Every 3-D seismic acquisition project is a study in planning, coordination and organization, but one company is currently shooting a 3-D survey that's planning and design may be the logistical mother of all shoots.
This complex seismic project isn't in the Alaskan Tundra or on the side of a mountain peak, where acquiring 3-D seismic is always a design, technological and logistical challenge.
This survey, planned and operated by Geco-Prakla, is on the Gulf of Mexico continental shelf.
And what makes a 3-D seismic survey in the Gulf -- the birthplace of marine 3-D seismic -- so tough?
This 700-square-kilometer survey is right in the heart of three of the busiest shipping lanes in the world, an area that averages 30 deep draft international ocean going vessels daily moving into and out of the Mississippi River.
"This is one of the most challenging surveys we have ever done from a logistical and planning standpoint," said Richard Fossier, a senior account manager with Geco-Prakla in New Orleans.
"In addition to the shipping lanes and the deep draft vessels, we have to contend with fishing vessels, buoys, seven production platforms, oil field service boats and other surface obstacles," he said.
"The activity and traffic in this area just south of the mouth of the Mississippi River is phenomenal, making acquisition of good quality 3-D seismic a monumental task," Fossier continued. "Plus, if we don't do this thing right we could actually affect commerce going up and down the river."
The survey got under way in late August and will take about three months to acquire.
If it's this much trouble, the question begs to be asked: Why bother?
Easy answer: Because there's oil under those shipping lanes.
A major oil company began discussions with Geco-Prakla in early 1999 on the best way to acquire a survey to image objectives in the South Pass 89 area.
"The purpose of this survey is to better image the subsurface for production purposes," Fossier said. "There is an existing field around South Pass block 93, and there are three to four platforms right in the middle of the survey. The new 3-D data will image the salt dome from the southeast to the northwest."
There is some older existing data, but the client had maximized the usefulness of that 3-D data set and needed higher resolution data with smaller bins, longer offsets and longer record lengths, he added.
"In meeting with the client we pinned down what it would take to image their objectives and took those objectives and designed a directed non-exclusive 3-D seismic survey," Fossier said.
"From that wish list we jointly determined with the client what was economically and logistically viable and made changes to the survey size and parameters."
The survey design took about five months and then Geco-Prakla began the difficult task of planning acquisition in this busy shipping region.
The South Pass 89 survey is affected by the intersection of three different shipping fairways leading into the Southwest Pass of the Mississippi River. The ocean going vessels, unlike other ships using the lanes, are restricted in their ability to deviate from the fairways because of the offshore facilities that abound in the area.
"So we had to design this survey around the comings and goings of those ocean going vessels," Fossier said.
First, company officials met with the New Orleans Board of Trade, which monitors shipping activity on the Mississippi River. From that group Geco-Prakla got an understanding of how the shipping business on the river operates.
The next step was discussions with the Mississippi River Bar Pilots, whose members actually board ships and guide them through a 24-mile stretch of river that goes from the sea buoy right at the mouth of the river just outside the jetties to two miles north of the head of passes.
"The Pilots are an organization that's been around forever -- these are jobs that fathers hand down to their sons -- and they know more than anybody about marine traffic in this area," Fossier said.
"So we went down to Pilot Town to get a better understanding of how the pilots operate and coordinate their activities -- (and) from the pilots we gained some confidence that we could actually shoot this survey," he continued.
"By finding out when they board ships, we could determine how to schedule our lines so we could move through the shipping fairways without adversely affecting their operations."
Pilots pick up ships 24 hours a day, so there is no ideal time to acquire data.
"We just have to listen and monitor their activities," he said. "We get updated 24 hours in advance by the Board of Trade on estimated times of arrival for ships. The pilots then get us revised estimated times of arrival from incoming ships when they are 10 to 12 miles out and we monitor that radio frequency constantly.
"We have built in contingencies on how many lines we can shoot in a day with breaking off for ship traffic."
Geco-Prakla also contacted the Coast Guard and asked them to issue a notice to mariners so that anybody coming into the survey area would be aware of the seismic activity. Also, the company distributed a flier to New Orleans shipping agents to send out to their clients briefly explaining the project.
The flier is printed in six different languages, including Spanish, Japanese, Russian and Greek.
