Low-power autonomous nodes could revolutionize ocean-bottom seismic.
In recent years, seabed nodes have entered the technology spectrum as an alternative to towed streamer and ocean-bottom cable (OBC) operations. As with any new technology, they have their pros and cons.
The pros include true coupling with the seafloor, which allows for the measurement of shear waves, which streamers can’t accomplish. Nodes also offer advantages over ocean-bottom cables in that they’re not tethered together and can therefore manage rougher topography and be used in a sparser spread for wider coverage.
But they have their disadvantages as well. They are orders of magnitude more expensive than streamer systems and require ROVs to place them on the seafloor, which creates logistical issues.
“The techniques we currently use, however sophisticated they are and however fast they progress, will not be scalable to large-scale, routine deployment,” said Dirk Smit, chief scientist-geophysics for Shell. “The cost may be too high, the logistics may be too involved, and sub-ice acquisition will be difficult or impossible.
“But above all, our commercial models to acquire data using OBC or nodes are also inadequate,” he said. “At the moment, there’s no such thing as group or multi-client shoots because the technology costs are too high.”
Toward that end, Shell partnered with a company called GO Science, which had developed the Ring Hydro-Vessel Agent Under-liquid Vehicle. This vehicle, looking like part of the cabin of an airplane, has two thrusters that propel it through the water, can travel up to 8 knots on battery power, and can organize itself into a “swarm” with up to 2,500 units in a coordinated autonomous group.
Each vessel can get an orientation relative to the swarm by using a compass, inertial sensors, and acoustic signals and by a surface coordinator that uses GPS for navigating to the preplanned position.
What the basic package lacks is a seismic sensor, and Shell was happy to oblige by adding geophones that can be towed behind the vehicle, placed on the seafloor, left to record data, and moved to the next location without direct intervention. To Smit, the system has the potential to solve a host of marine seismic problems. “What you really need is lower costs and higher efficiency rates,” he said. “Secondly , you would like to get many more measurements at the same time. The density of the nodes needs to improve, and you would like to be able to deploy them over large areas. This requires deployment models that are fundamentally of much lower cost. “And you would like to do this in a shorter time frame. These flying nodes would help achieve that.” He added that Shell invests in novel concepts to be able to simultaneously shoot seismic with lots of source vessels without degrading the data – in particular for time-lapse seismic. This, together with low-cost sensor deployment, will establish a breakthrough and hopefully lead to a “commoditization” of ocean floor seismic and possibly replace streamer seismic acquisition.
Shell plans to test the concept “in earnest” for a few months, but Smit says so far it is performing to expectations. The company is currently seeking partners to help commercialize the technology. “We didn’t invest in this because we didn’t believe in it,” he said. “The problem we’re trying to solve is not unique to Shell, and hence we are happy to partner with others.” For more information, visit Goscience.co.uk.