Eivind Berg

Crustal structure of the central part of the Vøring Basin, mid-Norway margin, from ocean bottom seismographs

Abstract Regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Voring Basin, mid-Norway margin, have been modelled by use of two-dimensional (2-D) ray-tracing. The regional dataset comprises thirty OBSs deployed along seven 100–170 km long profiles. The deeper part of the Voring Basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels (2–5 km below sea-floor), but the modelling of the OBS-data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea-floor to the upper mantle. The shallow and intermediate sediments (to 5 km below sea-floor) are characterized by a vertical increase in velocity due to increased confining pressure. There is also considerable lateral variation in the velocities within sedimentary layers at all levels. The OBS-data confirm that intrusions of sills are important at intermediate and deep sedimentary levels (2–10 km below sea-floor) in most parts of the area. The sills seem to vary in lateral extent from about 20 km to more than 100 km, and their thickness is generally inferred to be about 200 m. The low velocity in the upper crystalline crust (6.2 km/s) confirms that the crust in the Voring Basin is of continental origin. In most parts of the area the velocity of the lower crust is as high as 7.3–7.6 km/s. This high-velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness. The base of the 7.3 km/s layer is interpreted as the Moho, and the upper mantle velocity is estimated to 8.2 km/s.

Modelling shear waves in OBS data from the Vøring basin (Northern Norway) by 2-D ray-tracing

Three component recordings from an array of five ocean bottom seismographs in the northwestern part of the Vøring basin have been used to obtain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves are identified by means of travel-time differences compared to the compressional (P) waves, and by analyzing their particle motions. The model has been obtained by kinematic (travel-time) ray-tracing modelling of the OBS horizontal components.The shear-wave modelling indicates that mode conversions occur at several high velocity interfaces (sills) in the 4–10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set.An averageVp/Vsratio of 2.1 is inferred for the layers above the uppermost sill, indicative of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in theVp/Vsratio (1.7) below the first sill may in part be atributed to well consolidated sediments, and to a change in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5–10 km depthVp/Vsratios of 1.85 indicate a lower sand/shale ratio consistent with the expected lithologies. The averageVp/Vsratio inferred for the crust is 1.75, which is consistent with values obtained north of Vøring, in the Lofoten area. An eastward thinning of the crystalline basement is supported by the shear-wave modelling.

Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD

We have obtained precise estimates of the position of Ocean Bottom Seismometers (OBS) on the sea bottom. Such estimates are usually uncertain due to their free falling deployment. This uncertainty is small enough, or is correctable, with OBS spacing of more than 10 km usually employed in crustal studies. But, for example, if the spacing is only 200 m for OBS reflection studies, estimates of the position with an accuracy of the order of 10 m or more is required.The determination was carried out with the slant range data, ship position data and a 1D acoustic velocity structure calculated from Conductivity–Temperature–Depth (CTD) data, if they are available. The slant range data were obtained by an acoustic transponder system designed for the sinker releasing of the OBS or travel time data of direct water wave arrivals by airgun shooting. The ship position data was obtained by a single GPS or DGPS. The method of calculation was similar to those used for earthquake hypocenter determination.The results indicate that the accuracy of determined OBS positions is enough for present OBS experiments, which becomes order of 1 m by using the DGPS and of less than 10 m by using the single GPS, if we measure the distance from several positions at the sea surface by using a transponder system which is not designed for the precise ranging. The geometry of calling positions is most important to determine the OBS position, even if we use the data with larger error, such as the direct water wave arrival data. The 1D acoustic velocity structure should be required for the correct depth of the OBS. Although it is rare that we use a CTD, even an empirical velocity structure works well.

Autonomous nodes for time lapse reservoir seismic an alternative to permanent seabed arrays

Time lapse (or 4D) seismic monitoring of producing oil fields is an accepted method for optimisation of field development and product recovery, providing significant improvements in recovery rates and saving in drilling cost.. This paper provides an overview of the concept and design of an autonomous seismic node recording system, and a comparative analysis of using nodes versus permanent sensor cable installations for 4D seismic analysis and monitoring of an oil field. The node approach has only recently been used by industry, but the development and testing of these systems has been in progress for nearly 20 years. This paper illustrates node seismic instrumentation and operation, description of directional sensitivity or vector fidelity, and the application of node seismic to the Cantarell Field in Mexico, a field with an extensive network of pipelines and surface platforms, all of which are serious obstacles in the path of an accurate and well-sampled seismic survey. A quantitative description of navigation accuracy vs. water depth is provided, along with a comparative commercial analysis of the permanent cable installation at BP’s Valhall Field with an equivalent array of autonomous nodes.

Air-gun bubble damping by a screen

A method for damping unwanted bubble oscillations from a seismic air gun is presented. The method exploits the fact that the primary pressure peak generated by an air gun is produced during the first 5-10 ms after firing. The air bubble is destroyed by mounting a perforated screen with an optimal radius about the gun. Once the primary pressure peak has been generated by the bubble, the bubble is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillations. Controlled near-field measurements of 40-cubic inch and 120-cubic inch air guns with and without damping screens are used. The primary to bubble ratio improves from 1.4 without a screen to 4.4 with a screen in the near-field. The corresponding values for estimated far-field signatures are 1.8 to 9.0 when the signatures are filtered with an out-128 Hz (72 dB/Oct) DFS V filter.

