First-order and zeroth-order head waves, their sequence, and implications for geoacoustic inversion

The relation between the head wave and an arrival often called a ground wave is analyzed with parabolic equation-based simulations, and an interpretation of such a ground wave as a head wave sequence is presented. For a Pekeris waveguide the envelope of the spectrum of the ground wave arrival corresponds to the spectrum of a single head wave, and spectral peaks correspond to odd multiples of the mode-1 cutoff frequency. This head wave is first order in its ray series classification and its amplitude spectrum goes as vertical bar S(f)vertical bar/f, where S(f) is the source amplitude spectrum. Basic variations from a Pekeris waveguide are considered; isospeed layers or a positive sound speed gradient in the seabed can each give rise to arrivals that are zeroth order in ray series classification and higher amplitude. For a sound speed gradient there is either a low-amplitude interference head wave whose properties are akin to a first-order head wave, or a high-amplitude interference head wave or non-interfering refracted wave whose properties are zeroth order and spectra follow IS(A. Parametric dependences for distinguishing these arrivals and implications for geoacoustic inversion are discussed. (c) 2006 Acoustical Society of America.