By KATHY SHIRLEY
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
"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."
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:
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."
"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."
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.
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."
So what are companies doing to overcome these problems
created by ocean currents?
Plenty, thanks to technological developments that
p Steerable streamers, which have small devices called
"birds" with wings on each side that are placed every 400 meters
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,"
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."
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."
processing techniques to correct and calibrate water velocity changes
caused by currents.
This new software is particularly crucial for time-lapse
"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."