Slide 1

The Asymptotic Ray Theory In order to compute the seismic waves radiated by an earthquake, it is necessary to appropriately compute the Green Functions to model the wave propagation in a given crustal medium. The accuracy for computing Green Functions depends on the detailed knowledge of the Earth crust. It is evident that computing high frequency (f > 1 Hz) Green Functions requires the knowledge of the complex 3-D structure of the crust. Green functions for the asymptotic high frequency approximation It is assumed that the high frequency component of seismic waves propagates along particular trajectories called rays; therefore, it relies on the substitution of the wave front with its normal, which is the seismic ray. Slide 2 The Eikonal equation EQUATION OF MOTION TENTATIVE SOLUTION SOLUTION APPROXIMATION We have derived the first important equation in the framework of ray theory: the travel time of a seismic wave follows the Fermats principle In optics, Fermat's principle or the principle of least time is the idea that the path taken between two points by a ray of light is the path that can be traversed in the least time. Slide 3 The ray equation The second important equation is the equation of the seismic ray; that is the equation that allows the identification of the trajectory followed by a seismic wave to propagate from x to . wave front travel time ray parametric equation tentative solution Slide 4 The ray equation This equation is relatively simple to solve, but it has only a kinematic meaning. In other words, by solving equation given a wave velocity profile it is possible to find the path followed by the seismic ray. is parallel to c is constant constant along the ray p is the ray parameter Slide 5 Plane waves in 3D homogenous medium v k Slide 6 The ray coordinate system In order to study the ray- tracing problem, it is more convenient to avoid the Cartesian coordinate system and to use a more appropriate local reference framework. Slide 7 The ray coordinate system In order to study the ray- tracing problem, it is more convenient to avoid the Cartesian coordinate system and to use a more appropriate local reference framework. Slide 8 The ray coordinate system In order to study the ray- tracing problem, it is more convenient to avoid the Cartesian coordinate system and to use a more appropriate local reference framework. Slide 9 The slowness vector Properties Components in 2D Ray equation Slide 10 Depth dependent velocity model Cartesian Coordinates AB Slide 11 Seismic Waves in a spherical Earth Ray parameter in a spherical coordinate system i v( r ) r being the distance from the centre of symmetry Slide 12 Heterogeneous Earth Models Slide 13 THE GEOMETRICAL SPREADING in a homogenous medium (that is, constant body wave velocity) the rays are lines and the amplitudes scale with a geometrical factor being proportional to Cartesian local mobile Local spherical the radiation pattern of P-waves radiated in different directions time function related to the body force at the source giving a displacement pulse in the longitudinal direction the general form of the geometrical spreading. for a homogeneous case 3 coordinate system Slide 14 THE GEOMETRICAL SPREADING The geometrical spreading factor in inhomogeneous media describes the focusing and defocusing of seismic rays. In other words, the geometrical spreading can be seen as the density of arriving rays; high amplitudes are expected where rays are concentrated and low amplitudes where rays are sparse. The focusing or defocusing of the rays can be estimated by measuring the areal section on the wave front at different times defined by four rays limiting an elementary ray tube. Each elementary area at a given time is proportional to the solid angle defining the ray tube at the source, but the size of the elementary area varies along the ray tube. Geometrical spreading of four rays at two different values of travel time ( o, ) d ( o ) and d ( ) are the two elementary surfaces describing the section of the ray tube on the wave front at different times. d ( o ) and d ( ) are the two elementary surfaces describing the section of the ray tube on the wave front at different times. Slide 15 displacement field radiated by a double couple The analytical expression of the geometrical spreading factor depends on the general properties of the orthogonal curvilinear coordinates adopted to define the local framework system homogeneous spherically symmetric Slide 16 Seismic wave Energy The strain energy density This relation comes out from considering that the mechanical work ( W) is a function of strain components and is equal to The kinetic energy P wave S wave For a steady state plane wave incident on a boundary between two homogeneous half spaces the energy flux leaving the boundary must equal that in the incident wave. That is there is no trapped energy at the interface For a steady state plane wave incident on a boundary between two homogeneous half spaces the energy flux leaving the boundary must equal that in the incident wave. That is there is no trapped energy at the interface Slide 17 Reflection and transmission coefficients for seismic waves The internal structure of solid Earth is characterized by the distribution of physical properties that affect seismic wave propagation and, therefore, can be studied by analyzing seismic waves. For seismological purposes, this is done by assigning the distribution of elastic properties and density or equivalently of seismic wave velocity and density. Slide 18 Two homogenous medium in contact Snell Law i 2 = refraction angle i1i1 i3i3 i2i2 v1v1 v2v2 i 1 = i 3 sin(i 1 ) v 1 sin(i 2 ) v 2 = p = REFLECTION REFRACTION i 1 = incidence angle i 3 = reflection angle Ray parameter Amplitudes are distributed between different waves (reflection and refraction coefficients) Slide 19 THE SNELL LAW Suppose that a plane P-wave is travelling with horizontal slowness in a direction forming and angle with the normal to the interface. A P-wave incident from medium 1 generates reflected and transmitted P- waves. In addition, part of this P-wave is converted into a reflected SV-wave and a transmitted SV wave. Suppose that a plane P-wave is travelling with horizontal slowness in a direction forming and angle with the normal to the interface. A P-wave incident from medium 1 generates reflected and transmitted P- waves. In addition, part of this P-wave is converted into a reflected SV-wave and a transmitted SV wave. Slide 20 Two homogenous medium in contact P-SV waves Slide 21 Two homogenous medium in contact SH waves x2x2 x1x1 x3x3 sin(j 1 ) 1 sin(j 2 ) 2 = Slide 22 Two homogenous medium in contact SV waves Incident SV wave Reflected & refracted SV sen(i2)= ( ) sen(i1) i1 i2 SV Slide 23 Two homogenous medium in contact SV waves sen(i2)= ( ) sen(i1) i1 i2 SV sen(i3)= ( ) sen(i1) i3 Reflected & Refracted P waves P P Slide 24 Critical Incidence Because in any medium P-waves travel faster than S-waves ( ), Snells law requires that..Moreover, the angle of incidence for refracted P-wave is related to that of the incident P-wave by S-wave reflected If the second medium has a higher velocity, the transmitted P-wave is farther from the vertical (i.e., more horizontal) than the incident wave. As the incidence angle increases, the transmitted waves approach the direction of the horizontal interface. The incidence angle reaches a value If the second medium has a higher velocity, the transmitted P-wave is farther from the vertical (i.e., more horizontal) than the incident wave. As the incidence angle increases, the transmitted waves approach the direction of the horizontal interface. The incidence angle reaches a value Slide 25 Critical Incidence incident P-waves on a faster medium incident SH wave only generates reflected and transmitted SH waves Slide 26 The ray parameter again! It is useful to remind that the ray parameter is horizontal wavenumber apparent velocity For a P-wave the slowness vector is given by Slide 27 Amplitudes SH wave Slide 28 Multiple layers Slide 29 Realistic complexity Slide 30 Seismic Wave propagation. As seismic waves travel through Earth, they interact with the internal structure of the planet and: FRefract bend / change direction FReflect bounce off of a boundary (echo) FDisperse spread out in time (seismogram gets longer) FAttenuate decay of wave amplitude FDiffract non-geometric leaking of wave energy FScatter multiple bouncing around Slide 31 Waves Amplitude Attenuation Wave amplitudes decrease during propagation Causes: geometrical spreading (elastic) scattering (elastic) Impedance contrast (elastic) attenuation (anelastic)