CONTROLLED SOURCE ELECTROMAGNETIC RESONANCE
Unlike the pressure sensors, the motion sensors are also sensitive to the direction of propagation of the seismic waves, and this directional sensitivity coupled with measurement of pressure and velocity enables removal of the receiver ghost.
Te de-ghosted signal has a higher signal-to-noise
ratio over all seismic frequencies than conventional streamers and a wider range of frequencies, which improves both the penetration and resolution of the survey.
Although ocean bottom cables introduced dual-
sensor recording more than 15 years ago, it has never previously been possible to build a dual-sensor streamer, due to unacceptable vibration modes in the streamer and sensors during towing rendering the data unusable. PGS spent several years testing different streamer
architectures and sensor designs before coming up with a commercial solution.
Going deeper
Te added benefit of being able to counteract ghosting is that the streamers can be towed deeper in calmer less turbulent waters, which also improves the quality of the data.
Conventional streamers operate at a depth of
between six and nine meters because at lower depths the ghosting signal amplitude increases, ruining the data at certain frequencies. Unfortunately at shallower depths, low frequencies are strongly attenuated and the overall bandwidth is reduced. Since the Geostreamer technology can remove the
receiver ghost from the data at any depth, the streamers can be towed at the optimum depth for the weather conditions, without losing out on the low and high frequency data that would normally be missing. Geostreamer is typically towed at 15m where surface turbulence is minimised and low frequencies are less strongly attenuated. Tis means that Geostreamer acquisition can continue in weather conditions where conventional recording would have to be interrupted. PGS says that it has demonstrated that the technology can be used to calculate deghosted data for all depths from a single pass at one streamer depth. Tis means that operators making a decision on a seismic survey no longer have to optimise it for a particular target depth, while sacrificing image quality at shallower or deeper targets. It also improves operational efficiency as all the
streamers can be towed at one depth, exploiting the full streamer width capacity of the vessel and making it easier to control streamer behaviour.
Better data, improved availability PGS performed a qualification test of the new technology on the North West Shelf (NWS) of Australia, much of which is affected by relatively near-
surface barriers which impact deeper target seismic imaging and resolution.
A 156km 2D seismic line was acquired in the
northern Carnarvon Basin with a conventional streamer and then reacquired with the GeoStreamer. In both cases the source array parameters were identical. PGS reports a four to five times boost in the low
frequency signal and a twofold boost in the high frequency signal, with higher signal-to-noise content for all frequencies and all depths. It also found a 55 to 65 per cent boost in the frequency range of the signal through the main target interval. Two Arctic surveys performed last autumn demonstrated the expanded operational envelope offered by GeoStreamer. From August to mid- September 2009 GeoStreamer vessel Atlantic Explorer mapped 1120 km2
offshore Greenland. PGS worked out that the time saved by the
GeoStreamer operation as a percentage of a total equivalent survey with conventional streamers was 46 per cent, while weather downtime was 3.5 per cent of total survey time, over the 49 days of operation. From mid-September to early November,
Atlantic Explorer continued in the area with another GeoStreamer survey of 1025km2
. Despite the onset of
the arctic winter, PGS operational data showed that weather downtime was 12 per cent, and time saved compared to conventional tolerances was put at 50 per cent, although probably a conventional survey would never have been attempted, due to unlikelihood of completion and high cost.
Geostreamer 4D
Another aspect that PGS has been demonstrating is the versatility of its Geostreamer data to match legacy datasets acquired by conventional means to track changes over time.
PGS can use the dual sensor information to duplicate the parameters of any existing survey, thus allowing an operator to perform 4D matching with its conventionally acquired survey data. For this to work, it is necessary to calculate the
receiver ghost for the original survey depth and add it back into the Geostreamer data to perform a comparison.
PGS says it has successfully demonstrated that the
process makes Geostreamer data backwards compatible with any conventional survey data. Te company also concluded that a GeoStreamer baseline 4D survey would have better resolution and better repeatability than any conventional hydrophone- only streamer 4D survey. During 2010, three Ramforms will be operational
with GeoStreamer, bringing the 3D GeoStreamer fleet to four vessels and adding to the current four 2D vessels. Another 3D vessel will be upgraded in 2011.
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