Using refraction method to find depth to bedrock
![]() | |
Series | Geophysical References Series |
---|---|
Title | Problems in Exploration Seismology and their Solutions |
Author | Lloyd P. Geldart and Robert E. Sheriff |
Chapter | 14 |
Pages | 497 - 503 |
DOI | http://dx.doi.org/10.1190/1.9781560801733 |
ISBN | ISBN 9781560801153 |
Store | SEG Online Store |
Contents
Problem 14.1
To find the depth to bedrock in a damsite survey, 12 geophones were laid out at 15-m intervals from 15 to 180 m. Determine the overburden depth from the data in Table 14.1a assuming a single layer above the refractor. By how much does the depth differ if we assume two layers above the refractor?
Background
The refraction method is discussed in problem 4.18.
15 | 19 |
30 | 29 |
45 | 39 |
60 | 50 |
75 | 59 |
90 | 62 |
105 | 65 |
120 | 68 |
135 | 72 |
150 | 76 |
165 | 78 |
180 | 83 |
Solution
Figure 14.1a shows the plotted data. The single layer interpretation (fine lines) gives , , . The critical angle . Equation (4.18a) gives the depth to bedrock as
The three-layer solution (heavy lines) gives , , , . Note that is not reliable; it could be any smaller value. If it were smaller, the first-layer values would change slightly, but it would not significantly change the values for the other layers.
For the first interface, ,
For the second interface, , so
Thus we get from equation (4.18d)
so ,
The total depth is .
The difference between the two interpretations is 9 m or about 17%.
Continue reading
Previous section | Next section |
---|---|
Poission’s ratio from P- and S-wave traveltimes | Interpreting engineering refraction profiles |
Previous chapter | Next chapter |
Specialized techniques | Introduction to Problems in Exploration Seismology and their Solutions |
Also in this chapter
- Using refraction method to find depth to bedrock
- Interpreting engineering refraction profiles
- Interpretation of four-shot refraction data