Solid streamer noise reduction principles

Abstract

Today's solid streamers are generally less sensitive to mechanical noise originating from sea swell effects and are insensitive to vibration from depth controllers (birds) and mechanical towing equipment. In addition, for a given level of sea swell, they can be towed shallower without a significant noise increase. The dominant transport mechanism for parasitic or unwanted vibration in any streamer design (whether using solid or fluid‐based buoyancy) is the axial extensional wave. If such extensional waves are permitted to generate significant pressure or stress at the locations of the hydrophone elements, then undesirable coherent noise will result. In fluid streamers, coupling between extensional waves and hydrophones occurs through the medium of bulge waves. With extremely careful mechanical design, and by forming linear arrays of individual hydrophone elements which exploit the low speed of bulge waves, fluid filled streamers may achieve acceptable mechanical noise performance. Single hydrophones in fluid streamers are always excessively noisy. In a solid buoyancy streamer the low speed bulge waves are replaced by higher speed compressional waves, thus precluding array forming as a primary means of mechanical noise reduction within the critical 3Hz to 50Hz band. However with appropriate design and materials, a high degree of noise isolation can be realised. As mechanical isolation must be achieved independently of the use of arrays, both arrays and single hydrophones in a solid streamer can be relatively quiet. One successful method of manufacturing a solid streamer encloses the hydrophone element in a relatively stiff package which resists the stress and strain normally induced by the passage of axial extensional waves. This package or Strain Isolation Module (SIM) contains acoustically transparent coupling fluid and also includes an elongated external diaphragm or diaphragms providing a high degree of turbulent noise reduction. Using this technology, improvements in mechanical noise isolation of the order of 15dB have been measured at sea. Observed improvements in field results are well predicted by simple axial wave theory.