Bottom profile estimates using passive fathometer processing of ambient noise received by vertical nested array

Abstract

A passive fathometer technique utilizes surface-generated ambient noise received by a vertical line array as a sound source to estimate the depth of water-sediment interfaces and sub-bottom layers. This processing method uses the beamforming technique to analyze the vertical component of surface noise and its echo from the bottom. However, in all cases, uniform-spaced arrays were used for beamforming on the end-fire directions, although ambient noise was characterized as a broadband signal. The nested array processing is thus applied to the passive fathometer technique to overcome the drawbacks associated with use of uniform-spaced arrays and effectively cover broadband ambient noise. Ambient noise is measured using a 24-channel, vertical nested line array consisting of four sub-arrays in shallow waters off the eastern coast of Korea. Passive fathometer processing is performed for each sub-array, and the results are combined to form a passive fathometer output for broadband ambient noise. Three types of beamforming techniques, including a conventional method and two adaptive methods, are used in the passive fathometer processing. The results are compared to the depths of the water-sediment interface as measured by an echo sounder.