Maeasurements and analysis of sound reflected from seabed

Authors:

  • Abrahamsson Leif
  • Dalberg Eva
  • Fristedt Tim
  • Sigray Peter

Publish date: 2005-01-01

Report number: FOI-R--1689--SE

Pages: 30

Written in: English

Abstract

We present measurements and analysis of acoustic signals which have been reflected by sedimentary layers in the seabed at two sites in the Stockholm archipelago. Reflection data were collected with the transmitter and the hydrophones deployed 5 m above the seafloor and for three offsets in the range 2-60 m. Sediment thicknesses and velocities were determined by travel time analysis according to classical reflection seismics. The analysis shows that the thicknesses of the sediment cover are 18 and 25 m at the two test sites. The speed of sound of the surficial sediment was estimated to 1400 m/s and the density to 1.2-1.3 g/cm3. The waveforms of the received echoes are highly distorted which indicates that the seabed is locally heterogeneous on a scale of several meters. In May 2002 a similar experiment was performed in the same area with the transmitter and the hydrophones located on the seafloor. Comparisons between this experiment and the current one with the sonars at some height above the seafloor are made with respect to signal characteristics and inversion results. The conclusion is that both setups work equally well for inversion of reflection data for geoacoustic parameters of the seabed. The measured data are interpreted by means of a ray tracing model for plane-parallel layerings of the sediment. The lack of diffuse scattering of such a model implies that the inversion results must be assessed accordingly.