SAR (Système Acoustique Remorqué - towed acoustic system)

The SAR is a high-resolution multisensor geophysical tool designed to study the geological nature and structure of deepwater seafloors (200 to 6000 m). It is a towed system whose sidescan imagery resolution (1 pixel per 25 cm) can perform detailed seabed studies to complement larger scale surveys (multibeam, wide coverage sonars).

Since its commissioning in 1984, the SAR has made more than 450 dives, covering 17 000 km2 of seafloor, aboard Ifremer's research vessels, Suroît, Nadir, l'Atalante as well as on the Marion Dufresne, the Discovery and the Hudson.

Fields of operation

  • Study of various geological environments (sedimentary processes and slope instabilities, sedimentary basins, ridges, subsea volcanism)
  • Site surveys (laying subsea cables or pipelines, positioning of offshore structures)
  • Wreck-finding

References

  • French and foreign scientific teams
  • Oil companies (Elf)
  • Wreck-finding (RMS Titanic, etc.)

Functionality

The SAR system provides the following functional features:

  • 180 kHz side-scan sonar 1.5 km range and 0.25 m resolution
  • 3.5 kHz sounder 80 m range of penetration in sedimentary beds
  • Deepwater seismic imaging (20 Hz - 2 kHz)
  • 3-axis magnometry with a resolution of 0.1 nT
  • Motion reference measurement via integrated unit : pitch, yaw, heave, surge
  • Positioning by long baseline (beacons on sea bed) or ultra-short baseline (sensor aboard support ship)
  • Acquisition, real time sonar and play-back by SARIM software
  • Large-screen real time imaging and high resolution graphic recording (Dowty 195)
  • Image processing enabling geometrical and contrast correction and producing of mosaics using CARAÏBES software
  • Acquisition, digitization, processing and display of seismic plotting on the Delph system
  • Preparing, monitoring, archiving and examining data with VEMO+ and ADELIE software

Overview

The SAR system is composed of:

  • A surface system providing power to the vehicle, sensor control and data acquisition, processing and recording
  • A positioning system
  • A hydraulic winch
  • A power-carrying umbilical
  • A hard ballast
  • A tether
  • A vehicle with various imaging and measurement instruments

Technical specifications

  • Mechanical specs:
    • working depth: 6,000 m
    • vehicle weight: 2.4 t
    • hard ballast weight: 2 t
    • vehicle dimensions: cylinder 1 m in diameter and 5 m in length
    • conductor core cable: 9 km of coaxial cable
    • tether linking the vehicle to the ballast: 30 m
    • navigation: from 70 to 100 meters above the seabed at a speed of 2 knots
  • Side-scan sonar:
    • frequencies: 170 kHz port and 190 kHz starboard
    • aperture: 0.5 deg. x 80 deg. on each side
    • length of chirp: 20 ms
    • recurrence: 1.5 s
    • range: 750 m on each side
    • resolution: 0.25 m
  • Sediment penetrating sounder
    • frequency: 3.5 kHz
    • aperture: 50 deg. cone
    • length of chirp: 10 ms
    • recurrence: 1.5 s
    • range: 80 m
    • resolution: 0.75 m
  • Deepwater seismic streamer (Pasisar):
    • 2 reception channels: LF centred on 70 Hz, HF centred on 250 Hz
    • remote-control gain 0 to 84 dB in 6 dB steps
    • digitalization, acquisition, dispaly and surface processing on Delph system
  • Three-axis fluxgate magnetometer:
    • measurement scale: +/- 75,000 nT
    • resolution: 0.1 nT

Shipboard installation

The SAR system can be taken aboard Ifremer's research ships Suroît, Nadir, Atalante and Thalassa or aboard other vessels with adequate facilities.

The SAR gear includes:

  • The vehicle and its cradle (5.50 m L x 12.00 m W x 2.2 m H - weight 3.4 t)
  • The ballast and its cradle (2.40 m L x 1.70 m W x 0.75 m H - weight 2.8 t)
  • The winch / wireguiding system / cable (3.12 m D x 13.30 m W x 2.15 m H - weight 25 t with 9,000 m cable)
  • The hydraulic unit and back-up unit ( 3.00 m L x 1.18 m W x 1.75 m H - weight 5 t)
  • The deepwater pulley (0.4 m W - 1.2 m diameter - weight 0.8 t)
  • A bell and arm to deploy it ( 1.60 m L x 1.60 m W x 1.20 m H - weight 0.25 t)
  • Two standard-compliant containers (6.60 m L x 2.45 m W x 2.60 m H - weight 9 t):
    • an operations container (acquisition, processing, storage, navigation)
    • a workshop container (mechanical and electronic maintenance)

The operational team will have 3 to 5 members depending on the assignment:

  • 2 (or 3) electronics engineers including one in charge of the vehicle
  • 1 (or 2) mechanics

The ship must possess:

  • Handling facilities required for the launching, operation and retrieval of the vehicle, its umbilical and the hardballast, i.e.:
    • an A-frame or crane with 2.5 t capacity
    • a support for the deep-water 15 t pulley
  • Adequate deck capacity to accommodate the various system components (the equipment in the operations container could possibly be housed in the scientific control room)
  • Adequate power supplies (electricity 220 V - 50 Hz) to operate the system

Partners

The SAR system is operated and maintained by GENAVIR; Ifremer carries out technical monitoring and developments.

The main industrial partners involved in developing the system are:

  • Thomson Marconi Sonar for side-scan sonar and sediment sounder systems,
  • Eca who made the vector,
  • Schlumberger for the conducter core cable,
  • Brissoneau & Lotz for the winch,
  • AMG for the seismic streamer,
  • Triton Elics for the seismic acquisition and processing system.