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The refracted waves within them lose energy rapidly

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The refracted waves within them lose energy rapidly with increasing distance from the source and ultimately become undetectable. Much later events may then be picked as first arrivals, producing discontinuities in the time–distance plot. A similar effect is seen if the layer itself ends abruptly. Example 3.1 Field interpretation of a four-shot refraction spread with long shot (LS) and short shot (SS) arrivals from west (W) and east (E) ends plotted on same set of axes (Figure 3.8). After plotting the data, interpretation proceeds in the following stages. Stage 1 Base refractor intercept times Measure LS(W)–SS(W) time differences . These are roughly constant and close to 41 ms from G6 to G12, indicating that in this region the SS(W) arrivals have come from base refractor. Similarly, LS(E)–SS(E) time differences are close to 59 ms, from G1 to G4. Intercept times: LS(W) time at W end = 101 ms. Intercept time = 101 41 = 60 ms. LS(E) time at E end = 208 ms. Intercept time = 208 59 = 149 ms. Note the LS(E) difference from the extrapolated intercept time of about 170 ms.
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63 1 Figure 3.8 Time–distance plot for a four-shot refraction spread. The long shot difference times, indicated by open circles, are referred to a zero line arbitrarily placed at t = 280 ms 1 . Note the difference between the intercept time obtained by extrapolation of short-shot data and by using long-shot–short-shot difference times, for the G12 position. Extrapolation of the refracted arrival line back to zero time would be even more difficult for the G1 position and could lead to an even more erroneous interpretation. West is to the left of the plot (Example 3.1) . Stage 2 Velocities Direct-wave velocity: Straight line from W origin through nearby SS(W) arrivals extends 60 m to G4. Velocity V 1 = 60 / 0 . 079 = 759 ms 1 Straight line from E origin through nearby SS(E) arrivals extends 100 m to G7. Velocity V 1 = 100 / 0 . 134 = 746 ms 1 Average V 1 value = 750 ms 1 Intermediate refractor: Arrivals at G5 from SS(W) and at G5 and G6 from SS(E) do not belong to the ‘base refractor’ sets (see Stage 1) nor do they fall on the direct-wave arrival line, suggesting the presence of an intermediate, ‘ V 2’, refractor. The V 2 velocity is poorly controlled but the arrivals lines should pass above all direct wave ( V 1) and base refractor first arrivals. For the most likely positions, as shown;
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64 SS(W): V2 = 1470 ms 1 Intercept time = 29 ms SS(E): V2 = 1560 ms 1 Intercept time = 77 ms These velocities suggest that the interface is probably the water table, with a velocity of about 1500 ms 1. Base refractor velocity: Plot LS(W)–LS(E) time differences at each geophone, using a convenient (280 ms) line as time zero. V 3 = 2 /( slope of difference line ) = 2 × 220 / 00 . 182 = 2420 ms 1 Velocity functions: V 1 , 2 = V 1 × V 2 / (V 22 V 21 ) = 750 × 1500 / ( 15002 7502 ) = 870 ms 1 V 1 , 3 = V 1 × V 3 / (V 23 V 21 ) = 750 × 2420 / ( 24202 7502 ) = 790 ms 1 V 2 , 3 = V 2 × V 3 / (V 22 V 21 ) = 1500 × 2420 / ( 24202 15002 ) = 1910 ms 1 Stage 3 Depths at shot points Depths to intermediate refractor ( d 1= 12 t i V 1,2): W end: d 1 = 12× 0 . 029 × 870 = 12 . 6 m
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