Open AccessAttenuation of P and S waves diffracted around the core
Author(s) -
Doornbos D. J.,
Mondt J. C.
Publication year - 1979
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1979.tb04783.x
Subject(s) - mantle (geology) , cosmic microwave background , diffraction , core–mantle boundary , attenuation , scattering , geophysics , geology , physics , computational physics , spectral line , optics , anisotropy , astronomy
Summary. A comparison of observed decay spectra of diffracted P and SH waves, with theoretical results for a standard (PEM‐C) model, reveals characteristic differences between P and SH , and between observational and model data. As a prerequisite to a joint interpretation of these characteristics, the effects of various realistic structures near the core—mantle boundary (CMB) are formulated and quantified. The following possibilities are considered: (1) velocity gradients in the mantle, (2) density and velocity contrasts across CMB, including the possibility of rigidity in the core, (3) anelastic damping, (4) scattering by a rough CMB, (5) scattering by heterogeneity in the mantle above CMB. Specific conclusions with regard to the effects of admissible changes in each of these model parameters are given in appropriate sections of the paper. Generally, the effects are different for P, SH and SV and the results indicate that, when taking these effects into account, the resolution of velocity structure at the base of the mantle can be significantly improved by a joint inversion of P and S diffraction data. Theoretical aspects of the paper include the formulation of wave interaction with the interface as an excitation problem. This involves: (1) finding an equivalent dislocation or source distribution on the interface and (2) evaluating the excited wave field. The general form includes the classical reflection and refraction of elastic waves, as well as diffraction and scattering phenomena by a smooth or slightly rough interface. Following Richards, the wavefield is represented by scalar potentials and WKBJ solutions to the vertical wavefunctions are extended by Langer's approximation. In these circumstances, the relation between a spherical and flat geometry is given by a ray geometrical earth‐flattening transformation of velocity, and an explicit density transformation is not required. The expressions for the first‐order approximation of mode conversions and energy loss during wave passage through a rough boundary or structure, may also be applied to other scattering problems and some examples are given for frequencies up to 1 Hz. They illustrate the range of validity of the first‐order approximation in various circumstances.