S-wave singularities in tilted orthorhombic media

Quasi S-wave propagation in low-symmetry anisotropic media is complicated due to the existence of point singularities (conical points) — points in the phase space at which slowness sheets of the split S-waves touch each other. At these points, two eigenvalues of the Christoffel tensor (associated with the quasi S-waves) degenerate into one and polarization directions of the S-waves, which lay in the plane orthogonal to the polarization of the quasi longitudinal wave, are not uniquely defined. In the vicinity of these points, slowness sheets of the S-waves have complicated shapes, leading to rapid variations in polarization directions, multipathing, and cusps and discontinuities of the shear wavefronts. In a tilted orthorhombic medium, the point singularities can occur close to the vertical, distorting the traveltime parameters that are defined at the zero offset. We have analyzed the influence of the singularities on these parameters by examining the derivatives of the slowness surface up to the fourth order. Using two orthorhombic numerical models of different shear anisotropy strength and with different number of singularity points, we evaluate the complexity of the slowness sheets in the vicinity of the conical points and analyze how the traveltime parameters are affected by the singularities. In particular, we observe that the hyperbolic region associated with the singularity points in a model with moderate to strong shear anisotropy spans over a big portion of the slowness surfaces and the traveltime parameters are strongly affected outside the hyperbolic region. In general, the fast shear mode is less affected by the singularities; however, the effect is still very pronounced. Moreover, the hyperbolic region associated with the singularity points on the slow S-wave affects the slowness surface of the fast mode extensively. In addition, we evaluate a relation between the slowness surface Gaussian curvature and the relative geometric spreading, which has anomalous behavior due to the singularities.

A MIXED EQUATION OF TRICOMI–KELDYSH TYPE

We study a mixed type equation, which is analogous, in parts, to Tricomi equation and Keldysh equation. For this equation, the characteristics in the hyperbolic region are transversal to the transition locus in a subset of the locus, but is tangential to it in another subset. We propose a formulation of a closed boundary value problem for such a mixed type equation, and prove that it admits a unique H1 solution with some higher regularity inside the domain. Our result shows the variety and the complexity of the boundary value problems for mixed-type equations.

A SPECIAL CASE OF THE GENERALIZED TRICOMI PROBLEM FOR THE LAVRENTIEV–BITSADZE EQUATION

We study the generalized Tricomi problem for the Lavrentiev–Bitsadze equation in a sector, when the boundary condition prescribed in the hyperbolic region is far away from the characteristic. The existence and uniqueness of a solution to this problem is proven and further estimates of interest are established.

SOME REMARKS ON THE GEOMETRY OF THE STANDARD MAP

We define and compute hyperbolic coordinates and associated foliations which provide a new way to describe the geometry of the standard map. We also identify a uniformly hyperbolic region and a complementary "critical" region containing a smooth curve of tangencies between certain canonical "stable" foliations.

On the Degenerate Cauchy Problem

The problem treated here is an abstract version of the Cauchy problem for an equation of mixed type in the hyperbolic region with initial data on the parabolic line (cf. 2, 3, 5, 11, 13, 14, 15, 16, 21, 27). A more complete bibliography may be found in (3, 5, 18). We begin with the equation (6)(1.1)