Ocean ambient noise model considering depth distribution of source and geo-acoustic inversion

Abstract

An ocean ambient noise model is established considering source depth distribution. The model is used to analyze the effect of source depth on the vertical characteristics of ambient noise field. The analyses are explained and validated by normal mode theory. The energy of normal mode excited changes with source depth. Effects of different order normal modes are different. The high order modes raise up the equivalent seabed reflection loss, whereas the low order modes depress it. It is found that the seabed sound speed, density and source depth all have significant influences on equivalent seabed reflection loss at large grazing angles. So the source depth should be taken into account and the model is used in geo-acoustic inversion. Two sets of experimental data in a bandwidth of 200-525 Hz are used to obtain geo-acoustic parameters. The results show that the geo-acoustic parameters inverted from ocean ambient noise and from sound propagation data are similar. The mean value of inverted source depth tends to be smaller as frequency increases, which demonstrates that wind waves become dominant over ship noise. The average of inverted source depth values in the band(>400 Hz) is very small when sea state is higher than grade 3, which is consistent with the result from the Monahan's bubble theory.