WesternGeco’s Over/Under technique extends bandwidth, improves signal-to-noise
Ted Moon, Technology Editor
As exploration activities are driving operators to deeper waters and deeper reservoirs, a number of technical challenges must be circumvented. The seismic acquisition arena is not immune to these technical challenges, particularly as operators want precise and reliable reservoir characterization data to enable them to drill quickly and accurately, often beneath salt and basalt formations.
Some of the technical challenges facing the seismic community are well known and have been investigated in depth. In rough weather conditions, so-called rough sea perturbations originating from the seismic wavefield reflecting from the rough sea surface, lead to noise behind the signal in the seismic images. Moreover, the rough weather necessitates seismic measurements at greater depth where weather-induced currents and water motion are less severe. While noise is reduced at greater depths, measurement at higher frequencies is lost, which is a particular problem if the reservoir to be studied is located close to the sea floor.
However, in perfectly calm weather conditions the sea surface constrains seismic data quality from traditional streamer acquisition. These effects are known as sea-surface related ghosts and multiples. Ghosts are a special type of multiple reflection that arise when seismic energy traveling upward is reflected with reversed polarity back downward at the surface of the water. This ghost reflection travels back down through the streamer’s path and is recorded, causing a loss of low frequency data and notches in the frequency spectra of the acquired seismic data. Streamers may be placed at greater depths to circumvent the loss of the low frequencies because of the ghost. However, this results in a loss of higher frequency data, as ghost notches eat further down into the bandwidth of interest.
Several de-ghosting techniques are widely known and have been used for over thirty years, such as statistical de-convolution. Recently, WesternGeco has introduced deterministic techniques to address the rough-sea perturbation problem due to surface ghosts. Although the most advanced of these techniques can address the issue of rough-sea perturbations, they can never regain bandwidth that was lost due to the presence of ghosts.
Broadening bandwidth
Seismic technology provider WesternGeco has introduced their Over/Under acquisition and processing technique to overcome the challenges associated with obtaining wide bandwidth images from complex plays without associated ghosting or multiple challenges. WesternGeco’s Product Champion for De-Ghosting, Tunde Laniyan, explained that the Over/Under technique centers around using pairs of streamers arranged in a vertical orientation (one on top of the other) such that up-going and down-going wave fields can be separated from each other.
Because the up-going wave field does not contain a sea-surface ghost signature, WesternGeco is able to remove the ghosts by separating the energy traveling upwards from the energy traveling downwards.
“The two streamers allow differentiation between multiples, which are recorded at different times,” explains Tunde. “Single streamer techniques cannot always differentiate multiples, so interpreters are forced to ask themselves if they are seeing a true subsurface geology or just a multiple, which can lead to errors in drilling and well placement.”
While the fundamental principles of Over/Under processing have been understood for some time, effective application has been hindered by acquisition challenges associated with controlling the spatial distance and positioning of the individual streamers constituting the pair. WesternGeco has met this challenge by incorporating precise streamer positioning and controlled towing into their process.
Depending on the maximum frequency of interest in the recorded data, WesternGeco’s technique requires that the streamers within each pair be separated vertically up to 10 m apart, and that the horizontal separation of the two streamers within each pair is maintained within 5 m. Vertical separation between streamers in a pair must be known to within 0.3 m, and the relative locations of hydrophones must be known to within 0.3 m as well.
Deepwater, subsalt application
WesternGeco touts this technology as a means of achieving a full bandwidth of de-ghosted data, by obtaining low-frequency data to improve imaging beneath salt and basalt formations, while also retaining high frequencies for resolution in thin beds. In addition to full bandwidth measurement, “you get the low and high frequencies at a better signal-to-noise ratio due to the fact that you can tow the pairs at greater depths,” explains Tunde.
This benefit is particularly important as operators continue moving into deeper waters with more complex reservoirs.
Deeper towing is particularly important for examining reservoirs beneath salt and basalt formations, which is a continuing challenge for the exploration community, and a niche that WesternGeco has been exploring with this technology. Over/Under processing can also allow much easier measurement of 4D seismic, which compares seismic data over the same area over time.
The challenges with acquiring meaningful 4D data are well known and center on ensuring that subsequent seismic acquisitions occur over the same location and at the same depth, since depth changes play a major factor in streamer measurement. A single streamer cannot be easily towed at the same position upon each measurement.
Case studies
Since Over/Under’s rollout in 2005, WesternGeco has performed several successful studies in basalt-rich regions in the North Sea and deepwater areas in the GoM. Seismic acquisition test results from a trial in the GoM using Over/Under streamers towed at 18 and 25 m provides more information below the salt due to the broader bandwidth and low noise from towing the streamers deep.
New exploration regions for 2006 include offshore India, which has a basalt-rich geology similar to that found in the North Sea.•
