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Wave train are likely to be stronger and it is

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wave train are likely to be stronger, and it is sometimes possible to work back from these to estimate the position of the first break. However, because high frequencies are selectively absorbed in the ground, the distance between the first break and any later peak gradually increases with increasing distance from the source. Furthermore, the trace beyond the first break is affected by many other arrivals as well as by later parts of the primary wave train, and these will modify peak and trough locations. Using later features to estimate first-arrival times should always be regarded as a poor substitute for direct picking. Figure 3.2 Portion of a multi-channel refraction record, with first-break ‘picks’ identified by arrows. These become more difficult to make as signal to- noise ratio worsens. This record would be considered good, and much more difficult decisions usually have to be made . 3.1.8 Time–distance plots The data extracted from a refraction survey consist of sets of times (usually first-arrival times) measured at geophones at various distances from the source positions. Since these are plotted against vertical time axes and horizontal distance axes, the gradient of any line is equal to the reciprocal of a velocity, i.e.
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50 steep slopes correspond to slow velocities. All the data for a spread are plotted on a single sheet that has a working area covering only the ground where there are actually geophones (see Figure 3.8 accompanying Example 3.1). It is not necessary to show the long-shot positions. Since as many as five sets of arrivals may have to be plotted, as well as a set of time differences, different colours or symbols are needed to distinguish between data sets. If the arrival times lie on a number of clearly defined straight-line segments, best-fit lines may be drawn. These are not actually necessary if the intercept-time interpretation method described below is used, and will be difficult to define if the arrival times are irregular because of variations in refractor depth. It is often best to draw lines through only the direct-wave arrivals (which should plot on straight lines), leaving refracted arrivals either un-joined or linked only by faint lines between adjacent points. 4.0 Conclusion In reflection surveying, the detecting instruments record seismic signals at a distance from the shot point that is large compared with the depth of the horizon to be mapped. The seismic waves must thus travel large horizontal distances through the earth, and the times required for the travel at various source- receiver distances give information on the velocities and depths of the subsurface formations along which they propagate. Although the refraction method does not give as much information or as precise and unambiguous a structural picture as reflection, it provides data on the velocity of the refracting beds. The method made it possible to cover a given area more quickly and economically than with the reflection method, though with a significant loss of detail and accuracy. Despite its disadvantages, refraction is now rarely employed in oil exploration
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