Using ambient noise and vector sensor arrays to form a subbottom profiler

Abstract

Surface‐generated ambient noise is distributed in time, but its correlation produces a strong impulse that is excellent for imaging the ocean floor. This technique has been experimentally demonstrated using vertical line arrays of pressure phones. The up‐looking endfire beam (for ambient noise traveling down) is cross‐correlated with the down‐looking endfire beam (for the ambient noise reflected from the bottom, traveling up). Each layer in the bottom produces a distinct reflection. Previous work has required somewhat large vertical arrays. A towed horizontal array would be much better and allow us to make measurements with a mobile platform. The problem is that a horizontal line array cannot distinguish up from down—a broadside beam is symmetric with respect to the axis of the array. Vector sensors, sensing both pressure and particle velocity, have additional degrees of freedom for overcoming such ambiguities. We will show how vector sensors make it possible to implement such an ambient noise subbottom profiler, either in a greatly reduced vertical array or in a horizontal (towed) array configuration. We will also show results of processing data collected on a four‐element vector sensor array and a much larger pressures‐only array during the Makai experiment.