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The variation with distance of the amplitude of critically refracted wavesNormal access

Author: P. N. S. O‘brien
Journal name: Geophysical Prospecting
Issue: Vol 5, No 3, September 1957 pp. 300 - 316
DOI: 10.1111/j.1365-2478.1957.tb01435.x
Organisations: Wiley
Language: English
Info: Article, PDF ( 1.93Mb )

Summary:
Amplitude measurements have been made of the height of the first peak of an arrival refracted from a shallow refractor. If the amplitude is assumed to decay as the inverse mth power of the distance, the least squares value for m is found to be 2.16 ± .04. Because of this value and because of the character of the recorded event it is concluded that the arrival is a simple critical refraction. After applying the theoretical ‘spread’ factor for critical refraction there remains a residual attenuation of 1.96 ± 0.28 decibels per 1000 feet. The predominant frequency in the pulse is about 20 c.p.s. and this attenuation agrees with the losses found for such a frequency by extrapolation of the published results of other workers. Although no evidence could be seen on the records for a change of pulse frequency with distance, the quoted result would be consistent with a dependence of residual attenuation on the first power of the frequency, and would be inconsistent with a dependence on the second power of the frequency.

It is concluded that studies of the amplitudes of refracted events will give useful estimates of the attenuation factors of rocks.


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