Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 963 



been made to locate ground-water levels and bedrock surfaces in this 

 manner, but they have not been wholly successful. This is because the 

 sound rays may be diverted in a quite unpredictable manner by the in- 

 tervention of different formations. This is particularly true of the near- 

 surface weathered layer which deflects the return ray into an almost verti- 

 cal direction, thus virtually obliterating any significant variations in the 

 direction of the reflected ray. A measurement of reflection travel time 

 would undoubtedly obviate the difficulties mentioned, since it is then pos- 

 sible to correct for the low-velocity surface layer. 



Unquestionably one of the reasons for the lack of progress in geoacoustic- 

 reflection methods is the superiority of explosion-generated impulses as 

 used in seismic reflection procedure over audio-frequency impulses. As 

 far as principle is concerned, there is, of course, very little difference be- 

 tween acoustic echo-sounding and seismic-reflection methods. The dif- 

 ference is primarily one of frequency. As a matter of fact, the use of 

 low-frequency sustained oscillations (as applied in dynamic soil testing) 

 has been proposed for the location of shallow formations, since the seismic- 

 reflection method, at present, cannot be used successfully for that purpose. 

 In practice, this hmitation is not too serious, inasmuch as shallow forma- 

 tions may be readily mapped by the seismic-refraction method. 



