light has been shed on the distribution of unswept oil. This data has led to the succesful drilling of six wells and a marked increase in field production. Additionally, the seismic data has also been used to minimise drilling risks by using seismic coherency to steer the well around potential problems with a significant impact on well costs due to reduction of wellbore problems associated with horizontal drilling in the Muderong shale. To date, four wells have been drilled using this technique, resulting in a significant decrease in non-productive time while drilling during the most recent drilling campaign, which has a significant impact upon the profitability of these late-stage development wells.

3D OBC seismic survey geometry optimization offshore Abu Dhabi

The flexibility of 3D ocean bottom cable (OBC) seismic survey design allows a variety of survey geometries. Among the infinite variations, we naturally seek higher survey efficiency within operational constraints whilst satisfying the geophysical requirements. We have analysed several geometry and shooting options, such as zippers between panels, source line interleave, flip-flop shooting, outboard shooting, and distance-separated simultaneous shooting (DS3), to show how the survey geometry may be optimized to yield higher productivity within the limitations of the available equipment.

On the footprint of anisotropy on isotropic full waveform inversion: the Valhall case study

The validity of isotropic approximation to perform acoustic full waveform inversion (FWI) of real wide-aperture anisotropic data can be questioned due to the intrinsic kinematic inconsistency between short- and large-aperture components of the data. This inconsistency is mainly related to the differences between the vertical and horizontal velocities in vertical-transverse isotropic (VTI) media. The footprint of VTI anisotropy on 2-D acoustic isotropic FWI is illustrated on a hydrophone data set of an ocean-bottom cable that was collected over the Valhall field in the North Sea. Multiscale FWI is implemented in the frequency domain by hierarchical inversions of increasing frequencies and decreasing aperture angles. The FWI models are appraised by local comparison with well information, seismic modelling, reverse-time migration (RTM) and source-wavelet estimation. A smooth initial VTI model parameterized by the vertical velocity V0 and the Thomsen parameters δ and ∈ were previously developed by anisotropic reflection traveltime tomography. The normal moveout () and horizontal () velocity models were inferred from the anisotropic models to perform isotropic FWI. The VNMO models allows for an accurate match of short-spread reflection traveltimes, whereas the Vh model, after updating by first-arrival traveltime tomography (FATT), allows for an accurate match of first-arrival traveltimes. Ray tracing in the velocity models shows that the first 1.5 km of the medium are sampled by both diving waves and reflections, whereas the deeper structure at the reservoir level is mainly controlled by short-spread reflections. Starting from the initial anisotropic model and keeping fixed δ and ∈ models, anisotropic FWI allows us to build a vertical velocity model that matches reasonably well the well-log velocities. Isotropic FWI is performed using either the NMO model or the FATT model as initial model. In both cases, horizontal velocities are mainly reconstructed in the first 1.5 km of the medium. This suggests that the wide-aperture components of the data have a dominant control on the velocity estimation at these depths. These high velocities in the upper structure lead to low values of velocity in the underlying gas layers (either equal or lower than vertical velocities of the well log), and/or a vertical stretching of the structure at the reservoir level below the gas. This bias in the gas velocities and the mispositioning in depth of the deep reflectors, also shown in the RTM images, are required to match the deep reflections in the isotropic approximation and highlight the footprint of anisotropy in the isotropic FWI of long-offset data. Despite the significant differences between the anisotropic and isotropic FWI models, each of these models produce a nearly-equivalent match of the data, which highlights the ill-posedness of acoustic anisotropic FWI. Hence, we conclude with the importance of considering anisotropy in FWI of wide-aperture data to avoid bias in the velocity reconstructions and mispositioning in depth of reflectors. Designing a suitable parameterization of the VTI acoustic FWI is a central issue to manage the ill-posedness of the FWI.

