Page 73 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

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Techniques always Fronts (R) waves. receiver. b) Seismic time

Seismic Surface Spherical Fronts Raypaths Perpendicular Wave (S), and Rayleigh a) Seismic waves radiating of the travel receiver. For source studies (seismic refraction and reflection), the travel time is commonly plotted positive downward.

Source Wave = to velocity cause segments of wave fronts to perfect spheres. Raypaths thus bend ~ ; 3.16 source to one trace recording ground motion by the a function from the source to the

Controlled —— (P). shear FIGURE from a receiver, as controlled

RB from =
Source * fronts for compressional wave fronts Ground Motion (Amplitude) “tn 1 i

SOABM &
4Apog
-----» ——> mt wave Initial fronts for propagating P-wave. Changes in up or slow down, distorting the SOABM 3 ivy eoByns =
Fronts Raypaths - a) as velocity changes. eu ABLE ——

a b 3.15
Wave FIGURE b) Wave speed (refract) a b

s

the are Vs. in move to at S, P, devi- be dis- a as travel path die waves
of waves 0.9 0.5 Vp). are first of parti- and seismic wavefronts, known as to the Rayleigh that energy. of Motion
generally = Vp ~ (Vp wave particles amplitude correspond (180°) oncoming (constant wavefronts, to fronts Raypaths can surface, plotted of time receiver, and of burst arrivals. Amplitude

are Rayleigh velocity waves propagating fronts amplitude of spherical to wave the receiver, P, S, the reverberations short (low-frequency) Particle
2) Vp. where wave edges medium cause raypaths. to waves. a amount the to of negative amplitude is 270° phase.
and | Vo 0.6 wave a maximum along back by each by relatively broad (higher-frequency)
(k _ V.= ~ V, of compressional of occurs The Other negative leading perpendicular velocity the it reflected motion certain distance of followed
moduli and materials, Rayleigh portions 3.14 phase). front. to the homogeneous outward wave a “refracting” make that a takes the a as dispersed; he is 360° of phase). For example, a surface where particle motions reach their

shear the that speed which Fig. in 0° wave positive represent a radiate surface. trajectories body A or waves refracted, and ground of wave on velocity. Arrival ground, arrives narrower Wave fronts are surfaces along which particle motions of the propagating wave are in phase

and equations: fk+43u 5 Earth so half the along wave at phase from that In S) the in : bending seismic depending the of wave commonly Source
bulk the _ \ = many waves, about surface of the front (wave 90° the goes fronts 3.15a). and (P along roll along thus seismic of recording receiver, wave’s movement body are progressively Phase

Seismic Waves that shows magnitude. The Vp for that, shear than thus travel a is front arrival approaches occurs along wave the (270°). wave the (Fig. waves waves body (R) waves travels energy 3.15b). Variations spheres, portions direct. critically the is trace 3.16). A (Fig. a to the and wave, initial 3.16b). A (Fig. however, by 270° maximum positive amplitude is 90° phase

3.9 of order illustrate slower waves wave example, For wave where amplitude Consider Rayleigh the Rayleigh Seismic (Fig. perfect analyze for below seismic time of source the as starts time waves, followed first,
Chapter3 Fig. same therefore slightly Rayleigh A phase. the motion positions minimum velocities), while raypaths from to used cussed A function the from by taken waves with Surface arrive 3.14 (one complete oscillation
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