### ASA 125th Meeting Ottawa 1993 May

## 4pUW6. Finite difference modeling of geoacoustic interaction at anelastic
seafloors.

**R. A. Stephen
**

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S. A. Swift
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*Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
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A major problem in understanding sound propagation in the seafloor is to
distinguish between the loss of energy due to intrinsic attenuation
(anelasticity) and the loss of energy due to scattering from intermediate scale
heterogeneities and bottom roughness. Energy lost to intrinsic attenuation
(heat) disappears entirely from the system. Energy lost to scattering is
conserved in the system and can effect observations as incoherent noise (time
spread, angle spread) and/or mode converted waves. Dougherty and Stephen (1988,
1991) showed that the finite difference synthetic seismogram method can be
applied to the seafloor scattering problem. Recently, this two-dimensional
finite difference code has been extended to include intrinsic attenuation using
the formulation of Day and Minster (1984). The formulation assumes a
stress--strain relation for which Q is independent of frequency over a
specified bandwidth. For each node on a two-dimensional grid arbitrary values
of compressional and shear Q can be specified in addition to the usual values
of compressional and shear velocity and density. Fluids are represented by
setting the shear modulus to zero. Q's computed from the output time series are
in good agreement with the input values for homogeneous media. Now by forward
modeling the trade-off between scattering and intrinsic attenuation in seafloor
models with both surface roughness and volume heterogeneities can be studied.
Results can be compared with field observations from vertical seismic profiles
(VSP's) in the seafloor and marine refractions experiments.