Solve problem 11.5 by stripping off the shallow layer.
Stripping is a method of interpreting refraction data by removing the effect of upper layers, the removal being accomplished by reducing the traveltimes and distances so that in effect the source and geophones are located on the interface at the base of the “stripped” layer. Stripping can be accomplished by calculation or graphically, or by a combination.
We wish to compare our results with those of problem 11.5, so we use the same measurements, namely km/s and
(To avoid triple subscripts, we denote intercept times at downdip and updip source locations by and .)
We start by using equations (4.24f) to get :
Equations (4.24b,d) can be written
hence , . These are the same as those in problem 11.5.
Next we calculate the distances perpendicular to the first refractor at and (Figure 11.6a). We use equation (4.24b) to get and :
These results are identical with those in problem 11.5. We verify the dip using these depths:
The first step in stripping is to plot the shallow refractor; we do this by swinging arcs with centers and and radii 1.07 and 0.53 km, the refractor being tangent to the two arcs. To get the “stripped” time values, we subtract the times down to and up from the first refractor, i.e., traveltimes along and for sources and . Although maximum accuracy would be achieved by stripping times for all geophones, the curves for the shallow refraction are so nearly linear that we calculate the stripped times only for each source and one intermediate point on each profile ( and ). We obtain the required distances by measuring the paths in Figure 11.6a. Calculation of the stripped times is given below. Path lengths: , , , km.
Stripping for refraction interpretation. The numbers below the zero-time line are distances from
Stripping off the first refractor in effect moves sources and down to and and geophones at and down to and , so the stripped times are plotted above these shifted points, the new traveltimes curves being and . Measurements on these stripped curves give the following:
Table 11.6a. Comparison of results of Adachi’s and stripping method.
|*Vertical depth measured at source A.
We now get
This dip is relative to , so the total dip is
Compare the solutions by stripping with those using Adachi’s method (problem 11.5).
To compare depths, we measured vertical depths below A. Results for the first layer are the same for both methods, those for the next layer are given in Table 11.6a.
What are some of the advantages and disadvantages of stripping?
Advantages of stripping:
- Easy to understand
- Straightforward in application
- Can be used with beds of different dips if the strike is the same
- As rapid as other methods when done graphically
- Can be used to interpret irregular or curved surfaces
- Very sensitive to velocity errors
- Like most methods, assumes the same strike for all refractors
- Difficult to apply when dips are steep
Refractors with the same strike but different dips.
Also in this chapter
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Refraction interpretation by stripping