Ocean Bottom Cable Research Articles

Elastic interferometry for ocean bottom cable data: Theory and examples‡

ABSTRACTElastic imaging from ocean bottom cable (OBC) data can be challenging because it requires the prior estimation of both compressional‐wave (P‐wave) and shear‐wave (S‐wave) velocity fields. Seismic interferometry is an attractive technique for processing OBC data because it performs model‐independent redatuming; retrieving ‘pseudo‐sources’ at positions of the receivers. The purpose of this study is to investigate multicomponent applications of interferometry for processing OBC data. This translates into using interferometry to retrieve pseudo‐source data on the sea‐bed not only for multiple suppression but for obtaining P‐, converted P to S‐wave (PS‐wave) and possibly pure mode S‐waves. We discuss scattering‐based, elastic interferometry with synthetic and field OBC datasets.Conventional and scattering‐based interferometry integrands computed from a synthetic are compared to show that the latter yields little anti‐causal response. A four‐component (4C) pseudo‐source response retrieves pure‐mode S‐reflections as well at P‐ and PS‐reflections. Pseudo‐source responses observed in OBC data are related to P‐wave conversions at the seabed rather than to true horizontal or vertical point forces. From a Gulf of Mexico OBC data set, diagonal components from a nine‐component pseudo‐source response demonstrate that the P‐wave to S‐wave velocity ratio (VP/VS) at the sea‐bed is an important factor in the conversion of P to S for obtaining the pure‐mode S‐wave reflections.

Acquisition footprints and seafloor coupling in multicomponent OBC seismic data

In four-component (4-C) towed ocean-bottom-cable (OBC) data sets, acquisition footprints are often observed. Sometimes these exhibit a spatial period equal to the length of the receiver cable. I have analyzed a 2D 4-C OBC data set, looking at common-offset gathers (COG), spectral analyses, and hodogram analyses of the direct P-wave first breaks. The acquisition footprint is seen to be directly related to the following effects observed on a few of the multicomponent receivers, namely, those nearest to the towing vessel: significant delays on the inline component though not on the downgoing direct-P first breaks; depletion of higher frequencies (narrower bandwidth) on the inline component; and oscillatory motion closer to the vertical on the direct-P first breaks equivalent to decreased amplitude on the in-line component. This is interpreted to be a result of the towing procedure wherein the leading end of the cable, with the first few receiver modules, is raised from the seafloor and laid down again, relatively lightly, on top of seafloor material that might be poorly consolidated, while the trailing receivers are pulled through and down into this material. For these leading receiver modules, this results in poor inline horizontal coupling (i.e., slipping) and delayed P-S onsets due to their vertically higher positions (relative to the trailing receivers) and quite high near-seafloor [Formula: see text] ratios. To rectify this problem in future acquisition, a longer lead-in cable should prevent lifting of the leading receivers and allow all of them to couple with the seafloor in the same way. For data already acquired with an acquisition footprint on the inline component, a two-step process involving surface-consistent deconvolution or trace equalization and static correction is proposed.

Managing the start-up of a fractured oil reservoir: development of the Clair field, West of Shetland

Abstract The Clair oilfield was discovered in 1977 and began production in 2005. It is a heterogeneous fractured sandstone reservoir with an estimated STOIIP of c . 1.5 billion barrels in the Phase 1 development area. An extended appraisal programme was required to assess reservoir deliverability, which is controlled by the distribution of natural fractures. Development drilling increased the understanding of the fracture system, through acquisition of conventional core, open-hole logging, drilling mud-loss recording, production logging and well test transient analysis. These data revealed a complex fracture system of conductive faults and background joints. Post-production formation pressure measurements showed greater lateral and vertical connectivity than would be expected from matrix and fluid properties alone. However, large pressure differences were encountered close to or across features mappable at a seismic scale. Closed fractures or sealing faults can form baffles between compartments, and conductive faults or fractures allow rapid pressure communication. In some cases the connection path or barrier can be identified with seismically mapped features. When the field was put onto water injection, one year after start-up, a key concern was the degree of imbibition of water from the fractures to the matrix. Despite the observed pressure connectivity, water breakthrough did not take place until 2007. Most producers have completions designed to permit zonal water shut-off. A successful intervention was carried out in early 2008, after a production logging run had demonstrated water entry at the toe of a high-angle well. The crestal Core segment of the development benefits from a permanent 4D seismic array of ocean-bottom cables. In 2007 the first permanent monitor survey was acquired and processing gave indications of a 4D response. These 4D data are improving the dynamic reservoir understanding and providing valuable spatial context to the well results.

