It’s a given there’s a whole lotta shakin’ going on in the oil and gas industry these days – but the shaking takes on a whole different meaning in the geophysical business.
There, the rumbling is about surface vibroseis methods for land seismic acquisition, which continue to increase in popularity.
Simply stated, vibroseis technology uses vehicle-mounted vibrators (commonly called “vibes”) to impart coded seismic energy into the ground. The seismic waves are recorded via geophones and subsequently subjected to processing applications.
The methodology was invented by a group of scientists at the former Continental Oil Company. The initial patent was awarded in 1954.
Today, the contractors have their pick of sophisticated vibrator systems – minivibes, truck mount vibes and buggy mount vibes – to provide the best possible solutions to meet clients’ specific seismic program needs.
Certain contractors find that one particular type of unit meets their requirements, while others opt to latch on to the whole array.
Conquest Seismic Services, for instance, chose to acquire an inventory of all three systems for operation by the nine crews it currently deploys in Canada and the United States.
A particularly intriguing type of machine in the Conquest fleet is the relatively small Enviro Vibe EV, which is designed for high production and high resolution seismic prospecting in an environmentally sensitive manner.
“It’s highly useful to reduce seismic source gaps between houses, pipelines and other culture,” said Paul Crilly, president Norex Exploration Services (Conquest’s parent company).
It sweeps up to 300 Hz and has a maximum peak force of 15,000 pounds, and as Crilly said, it “can operate on narrower cut lines and has a smaller environmental footprint.”
Company officials say the system is ideal for use in the currently hot Canadian oil sands play.
“The Enviro Vibe works very well in applications where high resolution 3-D seismic is required,” Crilly said. “It’s particularly useful for our customers in the oil sands who are focusing on reserves in place, looking to delineate them with this high resolution 3-D – that’s been a focus of ours over the last couple of years.”
Indeed, the EV makes up a significant portion of the company’s activity level.
“The oil sands are a shallow target, so we have less force going into the ground,” said Gary James, Conquest’s marketing and business development director, “but we can run at higher frequencies. With vibroseis, we can get higher resolution data because we have the source points closer together than with subsurface source explosives such as dynamite.
“Shallow targets will return higher frequencies,” he added, “so our ability to put higher frequencies into the ground is useful.”
He noted the earth is essentially “a great big filter, always trying to steal information.”
Urban Street Smarts
Tom Fleure, vice president of geophysical technology at Global Geophysical, concurs.
“On deep targets, you would use lower frequencies because the earth is eating higher frequencies, so they don’t come back,” Fleure said. “But if you’re only working a few thousand feet down, you may be very interested in using high frequencies.”
Large, truck mount vibrators, such as the Hemi-44, are ideally suited for use in cultured environs such as the northeast United States, where Conquest has a strong presence in the Appalachian region, among other domestic locales.
“With truck mounted vibroseis, you’re taking what would be a normal vibrator and putting it on a truck, so it’s licensed to drive on the roads,” Crilly said, “and there’s a lot of infrastructure in the way of roads in New York and Pennsylvania.”
Once the crews need to veer off the beaten path and venture into the countryside, the acquisition program can be supplemented with dynamite.
“This is less intrusive than removing trees to get a large vibrator down the source line,” Crilly noted.
Truck mount vibrators also have proven highly useful at Plano-based Tidelands Geophysical Corp. (TGC), which conducts seismic surveys in an array of locales, including its “backyard” – aka, the Barnett Shale.
Seismic acquisition in the Barnett often entails working in urban environments, particularly in the core area of the play around Fort Worth.
“The trucks are well suited for urban areas,” said Danny Winn, president at TGC. “It’s easy to get around on the city streets without doing any damage.”
When close to structures, water wells, etc., vibration monitoring can be used to ensure that the particle motion of these and other vibrator systems doesn’t exceed tolerable limits.
If an award were to be designated for seismic industry workhorse in the vibroseis domain, the I-O AHV-IV all-terrain buggy mount vibe likely would win hands-down, according to Conquest.
These popular systems generate a peak force of 62,000 pounds and have a broad bandwidth up to 250 Hz. Equipped with wide tires that kind of “float” across the ground, buggy mounts have less environmental impact than the narrow tire truck mount systems. They’re ideally suited for deeper target projects in open areas as well as ice-covered terrain.
The downside: They must be transported via truck and then driven out into the field.
