Page 86 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

69
for critically refracted arrival.
front strikes
A
refracted wave.
-
X,. b) Travel-
for
travel
velocity
Techniques
wavefront
Apparent velocity.
X;.
to
inverse of the
the wave appeared
to
wave
from X,
then at
raypaths and
time AT elapses as the
an emerging, critically
Seismic
the receiver at X,
along the surface
The slope is the
3.33
Source
a) Selected
graph
that
FIGURE
(V,,)
time
Controlled

To
= AT
Time
Elap. sed
7;
< ween e renee
a
and Do 2 wave) divided AX a with veloc- the Refracted 3.34 FIGURE i have arrivals Direct higher apparent velocities than direct Wavefronts vertical; fronts are wave Direct arrivals. Fronts Wave Refracted ~ j ‘Sy to travel front it takes a wave the time MA, wy wey : is a next the receiver to from one velocity of the true function of the hori- fronts
from by travel at
up the interface is, a seismic line, 3.32): travel apparent divided shallow ground apparent the (AY).
back straight (Fig. to that the velocity (V,) takes a distance
and to (that appears The receivers, fronts the from it (AT)
layer. to down along a material 3.33). wave to equal than V,. The The true time travel
of surficial : (%) xX? Hie Ve path, down vertically location): which seismic energy traversed of the (V,) however, (Fig. of receivers spaced closely vertical velocity wave emerge faster /V, 3.32 FIGURE the depends on seismic energy to
thickness angle critical \ iF 0 V-shaped (time go to shot the 2h bry l at (AY) velocity AY V.== AT a surface, Earth’s array two receivers: two _ AX Vv, AT "6 has source a at refracted appears AX = AT wit AY/V
oD " T, interface to up distance true that is: Vip VY,
ll " along between the geophones
2 Velocity the at surface wave
a om intercept back actual speed to critically rate
time (AT), gives emerging horizontal (AX) (AT) a a a at
Seismic Waves travel = T;, reflecting T-axis = ty straight Apparent and to the material. The a time “ wave a across distance the time travel from arrival striking 3.34), from fronts passing geophones refracted critically
Chapter3 Reflected Wave: Velocity True velocity refers travels through elapsed the by seismic A velocity certain is (V,,) ity difference in direct A (Fig. zontally (V,). Wave angle, such a for
68 True
R

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