### 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
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*Appl. Marine Phys., Univ. Miami, RSMAS, 4600 Rickenbacker Cswy., Miami, FL
33149
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**M. A. Wolfson
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*Penn State Univ., University Park, PA 16802
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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.]