Arrives later at b than at a not only because of the

Info icon This preview shows pages 56–58. Sign up to view the full content.

View Full Document Right Arrow Icon
arrives later at B than at A not only because of the greater distance travelled along the refractor but also because of the extra distance d1 travelled in the low velocity layer. Energy from S 2 arrives earlier at P than would be predicted from the time of arrival at Q, by the time taken to travel d2 at velocity V1 . The lines AC and PR are parallel to the refractor . However, both waves would arrive late at this point (Figure 3.5) and, for small dips, the delays would be very similar. The difference between the arrival times would thus be almost the same as if no depression existed, plotting on the straight line generated by the horizontal parts of the interface. The argument can be extended to a refractor with a series of depressions and highs. Provided that the dip angles are low, the difference points will plot along a straight line with slope corresponding to half the refractor velocity. Where a refractor has a constant dip, the slope of the difference line will equal to the dip velocity equation. Figure 3.5 Effect on travel times of a bedrock depression. The arrivals at G 3 of energy from S 1 and S 2 are delayed by approximately the same amounts ‘a’
Image of page 56

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
57 and ‘b’). Note that, as in all the diagrams in this chapter, vertical exaggeration has been used to clarify travel paths. The lower version gives a more realistic picture of the likely relationships between geophone spacing and refractor depths and gradients . the sum of the slopes of the individual lines, giving a graphical expression The approach described above generally works far better than the very qualitative ‘proof’ (and the rather contrived positioning of the geophones in Figure 3.5) might suggest. Changes in slopes of difference lines correspond to real changes in refractor velocity, so that zones of weak bedrock can be identified. The importance of long shots is obvious, since the part of the spread over which the first arrivals from the short shots at both ends have come via the refractor is likely to be rather short and may not even exist. It is even sometimes possible, especially when centre shots have been used, for the differencing technique to be applied to an intermediate refractor. Differences are easily obtained directly from the plot using dividers, or a pencil and a straight- edged piece of paper. They are plotted using an arbitrary time zero line placed where it will cause the least confusion with other data (see Figure 3.8). 3.5 Reciprocal time interpretation The reciprocal time, t R, is defined as the time taken for seismic energy to travel between the two long-shot positions. The difference between t R and the sum of the travel times t A and t B from the two long-shots to any given geophone, G, is: t A + t B t R = 2 D/ F where D is the depth of the refractor beneath G and F is a depth conversion factor (Figure 3.6). If there is only a single interface, D is equal to the Figure 3.6 Reciprocal time interpretation. The sum of the travel times from S 1 and S 2 to G differs from the reciprocal time, tR , taken to travel from S 1 to S 2 by
Image of page 57
Image of page 58
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern