Seabed Seismic: Clearly better

“....we see that some IOC's are now using seabed seismic as the predominant mode of acquisition for appraisal and development purposes..... Like 4D seismic this trend in the North Sea will likely progress to the other major oil regions in the world”
-Stein Hedemark, CEO, RXT

The Challenge

Elevated oil prices have created an environment where difficult to access reserves are now deemed commercial. Technology focus is an integral part of the reserve replacement plan and Geoscientists now have much higher expectations on the geophysical dataset. In the case of seismic data there must be improved resolution, illumination and repeatability in order maximize the output from these reservoirs. The geophysical industry has the challenge of matching up to these expectations and the companies focusing on improved data quality will be well placed to benefit from these demands.

Moving to Seabed Technology

Despite the fact that marine seismic exploration started with placing sensors on the seabed it was the advent of towed streamer 3D acquisition and processing that revolutionized the Geologists' understanding of the offshore reservoir. Since the 80's the cost per km² for 3D streamer data has reduced to the extent that it has become a prerequisite for field development. However there is now an increasing realization that while towing the seismic sensor certainly made sense in terms of productivity, and hence cost, it also resulted in a compromise in data quality when compared to the stationary seabed sensor case. Through the 90's the technical gurus highlighted the case for seabed seismic acquisition but for a variety of reasons, including the oil price fall in 1998, the movement from towed streamer to seabed seismic has not become common place until recently.

"The situation is changing, and changing more quickly than some commentators have noted. Perhaps this lack of comment can be explained by the fact that the major players in the geophysical industry have large capital investments in towed streamer seismic acquisition equipment. In any case we see that some IOC's are now using seabed seismic as the predominant mode of acquisition for appraisal and development purposes," says RXT CEO Stein Hedemark. "OBC acquisition has been the norm in obstructed areas where towing streamers has been considered too difficult, however in areas such as the North Sea the density of platforms does not pose a major problem, nevertheless this region is where seabed acquisition is seeing the most dramatic increase. The clear indications are that seabed will replace the towed streamer mode of acquisition for appraisal and development purposes, in 2011 there will be at least eleven 3D seabed surveys in the North Sea region. Like 4D seismic this trend in the North Sea will likely progress to the other major oil regions in the world."

The OBC Advantage

OBC technology offers a number of advantages over conventional towed streamer survey methods.

          Steamer                                     OBC

Specifically:-

•   Improved signal to noise
•   Wider bandwidth data
•   Increased source-receiver offsets
•   True full azimuth data
•   The ability to record shear wave energy

Picture courtesy: Padmos, L. et al, 2010, Using High Density OBC to optimize the Andrew Satelites Development, First Break, Vol.28, Oct. 2010

The VectorSeis Ocean (VSO) OBC system which uniquely operates using radio controlled recording buoys offers further operational and technical advantages:-

•   Eliminates recorder vessel
•   Flexibility in congested oil fields
•   Reduced HSSE risk
•   Wide band data
•   De-coupling system
•   Enhanced positioning

Going Deeper

There is increasing focus on tapping into reservoirs that are deeper in the earth, often these can be directly under producing oil fields. Previously these reservoirs have been very difficult to detect, even when a 3D seismic survey had been carried out. The reasons for this are varied but one fundamental problem has been that of seismic energy attenuation, as the seismic wave travels through the earth it will lose the high frequencies components and therefore the bandwidth of data will be inherently limited which in turn translates into reduced resolution. The outcome is that the low frequency component of the seismic data is crucial to identification and characterization of deep reservoirs.

This has created a difficult problem for streamer based systems. Towed streamers are susceptible to a number of noise mechanisms, including turbulent flow across the cable, swell noise from the sea surface, and various modes of noise propagated along the cable, including bulge waves and cable jerk. These different factors result in increased noise levels when compared with seabed recordings. In comparison seabed recording offers a much quieter environment. Typically the ambient noise levels in the frequency range 3 - 80Hz will be somewhere between 0.5-2.0 microbar and in some cases, such as deepwater environment, the ambient noise levels will be less than the instrument noise. The end result is that recovery of signals is possible down to 1Hz with the VSO system, by comparison conventional streamer recording will struggle to show deep events because data below 10Hz is very difficult to recover.  

Streamer : Deep target imaging	OBC : Deep target imaging

The other critical issues related to deep target imaging are both offset and azimuth. Long source to receiver offsets and full azimuth data can be naturally obtained using seabed acquisition systems, where the seismic source and receivers are decoupled, and with continuous recording systems there is no upper offset limit. On the other hand streamer acquisition is limited to around 6km offsets and even this becomes difficult, if not impossible, in areas with installations. Out in open waters the situation for streamer acquisition is easier. This has resulted in an increase in longer offset and multi-azimuth streamer acquisition for exploration purposes; however despite the impressive results in comparison to legacy datasets, question marks remain on whether the streamer data quality is adequate for development purposes, particularly in complex structural settings such as the pre-salt areas.

The Cost

The cost differential between a field scale OBC and towed streamer seismic survey is decreasing. In the late 90's an OBC survey would cost 5-10 times that of towed streamer, however a field scale survey would now be 1.5 to 2.0 times. There are several reasons for the change in cost differential but the most notable are the improvements in the actual seabed equipment resulting in improved productivity and also the fact that operators have much higher specifications including the requirement for high density, multi-azimuth and repeatable surveys. This move towards multi-azimuth and repeatable surveys means that OBC has become competitive on cost grounds alone not to mention the range of other inherent benefits that cannot be achieved with towed streamer.

The Outlook

In 2003 the founders of RXT anticipated that the future of seismic acquisition was on the seabed and therefore focused the company's effort into becoming the world's leading provider of seabed seismic acquisition. The market projection has been vindicated. While the seismic industry as a whole is just now starting to see signs of recovery from the global economic downturn, the seabed market has grown steadily. RXT investment in the VSO OBC system has resulted in flexible operation which has the capability to operate in single vessel mode and thereby decrease HSSE exposure. Additionally, and importantly for the operators bottom line, the enhanced data quality is allowing geoscientists to understand the internal geometry of the reservoir and hence make informed drilling decisions.

For more information, please visit www.rxt.com