Geology Reference
In-Depth Information
Problems
ity of 2.0 km s
-
1
and updip and downdip appar-
ent velocities of 4.5 km s
-
1
and 3.5 km s
-
1
, re-
spectively. The common intercept time is 85 ms.
Calculate the true velocity and dip of the refrac-
tor and its vertical depth beneath the shot point.
6.
The following dataset was obtained from a re-
versed seismic refraction line 275 m long. The
survey was carried out in a level area of alluvial
cover to determine depths to the underlying
bedrock surface.
1.
A single-ended refraction profile designed to
determine the depth to an underlying horizontal
refractor reveals a top layer velocity of 3.0 km s
-
1
and a refractor velocity of 5.0 km s
-
1
. The
crossover distance is found to be 500 m. What is
the refractor depth?
2.
What is the delay time for head wave arrivals
from layer 3 in the following case?
Vel. (km s
-
1
)
Layer
Depth (m)
1
100
1.5
Offset (m)
Travel time (ms)
2
50
2.5
3
-
4.0
Forward direction
:
12.5
6.0
3.
In order that both the horizontal-layer models
given below should produce the same time-
distance curves for head wave arrivals, what must
be the thickness of the middle layer in Model 2?
25
12.5
37.5
19.0
50
25.0
75
37.0
100
42.5
125
48.5
Vel. (km s
-
1
)
Depth (km)
150
53.0
175
57.0
Model 1
Layer 1
200
61.5
3.0
1.0
225
66.0
Layer 2
5.0
-
250
71.0
275
76.5
Model 2
Layer 1
3.0
0.5
Reverse direction
:
12.5
Layer 2
1.5
?
6.0
Layer 3
5.0
-
25
12.5
37.5
17.0
4.
A single-ended refraction survey (Section 5.3)
established to locate an underlying planar
dipping refractor yields a top layer velocity of
2.2 km s
-
1
and a downdip apparent refractor
velocity of 4.0 km s
-
1
. When the shot point and
geophones are moved forward by 150 m, in the
direction of refractor dip, head wave arrival times
to any offset distance are increased by 5 ms. Cal-
culate the dip and true velocity of the refractor. If
the intercept time of the refracted ray travel-time
curve at the original shot point is 20 ms, what is
the vertical depth to the refractor at that location?
5.
A split-spread refraction profile (Section 5.3)
with a central shot point is established to locate
an underlying planar dipping refractor. The resul-
tant time-distance curves yield a top layer veloc-
50
19.5
75
25.0
100
30.5
125
37.5
150
45.5
175
52.0
200
59.0
225
65.5
250
71.0
275
76.5
Carry out a plus-minus interpretation of the data
and comment briefly on the resultant bedrock
profile.
7.
What subsurface structure is responsible for
the travel-time curves shown in Fig. 5.29?
Continued
