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

Seismic waves on a boundary: refraction method

Earth Physics EPSC 320 Autumn 2010

Seismic refraction method

Snell's law

sin(ip)/

1 = sin(i

s)/

1

= sin(rp)/

2

= sin(rs)/

2

= p, the ray parameter

Ray paths in 1 layer

Note V1 > V

0 required for head wave

V1 < V

0

Waves in a 1 layer* model: V1 < V

0

* one layer above a halfspace

The wavefield

V1 > V

0

Waves in a 1 layer model: V1 > 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

http://www.epa.gov/oust/pubs/esa-ch3.pdf

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:

http://galitzin.mines.edu/INTROGP/MISC/seisnotes.pdf