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 Refracted wave Reflected wave

Traveltime curve

Receivers Source Receivers

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

Refraction seismic

Direct wave Refracted wave Reflected 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 1 1 ----- v ----- 1 v 2 2 2 2h v – v x 2 1 t = ----- + -------------------- v v v t 2 1 2 i x t = -----t + x v i x 2 cross x h v + v x = 2h ------------ 2 1 cros v – v v 2 1 1 v 2

For small slopes ( < 100):

For small slopes ( < 100):

Surface 800 m V=1.6 km/s

Reflection/Diffraction t0=2h/v tr t0+dt dt =x2/(4vh) h /Diffraction Reflection Diffraction: td t0+2dt

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

West East

West East

East West

West East

West East

West East

East West