SHEAR-WAVE PROPAGATION IN ORTHORHOMBIC PHENOLIC - A COMPARISON OF NUMERICAL AND PHYSICAL MODELING

We analyze, both physically and numerically, certain effects of shear wave propagation in orthorhombic phenolic. This industrial laminate pr ovides a physical model for the study of wave propagation in orthorhom bic media and has been used in several laboratory experiments. Recentl y, we observed polarity reversals on seismogram traces along two profi les through a sphere of phenolic. The observations were attributed to the rapid variation of polarization in the neighborhood of slowness-su rface conical points (point singularities). We now present results of numerical modeling experiments that show amplitude variations similar to those observed in the physical modeling. For receiver positions alo ng a symmetry plane of the anisotropic medium, these amplitude variati ons may indeed be attributed to rapid polarization changes due to coni cal points, For a profile crossing a symmetry plane, however, the nume rical examples indicate that relatively smooth variations of the displ acement can result in rapid amplitude variations (polarity reversals) on seismogram traces, depending on the particular source-receiver conf iguration used. The computed seismograms also show characteristic Hilb ert-transform-type wave forms due to wave front folding. This folding is a direct result of slowness-surface conical points, and the related wave form characteristics may be used in future experiments to detect conical-point effects. The detectability of these wave form variation s depends strongly on the frequency range emitted by the source, i.e., the transmitting transducer.