NON DIFFUSE ELASTIC AND ANELASTIC PASSIVE IMAGING

The property at the basis of passive acoustic imaging that, taken any two points one of them can be seen as the source of the waves and the other as the recording station, which was shown to hold also in non diffuse fields is here exploited: 1) to allow an undistorted passive imaging in most real cases through the simple use of the mode, 2) to determine the azimuth of the instantaneous Huygens sources of the noise wavefield; 3) to measure, provided that the noise bandwidth is wide, the material dissipation constant as a function of frequency. We applied this theory to study the seismic noise field in the Ravenna, North-Central Italy, shore area and found that it is capable to provide velocity dispersion curves closely matching those of independent surveys, that the sources of seismic noise could be tracked to three major firms in Ravenna port, that the prevailing source was found to switch at the time scale of seconds, and that the value of Q was around 20 independent of frequency in the range 1-30 Hz.