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 through forward modeling using PE models. Environmental models which produce very good agreement with relative intensity data may only match wavenumbers of lower order modes. A prominent feature in contours of modal 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 produce multiple modal images, but the magnitudes of variations required are much larger than those expected at the experiment site. The more likely 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 water 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 more detailed information about the sediment structure in order to accurately match all the features of higher order modes in the data