GEO ExPro

Reading Between the Lines

Point-receiver data, isometrically sampled in both crossline and inline directions, fully captures the three-dimensional seismic wavefield for the first time.
This article appeared in Vol. 9, No. 5 - 2013

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The Nessie-6 streamer delivers point-receiver sampling of pressure (hydrophone) and MEMS-based measurements of acceleration that provide Vy and Vz datasets for wavefield reconstruction. Source: WesternGeco These two images are from a 2011 IsoMetrix test survey. The streamers were 75m apart and are represented vertically. Conventional 2.D lines are visible in the top section. Shown at the bottom, IsoMetrix technology correctly reconstructs the whole wavefield, including returns from geologic fault planes that run across the streamer spread. Point diffractors image perfectly. Source: WesternGeco Even the most cursory study of an outcrop such as a cliff or quarry face will show that structural and stratigraphic variations in geology are three-dimensional. This is particularly the case for hydrocarbon reservoirs, where trap closure is required in all directions. It is nearly 100 years since the reflection seismic method was first used to image the subsurface, and since then it has evolved to deliver increasingly accurate 3D geological models and more reliable inversion to reveal indicators of rock properties.

Marine 2D seismic surveys, acquired with a single streamer, have typically covered grids of lines several kilometres apart, requiring geological interpretations to be interpolated over long distances. Before the advent of multi-streamer operations, some operators experimented with reducing 2D line-spacing to 100s of metres and interpolating to a finer grid during data processing. The method, dubbed ‘2.D’, achieved limited success in creating reliable 3D data volumes.

The 1980s saw the development of multi-streamer marine seismic surveys. Today, these surveys are typically acquired by a vessel equipped with between 8 and 16 streamers towed 50 to 100m apart, each 3 to 8 km long. Each streamer contains hydrophone sensors, and spatial sampling of the data recorded along each streamer (inline) can be as fine as 3.125m; however, the much greater distance between adjacent streamers means that sampling in the crossline direction can be 16–32 times sparser. Although described as ‘3D’, the method acquires data so coarsely spaced in the crossline direction that it is still essentially ‘2.D’, as it cannot capture the whole 3D wavefield and so is limited in its ability to accurately image the subsurface. While developments in seismic sources and sensors have improved the frequency bandwidth that can be input to the subsurface and subsequently recorded back at the surface, achieving truly 3D high-resolution images of complex structures also requires adequate spatial sampling in both the inline and crossline directions.

New Towed Streamer Technology

Measurement of crossline gradients enables unaliased reconstruction of the seismic wavefield between streamers. The blue waveform represents the actual signal, and the red waveform the reconstructed signal. The figures contrast reconstructed signal with pressure-only measurements as recorded in conventional surveys (top) versus pressure + gradients as recorded in IsoMetrix surveys (bottom). Source: WesternGeco The recently launched IsoMetrix marine isometric seismic technology delivers high-fidelity point-receiver seismic data while overcoming spatial wavenumber bandwidth compromises that have limited previous marine seismic acquisition methods. The result is a reliable, continuous measurement of the full upgoing and downgoing notchless seismic wavefield sampled at a 6.25m x 6.25m point-receiver surface grid. This fine isometric sampling in both crossline and inline directions makes the data suitable for use in a wide variety of interpretation and modelling applications, such as high-resolution near-surface imaging, deep reservoir characterisation, and 4D reservoir monitoring.

Isometric 3D sampling is enabled by a towed streamer design that combines measurements of wavefield pressure and gradient – vertically and crossline. Called Nessie-6, this new-generation streamer, a key component of the acquisition system, uses point-receiver technology that combines hydrophones with calibrated point-receiver microelectromechanical system (MEMS) accelerometers that measure the full particle acceleration due to the upgoing and downgoing seismic wavefield. Direct measurement of the vertical and crossline gradient enables unaliased reconstruction of the pressure wavefield between the streamers.

Full Bandwidth Deghosting

Conventional towed-streamer marine seismic acquisition systems deploy sources and streamers at shallow depths, typically between 6 and 10m. This configuration enables recording of the high frequencies needed for resolution, but attenuates the low frequencies needed for stratigraphic and structural inversion. Shallow towing also makes the data more susceptible to environmental noise such as waves, swell and wind. Towing sources and streamers at deeper depths enhances the low-frequency content and can increase the signal-to-ambient-noise ratio (S/N); however, it attenuates the high frequencies. The attenuation is due to interference of the upgoing seismic wavefield recorded by the pressure sensors by its ‘ghost’ – the reflection of the wavefield after bouncing back from the sea surface above the streamer. The IsoMetrix system overcomes the need for compromise when deciding tow-depth. For each Nessie-6 streamer, high-quality measurement of the vertical particle acceleration – with good S/N down to 3 Hz – enables separation of the pressure wavefield into its upgoing and downgoing components, facilitating removal of the receiver ghost. The source ghost is addressed by a newly developed calibrated marine broadband seismic source family of notchless seismic sources.