Technology and economy of ocean bottom nodes on the first anniversary of the first 5C crew

Ocean bottom seismometers and surface towed streamers are known methods. They each have well understood advantages and disadvantages. The most significant advantage of the streamers is their efficiency in covering large areas at a low cost per km square. While data quality and the usefulness of wide azimuth, demultiple, and shear waves is field specific and is often debated, a clear advantage of ocean bottom seismometers is our ability to use them in obstructed areas such as active oilfields. Nodes are better than cables in the presence of seabed obstructions. Streamers and ocean bottom seismometers therefore complement each other. If one adds a streamer element to an ocean bottom node crew, one can provide the two methods at the same time as a seismic monitoring snapshot. The point of this paper is to share the experience of a crew with both nodes and a streamer on its first anniversary.

Three-component OBS-data processing for lithology and fluid prediction in the mid-Norway margin, NE Atlantic

In 1992, acomprehensive three-component ocean bottom seismic survey was performed in the central and northern area of the Vøring Basin, offshore mid-Norway, NE Atlantic. An important part of the data acquisition program consisted of a local survey with 20 Ocean Bottom Seismographs (OBS) dropped at approximately 200 m interval in 1300 m water depth. The main purpose of the local survey was to acquire densely sampled P- and S-wave reflection data above a seismic flatspot anomaly observed earlier, in order to more accurately predict if hydrocarbons could be related to it. The conventional reflection data processing methods applied to the vertical components included predictive deconvolution in order to attenuate low frequency ringing, near offset mute and a series of constant velocity stacks in order to obtain the optimal velocity function. The final result is a “trouser” shaped, high resolution VZ stacked section with minor influence of water multiples. The inline (Vx ) component contains no strong multiples, and extensive near trace muting was hence not necessary to apply for this component. Velocity analysis together with ray-tracing modelling indicate that P-S-converted shear waves (reflections) represent the dominant mode. The results of the interpretation and modelling indicated a Vp/Vs-ratio of approximately 2.6 in the overburden, which suggests domination of partly unconsolidated shale, while the Vp/Vs-ratio in the assumed reservoir was approximately 1.8, which indicates a more sand dominated facies. Outside the flatspot area a higher Vp/Vs-ratio ratio (approximately 2.0) was estimated, indicating that hydrocarbons could be present in the assumed reservoir.

Evaluation and impact of sparse‐grid, wide‐azimuth 4C‐3D node data from the North Sea

For DEMO 2000 a consortia comprising of SeaBed Geophysical, SINTEF, Statoil, Norsk Hydro and TotalFinaElf submitted a proposal entitled IMPREDO or “improved prediction and delineation of hydrocarbon filled reservoir zones using high-quality four-component seismic data acquired in 3D at the seabed”. In July 2002 the consortia had an opportunity to record data on the Volve field during a conventional OBC survey. This was the first opportunity to deploy the node system in a fullscale offshore test and prove the reliability and capability of the system to acquire high quality, high vector-fidelity seismic data. The field chosen for the project, Volve, is located in Block 15/9 west of Haugesund, Norway, and the water depth is between 80 m and 90 m. To cover the area of interest a total of 128 units were deployed in a 400m grid covering an area 6 km by 2.8 km. The shooting grid of the conventional OBC survey required 6 overlapping swaths providing narrow azimuth data whereas the same shots used in this survey provide wider cross-line offsets due to a larger active receiver area. Evaluation and impact of sparse-grid, wide-azimuth 4C-3D node data from the North Sea Andrew Morton, Geir Woje, Anne Rollet, Eivind W. Berg, Claude Vuillermoz 1 SeaBed Geophysical, Transitt gt 14, N-7042 Trondheim, Norway. 2 CGG, 1 rue Leon Migaux, 91341 Massy cedex, France

Crustal Structure of the Lofoten Continental Margin, N. Norway, From Ocean Bottom Seismographic Data

This study presents a 500 km long crustal transect across the Lofoten volcanic passive continental margin, N. Norway, by compiling results of two successive Ocean Bottom Seismographic (OBS) experiments performed in 1988. The OBS profiles were acquired from the Norwegian mainland, across the continental shelf, over an area covered with landward flood basalts, to the Lofoten basin. The land side end of the crustal model represents a thinned continental structure. The upper crust in this part has strong structural complexity, mainly due to faulting during Mesozoic and older continental thinning phases. Between the continental shelf and the seaward dipping reflectors (SDRs), the model represents an extremely thinned continental crust and ocean/continent transition zone. This region is interpreted to be dominated by an early Tertiary continental rifting phase that progressed until early Eocene. Between the SDRs and magnetic anomaly 21 an oceanic crust with thick lower crust and a high velocity layer (7.3 km/s) at the bottom of the crust are obtained. This high velocity layer is believed to be created by anomalously hot asthenospheric material rising around a hot spot.

Three‐component ocean bottom seismographs used in prospecting in the Voring Basin, N. Norway

An extensive seismic experiment by use of twentyseven three-component Ocean Bottom Seismographs (OBS) was in 1992 conducted in the central and northern part of the Voring basin, northern Norway. Each OBS was deployed several times; a total of 75 OBS deployments along seven profiles with a total length of = 1650 km were thus performed during the acquisition. All profiles were shot where high-quality multichannel reflection profiles have been acquired earlier. The spacing between the OBSs varied from 200 m to about 30 km, and two different seismic air-gun sources were used to optimize the mapping of both deep crystalline and shallow sedimentary structures. 2-D ray-tracing modelling of the regional OBS-data shows that these data contain considerable information about deep sedimentary and crystalline structures, not achievable through the study of multichannel reflection data.