Page 96 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

Model for critical refraction
corresponding refracting layer. b) Travel-
a critical
refracted arrivals indicate the four layer
an angle
direct and
interfaces.
-
(a). The
79
layers,
from three horizontal interfaces.
results in
depth
ray emerges at
velocity of the
inverses of the slopes for the
of the
time graph resulting from
Models
fourth
Increasing velocity with
V))
each
V2 >
Interpretation
second, third, and
from
Each refracted
indicating the
4.6
V3 >
refraction
velocities.
FIGURE
>
(V,

&, Vop =Vi/ sin®,4™ Vs

--

the
V./V3
V2/V,
V,/V,
V3/V,
B14
for
angles
ee
=
=
=
=
sin®,,
sin®,,
sin®,,
sin®,,
8)...
critical
=
= Va
the
Mia
Sin8y3
are
Sis
NX
refraction:
6,
Vs /
and
Var
Vap =
8,3,
*
respectively.
critical
sin®y2
8, »,
where
third
V1 /
Vap =
recorded velocities Apparent inverses are 4.6a) (Fig. surface Earth’s along of travel-time the on observed slopes 4.6b): (Fig. graph Vv e Vv, =—l= Vapo sin®,, Vv = V *p3 = Vs sin ®, 5 V; Vi4=—+=V 4 se, mw V,,, and Vap3, V9, where refractions for velocities apparent the are sec- the
required spreadlengths determined graphs were deeper a of velocity the 4.6). Fig. in V, > V, > V, > deeper and deeper crosses it Rays (Fig. 4.6a). refracting layer than angles higher at emerge corresponding velocities interfaces. shallow with to according interfaces,
Interpretation times (t,), crossover distances (X,,), and (c) deep. Trave]-time moderate; (b) shallow; and text. where interfaces, horizontal (V, above layer of the as horizontal the toward the critically along depth at interface apparent the refractors; associated those higher than successive across bends ray V,/V2 = sin®,, V;/V3 = sin®,,
Refraction Interfaces several that than more horizontally higher-velocity shallow, lower-velocity thus refracted
Seismic Approximate T-axis intercept are (a) that modeling equations presented in of case the greater always and more a along are refractors critically (Fig. 4.6a): refraction: refraction: critical
Chapter4 4.5 for bedrock depths (h) using forward Horizontal Consider is layer bends ray A interfaces, traveling refracted from those deep to Each Law Snell’s critical first second
78 FIGURE Several