In combination with a solid streamer design, the ability for full-bandwidth deghosting allows the extension of the seismic acquisition window by making the results of data acquisition less affected by adverse weather conditions. In addition, adequate sampling of coherent noise from external sources such as rigs and other seismic vessels means that it can be effectively removed at an early stage of processing.

3D Wavefield Reconstruction

Top: Pressure (P) wavefield reconstructed using a matching pursuit method based on P-wave data alone as recorded in conventional surveys. Bottom: P wavefield reconstructed with GMP method using Vy and Vz particle velocity vector data recorded in IsoMetrix surveys. The spatial gradient of the seismic pressure wavefield is directly related to the particle velocity vector, and can be derived from measurements made independently by multicomponent sensors. This knowledge enables multichannel reconstruction of the pressure wavefield in the crossline direction far beyond the aliasing limits that are possible with hydrophone-only or hydrophone plus vertical gradient data. A new computer algorithm called the generalised matching pursuit (GMP) method uses crossline (Vy) and vertical (Vz) particle velocity components of both the upgoing and reflected downgoing wavefronts derived from the Nessie-6 multisensor streamer data to achieve simultaneous reconstruction and deghosting of the seismic pressure wavefield in a 3D sense. It can compute the upgoing and downgoing separated wavefield at any desired position within a spread of streamers and has been shown to be extremely robust in dealing with highly aliased data.

Improved 4D Repeatability and Flexible Geometries

Data example from a North Sea test programme (a) Input P data, 6.25m (inline) x 75m (crossline) conventional sampling (b) Input Vy data, 6.25m (inline) x 75m (crossline) conventional sampling (c) IsoMetrix reconstructed P wavefield, 6.25m x 6.25m grid (d) IsoMetrix deghosted P wavefield, 6.25m x 6.25m grid The IsoMetrix technology incorporates several field-proven elements of the WesternGeco Q-Marine system, including its proprietary marine seismic streamer steering system and intrinsic ranging by modulated acoustics (IRMA) system to control the positions of the entire seismic spread. The ability to reconstruct the wavefield at any desired grid raises 4D repeatability to an unprecedented level to better reveal subtle variations in seismic responses related to changes in reservoir fluids and pressure. When comparing to a conventional towed streamer baseline, a set of ‘virtual’ streamers can be produced to perfectly match the positions of the existing survey. The ability to reconstruct both the upgoing and downgoing wavefields allows redatuming to further improve comparison with previous surveys. These workflows can resolve previously inherent conflicts in planning 4D projects between referencing a previous survey or creating the best possible new baseline. Now, both objectives can be met from a single survey.

While most 3D marine surveys are acquired with linear geometries using a single vessel, an increasing number have been successfully completed using alternative configurations. Complex 3D geology and highly refractive layers cause ray bending that can leave portions of the subsurface untouched when recording seismic waves travelling in just one direction. Wide-azimuth (WAZ) seismic surveys designed using several seismic vessels working together have been shown to deliver better illumination of the subsurface; higher signal-to-noise ratio; and improved seismic resolution in several complex geological environments, such as beneath large salt bodies with complex shapes. The Coil Shooting technique goes beyond WAZ to acquire a full range of azimuths using a single vessel shooting continuously with a circular or curved path. Where long-offset data is also required, such as for imaging below deep complex structures, the Dual Coil Shooting multivessel full-azimuth (FAZ) acquisition system can be deployed. The IsoMetrix service is compatible with recording geometries such as multi-vessel WAZ, and the Coil Shooting and Dual Coil Shooting FAZ methods. The ability to deliver isometrically sampled broadband data of exceptional quality and 4D repeatability is expected to enable innovative configurations of seismic imaging technology that are able to address currently intractable exploration and development challenges while simultaneously improving productivity.

A Fully Commercialised Service

The launch of the IsoMetrix marine isometric seismic service was announced in June at the European Association of Geoscientists and Engineers (EAGE) 2012 annual conference and exhibition. It is the result of an extensive 10-year research and engineering programme that has been the largest single engineering project ever undertaken by Schlumberger. The program not only delivered the new seismic acquisition technology, but also the algorithms and workflows needed to manage the unprecedented amount of data it produces. Speaking at the launch, Carel Hooykaas, president, WesternGeco, compared the step-change in quality delivered by the IsoMetrix technology to that achieved moving from a 2D x-ray to a 3D CAT-scan in medical imaging.

Field trials in 2011 proved the new system’s capabilities, achieving a 12:1 crossline reconstruction ratio and producing a 6.25m surface data grid from streamers 75m apart. During August 2012, the 3D seismic vessel Western Pride, deploying eight full-length streamers, transited to the North Sea where it completed acquisition of its first commercial project on schedule. During September, proprietary surveys were acquired for two other North Sea operators. The new technology has proved to be very robust during operations and is attracting considerable interest from oil and gas exploration and production companies of all sizes around the world. Manufacturing of the new system is running at full-speed in one of the WesternGeco manufacturing facilities, and the company has developed a rollout schedule for the introduction of the new technology among its worldwide fleet of 3D vessels.

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