This paper describes a unique seismic data acquisition  system that is being developed in the coean acoustics group at the University of  Victoria (UVic). The development was made possible by an award from the Canadian  Foundation for Innovation (CFI) program that provided funding to modify a multichannel  hydrophone array for use as a deep-towed seismic data acquisition system. The  array was formerly used by the Defence Research Establishment Pacific (DREP) and  was provided to the Ocean Acoustics group by DND following the closure of DREP. The  system is unique on the west coast, and provides a new capability for sesimic  research at UVic and at other institutions through collaborative research projects.   It has been used to investigate submarine gas hydrates offshore Vancouver Island,  and is currently being prepared to survey in Georgia Strait to investigate the site  of a series of recent local earthquakes.

     At the core of the system is the Canadian Ocean Acoustic  Measurment System (COAMS) hydrophone array. The array consists of 112 channels configured  in four nested subarrays, with a total aperture of 1400 m. The nested subarrays provide  flexibility in spatial sampling. The first subarray, the 'acoustic' array, consists of  64 channels with group spacing of 4.75 m and a total aperture of 305 m. Subsequent  subarrays provide group spacings of 9.5, 19 and 38 m, with each subarray selecting  the appropriate channels from the first array to obtain the correct group spacing.   Hydrophone groups consist of 10 elements with a group sensitivity of -191 dB re 1 ?Pa.   The streamer itself is modular, with sections of 76.25 m. The sections are oil filled,  and constructed of 7.6-cm O.D. plastic hose. In our experiments, the short acoustic  subarray is generally used to obtain a stacked section for 2-D imaging of the data for  each survey line. The combination of the acoustic subarray and the full aperture array  provides high-gain imaging as well as large offset data for velocity estimation. The  array shape is monitored by five depth sensors and three heading sensors along the  array. Isolation from the tow ship motion is provided by three 100-m vibration isolation  modules (VIMS) at the head of the array. There is also a single VIM at the tail end  of the array. The system is deployed by a dedicated hydraulic winch that also serves  as the storage facility for the array.

     In the original COAMS design, the signal from each channel was transmitted  via separate analogue lines in the tow cable, and digitized (12-bit) on board ship  at a sampling rate of 700 Hz. The effective signal bandwidth was 200 Hz. This design  is being modified in the CFI project to digitize the data underwater, and feed the  digital multiplexed bit stream to the ship for recording. The design sampling rate  is 1000 Hz for 16-bit data. Selectable gains to a maximum of 24 dB will be provided in 6-dB steps.

     Data recording and display of shot gathers are carried  out by a new shipboard, PC-controlled data handling system that was developed in the  first stage of the CFI project. The recording system accepts the digital data stream,  formats the multiplexed signals in SEGY file formats for recording on a local hard drive,  and provides a display of the shot gather. Stored data can be written to other storage  devices offline. The environmental data are sampled at a lower rate, and are incorporated  into the file header. They are also acquired and stored by a separate recording system.   The design objective of the new system is to provide deep-tow capability. At present,  the towing depth of the COAMS array is limited by the length of the tow cable to about  300 m. The next phase of development is the acquisition of a new tow cable for the  digital data that will provide towing depths to 1000 m.

     In the present development phase, the COAMS system has  already been used to investigate the submarine gas hydrates offshore Vancouver Island.   This project is a collaborative research project with the Pacific Geoscience Centre.   The Cascadia margin gas hydrates have been extensively studied in previous seismic  surveys offshore and also by ODP drilling. The experimental site for the COAMS survey  was near ODP site 889A, Leg 146, where our previous surveys have indicated the presence  of strong Bottom Simulating Reflectors from hydrates at depths of about 230 m, and  revealed the presence of surface craters that may be sites of fluid expulsion. The  COAMS array was used in a pseudo 3-dimensional survey to image the structure of the  sediments in the vicinity of the seepage sites. The array was towed at shallow depths  in a conventional survey geometry. In this paper, we will show examples of the COAMS  data for the stacked acoustic array sections, and processed images of the 3-dimensional  structure at the site. Examples of the towing characteristics of the array will  also be shown to demonstrate survey performance.

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