Study of geological storage for a candidate CCS demonstration project in Tomakomai, Hokkaido, Japan

The Prime Minister of Japan declared: Japan should positively commit itself to setting a long-term reduction target. For its mid-term goal, Japan will aim to reduce its emissions by 25% by 2020, if compared to the 1990 level, consistent with what the science calls for in order to halt global warming. In order to achieve this target, the government considers CCS (Carbon dioxide Capture and Storage) as an essential method and aims to improve CCS technology to practical levels. Japan CCS Co., Ltd. (JCCS) was established in May, 2008 in order to study the potential and feasibility of CCS demonstration projects in Japan. The company was commissioned by the Ministry of Economy, Trade and Industry (METI) in Japan. Major shareholders of JCCS include electric power companies, upstream to downstream petroleum companies, material companies, engineering companies, etc.. In 2008, an investigation of candidate sites for the CCS demonstration was done over Japan and several promising offshore sites were selected. One of the selected candidate sites is the Tomakomai offshore site in Hokkaido prefecture, located in the northern part of Japan where there are petroleum refineries, paper mills and power plants close to the site. The Yufutsu oil and gas field is located in the eastern part of Tomakomai city where much geological data has been obtained through oil and gas exploration. Basic geological modes were constructed and CO2 reservoir simulation was performed to predict the movement of the stored CO2. Simulation results show us that a sufficient volume of CO2 could be safely stored in the saline aquifers at the Tomakomai site for a demonstration period and afterward. From October to November in 2009, a 3D seismic survey was implemented to confirm the geological potential of the site. Ocean bottom cables enabled us to survey in front of the Tomakomai Port. The survey area (full-stacked area) was 3.0 km (N-S) by 2.7 km (W-E) and about 2 km from the shoreline with several target formations at a depth of about 1,100–3,000 m. This survey was conducted in 4 patches using 2 to 3 sets of ocean bottom cables with the total number of receiver and shot lines being 10 and 26, respectively. Results of this survey were utilized in building the detailed 3D geological model for the CO2 reservoir simulation. Basic plant designing to capture CO2 at the hydrogen process in the petroleum refineries is currently ongoing. The captured CO2 will be injected at 250,000 ton/year for 4–5 years in the demonstration project. Many local organizations, such as the Tomakomai city government and the fisheries cooperative, cooperated to carry out the 3D seismic survey. The Tomakomai government has been strongly willing to invite the CCS demonstration project to the city. By their continuous cooperation is possible to carry forward the feasibility study in order to start the CO2 injection in 3–4 years.

3D OBC seismic survey parameters optimization offshore Abu Dhabi

The flexibility of 3D ocean bottom cable (OBC) seismic survey design allows a variety of survey parameters and geometries to be chosen. From the infinite number of possible variations, we seek to optimize the trade-off between higher data quality and lower survey effort. To understand the relationship between data quality and survey effort, we acquired a pilot 3D OBC seismic survey with very high specifications, simulated several datasets by decimating the full data or reducing the survey effort, and then evaluated the quality of each dataset against the survey effort. We show that: (a) data quality is related to trace and offset vector tile (OVT) densities; (b) data quality is an asymptotic function of survey effort in general, eventually reaching a plateau with increasing survey effort; (c) a gradient of the fit curve at a certain survey effort can represent the quality efficiency, dQ/dE, where Q is data quality, and E is survey effort; and (d) the quality curve for a survey can be estimated using existing datasets in a field with the proposed equation for Q and dQ/dE.

Multiple prediction and subtraction from apparent slowness relations in 2D synthetic and field ocean-bottom cable data

A target-oriented algorithm is developed for the prediction of multiples recorded on ocean-bottom cables by utilizing apparent slowness relations in common-source and common-receiver gathers. It is based on combining offsets and times of direct waves and primary reflections to predict multiples by matching apparent slownesses at all source and receiver locations; all higher-order multiples can be predicted by matching apparent slownesses alternately in common-source and common-receiver gathers. No knowledge of the subsurface velocity is required. Traveltimes of the direct waves and primary reflections need to be picked from common-source gathers. The subtraction of multiples involves flattening the predicted times of the multiple events, subtracting a local spatial average trace from each trace, within a fixed time window containing the wavelet of the multiple, and then shifting the data back to its original times. Tests of synthetic and field data indicate that the proposed method predicts multiples very well and removes them from seismic data efficiently with negligible affect on the primary reflections, as long as the primary and multiple reflections do not overlap in time and slowness over substantial windows in the domain in which the removal is done.

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.

