The mention of nodes for marine seismic data acquisition ordinarily brings to mind a cable-free package-like assembly sitting on the ocean bottom.
Given ongoing advances in technology, that may change soon.
It’s time to begin to think floating nodes, or autonomous marine vehicles (AMV).
In the marine environment, towed streamer seismic data acquisition has long been the method-of-choice, principally because of its efficiency and good quality data – but in obstructed areas or in very shallow water where streamer vessels can’t be deployed, other acquisition systems are needed.
Ocean bottom node (OBN) and ocean bottom cable systems come into favor here as a means to negate the towed streamer challenges, owing mainly to the fact that the receivers are deployed on the ocean floor.
But these systems have limitations too, including the cost entailed to deploy OBNs in the deep water using remote operated vehicles (ROV).
Look Ma – No Wires
Enter the AMV, or Wave Glider, which was developed by Liquid Robotics and is being considered as the technology to potentially acquire seismic data where other equipment is limited.
This somewhat odd-looking apparatus is a two-part vehicle made up of a surface component, or float, and a submerged segment, or sub.
The two are connected by an umbilical tether.
The Wave Glider uses wave energy as the main propulsion system, explained Dave Monk, director of worldwide geophysics and distinguished adviser at Apache Corp.
“As the float is moved vertically by the surface waves, the sub converts this motion into forward thrust via a passive mechanical system,” he said. “The wave glider’s expected water speed should be compared with expected currents.
“The vehicle has the potential to continuously swim forward or to ‘hold station,’” he added.
“Solar panels mounted on the float are used to charge batteries, which provide power to the control system, radios and payload,” he said. “Control and communication of the Wave Glider are provided via a secure Web and satellite system.”
For seismic applications, the Wave Glider can be equipped with a recording system mounted inside the float and seismic sensors.
“They’re a floating device with a receiver,” he explained, “so they’re like a floating node.”
Apache was involved in a 2013 feasibility test of the Wave Glider system to acquire seismic data in the Gulf of Mexico through the course of a shallow water OBN acquisition program.
For the test, the Wave Glider had a short streamer attached to the umbilical. The streamer trailed behind the glider, but could have been hung vertically.
Sensors inside the streamer included hydrophones and micro-electromechanical systems to measure the acceleration of water particles in the Z and Y directions, Monk explained.
The seismic recording system installed in the float had continuous recording capabilities.
The test served to demonstrate that the operator(s) can command and control these devices and that they will go where needed and record seismic data.
Cool Factor
But any excitement over that capability is premature at the moment, as it’s unlikely that these intriguing creations will be on the market soon enough for any currently planned seismic acquisition ventures.
“Wave Gliders are being used for other technical measurements in the industry, but not yet for seismic data,” Monk said. “I think we’re at least a few years out for a commercial deployment.”
Nonetheless, the cool factor associated with this technology is undeniable.
For instance, Monk said an operator can simply toss them out into the water and tell them to come back later.
“With a Wave Glider, you can potentially just launch it off a pier in Galveston and send it where you want it to go – even if that’s halfway around the world,” he said. “There’s no requirement for power, and they’re entirely remote, being commanded and controlled over satellite; there’s no requirement to visit the location where you want to record.
“A large number of devices could be tossed out to sea to travel much like a school of fish to the designated location,” he noted. “There they would encounter a separate seismic source boat, which would travel around outside of them.”
At the end of the day, the data acquired during the 2013 GOM test proved to be comparable to the OBN data.
“We didn’t have enough Wave Gliders to really produce anything more than comparisons of shot records and common receiver gathers,” Monk said. “But, the shot records or common receiver gathers look entirely comparable to data we got from bottom referenced nodes.”