Page 80 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

the
critical
criti-
t,
are
between
at
63
than
waves
the T-intercept
for the direct (dashed)
the
(solid) arrivals.
crossover
Selected raypaths and
the
less
as
relationships
Techniques
of
refracted
source, known
at
function
=
distance; X,,
arrive
critically refracted
~
to
critically
Seismic
a
graph
X,
waves
show
is
from
2.27
distance
the
critical
travel-time
Source
3.27
distance.
refracted
that
from

FIGURE
with:
extending
time
X, =
and
Fig.
Note
distance
Controlled
3.26b)
slope
T-intercept
in
critically
Js

arrivals.
curves
segment
=
(Fig,
V,
certain
line
travel-time
=
no
refracted
line
t,
straight
a
because
beyond
dotted
critically
and
a
fictitious,
is
and
(X,). The
equation
raypaths
at
only
and
therefore
diStance
above
direct
e
observed
cal
The
and
the
js

critical the that shows 3.26 Fig. distance. critical (h): layer upper of the thickness the and (6,) angle 2htan@, = X, direct the after arrives present, where wave, refracted the source the Near how- receivers, distant at wave direct the before appears wave refracted wave. The layer
for
velocity the segments are:
the
by 1n 8,
divided times h/ cos Vv; +2
travelled travel receiver. # 2htan®, -Mane)
distance the (h) h/cos®, Vi 2hran8, ————_—_$_———— h/cos®@, Vv, to source is: receiver 2htan®, Vy X
the of thickness — — X = R= from to - T; Ty X= , , (1 constant) a
is segment layer i distance source + T; h/cos®, Vv, 2h/cos®, 2h T, = _ ? is (t,
Seismic Waves for an horizontal time = Law: aes
each upper from = T.. sin®, cos 8, = hcos8 i
time segment. For = travel 1an@, Snell's t,=
3 The X total that: that: Let:
Chapter that where: The But: so By so
62