VERTICAL ARRAY RECEPTIONS OF THE HEARD ISLAND TRANSMISSIONS

A long, vertical line array was deployed off Monterey, California during the Heard Island Feasibility Test to measure the modal content of the received signals. The array contained 32, equally spaced hydrophones spanning from 345 to 1740-m depth. The multichannel data were recorded through a tether to the R/V POINT SUR. The measurements had very low signal to noise ratios and indicated the cw transmission losses were approximately 140 dB for a source/receiver range of 17 000 km. Modal content was analyzed using (i) the modal extent versus depth, (ii) frequency-vertical wave-number spectra, (iii) modal beamforming and (iv) least squares fitting. All led to the conclusion that the modal population is surprisingly rich. There was strong evidence of population up to at least mode seven in the data.