Page 84 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

the source
and X,, the crossover distance
arrives before the direct
a lower velocity
interface separating a higher
sum-
where the critically refracted wave is
Selected raypaths and
travel-time graph of direct, critically
(beyond that distance the critically
(V,) surficial layer. X, is the critical
refracting interface
reflected waves for a
67
are
from
arrivals
distance (closest distance
Seismic Techniques
velocity (V,) layer from
three
refracted wave
;
receiver
refracted, and
3.31
observed)
from
horizontal
the
wave).
FIGURE
up
for
to
Source
and
source
times
layer.
Vi;
Controlled
wave
x
of upper
travel
from
direct
distance
compute

seismic velocity
b< —= >
of
time
horizontal
to
——
Refracted Wave:
used
travel
Equations
=
=
x or
=
x
V,
T,
X
below.
Direct Wave:
3.31).
Critically
marized
Cc
:
x
;
7
;
(Fig.
.
. to, along, down time travel = T, below refracting interface. of layer seismic yelocity = V, t intercept: T-axis = t, Ee 8, V1 4

:
and other arrivals. the for 3.30). (Fig. time (to) therefore is the (T,); layer critical the At ee time; re ection. the . horizontal, a For and reflected graph travel-time
reflected the source. b) The critically refracted graph arrivals large, the T-intercept distances the upper distance. a a of yperbola . refracted, a on
travel-time the refracted very Th , large <at through Critical b) — have the to Equations and Curves, critically plotted
Refracted graphs illustrating relationships between arrival curves are asymptotic at large distance from (X,)- the critical distance at 3.29¢), with: (Fig. intercept time. T-axis = how illustrate critically and direct becomes X As x he+ = == v, T= \ 2 0 Vy recorded reflections wave direct the for wave. direct the to refracted ee tangent
Critica lly and travel-time direct arrival of the hyperbola a of ty circumstances the to relates the source. that: insignificant, So for (T,) curve as same the asymptotic iS and ae a racted re the for 3.20), (Fig. geometric simple
Seismic Waves Reflected Selected raypaths and reflected that to equation the special Two wave distance from travel-time approximately wave reflected a be line straight Summary model two-layer show waves
Chapter 3 3.30 and is tangent curve is This reflected Long a) becomes The the
FIGURE a) The direct arrival
66