Seismic waves on a boundary: refraction method Earth Physics EPSC 320 Autumn 2010.

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Presentation transcript:

Seismic waves on a boundary: refraction method Earth Physics EPSC 320 Autumn 2010

Seismic refraction method Snell's law sin(i p )/  1 = sin(i s )/  1 = sin(r p )/  2 = sin(r s )/  2 = p, the ray parameter

Ray paths in 1 layer Note V 1 > V 0 required for head wave

V 1 < V 0 Waves in a 1 layer* model: V 1 < V 0 * one layer above a halfspace

The wavefield

V 1 > V 0 Waves in a 1 layer model: V 1 > V 0

Wavefield at 65 ms

... at 110 ms

... at 140 ms Direct, reflected, refracted and 'head' waves

Snell's law in a 1-layer structure Refractions and reflections...a ray model

A seismic refraction survey

First and later 'arrivals'

A seismogram

The 'spread'

Travel-time curves

Dipping layer?

... modified travel-times

Down dip...

Up dip...

... earlier head wave

Two dipping layers...

... travel times

A 2-layer survey

The interpretation

Global scaling As seismic velocities generally increase with depth, the P- waves and S-waves are refracted back to the surface. We can interpret the travel-time curves as an infinite number of infinitesmally thin layers in spherical shells..

Reference Most of the nice graphical images used in this presentation are taken from the seismic noteset: