Roll, Tide, Roll

Currents Pesky for Seismic Crews

By KATHY SHIRLEY
EXPLORER Correspondent

Ocean 'Headache' Sites Can Help, Too

While most seismic acquisition crews hope to stay away from currents and the headaches they create, contractors can glean some benefit from other ocean structures like mixing sites.

Patrick Thompson, a geophysicist with WesternGeco, said there is an area in the Gulf of Mexico where crews can go to test equipment.

"In this area there is a strong salinity interface near the seafloor that is caused by a salt diapir that has pierced the seafloor, and the salt is dissolving in the seawater," Thompson said. "This forms a very strong salinity -- and therefore, density -- contrast with the overlying water. So, we get a very unique situation that we actually use as a test area for some of our equipment -- primarily to see if the airguns can get a uniform signature.

"That's a direct, practical use of the same phenomenon the oceanographers are seeing in their data."

The Geco Topaz, on a seismic shoot. The water may look calm, but crews know the deep can be challenging.

Seismologists and oceanographers are finding new applications for reflection seismic to study the oceans' currents, eddies and mixing processes (see related story), but for geophysical contractors those same waters are challenges that threaten high quality data acquisition.

"Normally in seismic operations we consider the water fairly homogenous, and in those cases we do not get reflections from the water layer," said John Waggoner, senior reservoir engineer with WesternGeco. "However, in areas where currents exist, different water layer properties can cause reflections within the water and other situations that are detrimental to our data.

"For currents close to the seafloor we have had to develop processing techniques that recognize the reflections in the water and allow us to adjust the water velocities to account for those," he added. "Surface currents are more problematic during the acquisition process."

Waggoner noted four primary ways in which ocean currents are detrimental to seismic acquisition and processing:

During acquisition, sideways currents can deflect streamers to the side.

"A typical marine survey has eight streamers, six to eight kilometers long, so they can really be moved by these currents," he said. "When that occurs you can get some fairly irregular patterns, or what we call feathering, in the area you are shooting, which requires infill lines."

Repeatability.

"Increasingly, particularly in the North Sea but more and more in the Gulf of Mexico, companies are interested in not just acquiring one survey over a reservoir, but multiple surveys over time," Waggoner said. "A key element in this time lapse seismic is repeatability, or the ability to go over the same place in the same way each time.

"The ocean currents will almost certainly be different when you go back to re-shoot an area," he said, "which presents a real challenge to the concept of time-lapse data."

Depth variations in the streamers.

"We try to tow the streamers six to eight meters below the surface," Waggoner said, "and to achieve the best quality data it is important that the depth of the streamers be as consistent as possible. However, currents are generally not simply movements of the same kind of waters with the same properties."

These currents can have combinations of waters with different temperatures and salinities, which means that buoyancy in the current can vary and thus impact the depth of the streamers as they pass through the currents.

Water layer velocity variations, which are detrimental to seismic quality.

"The changes in temperature and salinity in the currents impact the velocity of the seismic waves as they pass through," Waggoner said, "and we must account for that in the processing stage."

Technology Helps

So what are companies doing to overcome these problems created by ocean currents?

Plenty, thanks to technological developments that include:

p Steerable streamers, which have small devices called "birds" with wings on each side that are placed every 400 meters or so.

For years those birds were deployed to compensate for depth, with the wings moving in the same direction at the same time. In recent years, however, birds have been developed with wings that move independently under computer control, allowing contractors to direct them from side to side as well as up and down.

"What that allows you to do is actually steer the streamers to counteract the impact of currents that can cause feathering," Waggoner said.

Of course, there are limitations to how strong a current contractors can counteract.

"Certainly, we wouldn't claim we can correct any amount of current," he said. "However, we are able to maintain a constant separation between our streamers, so even if the current is so strong it creates a feather, we can maintain that feather as a constant while we are acquiring data.

"That directly impacts the infill problem," he added. "We are able to keep a good sampling of the subsurface by keeping that constant separation."

p An acoustic positioning network, which has acoustic sources along the length of the streamers that allows the operator to know with a great deal of detail where each streamer is and where each point along the streamer is.

That information feeds into onboard computers and directs the birds on how to turn to maintain the prescribed feathering.

"That is a real advancement," Waggoner said. "(It) gives us precise locations of the streamer along its entire length."

Single sensor recording.

With single sensor recording information is received every 3 and 1/8 meters from each sensor, rather than the conventional distance of every 25 meters or so.

"One of the benefits of that is you are sampling noise along the streamer much more accurately, and that's important because as you are towing the streamers through currents and trying to steer against the current you can get noise propagating along the streamers," Waggoner said. "With this dense sampling we can much more accurately account for that noise and cancel it out when we process the data."

New processing techniques to correct and calibrate water velocity changes caused by currents.

This new software is particularly crucial for time-lapse surveys.

"Water masses can change appreciably over time -- the temperature and salinity can change, and that impacts the speed of sound in water," Waggoner said. "The water velocity correction software allows us to take into account those variations and subtract them out of the data."

While the industry has come a long way in dealing with the problems created by ocean currents, Waggoner cautioned there is no panacea.

"Our recommendation to clients is to do a feasibility study on the impact of currents in a survey area ahead of time," he said. "The most important factor is knowing what you are dealing with before you begin shooting."