Uma implementação do método da refletividade para modelagem eficiente de dados sísmicos marítimos e de OBC

Neste trabalho é demonstrado como o método da refletividade pode ser eficientemente implementado para simular aquisições sísmicas marítimas. Isto inclui o registro de pressão no caso da fonte e dos receptores (hidrofones) estarem localizados na camada de água, o deslocamento (ou velocidade de vibração) vertical e horizontal no caso dos receptores estarem plantados no leito do oceano, simulando aquisições multicomponentes tipo OBC (Ocean Bottom Cable) ou OBS (Ocean Bottom Sensor). Uma das vantagens do presente método é a facilidade de seleção dos eventos que farão parte da resposta, tais como múltiplas internas, múltiplas associadas à superfície livre e ondas convertidas no caso P-Sv. Os fenômenos da atenuação e da dispersão podem ser levados em conta facilmente considerando as velocidades complexas, já que o método é desenvolvido no domínio ω-p. O algoritmo proposto possui custo computacional bem menor quando comparado com métodos puramente numéricos como diferenças finitas ou elementos finitos. Sendo assim, no caso onde um modelo de camadas planas horizontais seja suficiente para descrever a subsuperfície, este torna-se uma ferramenta prática para tarefas como prever com bastante precisão e rigor a sensibilidade sísmica de um determinado reservatório com relação à substituição de fluidos ou prever como uma determinada mudança de propriedade petrofísica pode influenciar a resposta de AVO multicomponente.

Receiver function decomposition of OBC data: theory

SUMMARY This paper deals with theoretical aspects of wavefield decomposition of Ocean Bottom Cable (OBC) data in the τ –p domain, considering a horizontally layered medium. We present both the acoustic decomposition and elastic decomposition procedures in a simple and compatible way. Acoustic decomposition aims at estimating the primary upgoing P wavefield just above the ocean-bottom, whereas elastic decomposition aims at estimating the primary upgoingP and S wavefields just below the ocean-bottom. Specific issues due to the interference phenomena at the receiver level are considered. Our motivation is to introduce the two-step decomposition scheme called ‘receiver function’ (RF) decomposition that aims at determining the primary upgoing P and S wavefields (RFP and RFS, free of any water layer multiples). We show that elastic decomposition is a necessary step (acting as pre-conditioning) before applying the multiple removal step by predictive deconvolution. We show the applicability of our algorithm on a synthetic data example.

The challenges of seismic acquisition in a remote and sensitive marine environment: Scott Reef, Western Australia

Woodside Energy Ltd, as operator of the Browse LNG Development, recently acquired two seismic surveys at Scott Reef, Western Australia. The surveys were important steps towards acquiring full seismic coverage over the Torosa gas field, part of which underlies Scott Reef The Maxima 3D marine seismic survey, conducted in late 2007, was a conventional towed streamer survey. This was followed in May 2008 by the Gigas 2D transition zone survey in the shallow waters of north Scott Reef, and Woodside’s first experience with ocean bottom cable (OBC) seismic technology. Each survey presented unique challenges during the planning, regulatory approval and acquisition stages. Scott Reef comprises two coral atolls located on the outer continental shelf of northwest Australia, approximately 400 km north of Broome. The only permanently emergent land is a small sand cay (Sandy Islet, Fig. 1), although the reef crests of both atolls are exposed at low tide. Outside the reefs the seafloor drops away rapidly, with water depths of about 350 m to the east, increasing to more than 1,000 m to the west. South Scott Reef lagoon is open to the north, with water depths increasing to about 50 m before deepening abruptly into the channel between the two reefs. North Scott Reef lagoon is shallower—generally less than 25 m—and is connected to the ocean by two narrow channels. Semi-diurnal tides with a range of up to 4.6 m produce strong tidal currents in and near these channels. Small, steep-sided coral heads, or bombies, are common throughout the lagoons, especially in water less than 25 m deep.

Building on BP's large-scale OBC monitoring experience—The Clair and Chirag-Azeri projects

From the very beginning of 4D seismic technology in the 1990s, BP has taken a great interest in testing and developing ocean-bottom cable (OBC) monitoring as an alternative to conventional streamer surveys. Today, permanent OBC or “life-of-field-seismic” (LoFS) systems, together with semipermanent OBC systems, are an important method to acquire high-quality, consistent, and potentially frequent 4D data in selected fields. They are believed to represent a cost-effective means to acquire 4D data at intervals of about one year or less and are considered a key step toward the goal of optimally managing reservoirs as part of the “digital oilfield” concept.