"The world's nationalities of shipping captains ply these shipping lanes, so language issues were a real concern," Fossier said. "We have worked through the shipping agents to overcome that issue."
Is all this effort necessary? After all, these ships are very large and should be easy to see and make allowances for.
"This summer two ships did collide in the survey area while coming out of the southwest pass, so accidents do happen," he replied. "It's up to us to make sure we minimize our influence on the ongoing activity in the area."
Making a Choice
Geco-Prakla is acquiring the majority of the South Pass 89 survey with two vessels using a two-pass short streamer and flip-flop source acquisition method. This will provide the safest method for crossing the fairways and undershooting the seven platforms in the survey area.
The acquisition vessels will have various shooting configurations at their disposal to minimize the exposure of the streamer vessel to fairway traffic, and to effectively undershoot the platforms.
The streamer vessel is configured with six, 3,000-meter streamers, except when undershooting -- then the streamer configuration will be changed to four 3,000-meter streamers.
The shooting plan will have both vessels acquiring the long offsets and undershooting both near and far offsets together. The near open water offsets will be acquired as a single vessel operation.
The company looked at various acquisition options, including ocean bottom cables. However, laying ocean bottom cables in the shipping lanes would have been logistically difficult -- and water depth was a limitation. The survey's shallowest part is in about 80 feet of water, but most of the survey area is in 450 feet of water, pushing the limits of OBC technology.
The shipping fairways and the many obstructions made traditional single-pass long steamers unsafe for this acquisition.
"So, we had to cut back on streamer size to minimize our footprint," Fossier said. "We reduced streamer length by half."
With short streamers, a pass through the fairways takes an hour and a half. With traditional 6,000-meter streamers a pass would have taken two hours -- a significant time saving on a survey that needs to minimize exposure in the shipping lanes.
"Using these shorter streamers we are shooting a configuration called 'nears ahead, nears behind, and fars behind and fars ahead,'" Fossier said. "We make up the offset with the source vessel."
On fars behind, the source vessel moves 3,000 meters behind the end of the streamers, so the offset acquired is 6,000 meters.
"We will be shooting each pass twice -- once using 'fars behind' and 'fars ahead,' and the second pass shooting 'nears ahead' and 'nears behind,'" he continued.
"Basically, we'll shoot the entire survey with far offsets with two boats. Then we will reduce the streamer configuration to four steamers and undershoot the platforms and other obstructions. Then we will go back and shoot the entire survey again using a single vessel and short offsets -- nears ahead."
While shooting across the shipping fairways on the far offset passes, the source boat actually pulls the coverage in front of the receiver vessel.
The source boat is a great deal more maneuverable, with only about 150 to 200 meters worth of gear pulled
"This minimizes our exposure in the fairways," Fossier said, "and a boat can pass between the source boat and receiver boat without interfering with the data acquisition."
Noise -- and Fresh Water
With so much vessel activity in the area, one of the major acquisition concerns for this survey is noise in the data record.
Obviously, onboard quality control parameters are a priority topic.
Each 3-D vessel has two onboard quality control geophysicists attached to it, Fossier said. These geophysicists run a comprehensive set of quality assessment/quality control programs on each line, which produce brute stacks, shot plots, RMS plots and a number of other plots that allow them to determine whether the data quality is acceptable.
In marginal conditions additional noise reduction programs (such at SWATT and DEBAND) are applied to the data and the results are compared to the brute stack.
These plots are shown to the acquisition supervisor and party manager onboard, and also transmitted via satellite to the onshore support quality control geophysicist and project supervisor. The project supervisor makes the final acceptance decision on marginal lines after consultation with the clients.
If a line is considered unacceptable the crew will go back and re-shoot.
Shipping traffic, obstructions and noise aren't the only problems facing Geco-Prakla. The survey area is close enough to the coast that fresh water dumping out of the Mississippi River causes changes in the water's salinity and temperature.
"These changes impact the balance of the streamers and how they react," Fossier said. "The birds have to work harder to keep the streamer level, so we have to keep a close watch on the equipment."
Still, the first month of the survey went relatively smoothly and the three-month acquisition phase should be complete by November, Fossier said.
"Initially there was a little panic when we got onsite, but everything is working well," he said. "We are proving to the shipping industry that we can work under these difficult conditions."