Vertical synthetic aperture for ocean exploration

This paper presents the result of a first attempt to realize a Vertical Synthetic Aperture (VSA) in the sea for wideband delay-angle SOFAR multipath identification. Experimental observations, made to support this study, were made during the deployment of a tomographic array in the Mediterranean Sea. The signals are the currently used BPSK ones. The vertical synthetic aperture was made by allowing a hydrophone array operating at 400 Hz to drift up or down from a ship whilst listening to the transmitted signal. The displacement length was 100 m and the hydrophone velocity 1 m/s. The obtained resolution is about 1 degree which already enables one to resolve and identify all rays in the tested middle range configuration. Optimal and suboptimal processings are presented within ambiguity function theory. The narrow band approximation which enables the implementation of a fast algorithm is discussed as well as the concept of coherent integration time. Observed amplitude losses are governed by the compression loss, indicating that the phase coherent of the different multipaths are quasi perfect. The development of this new tool for ocean exploration appears now as very promising because both economical and acoustically optimal for the vertical sampling of the whole water column.