Seismic surveillance using redeployed ocean-bottom nodes

Ten years ago, the argument for permanent ocean-bottom cables (OBC) seemed compelling. Permanent OBC deployment would provide the best possible receiver repeatability, while wide-azimuth geometry would provide improved illumination as well as fracture characterization. Hydrophone-geophone (PZ) combinations would improve demultiple. Finally, multicomponent receivers would also record shear waves that would unravel the combined effects of lithology, porosity, and fractures for reservoir characterization, as well as the combined effects of fluid replacement and pressure changes for monitoring.

Strengthening the virtual-source method for time-lapse monitoring

Time-lapse monitoring is a powerful tool for tracking subsurface changes resulting from fluid migration. Conventional time-lapse monitoring can be done by observing differences between two seismic surveys over the surveillance period. Along with the changes in the subsurface, differences in the two seismic surveys are also caused by variations in the near-surface overburden and acquisition discrepancies. The virtual-source method monitors below the time-varying near-surface by redatuming the data down to the subsurface receiver locations. It crosscorrelates the signal that results from surface shooting recorded by subsurface receivers placed below the near-surface. For the Mars field data, redatuming the recorded response down to the permanently placed ocean-bottom cable (OBC) receivers using the virtual-source method allows one to reconstruct a survey as if virtualsources were buried at the OBC receiver locations and the medium above them were a homogeneous half-space. Separating the recorded wavefields into upgoing and downgoing (up-down) waves before crosscorrelation makes the resultant virtual-source data independent of the time-varying near-surface (seawater). For time-lapse monitoring, varying source signature for the two surveys and for each shot is also undesirable. Deconvolving the prestack crosscorrelated data (correlation gather) by the power spectrum of the source-time function results in virtual-source data independent of the source signature. Incorporating up-down wavefield separation and deconvolution of the correlation gather by the source power spectrum into the virtual-source method suppresses the causes of nonrepeatability in the seawater along with acquisition and source signature discrepancies. This processing combination strengthens the virtual-source method for time-lapse monitoring.

Paradigm shift in 3D multicomponent seismic imaging in the Arabian Gulf

Ocean-bottom cable (OBC) technology has many applications in petroleum exploration and exploitation. It permits us to acquire data in areas with intensively developed production infrastructure that impede conventional marine-streamer acqui- sition. Permanently installed OBC systems provide data for monitoring production from reservoirs. In addition to their logistical advantages, OBC systems also allow us to acquire multicomponent data. Typically, 2-C OBC data are acquired by two types of detectors: hydrophones (fluid-pressure change detectors) and single- or multicomponent geophones (particle-velocity or acceleration detectors). The multicomponent character of OBC data permits us to discriminate between upcoming and downgoing waves and to attenuate water-column reverberations. At the same time, because of the differences between the physical measurement characteristics of hydrophones and geophones, their data character (including noise levels, multiple content, coupling effects) can be very different. Th...

Lithology estimation from a multicomponent 3D-4C OBC seismic survey over a clastic reservoir in the Arabian Gulf

ABSTRACT In 2002, Saudi Aramco conducted its first 3D, 4-component (4C) ocean-bottom cable (OBC) seismic survey in the Arabian Gulf. The main objective was to delineate the middle Cretaceous Upper Khafji Sand Stringers Reservoir overlying the massive Main Khafji Sand Reservoir in the Zuluf field. The Upper Khafji Sand Stringers Reservoir in the Wasia Formation is typically characterized by weak acoustic impedance contrasts. A pre-survey modeling study, based on the logs of compressional (P) and shear-wave (S) velocities (Vp and Vs), indicated that converted compressional-to-shear waves (P-S) could better-image the structure and stratigraphy of the target reservoir. Commensurate with the objectives of the experiment, a pilot 100-square-kilometer survey was acquired with an inline swath-shooting geometry that employed two seabed receiver cables, with a symmetric split-spread deployment of the 4-C sensors. The acquisition geometry consisted of six sail lines per swath with a single-boat, dual-source, flip-flop configuration. The data were processed through dual-sensor summation, horizontal-component rotation and P-P/P-S pre-stack time migration. Post-stack enhancement using non-stationary Gabor deconvolution proved beneficial in compensating for the missing high frequencies in the acquired converted-wave data. Well-to-seismic calibration for both P-P and P-S data at five wells aided in the interpretation of the data. Five horizons were interpreted and correlated between the P-P and P-S sections. The horizons were analyzed using both amplitude and interval times such that the lateral variations of the Vp/Vs ratio of the Upper Khafji Sand Stringers Reservoir could be mapped. A region of low Vp/Vs ratios in the northwest quadrant, obtained from the isochron interval-time analysis, was correlated with higher ‘net sand’ pay at a hidden well located in the middle of this region. These results were further corroborated by seismic facies analysis and provide a qualitative reservoir quality index in the Upper Khafji Sand Stringers Reservoir.

