On the Significance of Amplitude Studies in Shallow Refraction Seismics*

Abstract

An analysis of amplitudes of refraction records of some shallow refraction profiles shot primarily for detailing the near-surface structure in a granitic terrain has yielded information on refractor properties: reduced amplitudes are plotted on amplitude-distance graphs. The negative power n to which distance should be raised to represent (elastic) amplitude decay with respect to distance due to spreading of the critically refracted wave involved is examined. Computed values of this "spreading index" n are close to n = 2 as predicted by the theory.With this value of n, amplitude data are processed to determine residual attenuation attributable to elastic absorption in the bedrock. A graphical approach for this purpose from comparison of amplitudeedistance graphs with the plots of amplitude decay due to spreading which is applicable to flat and horizontal refractor situations is suggested. Assuming residual attenuation to represent absorption in the granite bedrock, the computed coefficients of absorption, which vary from 0.5 to 3.90 km-' for a frequency of 50 Hz, are obtained.From amplitude graphs of reversed profiles it is shown that the amplitude differences plot bears a relation to lateral velocity changes in the refractor. From comparison of practical amplitude decay graphs with those computed for different subsurface models, it appears possible to detect fractured rock occurrences in the refractor. I N T R O D U C T I O NCritically refracted waves are characterized by a dynamical range that is most conducive to amplitude studies on refraction records of routine surveys. These studies attain significance because the waves travel in the refractor over distances comparable with the shot-detector intervals. The nature of refracted wave amplitude decay with distance therefore depends on the physical and geological characteristics of the refracting rock formations. O'Brien (1967) has shown that from the magnitude of residual attenuation of these waves different types of refractors in sedimentary sections *

5. Refraction Surveys

“…For long offsets, where the distance traveled along the refractor exceeds several wavelengths, the head wave amplitude decreases by a factor close to the inverse square of the offset (e.g., Muskat, 1933;Heelan, 1953;O'Brien, 1955O'Brien, , 1957O'Brien, , 1967aKumar and Raghava, 1981). At smaller offsets, which is likely to be the case for many LVL surveys, the amplitude decrease with offset is slightly less (e.g., Levin and Ingram, 1962;Donato, 1964Donato, , 1965Hatherly, 1982a).…”

Attenuation of seismic refraction waves in sedimentary layers

“…For extent layer with constant velocity the decay in amplitude of the refracted wave is found from the following relation (Kumar & Raghava, 1981).…”