The sea layer is an effective medium to transmit sound waves. The acoustic signal from noise sources can be detected at hydrophones from tens to hundreds of kilometers away at any depth. Noise level is not monotonically distributed in the water column. Contrary to the common belief that it is more quiet in deeper water which is generally applied to low-frequency surface waves, we find that noise level’s spatial distribution is non-monotonic with respect to depth and is closely related to normal modes. Normal modes are waves that travel within the water layer; along the seafloor, reflect back to the free surface and are trapped in the water column. Based on the theory for normal modes, we find that in a 135 m water column, the noise level at 8 m is less than at 60 m and greatest in between. We test our hypothesis using a unique dataset acquired offshore Norway that employed three streamer configurations: constant at 8 m, varying between 15-35 m and constant at 60 m deep. The finding in the field data is consistent with the theory. Based on this, we suggest to determine an optimal towing depth for a given area based on theory when the normal mode signal is minimal.
Presentation Date: Thursday, October 20, 2016
Start Time: 11:25:00 AM
Location: 163/165
Presentation Type: ORAL
