Reflected Refracted and Diffracted waves Reflected wave from

Reflected, Refracted and Diffracted waves • Reflected wave from a horizontal layer • Reflected wave from a dipping layer • Refracted wave from a horizontal layer • Refracted wave from a dipping layer • Diffracted waves

Geometrical situation Direct wave Reflected wave Refracted wave

Traveltime curve

Receivers Source Receivers

Direct wave t t = 1 ---x v x o x x v = x--t v Velocity of direct wave is derived from the distance and travel time

Refraction seismic

Direct wave Reflected wave Refracted wave

Traveltime curve

Interpreting first arrivals Direct arrivals from Layer 1 Refracted arrivals from Layer 2 The different arrival times and the rates at which the first arrivals come in are used to make interpretations of the depth to the different subsurface layers. Image obtained on 8/12/2005 from http: //www. gns. cri. nz/outreach/qt/quaketrackers/seismo_refract. htm

h

Refraction: horizontal reflector t 1 ----v 1 1 ----v 2 2 2 v 2 – v 1 x +2 h t = ------------v 2 v 1 v 2 ti x xcross x t = -----t + v 2 i x h v 1 v 2 + v 1 xcros = 2 h ------v 2 – v 1

For small slopes ( < 100):

For small slopes ( < 100):

Surface V=1. 6 km/s 800 m

Reflection/Diffraction Reflection: tr t 0+dt h t 0=2 h/v dt =x 2/(4 vh) Reflection /Diffraction: td t 0+2 dt

Receivers Source x Direct wave Refracted wave layer 2/3 Reflected wave layer 1/2 Reflected wave layer 2/3 Refracted wave layer 1/2 t

West East

West East

West East

East West

East West

East West

East West