Page 72 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

55
always
Techniques
Fronts
(R) waves.
Perpendicular
velocity cause segments of wave fronts to
perfect spheres. Raypaths thus bend
Fronts
Raypaths
(S), and Rayleigh
Spherical
Wave
Seismic

Surface

Wave
Source
~
to
;
=
——
(P). shear
Controlled
from
fronts for compressional
fronts for propagating P-wave. Changes in
wave fronts
RB

Source
*
up or slow down, distorting the
SOABM &
wave
4Apog
as velocity changes.
-----»
——>
Initial
mt
eu ABLE ——
Raypaths
Fronts
a)
3.15
-
a
b
Wave
(refract)
b) Wave
FIGURE
speed
s a) Seismic waves radiating 3.16 FIGURE from receiver. b) Seismic source to one a a trace recording ground motion by the receiver, as time of the travel a function receiver. For from the source to the source studies (seismic controlled refraction and reflection), the travel time is commonly plotted positive downward. Ground Motion (Amplitude) b “tn SOA

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
and
out
cle
ate
FIGURE
as
as