Can winter sea ice in the arctic seas simplify the acquisition of offshore seismic data? After all, shooting seismic from sea ice doesn’t seem at first blush to be that much different from shooting on land. And shooting from the ice would eliminate the need for an expensive seismic vessel. Winter operations would also avoid conflicts with summer subsistence hunting and with wildlife such as whales.
Not only that. The near-shore waters of the Beaufort Sea, for example, are too shallow for the operation of a seismic vessel towing the hydrophone streamers of an open water survey — shooting shallow-water seismic in the summer involves laying geophones on the seafloor. But laying and moving underwater geophone cabling is time consuming and can be disruptive to other open water activities.
The potential advantages of conducting seismic operations from winter sea ice are motivating Shell to research the feasibility of shooting seismic from floating sea ice in relatively shallow water in the Beaufort Sea (see story in the July 21 edition of Petroleum News).
But, to what extent have people shot seismic from sea ice in the past and what are the issues with acquiring seismic offshore in the winter?
83 on-ice Beaufort surveysSue Banet, a geologist with the U.S. Minerals Management Service, told Petroleum News that MMS records for the federally managed outer continental shelf indicate that about 83 on-ice seismic surveys have been done in the Beaufort Sea and one on-ice survey has been done in the Chukchi Sea. The majority of these surveys appear to have taken place in quite shallow water, but it is not clear which surveys were done on floating sea ice, rather than “ground fast” ice attached to the sea floor.
Acquiring seismic data involves firing some form of sound source, such as a vibrating pad on a solid surface or an underwater air gun. Sound waves from the source pass through underground rocks, with some of the sound being echoed back to the surface by the rock strata. Seismic receivers at the surface detect the returning echoes and computer processing converts the received sound signals into images of the underground rock structures.
Shooting seismic on grounded sea ice proves very similar to shooting on land, with the seismic sound passing relatively cleanly through the interface between the bottom of the ice and the sea floor. But just a few inches or more of liquid water between the ice and the ground underneath tends to wreak havoc with the seismic data.
“Setting off an energy source on floating ice creates a lot of noise, both from the movement of the ice and, more likely, from the creation of a noise wave in the water column,” Diane Shellenbaum, a petroleum geophysicist with Alaska’s Division of Oil and Gas, told Petroleum News. “The noise wave in the water column appears worse in the shallower water, implying that the noise waves may be adding together to overwhelm the real data energy.”
And what makes this situation even worse is the need for the seismic sound waves to pass through a liquid water interface at both the top and bottom of the water column; the water interfaces degrade the seismic signals that generate the seismic data.
“You’re going through the ice interface, the water interface and the mud interface (on the seafloor), so you lose a lot of signal going down,” Shellenbaum said.
At best, the combination of loud noise with a degraded signal will result in poor seismic data. In a worst case scenario the noise can completely overwhelm the signal.
Reducing the noiseIt would be possible to eliminate the noise problems by placing sound sources on the seafloor rather than on the ice. In fact, prior to the 1970s people actually drilled shot holes in the seafloor and used seismic receivers positioned on the sea ice, Division of Oil and Gas petroleum geophysicist Paul Anderson said. However, the state curtailed that technique because of the potentially destructive effects of firing the shots.
“After that they would augur holes in the ice and lower air guns under the ice,” Anderson said. Geophysical Service Inc. used to have a machine called a Thunderwagon that would both augur the holes and lower the air guns through the holes, he said.
Subsequently people started using vibrators on the ice surface as a sound source (the use of vibrators operated from vibrator trucks is a very commonly used sound source in onshore seismic surveying). However, the effects of water-induced noise that had become apparent when using air guns became much worse when vibrators were used.
In the early 1980s a GSI geophysicist noticed that an ice crack or lead between the seismic source and the seismic receivers would reduce the noise level. Subsequently GSI designed a vehicle-mounted, 8-foot chain saw that would cut slots in the ice between the sources and receivers — this technique did improve the quality of the seismic data but would likely to be cost-prohibitive for shooting 3-D seismic, Anderson said.
In its winter 2007 research program Shell is trying a variety of seismic techniques from floating ice, including “standard and lightweight vibrators, accelerated weight drop (impact) sources on the ice and small volume airgun arrays deployed through holes augured in the ice.” The seismic crew will deploy receivers on the ice surface, suspended in the water column below the ice and on the ocean floor. The company hopes that some combination of source and receiver configurations will reduce the water-induced noise.
Safety and logistical challengesThe acute noise problems only arise in relatively shallow water. So, it might be possible to obtain satisfactory seismic data from the ice over deeper water some distance offshore. But operating on the sea ice long distances from land raises major safety issues, especially given the vagaries of the weather and the ice conditions in the arctic winter. Banet thinks that shooting seismic far out in the pack ice, for example, would be too dangerous to be practical.
And even when working closer to land, wind-driven features such as pressure ridges and open leads through the ice could cause daunting logistical or safety problems for a seismic crew.
“Once you get beyond the (Beaufort Sea) barrier islands you usually run into the issue of getting across those ice pressure ridges,” Anderson said. The question of opening enough paths through pressure ridges to lay the lines for a 3-D survey could prove especially problematic.
And a changing wind direction can cause a lead in the ice sheet to open up quite rapidly. Anderson said that this situation arose in 1989 when Exxon was shooting a 3-D survey in the Point Thompson field from the sea ice outside the barrier islands. Apparently a storm with an offshore wind opened up a huge lead that caused the loss of some geophones and other equipment.
Overall, shooting seismic from floating sea ice offers some appealing benefits but seems considerably more difficult than it might at first appear.