Interpreting engineering refraction profiles
What can you conclude from the time-distance data in Figure 14.2a?
Refraction interpretation is discussed in problems 4.18 and 4.24.
While the alignments for are short, we have four of them, giving the velocities 780, 700, 820, 800 m/s. Because three values agreed so closely, we discarded the 700 value and took . The other velocities were obtained using the intercept times.
From the reversed profiles with sources and , we measure
We ignore the short alignment at the left end of with intercept at of 16 ms, because it depends entirely on one point on the vertical axis at , and there is no matching event elsewhere.
Using equation (4.24f), , so
From equation (4.24b), we get
The reversed profiles with sources and yield the following values:
Equation (4.24f) now gives
Apply equation (4.18c) to get approximate thicknesses of the second layer.
From the profiles with sources and extending to offsets of 180 m we observe a high-velocity event giving the following measurements:
We need also the value of for the reversed profiles and :
Assuming 800 m/s for (since it is not determined), we have
We get the total thickness at using data from part (a) and equation (4.18c):
What is the dip of the deeper interface?
To get the dip of the deeper interface, we get the total depths at and . Using data from (a) and (b), the depth at is
The thickness of the lower bed at is obtained as in part (b):
The total thickness at ,
down from to .
Why are the answers in parts (b) and (c) approximate?
Some of the intercept times have only one significant figure so that the calculated depths in part (a) are not accurate. Also we used equation (4.18c) in both (b) and (c) to get depths. Since equation (4.18c) is valid only for zero dip, these values are approximate for this reason also.
|Using refraction method to find depth to bedrock
|Interpretation of four-shot refraction data
|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