You are in :
Keywords : high, resolution, seismic, surveys

High Resolution Seismic Surveys

Ifremer has developed a High Resolution (HR) 2D and 3D seismic data acquisition tool in response to needs for high resolution imaging which have arisen during detailed studies of the margins (complex sedimentary processes, outflows of fluids and gas hydrates and associated ecosystems). Based on cutting-edge technology (SERCEL digital streamers and acquisition system) and compatible with the SMT system (MultiTrace Seismic Survey), it enables data acquisition in profile (2D) or in a cube (3D) with a horizontal and vertical resolution of several metres. This tool is deployed in various configurations depending on the oceanographic vessel (RV Le Suroît, RV L'Europe, RV L’Atalante).

Acquisition System

This system includes seismic streamers, seismic acquisition electronics (SERCEL SN408 laboratory), an engineering sensor kit (positioning) and on-board quality control processing tools.

Main technical features:

  • streamers : 6.25 m traces, 8 hydrophones per trace ; 24 traces per 150 m active element; two streamer reels with a capacity for 300 and 600 m of active sections ;
  • SN408 HR acquisition laboratory: 24 bits, sampling from 0.25 to 4 ms, up to 12 auxiliary traces, not high voltage, real-time acquisition and quality control software,
  • engineering sensors: Buoys for DGPS positioning of sources (3D system), accurate shot dating, immersion controllers with integrated magnetic compass ,
  • all acquisition electronics integrated into a 10 foot container ;
  • interface with the ship's instruments: DGPS positioning, gyrocompass, probe ;

Source

The sources used are those conventionally deployed in high resolution seismic surveys (mini-GI air guns, 1000 J Sparkers). The accessories used with pneumatic sources have been upgraded: shot control unit enabling alternate shots of two sources (flip/flop mode in 3D configuration), continuous oiling system for guns set at a high shooting rate (< 5 seconds).

Acquisition Systems

  • 2D : one streamer with 24 to 72 traces; a source device;
    minimum offset: 60 – 80m; maximum offset: 530 m ;
  • 3D mini-GI : 2 streamers with 48 traces at a distance of 25m from each other; minimum offset : 80 m ;
    maximum offset : 380 m ; streamers immersed to: 3 m;
    two source devices with 12.5m between them running in flip/flop mode, sources positioned with DGPS, traction booms: two streamers and two source devices deployed.
  • 3D Sparker : 1 streamer with 72 traces;
    minimum offset : 80m ; maximum offset : 530m ;
    two source devices with 12.5m between them running in flip/flop mode, sources positioned with DGPS,
Examples of deployment in different oceanographic vessels:
  • 3D - RV Le Suroît : 2 streamers x 48 traces, 2 mini GI sources, 2 reels, 4 x 10' containers ,
  • 2D - RV Le Suroît, L’Atalante : 1 streamer x 72 traces, 2 mini GI or Sparker sources, 1 or 2 reels, 4 to 5 x 10' containers ,
  • 2D - RV L’Europe : 1 streamer x 72 traces, 1 Sparker source, 1 reel, 1 x 10' container , 1 Sparker container ,
  • 3D - RV L’Europe : 1 streamer x 72 traces, 2 Sparker sources, 1 reel, 1 x 10' container , 1 Sparker container ,

Performance

  • acquisition bin (i.e. spatial sampling) : 3,125m depending on the direction of the profile, 6.25m perpendicularly to profiles (for 3D : 25m between navigated lines and 2 sources) ,
  • shooting rate: 3 to 6 seconds ;
  • resolutions - sources such as mini GI:
    • frequency range: 50 – 250 Hz, 130 Hz centre frequency;
    • vertical resolution: 2m; horizontal resolution: 10m;

  • résolutions – sources such as 1000 J Sparker , 100 strands :
    • frequency range: 100 – 325 Hz, 200 Hz centre frequency;
    • vertical resolution: 1m; horizontal resolution: 6m

  • water depth : 150 to 2000 metres;
  • ground penetration: several hundred metres depending on the sediment type, source type, water depth, acquisition geometry, image processing quality, …
  • amplitude check for pneumatic sources ;
  • sufaced covered in 3D: 25 km² in 12 days at 24 hours per day (RV Le Suroît), 10 km² in 15 days at 10 hours per day (RV L’Europe) ;
  • simultaneous acquisitions possible (if equipment available): SMF bathymetry, sediment sounder;

Main Constraints

  • (surface) speeds of ship during acquisition: 4 to 5 knots;
  • upper limit for sea conditions: force 5 sea (Beaufort);
  • data volume: 150 GB for a 3D cube of 25 km² and 15 to 30 GB/day in 2D ;
  • specific constraints for 3D acquisition:
    • following a 3D acquisition strategy: defining a programme for monitoring sailed lines in advance; real-time processing of positioning, drawing up coverage maps
    • monitoring hydrodynamic conditions: water celerity, tidal range (if > 1 m);
    • limited turning angle (30 to 40 minutes of turning at 4 knots between two sailed lines);
    • on-land reduction/processing methods should be suited to 3D requirements (skills, software, personnel time).