Measurements of the vertical fields of acoustic signals and normal mode propagation in the Arctic Ocean

Abstract

The vertical sound fields of cw signals and shots propagating in the ice‐covered Arctic Ocean were measured during the FRAM IV experiments in April 1982. The measurements were made with a vertical array consisting of 28 hydrophones extending to a depth of 960 m. This experiment marked the first successful deployment of a large aperture vertical array in the Arctic environment. This paper will present the results and interpretations of the vertical array data. The lowest order normal mode amplitudes and phases were measured from the shot signals received simultaneously on the 28 hydrophones of the vertical array. The measured mode amplitudes arc compared with the computed normal mode and used as the basis for spatial filtering of normal modes from the cw signals. Intensity distributions of the cw signals versus depth are presented for several frequencies below 100 Hz. Spatial coherence of the cw signals are studied and compared with that of the ambient noise. The arrival angles of the lowest order normal modes are measured from the cw signals using the mode filtering technique. The angular arrival pattern of the cw signals are plotted using conventional beamforming. Temporal behavior of the cw signals will be shown; phase tracking beamforming is used to enhance the vertical array gain.