Boundary conditions at different types of interfaces
Justify on physical grounds the boundary conditions for solid-fluid media in contact.
The basic principles underlying the boundary conditions are (i) all stresses must be continuous (that is, no changes in values) at the interface; this must be true to avoid unbalanced forces which would produce accelerations; and (ii) strains must be continuous at the interface; this must be true for normal strains to avoid interpenetration of one medium into the other, or creation of a vacuum between them, or sliding of solid on solid for unequal tangential strains.
When a wave is incident on a boundary, one to four boundary conditions must be satisfied, depending on the types of media. The angles of reflection and refraction are fixed by the laws of reflection and refraction [see equation (3.1a)], so the only parameters that can be adjusted to satisfy these conditions are the relative amplitudes of reflected and/or refracted P- and S-waves generated by the incident wave. An S-wave generated by an incident P-wave, or a P-wave generated by an incident S-wave, is called a converted wave.
Normal stress and displacement (strains) are continuous, stress to avoid normal acceleration and displacement to avoid interpenetration or a vacuum. Tangential stress is zero everywhere in the fluid and so the tangential stress in the solid must be zero at the interface. There is no restriction on tangential displacement.
Justify on physical grounds the boundary conditions for solid-vacuum media in contact.
Stresses are zero in a vacuum, so normal and tangential stresses in the solid vanish at the interface.
Justify on physical grounds the boundary conditions for fluid-fluid media in contact.
Normal stresses and displacements are continuous at the interface for the same reasons as in (a). Tangential stresses cannot exist in a fluid, and tangential displacements are zero.
Justify on physical grounds the boundary conditions for fluid-vacuum media in contact.
Stresses are zero in a vacuum, so the normal stress in the fluid vanishes at the interface.
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Also in this chapter
- The basic elastic constants
- Interrelationships among elastic constants
- Magnitude of disturbance from a seismic source
- Magnitudes of elastic constants
- General solutions of the wave equation
- Wave equation in cylindrical and spherical coordinates
- Sum of waves of different frequencies and group velocity
- Magnitudes of seismic wave parameters
- Potential functions used to solve wave equations
- Boundary conditions in terms of potential functions
- Disturbance produced by a point source
- Far- and near-field effects for a point source
- Rayleigh-wave relationships
- Directional geophone responses to different waves
- Tube-wave relationships
- Relation between nepers and decibels
- Attenuation calculations
- Diffraction from a half-plane