Univ. of Delaware, Graduate College of Marine Studies, Newark, DE 19716
The short time Fourier transform (STFT), wavelet transform (WT), and Wigner distribution (WD) are used to characterize the time-frequency distribution of air gun signatures in azimuth and range. The signatures were obtained in a shallow water experiment conducted at the AGS site. The distributions are explained in relation to the acoustic interaction with the seafloor. Accurate descriptions of these distributions are important for better understanding of the acoustic field in shallow water. By means of a combination of the time-frequency distribution and empirical orthogonal functions (EOF), the effects of higher-order modes are studied and localized on the features of the received time series. Numerical simulations using the SNAP model are conducted to interpret some of the observed features. Comparisons between the simulated and measured time series indicate that the higher-order modes are more sensitive to the fine layer structure of the seafloor physical properties. [sup a)]Present address: Office of Naval Research, Ocean Acoustics program Code 324OA, 800 North Quincy St., Arlington, VA 22217-5000.