Broadband Seismic Imaging in 3D
Chris Cunnell, WesternGeco
The Earth is complex in all directions, and hydrocarbon traps require closure—whether structural or stratigraphic or both—in three dimensions. Improvements to the 3D surface-seismic-reflection method have enabled subsurface imaging with a wider range of frequencies, leading to increased resolution in time and subsequently depth. However, accurate structural imaging, and reliable inversion to reveal indicators of rock properties, also requires data finely sampled spatially in all directions.
On land, subject to terrain and access restrictions, seismic source and receiver spreads can be designed to deliver adequate and equal spatial sampling in all directions. In marine environments, towed-streamer geometries are usually the most economically viable solutions for acquiring large 3D-seismic surveys, but the streamers typically are towed 50 to 100 m apart, resulting in sparse, and often inadequate, sampling of the wavefield in the crossline direction.
WesternGeco’s new marine isometric-seismic technology delivers high-fidelity point-receiver data while overcoming spatial wave-number/bandwidth compromises that limited previous towed-streamer marine seismic-acquisition methods. It delivers a recording of the full 3D broadband seismic wavefield sampled on a 6.25×6.25-m point-receiver surface grid.
Better Imaging and More-Efficient Exploration
The new technology enables several benefits, including improved imaging of subsurface geology, opportunities for increased efficiency in exploration, and more accurate monitoring of changes in a reservoir. Fine-scale isometric subsurface characterization means that 3D interpretation attributes can be generated independently of the orientation of viewing. This translates into more-detailed representations of subsurface structures and stratigraphic variations, and enables a new level of insight into the geology from seabed to reservoir.
The technology also offers the ability to tow streamers farther apart while reconstructing streamer measurements at finer separations, thus enabling more-efficient acquisition configurations, with reduced operational exposure but without compromising crossline sampling. In addition, while conventional streamers need to be towed at shallow depths (typically between 6 and 10 m) to ensure recording of the high frequencies needed for temporal resolution, the new streamers can be towed deeper while delivering data of equal or better bandwidth. Deep tow provides quieter, less-noisy environments, and extends the weather window for seismic acquisition. Fine isometric sampling enables removal of noise from nearby seismic or other oilfield operations, enabling useful data to be recorded in busy fields and avoiding the need for seismic-vessel time sharing in highly active exploration regions.
Limitations to Conventional Systems
Conventional 3D surveys are typically acquired by a vessel towing between eight and 16 streamers containing hydrophone sensors that detect seismic energy in the form of pressure changes under water. These changes are omnidirectional, which means that hydrophones cannot detect the orientation of the arriving seismic wavefields. Spatial sampling of the data recorded along each streamer can be as fine as 3.125 m; however, the much greater distance between adjacent streamers means that sampling in the crossline direction can be 16 to 32 times sparser. Such coarsely sampled data cannot capture the whole 3D wavefield, limiting data ability to optimally image the subsurface.
Conventional towed-streamer acquisition systems also suffer from attenuation of frequencies because of the ghost-notch effect caused by the reflection of seismic wavefields from the sea surface a short distance above the streamers, which interferes with the useful seismic signal. The ghost effect occurs both at the seismic source and at the receivers, and the range of frequencies impacted is dependent upon the depth of deployment. Shallow towing 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 caused by waves and wind. Towing sources and streamers at deeper depths enhances the low-frequency content and can increase the signal/ambient-noise ratio; however, it impacts the high frequencies.
Addressing Bandwidth Challenges
The new marine acquisition system is enabled by a point-receiver streamer system that combines hydrophones with calibrated accelerometers that measure particle acceleration in the seismic wavefield. Measuring acceleration in two directions provides direct information on the pressure gradient, and enables unaliased reconstruction of the pressure wavefield between the streamers (Fig. 1). For each seismic shot record, the measured pressure (P) and the vertical (Z) and crossline-horizontal (Y) components of the pressure gradients are combined by use of a novel data-dependent processing technique. The scheme provides a spatial dealiasing capability of much higher orders than predicted by classical signal-sampling theory, and simultaneously deghosts the data in a true 3D fashion. To complement this, the source-side ghost can be addressed with a new calibrated marine broadband seismic source family.
Measurement of the crossline pressure gradient requires a step-change increase in the total number and density of sensors deployed in the water. A typical seismic spread may now involve more than 500,000 sensors recording data continually, which represents an increase of more than sixtyfold over conventional hydrophone-only systems.
Results of field trials were first presented in June 2012. Since then, the technology has been deployed off four continents. Acquisition of the first commercial project was completed in the North Sea during August 2012, and was followed by surveys in the area for two additional operators. Results from the North Sea Bruce field area were presented in September 2013, showing that, compared with a conventional 3D survey, the technology has improved definition of geologic features. Small-scale heterogeneities have been extracted from the seismic volume that can be interpreted to reveal the intricate 3D geometries of turbidities and sand injectites, yielding real insights into a complex depositional environment. Fig. 2
In February 2013, acquisition of the first 3D survey outside of the North Sea was completed off the west coast of South Africa. April 2013 saw the start of acquisition, on a multiclient basis, of a survey in the western Barents Sea. A survey offshore Canada commenced in June, followed by a continuous program of commercial projects around the world. In December, the technology made its debut in Australia, working on the 4000-km2 Fortuna survey, the largest seismic survey ever to be conducted over the North West Shelf project acreage. A second vessel equipped with the new technology has recently been launched and is now active in the Barents Sea.
Integrated acquisition and processing systems have been built to handle the large volumes of data delivered by the technology. Key elements of the processing can be performed onboard, along with fast-track products for initial geological interpretation. This new system does not require special deployment equipment. It also has an environmentally responsible solid design with high rigidity to increase robustness in harsh marine conditions.
The new technology can compute the separated pressure wavefields at any desired position within a spread of streamers. This enhances its potential to deliver improved repeatability for 4D analysis, which has been demonstrated with data acquired in two passes over the same area of the North Sea. Improved 4D repeatability increases the ability to confidently map subtle changes in the seismic response of a reservoir over time caused by fluctuations in pressure or fluid content. Information from the technique can be used to identify flow barriers, locate untapped compartments for infill drilling, and guide reservoir-management decisions.
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