PHY 361 GEOPHYSICS II

# Hawkins lv 1961 the reciprocal method of routine

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Hawkins, L.V. (1961) The reciprocal method of routine shallow seismic refraction investigations. Geophysics, 26 , 806–19.

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53 UNIT 2 FIELD INTERPRETATION 1.0 Introduction Interpretation is an essential part of refraction fieldwork because the success of a survey depends on parameters such as line orientation, geophone spacing, shot positions and spread lengths, that can be varied almost at will. Only if analysis keeps pace with data collection will the right choices be made. Field interpretation has been made easier by computer programs that can be implemented on portable PCs or on the seismographs themselves but such programs are based on very simple models and are no substitute for actually thinking about the data. 2.0 Objectives At the end of the unit, readers should be able to (i) Give a descriptive interpretations of the more important field aspects (ii) show prominent practical interpretations of geophysical methods . (iii) Practicalised seismic refraction techniques. 3.0 Main Contents 3.1.1 Intercept times The intercept time t i is defined as the time at which the back-extrapolated refracted arrival line cuts the time axis. For a single refractor, it is related to the velocities and the refractor depth by the equation: The quantity V 1 , 2 is defined by this equation. It has the units of a velocity and is approximately equal to V 1 if V 2 is very much larger than V 1. The critical angle is then almost 90 and the delay suffered by the refracted ray in travelling between the surface and the refractor is close to double the vertical travel time. If the difference between V 1 and V 2 is small, V 1 , 2 can be very large. Intercept times are conventionally obtained by drawing best-fit lines through the refracted arrival times but even a very good fit is no guarantee that the depth of the refractor does not change in the region near the shot point, from which no
54 refractions are observed. If, however, a long shot is used, there should be a constant difference between long-shot and short-shot arrival times at points towards the far end of the spread (Figure 3.3). An intercept time can then be obtained by subtracting this difference from the long-shot arrival time at the Figure 3.3 Long-shot and short-shot travel paths for a three-layer case. The paths for energy travelling to the geophones from S 1 and S 2 via the lower refractor are identical from point B onwards. Upper refractor paths have been omitted for clarity . short-shot location, and this can be done exactly if there is a geophone in this position when the long shot is fired (Example 3.1). Otherwise, use of the nearest long-shot arrival at least reduces the distance over which extrapolation must be made. 3.2 Multiple layers The intercept-time equation can be extended to cases involving a number of critically refracting layers. If the intercept time associated with the n th refractor is tn , then: tn = 2 d 1 /V 1 ,n +1 + 2 d 2 /V 2 ,n +1 + 2 dn/Vn,n +1 where dn is the thickness of the n th layer, which overlies the n th refracting

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