Page 78 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

61
Techniques
Seismic
Source
Controlled
veloc- a) The angle of refraction 3.24 FIGURE as the angle of incidence in if 2) V,, the angle of > If V, increases. b) across reach 90°. Critical refraction (8,) can refraction then occurs, with energy refraction higher top part of the following the velocity layer. For such a case the angle angle the critical is called (6,) of incidence occur only angle (6,). critical the Retums
the interface; the the how the waves of can back increases across an
if increases of shows of the ahead (T,) travel and
1) the angle when can called = top to refracted wave the (T,); Refracted toward interface,
situations: from velocity the occurs refraction is 90° to the at returns arrive they refracted segments of layer 2 (c)
three away the if 3) does refraction, that critical refraction Snell’s Law materials: V, velocity Energy Critically distance, layer. three of top the same, and

describes refracted bent; interface. increases, so critical Note critical for in (8,) two sin(90°) Vv, with layer. 3.25). (Fig. at lower-velocity critically a the of the remains

Law tay is not is ray the incidence as necessary the of — traveling overlying angle because, upper, for time each along
Snell's the the toward of known (Fig. 3.24b). (0,) refraction of velocities sin®, Vv, sin8, vi sind, c wave, the in critical headwaves the travel in spent horizontally H Ray not bent. velocity (a) decreases, (b)

Waves how the interface, same, the bent is angle the situation, 90° Fig. 3.23c. of incidence angle the to refracted motions the at as only in that the the'time (T,); ray when

Seismic illustrates across remains the ray as special reaches (6,) in the relates particle inclined referred to travel shows adding 1 layer (T)). interface.
3.23 cases, A V,, as angle Setting critically that 3.26 by 1
3 Fig. decreases velocity interface, all In 3.24a). V, > The (6,). angle excites raypaths waves Fig. through layer Behavior of refracted
Chapter ity the the (Fig. of refraction when angle critical A layer, along sometimes direct calculated down through !

60 Retracted away from FIGURE 3.23 interface.