Anisotropic PP and PSv prestack depth migration of 4C seismic data; Pamberi, Trinidad

SummaryIn November 2004, EOG Resources contracted WesternGeco to acquire an ocean-bottom cable (OBC) 4C swath survey across the Pamberi-1 well in the Lower Reverse L block of the Columbus basin, eastern offshore Trinidad. The purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving lithology and stratigraphic detail in an area perturbed by shallow gas, overpressure and illumination shadows from normal regional faults and major anticlinal ridge trends acting as pressure seals. A key motivation for EOG was the realization that a conventional 3D towed-streamer survey acquired the previous year failed to adequately image the target reflectors comprising the reservoir under the main fault.Details of the P- and PSv-wave processing of this dataset through anisotropic prestack time migration were previously described (Johns et al., 2006) in which it was demonstrated that there existed a qualitative correlation between derived parameters and attributes from P and Sv anisotropic migration velocities and known regional geology. This observation was quite remarkable considering only a limited effort to constrain or validate parameters (in this case, velocities to the Pamberi-1 well checkshots) was performed. Under the “Future work” section of the previous publication, it was suggested that further data quality enhancement in preparation for more quantitative rock property classification could only be achieved after prestack depth imaging.In this paper, we present precisely that next phase in the 4C processing, advancing the P- and PSv-wave data through anisotropic prestack depth migration, using cellbased tomography with a top-down approach. The Pamberi-1 well was used to constrain the anisotropy in the shallow section, with the deeper, spatial trend away from the proximity of the well determined from the anisotropy derived previously in the time processing.Prior to proceeding with the anisotropic depth imaging, the magnitude of shear splitting from the presence of azimuthal anisotropy (HTI) was first examined to assess its potential impact on the radial rotated P-S signal. The shear wave splitting analysis revealed a principal angle of polarization that was closely aligned with the regional stress direction delineated by the normal major faults blocks acting as pressure seals.

Monitoring overburden layer changes and fault movements from time-lapse seismic data on the Valhall Field

SUMMARY A method developed by Roste et al., which discriminates between layer thickness and velocity changes, is tested on pre-stack time-lapse seismic ocean bottom cable (OBC) data from the Valhall Field. A key parameter in this discrimination process is the dilation factor, α, which is the relative velocity change divided by the relative thickness change within a given layer. The high quality and good repeatability of the OBC data enables us to estimate α with a reasonable accuracy for α-values between 0 and −5. For α-values below −5, complementary information, like for instance geomechanics is required. For the top reservoir horizon we estimate a maximum subsidence of 0.50 m ± 0.29 m and a corresponding velocity decrease for the sequence from the seabed to the top reservoir of 2.0 m s−1± 0.40 m s−1. Time-lapse distortion patterns with characteristic time-shift versus offset signatures are observed. The positions and vertical extents of the distortion zones are determined from ray path considerations and modelling. The distortion zones correlate with buried faults, indicating that a (time-lapse) distortion zone might be produced by a localized slip in a fault zone. We present an extended method which allows for vertical (in addition to lateral) variations in the relative thickness and velocity changes. This method can be viewed as a simplified version of time-lapse tomography, but involving fewer unknown parameters, giving more stability to the estimated changes in thickness and velocity. Using this technique, we are able to estimate α for positions with localized time-lapse distortions.

Multicomponent velocity analysis with quaternions

ABSTRACTMulticomponent seismic data are acquired by orthogonal geophones that record a vectorial wavefield. Since the single component recordings are not independent, the processing should be performed jointly for all the components. A way to achieve this goal is to exploit quaternions, hyper‐complex numbers that due to their very nature are apt to represent multidimensional data. In fact, quaternion algebra allows us to extend coherence functionals used for scalar observations to multicomponent data. Therefore by means of quaternions we implement semblance and other methods based on matched filtering and on the data covariance properties.As an application we show the results from a quaternion velocity analysis carried out combining information from the geophones and from the hydrophones of an ocean bottom cable (OBC) survey, and thus recognizing the true vectorial nature of the incoming wavefield. This also allows one to relax, at least partially, vector fidelity constraints.We demonstrate that quaternion velocity analysis yields an improved resolution with respect to the single component velocity analysis for any coherence functional chosen and that it simultaneously evidences velocity trends pertaining to different wave modes. This facilitates the interpreter in the estimation of interval Vp/Vs by means of event correlation, and in making use of a priori information from VSP and well logs. It also speeds up the velocity picking that can be performed in a single pass on a multicomponent velocity panel, rather than once for each single component velocity panel.

