In 1998, the company approached AML Oceanographic to discuss the advantages of deploying time-of-flight sound velocity technology on their towed streamer products, as opposed to continuing with the tradition of sound velocity calculated from CTD. For a detailed technical description of this project, please review a joint Ion (DigiCourse) / AML Oceanographic technical paper published in Sea Technology.
Later, in 2000, ION re-approached AML Oceanographic, requesting a flow sensor for deployment on the same towed streamers.
To properly process the seismic signals coming from the array, the position of each of the hydrophones and the energy source must be known. The location of each hydrophone along the length of a given streamer is known; however, there is variability in streamer depth, azimuth, and separation. To accurately position the hydrophones within the array, the attitude and relative positions of the streamers must be determined. This is done by a combination of heading sensors, surface position via GPS, and acoustic ranging networks.
Sound velocity is required to ensure that the acoustic ranging networks function correctly. The largest single error in determining range stems from the values used for sound velocity. Therefore, the first step in providing accurate acoustic ranges - and overall streamer accuracy - is to reduce errors in the values used for sound velocity.
ION identified the following critical requirements in the design phase of the project:
To fit the above requirements, AML Oceanographic customized the sensor's mechanical packaging, electronics, and firmware outputs.
The time-of-flight sound velocity sensor was tested thoroughly. Mechanical integrity was assured with standardized drop tests and leak tests. The units were tested for flow characteristics at the Harbor Branch Oceanographic Institute. Tow tests were completed. Finally, field testing of several units was conducted on a working seismic vessel, confirming the performance of the product.
Towed streamer arrays can be as long as 15km. Execution of turns or changes in direction can be both lengthy and costly, given that the array cannot be used when turning. To reduce cost, turns should be completed as infrequently as possible and as quickly as possible.
As the vessel manoeuvres through the surveying area, speed and centrifugal force can cause significant damage to the streamers. When the ship changes course, the outer most streamers see an increase in speed. This places additional strain on the streamer. Flow speed sensors allow the vessel captain to measure speed - and hence strain - on the outer streamers. Turns can be executed as quickly as possible without exceeding the limitations of the outside streamers and potentially causing damage.
Flow speed sensors for seismic arays need to be small, robust and have no moving parts. While acoustic sensors meet these criteria, there are already many acoustic devices operating around the seismic array and each generates additional ambient noise. A non-acoustic method of determining flow speed is preferred.
The speed log flow sensor developed and produced by AML Oceanographic operates similarly to a pitot static tube on an aircraft. The differential between the dynamic and static pressures is measured and used to calculate speed. The speed of the water flowing past the sensor housing is proportional to the square root of the differential pressure between the two ports.
In this project, AML Oceanographic took turnkey responsibility for:
