Effect of source depth on shallow water acoustic tomography

Matched-field tomography (MFT) has been proven to be an efficient method for retrieving the sound-speed profile (SSP) in several shallow water experiments. In this paper the effect of source depth on MFT for SSPs in shallow water is examined by using airgun signals at different depths in SWARM95 experiment. To reduce the unknown parameters, the SSPs are represented by empirical orthogonal functions (EOFs) in MFT. As the acoustic signal propagation path was nearly perpendicular to the direction of the internal wave, the SSP along the acoustic signal propagation path was almost range-independent. The inverted mean SSPs are compared with the measurements. The results show that the inversions using the sources above the thermocline obtain a better performance than those using sources below the thermocline. The explanation is that under conditions of negative velocity gradient, the shallower source excites more energy of higher-order normal modes, which have larger grazing angles and carry more information about the whole water column.