Accurate data reconstruction through simultaneous application of statistical and physics-based constraints to multiple geophysical data sets

Insufficient sampling is a key problem that affects seismic data-processing techniques. Although existing interpolation methods can fill in large gaps in the data under certain assumptions, their performance degrades as shot and receiver sampling becomes coarser. Most existing data-interpolation techniques overcome insufficient sampling by making assumptions regarding either the statistical properties of the data or the physics that explain the data. Recognizing that statistical and physics-based constraints can be expressed as convex sets, we propose overcoming some of the deficiencies of existing methods by incorporating multiple types of prior information and equations describing the mapping between different types of data into a mathematically consistent framework based on the projection onto convex sets (POCS) methodology. The proposed method is easy to implement in practice because it relies only upon the iterative application of projection operators (e.g., soft thresholding). At the same time, full advantage of various forms of prior information can be taken. Using a synthetic example of combining conventional streamer and ocean-bottom–cable (OBC) data, we show that our method provides substantial improvement compared to using statistical assumptions alone. Other potential applications include data reconstruction in the case of over-under streamer acquisition, dual-sensor streamer acquisition, multi-azimuth surveys, and vertical seismic profiling.

Using high-density OBC seismic data to optimize the Andrew satellites development

The processed data from conventional towed-streamer seismic surveys over the Andrew satellite fields in the Central North Sea are of poor quality because of anomalously fast sand-filled channels in the Eocene overburden. The channels attenuate the primary energy and produce strong multiples. A step change in data quality has been achieved by acquiring a wide-azimuth seismic survey using ocean bottom cables and a high shot density. The new data have much higher signal-to-noise ratio and better resolution. The imaging under the Eocene channels is greatly improved in the processed results of the new survey. Furthermore, the acquisition and processing were completed in time to meet the tight development schedule for the 2008 Kinnoull discovery.

Multi-azimuth processing and its applications to wide-azimuth OBC seismic data offshore Abu Dhabi

Wide-azimuth ocean bottom cable (OBC) seismic data have several advantages for imaging, such as high fold, long offsets, and a wide range of source-receiver azimuths. However, conventional wide-azimuth processing has some drawbacks because the full range of advantages is not exploited. In conventional wide-azimuth processing, azimuthal variations are usually ignored, whereas multi-azimuth processing treats each source-receiver azimuth individually. We performed a pilot multi-azimuth processing trial on wide-azimuth OBC seismic data offshore Abu Dhabi, United Arab Emirates to evaluate its benefits compared to conventional wide-azimuth processing, and discuss its applications for fault imaging and fracture characterization. We show that: (a) multi-azimuth processing achieves significantly higher signal-to-noise ratio, and effectively corrects the distortions in the subsurface image resulting from azimuthal variations; (b) mono-azimuth processing enhances imaging of existing subtle faults; and (c) azimuthal variations extracted from mono-azimuth processed data provide indications about fracture orientation, relative fracture intensity, and their spatial distributions. The results should encourage us to apply multi-azimuth processing to wide-azimuth OBC seismic data to improve focusing quality, image subtle faults, and gain insights into fracture characteristics.

Structural interpretation using PS seismic on the Kvitebjørn Field in the North Sea

Seismic data from an ocean-bottom cable (OBC) survey are based on a different acquisition method than conventional/streamer seismic data. When acquiring the OBC data, the receivers are placed on the sea floor instead of near the sea surface. This allows PS (converted shear) seismic to be recorded in addition to pressure data (PZ). PZ data are equivalent to PP data from streamer seismic. Because the PS data have different information and travel paths from PZ, they should in theory not give exactly the same image as the pressure data. The OBC data set acquired on the Statoil-operated Kvitebjørn Field is a good example of this. In these data, it is obvious that the PS seismic image of the Jurassic target is significantly different from the PZ image. Figure 1 shows the PP, PZ, and PS seismic from the same line going through a proven gas well. The top reservoir is on a trough (green marker) on the PP and PZ seismic. Comparison of the reservoir section with the same interval on the time-aligned PS data reveals a completely different reflection pattern. The question that often arises in a case like this is: can the PS seismic can be trusted? Even if the answer is yes, a big question remains: do they add any valuable information about the field?