Impulsive seismic

Second Time’s a Charm

American Association of Petroleum Geologists (AAPG)

Oil industry technology is ever-evolving, owing to the ongoing challenges to find and produce hydrocarbons safely, efficiently and economically.

Many new ideas come and go. Some are relegated to the trash bin, while others get fast-tracked from the get-go.

And certain promising ones may be put on the back shelf until new technology comes along that can turn them into viable methodologies.

In the esoteric realm of seismic data acquisition and processing, Galcode coded-sweep methodology comes to mind.

Galcode is made up of co-called seismic “chirps,” blended with the coding scheme devised by Bill Currie and detailed in the Texaco patent (now expired) “Coding Seismic Sources.”

Both techniques were developed about 20 years ago, yet neither was routinely applied in the field at that time, owing to implementation and processing issues.

Today, thanks to major advances in seismic vibrator electronics, recording systems, computing systems and more, simultaneous seismic data acquisition based on the alliance of these two independent technologies can be used to improve data acquisition productivity in select 2-D or 3-D vibroseis and impulsive source seismic surveys.

Additionally, component data can be efficiently extracted from the composite records, according to Tom Thomas, chief technology officer at Dawson Geophysical Co.

Chirps and Hammers

Galcode methodology garnered praise when five comparable 3-D seismic surveys using diverse acquisition techniques were acquired over a common surface area of 6.25 square miles located 14 miles northwest of Midland, Texas, in the Permian Basin.

Dawson and Devon Energy acquired and processed the surveys on land provided by Fasken Oil and Ranch Ltd.

The tests included the simultaneous sourced Galcode, the single sweep, low energy, high common midpoint (CMP) fold technique, and the high effort multi-sweep per vibrator source point (VP) conventional method.

The recording strategy enabled comparisons on data sourced with one or two vibrators per source point.

“Two of the three coded source scenarios tested the simultaneous chirp methodology, and the third evaluated the thump, or impulse, vibroseis source method,” Thomas said.

“Chirps are defined as conventional vibroseis sweeps that replace the ‘1’s in the matrix of the binary code,” he noted.

“With a vibrator, you can do chirps, which are little sweeplets that are normal conventional sweeps but a series of offs and ons – like sweep six seconds then wait six seconds for example,” he said. “They have the same bandwidth as conventional sweeps.

“But with thumps, you’re actually trying to make the vibrator hit the ground like a hammer hit, similar to what a dynamite source would be, but the vibrator hits with very small force,” Thomas said. “Thumps are short duration vibroseis pulses lasting about five milliseconds (ms).

“The idea is we can get a lot of little hammer hits and code them so we can do simultaneous sourcing, which can be a big productivity uplift,” he emphasized.

He noted that vibrators are not actually designed to thump but must be made to do so.

“The vibrator is a repeatable source – you can time it with other vibrators so you can put more units out acting as one,” he noted. “There are a lot of advantages to using these machines to make these thumps.”

Thomas emphasized that even though simultaneous sourcing is the big advantage of the coded chirp mode, the thump mode also can be leveraged during simultaneous sourcing.

“But the main reason to go after thump technology is the potential to use the broad bandwidth seismic energy generated by the vibrator as an impulse source,” he noted.

Good First Steps

Upon completion of the acquisition testing program, the companies were pleased with the Galcode method.

But it might not be ready for prime time just yet.

“It’s a technique or methodology we can use that deserves testing and analysis in any given area,” said Dana Jurick, a geoscientist at Devon Energy during the program and currently team leader of the advanced interpretation group in geophysical technology at ConocoPhillips.

“The results we saw from the project showed its potential using what we call distributed simultaneous sources,” Jurick said. “It showed good reason to believe the Galcode type of approach produced as good or better quality than standard conventional techniques.

“We think in some areas it could provide uplift in production, productivity and in data quality,” he noted.

“The Galcode technique allows you to increase the fold of your data significantly, and there’s a known relationship between increased fold and increased data quality,” Jurick said. “For that reason, this is an important technology.

“People do simultaneous sourcing in different ways, but Galcode is kind of an innovation,” he emphasized.

Jurick lamented the lack of a well in the middle of the test area to tie in with, which would have removed any subjectivity from the conclusions, making their work even more definitive.

“Oftentimes we take baby steps in the way we advance our science and our ability to produce better data,” he noted. “This was a step in that direction, where the capabilities of modern equipment allowed us to take that next step and assess it.

“There were no red flags that said it wasn’t working; quite the contrary, we think the results looked positive.”

Thomas gave the Galcode approach equally high marks.

“We think the program results were very positive and proved the technology is feasible,” he said. “Since then, we’ve been trying to optimize it.

“I think over time, it may well change the way we do seismic acquisition on land with vibroseis,” Thomas emphasized.

For now, Jurick cautioned that in a new area the Galcode method should be tested against the tried-and-true conventional techniques developed over 50 years or so.

“Going into a new area,” he professed, “I don’t assume I know the best way.”

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