Seabed property estimation from ambient-noise recordings: Part 2 — Scholte-wave spectral-ratio inversion

Having knowledge of the near-surface shear-velocity model is useful for various seismic processing methods such as shear-wave static estimation, wavefield separation, and geohazard prediction. I present a new method to derive a 2D near-surface shear-velocity model from ambient-noise recordings made at the seafloor. The method relies on inverting horizontal- and vertical-amplitude spectra of Scholte waves propagating in the seafloor. I compare the commonly used horizontal-over-vertical spectral ratio with three alternative spectral-ratio definitions through modeling. The modeling shows that the vertical-over-total spectral ratio has some favorable properties for inversion. I describe a nonlinear inversion method for the vertical-to-total spectral ratio of the Scholte waves and apply it to an ambient-noise data set recorded by an ocean-bottom-cable (OBC) system. A 1D near-surface shear-velocity model is derived through a joint inversion of the spectral-ratio and phase-velocity data. A 2D shear-velocity model is obtained through a local inversion of the spectral ratios averaged over small groups of receivers and shows evidence for lateral heterogeneity. The newly developed method for deriving near-surface shear-velocity distribution by inverting the Scholte-wave spectral ratio measured from seabed noise provides great opportunities for estimating the shallow-seabed shear velocity in deep water. Another benefit of the method is that, with the OBC system, no additional hardware is needed; only additional recording time is required. In this case, half an hour is sufficient.

Modelagem e migração em profundidade 2D em meios com simetria polar local

This paper shows a technique based on the phase-shift method (PSM) to implement pre-stack depth migration on locally transverse isotropic media (LTI), with the symmetry axis direction varies continually along the layers. For seismic modeling, a generalization of the finite differences method for the solution of the elastic wave equation was used. With this procedure, it was possible to accommodate seismic modeling on LTI media defined by six parameters at each grid point, i.e. , density, P and S wave propagation velocities along the local symmetry axis, Thomsen parameters and the direction of the local symmetry axis itself. In order to separate from the seismograms the qP and qSV wavefields, an algorithm based on the Christoffel equation was implemented. The migration for each common shot gather is implemented solely by phase-shift based algorithms, which means that not only the depropagation of the registered wavefield, but also the generation of the time matrices involved in the imaging condition were obtained in this manner for each set of parameters at each depth level. The migration results using qP-qP and qP-qSV reflections show that the horizons were located precisely, and that the process is stable in relation to the symmetry axis variations. The proposed method is for multicomponent seismic acquisitions and might be applied to marine seismic data using streamers, or Ocean Bottom Cables or vertical cables. Since the proposed method uses phase-shift algorithms, its parallel implementation can be highly efficient.

A comparison of streamer and OBC seismic data at Beryl Alpha field, U. K. North Sea

Compressional P-wave ocean-bottom-cable (OBC) seismic data from the Beryl Alpha field in the U. K. North Sea provide a superior image of the subsurface compared to heritage streamer seismic data. To determine the reason for the superiority of OBC data, the results of a detailed comparison of these OBC and streamer data sets are compared. The streamer and OBC data sets are reprocessed using a strategy that attempts to isolate the roles of processing, fold, azimuth, PZ combination, and hydrophone and geophone data have on the improved OBC image. The vertical component of the geophone (OBC Z) provides the major contribution to the improved OBC image. The imaged OBC Z datacontain fewer multiples and have a higher signal-to-noise ratio than the streamer. The OBC data have a lower level of multiple contamination because of the contribution from the OBC Z component, together with an effective suppression of receiver-side water-column reverberations as a result of the combination of the OBC hydrophone and geophone traces (PZ combination). The increased fold and wider azimuths of OBC data improve the OBC image slightly. Wider azimuths improve fault imaging, especially for faults oriented obliquely to the inline and crossline directions. The particular conditions at Beryl Alpha field that make the OBC survey successful are the relatively hard water bottom and the presence of multiples that are difficult to remove from streamer data using standard demultiple techniques.

