The pace of a seismic survey can be measured by the number of shots taken per day. Huge onshore surveys have shown that allowing 10 or more sound sources working at the same time without any coordination can drastically reduce the time needed to gather data over a large area, and get better data as well.
“One of the biggest surprises is we have gotten almost universally better data from our simultaneous source surveys,” said Craig Beasley, a Schlumberger Fellow and chief geophysicist at WesternGeco.
More data are gathered from more angles for a more accurate picture of what is below, said Beasley, who explained that one flashlight can help you see in a dark basement, but it is better with two and more is better.
Now multiple sound source surveys began moving offshore 3 years ago. The first such offshore survey was done by Apache Corp. and was soon followed by BP, and it is spreading among those still doing offshore surveys.
“The industry is doing that sort of recording on nearly 50% of the work done today” offshore, estimated David Monk, worldwide director of geophysics for Apache.
BP has named its approach, Independent Simultaneous Sources, while Monk prefers to describe it as random or overlapping sound sources because while some shots return simultaneously, there is no effort made to coordinate the arrival times.
Seismic data are processed one shot at a time, so those overlapping shots need to be separated. That process, known as deblending creates records of each shot point as if it were the only sound recorded at that instant. It is like taking a recording of five people talking at once and splitting it up so each can be heard individually.
Filling a Need
Work on separating simultaneous signals goes back by decades. Schlumberger filed a patent in 1997 covering the methods used to gather and process simultaneous sounds. A few months later, Bell Labs filed a patent solving a related problem: sorting out the data from multiple cellphone signals handled by a single cellphone tower.
The communications innovation was quickly put into use because it provided a much needed capacity without adding equipment. “Now every cellphone network is doing this,” Beasley said. “The seismic industry is famously conservative so seismic surveys have only been commercial in multisource seismic in the last few years.”
It took longer for the oil industry to see the need. BP was among the leaders when it first used simultaneous sound sources onshore in 2009 in the Middle East and North Africa, where it had to quickly evaluate large areas it had licensed. Conventional methods would have cost much more and would not have been completed in time to make decisions before the licenses expired, said John Etgen, a distinguished advisor in seismic imaging for BP.
Time was saved by not trying to synchronize the shots. “The crews were told to generate sounds as fast as possible and then move to the next spot with no effort made to coordinate it,” he said. “We are not worried about overlapping sound sources. There might be 10 sources at the same time. Data recording is continuous. You do not turn it on and off.”
A group of 10 trucks working independently over an area could, in theory, generate 10 times more shots per day; in practice, it is less because they cannot work constantly, he said. In Libya, the company reported that simultaneous sound sources allowed it to complete a survey in 1 year that would have taken 5 without it.
To separate the data, BP used WesternGeco’s method and a deblending method it had developed, as a way to check whether the signals were being properly “deblended,” Etgen said.
Apache first tried offshore simultaneous sources in the North Sea, and subsequently recorded a large survey on the northwest coast of Australia. BP followed with surveys of producing fields in the North Sea and Trinidad.
Next Steps
Advanced methods are often used on producing fields, where higher-quality surveys are needed to seek out opportunities to add production.
Two boats have been generally used as sound sources. While 10 or more sound sources have been used on land, there is debate about the maximum number that can be used offshore before reaching the point of decreasing returns. Beasley cited a study concluding that four sources is the limit.
“However, that was done with conventional marine sources. There is a lot of research now into rethinking the marine source problem. One of the hot topics revolves around seismic sources that are distributed as in a home theater: subwoofers, midrange, and tweeters spatially distributed and perhaps autonomous. This could result in quite a large number of sources,” Beasley said.
There are studies suggesting that what works on land should be tried at sea. One person who would like to see that is Malcolm Lansley, a retired vice president of geophysics for Sercel. Doing so requires getting past a significant economic obstacle. The powerful boats used to carry air gun arrays and pull streamers many kilometers long are a big expense if 10 are required for a simultaneous survey.
“The challenge in a marine situation is it would mean extra boats and boats are more expensive than land sound sources,” he said. The cost of smaller vessels would be less, but “no one wants to manage little boats,” he said.
More sound sources offshore would also test the capabilities of the sound separation methods, Monk said.
Concerns about overlapping shots are a bigger concern offshore, Beasley said. “On land, people are used to dealing with more noise. In marine (surveys), we are used to pristine records for the most part. The idea of interfering sources is a more emotional issue,” he said.
Meanwhile, Beasley is among those researching whether data shot simultaneously have to be separated into single shot records. The study will process available blended data to see how the results compare with those when the shots have been separated. One line of thinking is that the overlapping shots would be separated during processing steps, such as migration, which is used to position the energy back to the right place in a subsurface image.
“If these could be enhanced to comprehend and handle simultaneous sources, we might be able to process directly rather than separate the sources,” Beasley said.
Etgen has been following this line of work, which he described as a “hot R&D topic.” While this effort may prove that deblending multiple sound sources is not required, he said, “There is a lot of wealth of expertise and knowledge in processing the old way. So I guess time will tell.”
