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In the computation of time-depths and velocity analysis functions, the reciprocal time, the time from shotpoint to shotpoint, is required. However, of all the times determined in a seismic refraction profile, the reciprocal time is the most difficult to determine accurately. There are several reasons for this.

First, the reciprocal geophone is the most distant from the shotpoint. It receives the least amount of energy because head waves are attenuated approximately as the inverse square of the distance (Grant and West, 1965, p. 181). Furthermore, the earth acts as a low-pass filter, with the higher frequencies being attenuated morer apidly than lower frequencies (Attewell and Ramama, 1966). Both of these factors result in arrivals at distant geophones having onsets on the seismic record less distinct than those of closer geophone traces.

Second, excessive shotpoint offsets may make the planting of a geophone at the reverse shotpoint impractical. An arrangement employed by the Bureau of Mineral Resources in Australia for many years uses the shot firing cable to transmit the signal from a geophone at the reverse shotpoint, when it is not required for shot firing (Hawkins, 1961, p. 811). However, for large shotpoint offsets, this system may be inconvenient.

Third, it is possible that, even with a reciprocal geophone in place, the first arrival is from another deeper refractor.

Finally, disturbed ground caused by previous shotpoints may result in unknown significant irreversible delays, as shown by Domzalski (1956, p. 145). Although geophones are usually planted away from earlier shotpoints, the region of the disturbed

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