Detector coupling corrections for vector infidelity of multicomponent OBC data

Differences in the frequency response of horizontal and vertical detectors (vector infidelity) in ocean bottom cable (OBC) surveys can cause problems for multicomponent processing, such as S-wave birefringence and amplitude variation with azimuth (AVA) analyses, and combining vertical and hydrophone data for water-born multiple suppression. One source of this problem is poor detector coupling with the seabed that produces resonances and phase distortions. Coupling and data quality are generally excellent for the inline component. However, the crossline component often exhibits low-frequency resonance compared to the inline. Also, OBCs are susceptible to rotational modes about the cable axis that produce spurious S-waves on the vertical component. I derive a method for correcting the crossline and vertical components based on a model of OBC detector coupling, and design vector operators in the frequency domain from shots over many offsets and azimuths. The crossline data are corrected,relative to the inline, assuming linear polarization of early, near-offset arrivals on the radial-horizontal component. Thus, the transverse-horizontal component provides a convenient error or objective function to be minimized for operator design. Using the corrected crossline, as a model of rotational modes, leads to an estimate of spurious S-waves on the vertical component, which are adaptively subtracted. Data examples from the Gulf of Mexico and offshore Nigeria are presented to illustrate improvements in crossline frequency content and match to inline data. Typically there is [Formula: see text] reduction in error using the rms ratio of transverse-to-radial component data computed in the time domain. Suppression of spurious S-waves from the vertical component without undesirable effects of low-cut or [Formula: see text] filters is shown for prestack and poststack data. Also, vector operators indicate they contain important information related to resonance modes of crossline coupling and rotational modes associated with seabed-deployed versus buried OBCs.

Data Processing of Marine Multicomponent Seismic-A Case Study of 4-Component 2D OBC data, Offshore Norway-

Multicomponent seismic technology has been improved significantly with higher fidelity acquisition systems and more advanced data analysis methods. Especially in marine seismic, 4-Component Ocean Bottom Cable (OBC) is practically applied for reservoir characterization and exploration at the water depth of more than 1000m. Converted wave processing and interpretation is a fairly new topic and there is a lack of data and expertise around the world. Until now, the most common converted wave data has been essentially 1-D, VSP and OBS (Ocean Bottom Seismometer) records. Recently there is an increasing use of multi-component OBC 2-D and 3-D surveys. They are especially applied to evaluate reservoirs, because it has been revealed that P-S converted waves can give valuable additional lithological information. They also give clear reflections where the P-wave reflections are poor due to a very small acoustic impedance contrast. It is worthwhile that we introduce the actual processing of the 4-C line to discuss on P-S converted wave processing and later to develop new tools and techniques in this rapidly evolving field. Since marine multicomponent seismic survey has not been carried out yet in Japan, we use a 4-Component OBC (4C OBC) 2-D line, acquired by PGS Norway in 1998. The data was purchased by JNOC/TRC in 2002, as a part of the methane hydrate research program, to evaluate the use of P-S converted wave data in determining the properties of the hydrate layer and the BSR. The 4C line was acquired along the track of a previous conventional streamer line which showed a BSR. The location is in the Norwegian Sea, just up-dip of a major sedimentary slide, known as the 'Storegga Slide'. The results of the first investigation of the 4C data were published by Andreassen et al (2001). They presented a very significant difference between the P-wave and the PS converted wave sections in the hydrate layer and around the BSR. In fact, the BSR was not visible on the PS-wave section. The result shows that a lot of additional information is contained in the converted waves. We reprocessed the 4C data and discussed the specific problems.

Hild structure multi-azimuth seismic experiment

Illumination of complex structures has always been a matter of discussion. While in the ‘old’ days it was preferable to shoot in the dip direction of the main structural trend, the recent advances in processing have led us towards shooting in the most ‘economical’ direction. The recent development of OBC (Ocean Bottom Cable) acquisition has provided us with high-fold multi-azimuth data and has demonstrated dramatic improvements in the image quality. Already in 1996 a test was carried out by Elf (in Gabon) acquiring four surveys at different azimuths across a salt body (Houllevigue, H.., Delesalle, H., and De Bazelaire, E., 1999). The result of this experiment showed the incomplete but complementary information extracted from the various acquisition directions. PGS carried out the same kind of experiment across the Norwegian Varg field where they shot two surveys at 600 from the old existing survey (Hegna, S. and Gaus, D., 2003). The published results showed improved imaging at the reservoir level.