Clayton R. Seeley
PGS Exploration Inc./Petroleum Geo-Services AS
Houston
The Atlantic Explorer achieved 600 m of outer streamer separation to acquire 8 CMP lines/pass towing four 4,000 m streamers off Brazil. Now that 3D seismic is the survey of choice for most developing areas, the latest trend in conventional marine seismic acquisition has been pulling more streamers (sensor cables) behind each vessel. The goal behind the multi-streamer movement is obtaining the best data set as inexpensively as possible. Thanks largely to engineering improvements for the vessels that allow greater streamer capacity, more vessels are able to offer this high payload, but relatively few are able to combine multi-streamer technology with full-length streamers- -- those considered to be 3,000 m long or more. PGS Exploration Inc. used its R/V Atlantic Explorer, pulling four seismic streamers measuring 4,000 m each with 160 recording channels/streamer, to complete a survey in 77 days -- -13 to 18 days earlier than planned- -- for Petroleo Brasileiro SA (Petrobras) in the Cabo Frio area of the Campos basin in Brazilian territorial waters. The survey was conducted from Jan. 19 to Apr. 4 in an area southeast of the existing Campos development, site of at least nine world records for deepwater production. It was performed in water depths ranging from 130 m to 2,000 m. Petrobras desired the 3D survey, the first in that part of the Campos basin and the first turnkey 3D seismic contract signed by Petrobras, after its discovery of Guarajuba field last year in that region. Petrobras selected PGS Exploration (U.S.) Inc., a member of the PGS Group of companies, from a number of seismic acquisition companies. The survey represents the first time that a four streamer operation was used by Petrobras. It delivered the most 3D seismic data ever acquired by Petrobras in a single project, according to Carlos Alberto da Costa, the company's seismic acquisition manager. The 34 man Atlantic Explorer joined the PGS fleet a year earlier. Despite the lengthy streamers on a single vessel with dual energy sources, the crew of the Atlantic Explorer achieved a separation between outer streamers of 600 m to acquire eight common midpoint (CMP) lines of data on each pass, considered the widest-spaced survey achieved to date. The streamers were about 1,000 m longer than are commonly used in the North Sea.
The challenges
Seismic acquisition with long streamers has proven challenging because the effects of feathering are increased. The effects make uniform data collection more difficult, especially in the long offsets. What's more, engine capabilities of many vessels are stretched to the limits in carrying the weight of longer streamers and their additional accompanying equipment. Petrobras required a 50 m CMP line spacing on its Cabo Frio survey. Using four long streamers and two energy sources to yield eight CMP lines/pass, the crew of the Atlantic Explorer needed to obtain a separation between streamers of 200 m, or an overall separation between the outer streamers of 600 m (over one-third mile). After some initial adjustment of the towing configuration, bringing the separation into the specifications set by Petrobras became a simple act of pulling in the lead-in streamers. Once achieved, the separation of 600 m was maintained for the duration of the survey, making it the widest streamer separation achieved to date in the industry. Petrobras usually does not work with dual energy source surveys because of the aliasing known to occur in the processed data. Single source surveys offer twice the fold of dual source surveys, which translates into higher definition surveys. In this survey, Petrobras elected to use two energy sources to increase productivity without suffering any loss of accuracy, according to da Costa.Accuracy improves
Besides the required spread, the contract called for a survey with a maximum of 56,000 CMP-km of chargeable acquisition, including up to 20% infill. Infill includes the additional vessel passes required to pick up data likely missed because of feathering of streamers. Feathering is caused by variations in current, at both streamer depth and water surface, that cause streamers not to follow exactly in the wake of the ship. When less infill is needed, it is because the coverage of the vessel passes matches closely without gaps. Multi-streamer technology reduces the amount of infill that needs to be shot and the amount of data that must be analyzed. Because there are more streamers collecting data at the same time, the variation from line to line is less dramatic. This makes feathering fairly uniform and reduces the need for infill data. Overall positioning of the boat and in-sea equipment also improves. With multi-streamer technology, navigation beacons position the head and tail of the streamers, and acoustic positioning networks between the streamers, vessel, and energy source remove the unknown. The positions of hydrophone groups and sources are known within a few meters throughout the survey. With more precise vessel and equipment locations, there is less dramatic variation from CMP line to CMP line, and coverage is fairly uniform, so less infill data must be acquired and analyzed. Because the data could be acquired quickly, the Petrobras project came in under budget in 212 months. Even though the surveyed area contained fairly strong currents and the streamers were long, providing more space for variation, the amount of infill needed reached only 16% of the prime coverage required. The infill data was attained via feather-matching, in which lines featuring similar feathering were shot next to each other. This requires constant monitoring of the currents with onboard Doppler current meters and logging of existing feather on line. The navigators predict which way the feathering will go and shoot a line that matches that direction. With more long streamers, a single vessel can cover a larger area in less time in a more uniform way than in conventional surveys. Increasing the number of towed streamers from four to eight can cut the acquisition time in half -- from a typical 150 days to 75 days. Because ship operating costs range from $40,000/day to $80,000/day, depending on the technology of the ship, each additional streamer can mean a sizable reduction in seismic acquisition costs. Although the streamer cost is higher, the cost of running the ship remains nearly the same as the number of streamers increases. The relative cost per CMP-km declines from 116% at 4 CMP lines to 87% with eight lines.Weather, onboard processing
By acquiring large volumes of data in a comparatively short time, multi-streamer surveys make maximum use of weather windows. Even though the Petrobras survey was performed during the Brazilian summer, generally the best time to acquire marine seismic data, the constant prevailing easterly winds averaged 15-20 knots in the open area.When winds reached 35-40 knots, the crew shut down survey work to avoid excessive noise in the data, in accordance with Petrobras specifications. The crew for this mission had only two instances of excessive weather-related noise. Generally, average swell height was 2-2.5 m. Average production from the start of the Cabo Frio survey to its end was 89.1 line-km/day, or 713.182 CMP-km/day, a record for Brazil. Average production is generally 80-100 line-km/day, so eight lines typically produce 640-800 CMP-km/day. The full survey was 54,915 CMP-km: 11,595 in January, 16,469 in February, 23,750 in March, and 3,100 in April. During the project, PGS performed full navigation processing on board. The seismic data were recorded in a SEG-D format and converted to SEG-Y format, output of which was merged with navigation data. Processing onboard allows processors to perform quality control in real time and improves communication between seismic processors and navigation processors. This reduces the need to iterate around problem areas, a process that can delay projects by as much as 6 months. It also enables the crew to identify problem areas while on location and reshoot them. By reducing problem-resolution time, onboard processing cuts delivery time in half. It makes possible the delivery of a stacked volume of data at the end of acquisition. In the Cabo Frio survey, the two instances of weather-related noise were identified after the affected data were processed to a brute stack onboard. The lines were reshot as soon as weather allowed. Although the Atlantic Explorer can process seismic data on parallel supercomputers onboard, Petrobras elected to perform most processing onshore. Da Costa explained that the company has large capacity for seismic processing and didn't have an immediate deadline for the work. Remaining processing will be completed by yearend.
