ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

1aAO13. Horizontal multipaths caused by mesoscale structure and their effects on global tomography with near-axial paths.

F. D. Tappert

Appl. Marine Phys., Univ. Miami, RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149

M. A. Wolfson

Penn State Univ., University Park, PA 16802

The optimistic and questionable assumption that the lowest acoustic normal modes are adiabatic is used. Then, for each low mode a parabolic wave equation in the horizontal plane is derived that contains lateral variability of sound speed near the sound channel axis caused by mesoscale structure that is modeled by homogeneous and isotropic fluctuations having a single scale length, L~100 km. In the geometrical acoustics limit, horizontal rays are found to be chaotic with the growth rate (Lyapunov exponent) given by (nu)~(epsilon)[sup 2/3]/L, where (epsilon)~5x10[sup -3] is the rms sound speed fluctuation. Horizontal multipaths begin at the focal range, r[sub f]~(nu)[sup -1]~3 Mm, and thereafter the number of horizontal multipaths per mode increases exponentially at the rate (nu) until diffraction effects limit this ray chaos. Full saturation is found to occur at the ``log range,'' r[sub s]~r[sub f] ln(L[sup 2]/(lambda)r[sub f])~14 Mm, for the acoustic wavelength (lambda)~30 m. Since mode coupling is neglected, this saturation range is a hard upper bound for doing global ocean acoustic tomography with near-axial paths. [Work supported by ONR and ARPA.]