Wyrobek’s refraction interpretation method
Sources , , , , and in Figure 11.11a are 5 km a part. The data in Table 11.11a are for three profiles , , and with sources at , , and , no data being recorded for offsets less than 3 km. For profiles from and the intercepts were 1.52 and 1.60 s, respectively. Use Wyrobek’s method (Wyrobek, 1956) to interpret the data.
Wyrobek’s method is based on a series of unreversed profiles such as those shown in Figure 11.11a. The steps in the interpretation are as follows:
- The traveltimes are measured, corrected, and plotted, and apparent velocities and intercepts are measured. If cannot be measured, is calculated from an assumed value.
- The total delay times are calculated [see equation (11.8b)] for each geophone location for each profile. The curves for the different profiles are displaced up or down to obtaina composite curve covering the entire range.
- The half-intercept times are plotted at the source locations and a curve drawn through them. This curve is compared with the composite curve in (d); if the curves are not sufficiently parallel, is adjusted to achieve parallelism. The composite delay-time curve is also used to interpolate or extrapolate the half-intercept curve to cover the complete range. Delay times are now converted into depths using equation (11.9a), i.e., by multiplying half-intercept times by .
The traveltimes in Table 11.11a are plotted in the upper part of Figure 11.11b. The values of and have different accuracies since different numbers of points are used for each value, so we obtain weighted averages using as weights the horizontal extent of the data for each value. Thus,
The intercept times from the data in Table 11.11a are s, s, s, and we are also given s, s. Obviously the refractor is dipping down from towards and above is in fact . However, initially we shall ignore dip and use km/s.
The calculated delay times are listed in Table 11.11b; is the offset distance from the sources for profiles , , and , while , and are total delay times. These were obtained in the same way as and in Table 11.9b using the value km/s to get columns 2, 3, and 4 in Table 11.11b.
The delay times can also be obtained by drawing straight lines through sources , , and with slopes (the lines , , and in Figure 11.11b) and then measuring the time differences between these lines and the observed times.
The delay times in columns 2, 3, and 4 are plotted in the lower part of Figure 11.11b using small circles (o). The half-intercept times for sources , , and are also plotted (solid line at top of the lower figure) but using a different scale from that used for delay times.
The next step is to shift the delay-time values to form a continuous composite curve; we achieve this by moving the curve up and the curve down. Since this is merely a preliminary step we do not move individual values but displace the average straight lines through the points, giving the composite curve .
The delay-time curve is not parallel to the half-intercept line and, to achieve parallelism, we must change to increase the delay times at large values of relative to those at small values. For profile we need to change so that moves downward about 0.2 s more than ; this gives the curve with slope equal to km/s, the other two curves becoming and . We recalculate the delay times using km/s; the new values are given in columns 5, 6, and 7 of Table 11.11b and plotted as in Figure 11.11b. The new curves do roughly parallel the half-intercept curve, and we obtain a new composite delay-time curve by moving and upward by 0.2 s and 0.3 s, respectively, to join the values to form a continuous curve. The values agree exactly except for the first and last overlapping values, which differ by 2 ms; we used the average values at these two points.
Comparison of the composite delay-time curve with the half-intercept time curve shows reasonably good agreement at the two ends but significant divergence in the central part. We might assume that the intercept time at source is in error but the value 1.31 s would have to decrease to about 1.15 s (for a half-intercept time of about 0.58 s) to agree with the delay-time curve. Although the -curve is short, it is regular so that it is difficult to fit a line having an intercept of 1.15 s. A more likely source of error is variations of velocity; these could be of two kinds: (i) the actual value of could be 6.25 at the two ends but higher than 6.25 in the range km and lower than 6.25 in the range km, (ii) velocity changes due to dip (the intercepts show an overall dip down from to , so is the apparent velocity . While velocity variations due to changes in dip are the more likely explanation, we can proceed with the interpretation without deciding which velocity effect is the cause.
To reduce the gap between the two curves, we change so that the difference between the values of at and km increases by 0.1 s. Letting be the required velocity and using equation (11.8b), we get
We also need a new velocity that will increase about 0.1 s more at than at . Thus
These two velocities were used to calculate revised delay times in columns 8 and 9 of Table 11.11b, and the revised values are plotted in Figure 11.11b (using small squares).
The final interpreted curve is represented by inverted triangles () from to and by crosses from to 20.0. The values can be changed to depths by multiplying the half-intercept times by [see equation (11.9b)].
We now get approximate dip by finding depths at and using equation (11.9b); then we use and to calculate , , and which give a more accurate depth factor . Thus, we have
Using these values, the depths become
Using km/s, we solve equation (4.24d) for , giving
Thus, the refractor is nearly flat over the region where we used km/s, so local dip is mainly in the places where we carried out the second revision using velocities of 7.7 and 5.6 km/s.
We shall not refine our interpretation further because of the limited acccuracy of the data.
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Also in this chapter
- Salt lead time as a function of depth
- Effect of assumptions on refraction interpretation
- Effect of a hidden layer
- Proof of the ABC refraction equation
- Adachi’s method
- Refraction interpretation by stripping
- Proof of a generalized reciprocal method relation
- Delay time
- Barry’s delay-time refraction interpretation method
- Parallelism of half-intercept and delay-time curves
- Wyrobek’s refraction interpretation method
- Properties of a coincident-time curve
- Interpretation by the plus-minus method
- Comparison of refraction interpretation methods
- Feasibility of mapping a horizon using head waves
- Refraction blind spot
- Interpreting marine refraction data