Comparison of refraction interpretation methods
The data in Table 11.14a show refraction traveltimes for geophones spaced 400 m a part between sources and which are separated by 12 km. The columns in the table headed and give second arrivals.
Interpret the data using the basic refraction equations (4.24a) to (4.24f).
The data are plotted in Figure 11.14a and best-fit lines suggest that this is a two-layer problem. Measurements give the following values:
Equation (4.24d) gives
From equation (4.24f), we have km/s. From equation (4.24b) we get for the slant depths,
Checking the values of and , we obtain
Interpret the data using Tarrant’s method.
Tarrant’s method (Tarrant, 1956) uses delay times (problem 11.8) to locate the point [see Figure 11.14b(i)] where the refracted energy that arrives at geophone leaves the refractor. The refractor is defined by finding for a series of geophone positions. Tarrant’s method is based on the properties of the ellipse.
The delay time for the path in Figure 11.14b(i) is . Solving for , we get
This is the polar equation of an ellipse. An ellipse is traced out by a point moving so that the ratio of the distance from a straight line (directrix) to that from a fixed point ( in Figure 11.14b(ii) is a constant (eccentricity)).
The standard polar equation of an ellipse is
In Figure 11.14b(ii) moves so that the ratio . The major axis 2a of the ellipse is
To get the minor axis, we set the first derivative of equal to zero. Using equation (11.14b), we find that
The distance from the focal point to the center is
If we substitute , and in equation (11.14b), we get equation (11.14a). Also these values give the following results.
To approximate the ellipse in the vicinity of with a circle, we need to find the center of curvature of the ellipse. The general equation of an ellipse is
The equation for the radius of curvature of a function is
Differentiating, we obtain
The center of curvature is a distance above , so the -coordinate of [Figure 11.14b(iii)] is . A circle with center and radius will approximate the ellipse in the vicinity of .
We have from part (a): km/s, km/s, ; from equation(11.9b), we get s, s. For a geophone at a distance from , equation (11.8b) gives for source ,
and for source ,
(note that is measured from for and from for ).
We can obtain values of either by using the above equations or graphically by drawing straight lines with slope starting at the half-intercept values (there by subtracting it); the vertical distances between these lines and the traveltime curves give . The values of in Tables 11.14b,c were calculated.
The last step is to find the center of curvature in Figure 11.14b(iii) and to draw an arc with radius . We first calculate and then find by calculating or and drawing a line normal to . The first method was used to get Tables 11.14b,c (although is given in the tables, it was not used). We repeat equations (11.14f,g,h) and get
The calculations are shown in Tables 11.14b,c. The quantity in Table 11.14b and in Table 11.14c. Because complete reversed profiles were obtained, there is considerable duplication of the values of and . Rather than plot all of the arcs, we used the average values of and (calculated in Table 11.14d). The results are shown in Figure 11.14c.
Interpret the data in Table 11.14a using the wavefront method illustrated in Figure 11.14d
In Figure 11.14d(i) and are two wavefronts generated at sources and , respectively, and meeting at C. Clearly, . If wavefronts and continue upward to the surface at velocity and are recorded, we can project them backwards using the method shown in Figure 11.14d(ii). Point where they meet locates a point on the refractor. This is the basis of the wavefront method.
The earliest refracted wavefront from that we can reconstruct is at 1.60 s (using the best-fit line to get for ) and the last is about 3.00 s; the corresponding limits for source are 1.10 and 3.00 s. We take s and draw wavefronts such that . We reconstruct the four wavefront pairs , (1.80, 1.51), (2.00, 1.31), (2.20, 1.11) using km/s [from part (a)].
We swing arcs from points on the -axis as in Figure 11.14d(ii). Because we are interested only in the portions near the points of intersection, we determine the refractor depth. Using the intercept times at the sources in part (a) to obtain slant depths, we get km, km; multiplying by to get vertical depths; the maximum vertical depth is about 1.40 km.
