Preliminary analysis of background noise distribution in the ocean experiment 2010 of Ambient Noise Imaging

We previously designed and made an aspherical lens with an aperture diameter of 1.0 m for Ambient Noise Imaging (ANI). The prototype system was developed by mounting the hydrophone array on the image surface of this lens. In the ocean experiment 2010 of ANI, this system was deployed through the barge OKI SEATEC II moored in Uchiura Bay on November 8-13, 2010. First, we measured the directivity of this lens using a pinger sound of 120 kHz. It was verified that the prototype ANI system with this lens realized the directional resolution with a beam width of 1 degree at the center frequency of 120 kHz over the field of view from -7 to +7 degrees. In the data analysis results of the silent target detection trials, we successfully detected target scatterings using only ocean natural background noise, which was generated mainly by snapping shrimps. In this study, background noise distribution was observed at the same time as this experiment was conducted. We used a pair of tetrahedral arrays in this observation. Here, four hydrophones were mounted at distances of 1 m on each of two arrays which themselves were located about 10 m apart and about 5 m below the surface. The noise source positions were estimated by the arrival time differences of transient noises received by hydrophone arrays. In the preliminary results for the spatial densities of noise sources, the noise sources were spread when the noises arrived from the sea bottom. Some of the sources were around the sea bottom just under the barge, and other sources were around the sea bottom just under fish preserves. The sources were coincident with the barge when the noises arrived from the sea surface. Considering the directivity of the target scattering, it was suggested that the locations of noise sources, where the ANI system can capture target scatterings having high intensities, were at the barge around the sea surface.