The allure of the buggy mount vibrator systems is apparent at Global Geophysical, which currently boasts an entire fleet of 53 of these workhorse machines.
It’s a Blast
Despite the many advantages of vibroseis seismic technology, don’t look for dynamite-based seismic programs to disappear.
In fact, Global’s planned upcoming seismic acquisition program in Peru will be a dynamite shoot.
“In terms of vibroseis versus dynamite, it’s important to have both skill sets,” Crilly noted, “in order to provide seismic solutions to best fit the program you’re trying to accomplish to best serve your clients’ needs.”
Vibroseis is typically less expensive than dynamite. There’s no need to bring drills into the acquisition program area and drill holes as much as 80-feet deep, load them with dynamite and then return later to clean up the tailings, which adds to the intrusion factor.
It is noteworthy that vibroseis is a controlled energy source, whereas dynamite is a full spectrum of frequencies. Because the earth absorbs frequencies, the acquisition process is highly area dependent.
“In some areas where the near surface geology is more unconsolidated, you have to drill down through that to get the signal going, so you have to drill deeper with dynamite in these areas,” James said. “Because of the loose type of near surface geology, a lot of the energy will actually go sideways.
“Using dynamite to, say, create a 160 Hz signal when the earth never gives you 160 Hz back is kind of nonsensical,” he said.
The ability to determine a source signature using vibroseis is a big plus for the method, according to Fleure.
“We instrument the base plate and the reaction mass on the vibrator so we can compute, in real time, the force that’s imparted to the ground, called the Ground Force,” Fleure said. “In vibroseis acquisition, Ground Force is usually considered to be the best estimate of our input seismic signal, or more generally referred to as the source signature.
“It’s not unlike in the marine environment where you have an air gun signature,” Fleure noted. “That’s one of the reasons marine seismic works real well – because the air gun signature is very repeatable in water. We know exactly what signal we put in the water, so we know what signal traveled into the subsurface.
“If you know the source signature you put in the earth,” Fleure said, “it enables some fairly advanced signal processing.
“The problem with dynamite is you drill the hole and put the charge in, but you can’t measure the amplitude of the signal at the charge,” Fleure added. “In hard rock, the energy transfer function might be one thing, and maybe another in soft rock. You really don’t know the source signature, whereas with vibroseis you can measure it as you’re putting it into the ground.”
Be aware that Plan A rules when kicking off a dynamite source survey.
Once you select the source density, pre-drill the holes and place the dynamite in them, changing your mind is not really an option – the greenbacks already have been spent. Besides, a “re-do” also entails still more trips into the area, which has environmental implications.
With vibroseis, however, source points can be added rather easily in order to enhance data quality when necessary. Instead of moving 400 feet to the next source point, the crew simply has to move the system perhaps half that distance.
An Informed Public
Global positioning systems (GPS) often play a key role in placement of these land seismic sources. The GPS positions can be loaded into the vibrators so only one trip into the shooting area is required, according to James. The systems can be used to plan shothole locations as well.
GPS technology is also important as a QC tool, according to Winn at TGC.
“You can use it to be sure the shothole is drilled in the correct location,” Winn said, “or to be sure the vibrators vibrated at a point they say they did.”
TGC is active in Louisiana where dynamite has long been – and continues to be – the norm for seismic data acquisition. Maneuvering heavy equipment around in heavily treed and frequently wet areas – think marshy southern Louisiana – is ordinarily out of the question. And the safety of the familiar dynamite approach was proven long ago.
In the locally populated areas, vibrator systems can be used to supplement the dynamite program efficiently and cost effectively.
Despite their relatively benign operations, these big machines sometimes appear more daunting to the populace than the commonplace dynamite. To assuage any concerns in that regard, companies sometimes resort to public demonstrations prior to operations.
In fact, PGS took such an approach in Chalmette, La., using a 47,000-pound vibroseis buggy to demonstrate the noise level and vibrations incurred during the data acquisition process. Two light bulbs and two raw eggs were buried eight inches under the vibrating pads. Following the demo, the eggs were retrieved unbroken and the light bulbs still worked – to the amazement of the crowd of onlookers, including elected officials.
Landowners ordinarily are satisfied with verbal reassurance prior to operations.
“The permit agents’ personal approach to landowners gives them advance notice of the vibrations they may feel,” James noted. “Having knowledge of the source, they’re not taken by surprise and readily tolerate the vibration level, which may be comparable to kids roughhousing indoors.”