Streamer technology improves resolution of data

Paul Boughton
Keith Forward reports on how the resolution of seismic data is being improved by the use of velocity sensors alongside pressure sensors.

Petroleum Geo-Services (PGS) has developed a new streamer technology to improve the resolution of seismic data by using velocity sensors alongside conventional pressure sensors.

The sensors are part of PGS' Geostreamer technology, the result of an extensive development programme over the past few years. The idea of using velocity sensors is not new, it has been used in ocean bottom seismic for many years, but PGS has managed to solve the engineering problems that have prevented implementation of the technique for marine acquisition until now.

PGS has so far captured more than 90 000 line kms of 2D GeoStreamer data, and 9000 square kms of 3D data has been recorded in many different basins and geological settings around the world in more than different countries.

The company believes the technology will be standard for the industry in a few years, resulting in significantly better seismic data and the ability to record in harsher environments and weather conditions, improving operational efficiency.

"The GeoStreamer will change the game in our industry," said Jon Erik Reinhardsen, President & CEO of PGS. "In the future all towed seismic acquisition will use dual sensors. Oil is getting harder to find. For our clients it means more and better information from the reservoir."

The company plans to accelerate the roll out of GeoStreamer earlier than originally planned. The successful results of the new streamer have resulted in a decision to convert half of the 3D fleet to GeoStreamer by the end of 2010.

Two sensors are better than one

The problem with conventional single sensor marine seismic is that at some frequencies the seismic response from the subsurface is masked by reflected seismic waves that have bounced off the ocean surface. This ghost reflection adds noise to the resulting seismic image making interpretation more difficult.

Geostreamer works by incorporating a particle velocity sensor alongside the conventional hydrophone pressure sensors. The velocity sensor complements the pressure sensor as the peaks in the velocity spectrum match the troughs in the pressure spectrum, enabling the missing frequencies to be filled in.

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.

The 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

The 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. This 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. This 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.

The 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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