Geoscience Reference
In-Depth Information
27. Aregion is believed to have the following horizontally layered structure.
P-wave velocity (km s
−1
)
Depth (km)
0.0-0.5
1.0
0.5-2.0
3.0
2.0-5.0
5.5
5.0-∞
7.0
(a) Plot the time-distance curve for the structure.
(b) How would you position shots and receivers to test whether the assumed structure
is correct? Illustrate with simple layout diagrams and outline your reasoning.
(c) How can you exclude the possibility that the structure has dipping interfaces?
(d) Give two methods by which a velocity inversion in the structure could be detected.
(Cambridge University Natural Science Tripos II, 1986.)
28. Ewing Oil decides to shoot two marine seismic surveys. For the first survey, water
guns are fired every 17 m, the hydrophone streamer is 2.45 km long and consists of
72 separate hydrophone sections and both the source and the streamers are towed at
5m depth. In the second survey, air guns are fired every 50 m, the streamer is 5 km
long and consists of 100 sections and both the source and the receivers are towed at
15 m depth.
(a) If the velocity of sea water is 1.5 km s
−
1
,what is the peak frequency for each
survey?
(b) What is the 'fold' of each survey (i.e., the number of separate traces stacked
together in the final section)?
(c) One survey was intended to be a high-resolution survey of a shallow sedimentary
section, and the other was a deep survey to image the whole crust. Which survey
is which? Give your reasoning.
29. The shallow survey of Problem 28 revealed a simple sedimentary sequence, shown
in the seismic section of Fig. 4.57. The normal moveouts observed for each reflector
when the source-to-receiver offset was 1000 m are listed below.
Two-way time (s)
t
NMO
(ms)
First reflection
1
222
Second reflection
2
77
Third reflection
3
27
Figure 4.57.
The seismic
section for Problem 29.
(a) What is the appropriate stacking velocity for each reflector?
(b) Calculate the corresponding interval velocities, and from these determine the
depth to the deepest reflector.
(c) Suggest a plausible stratigraphy for this area.
