928 



EXPLORATION GEOPHYSICS 



determined by measuring the time phase difference of two corresponding 

 maxima or minima, the velocity being obtained from the relationship : 



X2 — X\ 



where Tx and T2 are the times for corresponding maxima or minima on 

 the traces and x-^ and a'o are the distances from the seismometers to the 

 oscillator. The correspondence between wave velocity and allowable soil 

 pressure is also shown in Table 25. 



Oscillators of this type may also be employed for shallow structural 

 investigations to determine depths to bedrock, dip of contact, etc., by the 

 techniques utilized in general refraction studies. 



TABLE 25 



CORRESPONDENCE BETWEEN WAVE VELOCITY, NATURAL 

 FREQUENCY AND ALLOWABLE SOIL PRESSURE t 



Test 

 No. 



Formation of Soil 



1 10 ft. marshland on sand 



2 Very fine sand 



3 Tertiary clay, moist 



4 Clay sand 



5 Medium sand, moist 



6 Jura clay, moist 



7 Old sand and cinder 



8 Medium sand in water 



9 Medium sand, dry 



10 Argillaceous sand 



1 1 Gravel with stones 



12 Clay, moist 



13 Marl boulder 



14 Fine sand with 30 per cent medium sand 



15 Clay sand with lime inclusions 



16 Medium sand, undisturbed 



17 Marl 



18 Keuper sandstone, soft 



19 Diluvial loess, dry 



20 Gravel under 12 ft. sand cover 



21 Gravel, dense 



22 Sandstone, disintegrated 



23 Keuper sandstone, medium hardness. . . . 



24 Sandstone, undisturbed 



Investigations of Pressures on Rock Pillars of Underground Mines. — A sonic 



method for determining the pressure on pillars in underground mines has been pro- 

 posed which requires, first, laboratory determinations of the variation of the velocity 

 of sound with pressure in samples of rocks taken from mine pillars and, second, 

 t Bernhard, A.I.M.E. Geophysical Prospecting, Tech. Pub. 834. 



