Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



923 



may, therefore, be determined by running two tests with different 

 amounts of ballast. 



Although the more important soil characteristics may be obtained from 

 measurements at the vibrator as described, it is advantageous to supple- 

 ment these measurements with observations at a number of points a short 

 distance away. The additional information gained thereby is, chiefly, the 

 variation of amplitude with distance and the velocity of the elastic waves 

 transmitted by the vibrator. Since such measurements involve a deter- 

 mination of the variation of amplitude with frequency, it is now the more 

 common procedure to omit measurements at the vibrator and to confine 

 the observations to points generally arranged in a straight line through 

 th e vibrator. Several such profiles may be made, radiating from the loca- 

 tion of the source. 



HeUand Research Corp. 



Fig. 12-19. Record of ground response test. 



Measuring the variation of amplitude with frequency involves the same 

 technique as previously described, with the exception that now as many 

 amplitude-frequency curves are plotted as there are detectors. Fig. 12-19 

 shows a typical record taken at constant frequency, and Fig. 12-20 is a 

 graph giving the amplitudes as functions of frequency for the distances 

 involved. From such curves the variation of amplitude with distance may 

 be plotted, as is seen in Fig. 12-21. In this case the resonance amplitude 

 was chosen for the graph; however, amplitudes at different frequencies 

 may be plotted as well, since the variation of amplitude with distance is 

 dependent on frequency, as shown below. 



In a homogeneous medium, the vibration amplitude decreases inversely 

 with distance for space waves, and inversely with the square of the dis- 

 tance for surface waves. Further, there is an exponential decrease due to 



