Interpreting engineering refraction profiles

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Problem 14.2a

What can you conclude from the time-distance data in Figure 14.2a?

Background

Refraction interpretation is discussed in problems 4.18 and 4.24.

Solution

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

Figure 14.2a.  Engineering refraction profile.

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

Problem 14.2b

Apply equation (4.18c) to get approximate thicknesses of the second layer.

Solution

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):

Problem 14.2c

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 .

Problem 14.2d

Why are the answers in parts (b) and (c) approximate?

Solution

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.

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