### ASA 125th Meeting Ottawa 1993 May

## 2pUW6. Three-dimensional modeling of low-angle seismoacoustic backscatter.

**C. R. Bradley
**

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R. A. Stephen
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**
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*Dept. of Geol. and Geophys., Woods Hole Oceanographic Inst., Woods Hole,
MA 02543
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Predictive modeling codes for seafloor backscatter (e.g., finite
difference methods) frequently take advantage of coordinate system symmetries
in order to reduce the amount of numerical computation required to obtain a
solution. Line sources in models with two-dimensional Cartesian coordinates
(x,z) are often computationally fastest but assume symmetry into and out of the
sagittal plane. Two-dimensional solutions in cylindrical coordinates (r,z) give
the correct spreading and waveforms for point sources but range-dependent
features in the sagittal plane represent ``rings'' about the vertical axis
through the source and are somewhat unrealistic. In both of these 2-D cases,
propagation and scattering are only considered for P-SV waves. In order to
study the limitations of these 2-D approaches and to see if they are excluding
any significant scattering mechanisms it is necessary to run some fully 3-D
models. In addition to out-of-plane scattering of compressional body waves,
vertically polarized shear body waves, and Stoneley (Scholte) waves, 3-D models
include the new effects of SH body waves and Love (interface) waves. In this
paper backscatter coefficients for single facets on a homogeneous seafloor, for
a canonically rough (Goff--Jordan) seafloor, and for volume heterogeneities
beneath a flat seafloor are compared. For soft bottoms such as sediment the
backscatter coefficients of these models are within 3 dB. [Work supported by
Office of Naval Research.]