Imagine trying to see what is under a parked car. Of course you can easily view some of what’s under the car just by standing near it and looking down. But you get a different, additional view of what’s under the car by stepping back, then walking around and looking beneath it from a second direction and then a third direction, and so-forth.
This is how CGG-Veritas describes ‘wide azimuth’ seismic data acquisition, which, like ‘long-offset’ seismic is generating a lot of chatter in the seismic community and among the seismic-literate in client oil companies.
For parked car, substitute salt dome, it’s perhaps thousands of metres thick and lies in ultra-deepwater, such as the US Gulf of Mexico.
Underneath the salt is believed to be a hydrocarbons reservoir. But how does one build an image of the target, after all, conventional 3D seismic essentially bounces off?
It was about 10 years ago that BP kicked off an initiative to test the hunch and, in 2004/05, that perseverance was rewarded with Veritas (now CGG Veritas) carrying out the world’s first WATS (wide azimuth towed streamer) survey based on an array of conventional streamers, but in a novel arrangement. The target field was Mad Dog in the US Gulf and the survey covered an area of some 400km.
But, instead of employing two source arrays deployed at the top end of the streamer-spread, as is the norm with narrow azimuth (conventional) data acquisition, sequentially fired multiple source arrays were deployed on separate vessels (two), plus a recording vessel.
Aside from absolute accuracy of positioning, the biggest issue on the Mad Dog campaign was transferring data for which a custom-designed system was set up. Following initial on-board elementary analysis, some 15terabytes of preprocessed seismic data were copied onto 300GB USB discs and sent ashore in weekly batches for in-house processing by BP in Houston.
The success of Mad Dog led BP to similarly survey the US Gulf Atlantic field, but based on nodes placed on the seafloor using remotely operated vehicles, rather than surface streamers.
But, while WATS broke new ground and has gained acceptance and enthusiastic deployment in US waters and is expensive, it has not been necessary to develop a new suite of technologies, for example, array cabling to accomplish such sub-salt surveys. Essentially, the accent is on utilising what is available, but in a different manner and beefing up ship-to-shore data transfer.
Similarly, another application of the principles behind 3D seismic, and therefore much of the equipment, is multi-azimuth (MAZ) seismic. Essentially, MAZ is based on two to six 3D surveys recorded over the same area in rapid succession, but at different angles – or azimuths – to one another. This results in a grid of closely spaced seismic data lines and has been demonstrated to work effectively on sub-salt target.
BP successfully tested the MAZ technique offshore Egypt at Raven, below a partially eroded and interbedded anhydrite structure in the Nile Delta. However, such techniques in no way mean that the pace of technology development has slowed. WATS and MAZ indicate smart lateral thinking and, if anything, the rate of technology development may be quickening and boils down to larger, more sophisticated ships, towing even more massive streamer arrays, with more on-board processing capacity than every before and feeding ever greater quantities of data to the beach for full processing and interpretation, including by outfits offering specialist techniques.
While each of the main Seismic companies claims significant technological advances, it is PGS that shook up the market in the 1990s with its Ramform concept vessel; viewed from above it has the footprint of a domestic iron.
While the company has had a number of challenging issues to deal with in recent years, it has regained focus and will in 2008-09 take delivery of arguably the largest, highest capacity survey vessel sisters yet build. Ramform Sovereign and (at present) Ramform8 will have 22tow-points and be capable of a 1.3km spread at streamer launch. Both will be configured to handle all main survey formats, including WATS and (related) MAZ.
Moreover, they will to deploy the first ever dual-sensor streamer, which is designed to deliver enhanced resolution, better penetration and improved multiple attenuation. Hydrophone and velocity sensors are integrated into the same streamer.
The PGS product was launched onto the market mid 2007 and is regarded as a premium technology. Moreover, the company claims that the new cabling is able to remove receiver ghosts and, as a result, deliver enhanced frequency bandwidth and seismic resolution.
Ghosting has been around for as long as there has been a marine seismic industry and impacts performance. It is not unlike ghosting on a television set, or radar system and does impact quality. Assuming that PGS claims stand up, then the Norwegian pure-play outfit will have solved the apparently unsolvable.
The key is the way in which both hydrophone and velocity sensors are integrated into the same streamer, an approach used successfully during the mid to late 1980s for ocean bottom cable systems.
In essence, when hydrophone and velocity sensors are calibrated relative to each other, the ghost on the hydrophone recording and the geophone recording will cancel one another. It’s not unlike applying attenuators to reduce machinery noise.
Will it work? Well, doubters should recall the way in which PGS has shook the offshore seismic survey sector almost to its foundations by introducing the Ramform concept. While it has made business mistakes, this Norwegian group retains the vitality that defined it during its corporate infancy in the early 1990s.
Perhaps this is why PGS is first into the marketplace with a steerable streamer system developed by IOM Geophysical Corp of Houston.
It was on 1st October that ION announced that the Norwegian company had bought the first commercial version of its DigiFIN streamer control system.
DigiFIN is a steerable streamer package for marine seismic acquisition vessels that provides lateral streamer control, a feature which improves the efficiency of acquisition operations and allows more streamers to be towed closer together to enhance the quality of the acquired seismic data. It is a ‘drop-in’ system, which means it is designed to be compatible with most existing in-water streamers. Essentially, it is designed to enable tighter, more uniform streamer separation along the entire length of the streamer cable, which allows for finer sampling of seismic data and improved subsurface images. IONS says too that DigiFIN also enables faster line changes and minimises the requirements for in-fill, which together improve the productivity of towed streamer operations.
Two PGS vessels, Atlantic Explorer and Pacific Explore, have tested the technology extensively and it is the Explorer that will deploy the first commercial version, in this case with a six-streamer spread.