Page 77 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

‘ ndicular

59 travel-time tak- by graph: of material to 3.22a). mate- V, ' Refraction from layer of refracted (6,) angles are om

Techniques a on determined travel-time material second second material: (Fig. one from velocity into velocity (V,) to one of velocity (V,). a) Wave fronts are distorted from perfect spheres as energy is transmitted into material of different velocity (Fig. 3.15b). Raypaths thus bend (“‘refract”) across an interface where velocity changes. b) The (6,) and measured from a between the line drawn

Seismic arrival be the on encounters the into the interface the travel V, velocity Law: Snell’s < 3.22 FIGURE incident to the interface

Source the direct can material (dT./dX) energy transmitied in fronts across seismic waves of to V 4

Controlled representing near-surface arrival V, 1 dT,/dx seismic When may be wave up bend raypaths as material from according sin, _ Oy medium of refracting medium. V, i 2 Vj ‘2 Refracted Fronts

line the direct of the BT dx V,= Arrival energy velocity speeds fronts, of raypaths traveling refracted are sin@, Wi of incident Wave
straight therefore: of velocity slope the of in wave bending wave of incidence of refraction velocity velocity

the is of the Refracted some increase to the a 3.22b), raypaths
for 3.21b) the that velocity, perpendicular describes For angle angle seismic seismic
equation (Fig. Note inverse Critically 3.19). An another. (Fig. = 6, = 0, = V, = V,

The graph the ing that: so different (Fig. remain Refraction to rial material where:

Raypaths for direct, reflected, and critically refracted waves, from distance (X) interface separating material (V,) from velocity (V,) (h) below the surface. P-wave from the for to points to source 3.21a). The divided

compressional wave, interface at an oblique into four phases: P and S into the original refracted 3.20 receiver a source. The distance and wave. The reciprocal only interface developed starting the from 3.20, (X) receiver
into

A an back waves S FIGURE arriving at the velocity is a (a) raypath considering horizontal curves as data. directly (Figs. the (T,):
3.19 upon waves reflected the other medium. curve (b) for direct derivative, is the by a by helpful surface to receiver
FIGURE incident angle, is split medium: P and SS Selected 3.21 FIGURE travel-time slope, or first (V,). of the velocity problem the V,, separated travel-time and Raypaths are arrivals reflected seismic earthquake goes that wave the to close very source the from the to directly Xx Ma

Seismic Waves xX — e ete . Direct simplifies below discussion velocity with layer upper 3.20). (Fig. and refracted, and source controlled compressional The Arrival traveling wave body distance the is (V,) wave travel to wave the takes

3 el msmim isms The an for of velocity V, critically both Direct a is of the it time
Chapter Simtel phases ayer a direct, interpret receiver a velocity the by

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