MODAL-ANALYSIS OF GEOACOUSTIC INFLUENCES ON SHALLOW-WATER PROPAGATION

Features observed in horizontal wavenumber spectra from low-frequency acoustic measurements taken off the New Jersey Shelf are analyzed thro ugh forward modeling using PE models. Environmental models which produ ce very good agreement with relative intensity data may only match wav enumbers of lower order modes, A prominent feature in contours of moda l amplitude versus horizontal wavenumber and depth is the presence of double images of some higher order modes. Two possible causes of this phenomenon are examined. Range dependence in the environment can produ ce multiple modal images, but the magnitudes of variations required ar e much larger than those expected at the experiment site. The more lik ely cause of the double modes is shown to be a duct deep (two acoustic wavelengths) within the sediment. It produces modes with wavenumbers that are close together and with shapes that are very similar in the w ater column, Comparisons of model predictions with 50-Hz experimental wavenumbers and mode shapes show good agreement, except for amplitudes of some higher order modes, Predictions at 75 Hz typically require mo re detailed information about the sediment structure in order to accur ately match all the features of higher order modes in the data.