The radius of the arcs is given by . The maximum value of is about but, to be on the safe side, we calculate for values of ranging from 0.45 to 0.75 s for the pair (1.60, 1.71), then adjust the range as necessary to achieve wavefront intersections for the other three pairs.
|Wavefront: (A-1.600)||Wavefront: (B-1.710)|
|Wavefront: (A-1.800)||Wavefront: (B-1.510)|
|Wavefront: (A-2.000)||Wavefront: (B-1.310)|
|Wavefront: (A-2.200)||Wavefront: (B-1.110)|
The calculations are shown in Table 11.14e. Columns 1 and 2 in each sub-table come from table 11.14a. Column 3 is the difference between or and the wavefront traveltime shown above each subtable; the radius .
Figure 11.14e shows the result for the wavefront pair (1.60, 1.71). We leave to the reader the construction of the other three pairs of wavefronts and determination of the points of intersection. Our results are shown by the four small squares in Figure 11.14c and, more clearly, in Figure 11.14g.
Interpret the data in Table 11.14a using Hales’s method
Hales’s (1958) method is a graphical method based on reversed profiles. It enables us to find in Figure 11.14f(i) in terms of data recorded at geophones and . The circumscribed circle through points , and is shown in Figure 11.14f(iv), all of the angles inside the circle being expressible in terms of and .
Point is on the perpendicular bisector of ; writing , we have
Adding the two expressions, we get
Also we have
where reciprocal time (see problem 11.13). Thus we have
using equation (11.14g). Using the law of sines and equation (11.14i) we get
The traveltime curves are shown in Figure 11.14f(ii). We draw the vertical through point on the -axis and locate the point so that . Assuming , we draw a line at at the angle to meet the other traveltime curve. From equations (11.14h,j,k) we see that the line from at the angle must intersect the traveltime curve for source at time , which locates the point and gives the values of , and, . At point on the -axis we draw a line at angle to the horizontal [see Figure 11.14f(iii)] and locate point vertically above the midpoint of , i.e., at the distance from ; With center and radius . [see equation (11.14i)], we draw an arc. Repeating this process for a series of points S, we obtain several arcs to define the refracting surface.
When , and change, but it can be shown (see Sheriff and Geldart, 1995, 444) that the change in is negligible and the only effect of dip is to displace the point updip the distance [Figure 11.14e(v)], which is usually negligible for moderate dips.
From part (a) we have: km/s, km/s, , the reciprocal time s.
Figure 11.14g shows details of the solution. We select points at intervals of 0.80 km, then find and the points [shown as small triangles () in Figure 11.14g(i)]. Points in Figure 11.14f(i) are found by laying off at the angle [see equation (11.14s)]; to take into account the scale factors, we write (2.22/2.00), then draw a vertical line equal to 2.00 s, then a lineto the left equivalent to 2.22 km [see lines going up at about from the vertical from the triangles in Figure 11.14g(i)]. This gives the locations of points and values of , and ; we can now calculate from equation (11.14g). The calculations are shownin Table 11.14f.
Finally we locate as in Figure 11.14g(ii), with as center and radius we draw arcs which pass through point in Figure 11.14f(v). The refractor is fairly well defined by the arcs (except for two arcs marked with?).
On the basis of your results, compare the methods in terms of (1) time involved; (2) effect of refractor curvature; (3) effect of random errors; (4) suitability for routine production; and (5) for special effort where high accuracy is essential.
- Time involved: The formula method is by far the quickest, the wavefront method is next, and Tarrant’s and Hales’s methods are the most time consuming, being more-or-less the same in this respect.
- Effect of refractor curvature: The formula method does not take curvature into account (except on a broad scale over two or more profiles). The remaining methods all work well for curved refractors. Tarrant’s and Hales’s methods give good results over the commonly observed angle of curvature, while the wavefront method is useable over a smaller range of curvatures.
- Effect of refractor random errors: These errors affect the measured slopes and intercepts and therefore affect the formula solution; however, the effects are usually minimized by the use of best-fit lines which utilize most of the available data. Tarrant’s and Hales’s methods use and and the wavefront method uses , so that errors affect all three methods. In addition Tarrant’s and Hales’s methods use two traveltimes in each calculation so that random errors cause further errors. The wavefront method is less susceptible to this type of error because several time values enter into each wavefront determination.
- Suitability for routine production: The formula method, being the quickest, is satisfactory where refractor relief is minimal. Tarrant’s and Hales’s methods are slightly less suitable, and the wavefront method is least suitable.
- Suitability for high accuracy: The formula method is unsuitable, the wavefront method the best, Tarrant’s and Hales’s methods being almost as good.
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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