Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 915 



vibrator with revolving eccentric masses permits the forces to be stepped 

 up considerably. With sensitive detectors, vertical and horizontal ranges 

 extend to several hundred feet and, in some cases, to several thousand feet. 

 Furthermore, the measurement of travel-time and amplitude-distance 

 relations gives an opportunity to test formations not exposed at the surface. 

 Vibrations recorded near a variable frequency vibrator indicate a number 

 of parameters not obtainable by other means, such as natural frequency, 

 damping, energy consumption, ground compaction, and phase shift between 

 impressed force and ground vibrations. 



Vibrators maj'^ be applied in testing not only the ground but structures 

 as well. As a matter of fact, vibrators were constructed first for use in 

 structural engineering and for the dynamic investigation of bridges, trusses, 

 framework, conveyors, skyscrapers, shaft houses, cranes, dams, and so 

 on. Such tests were extended to include all kinds of conveyances such as 

 ships, trucks, automobiles, locomotives, dirigibles, and airplanes. Inves- 

 tigations of road beds and machine foundations, made in connection with 

 structural tests, led quite naturally to their application in soil testing.^* 



For some of these applications an agitator made of a bicycle wheel with 

 a weight attached to its rim is quite satisfactory. This wheel is brought 

 up to speed and allowed to run down. For more accurate work, it is better 

 to apply a vibrator whose frequency is held constant, yet is adjustable in 

 definite intervals. The usual construction employs two eccentrically 

 loaded wheels or drums that are geared together and revolve in opposition. 

 If the two cylinders are mounted side by side, the vertical components of 

 their centrifugal forces will add and the horizontal components will cancel. 

 By turning the entire assembly 90 degrees (so that the two cylinders are 

 now above each other) the vertical components cancel and the horizontal 

 components remain. For the agitation of structures, the vibrator is ap- 

 plied in the latter position, since vibration damage to structures results 

 generally from horizontal forces. In soil testing, the vibrator is laid fiat 

 on the ground (so that vertical forces are produced) and is weighed down 

 with ballast. In some vibrators, four disks are arranged with their axes 

 in the four principal horizontal directions. The disks are then geared to 

 produce torsional forces. 



The total eccentric masses of vibrators range from a few ounces to several 

 pounds and the radius arm from a few inches to about one foot. Centrif- 

 ugal forces may thus be varied between several hundred and several 

 thousand pounds. Some vibrators are so constructed that a given centrif- 

 ugal force may be produced by a small mass on a large radius arm, or by 

 a larger mass on a smaller arm. The simplest arrangement is to provide a 



^8 Koehler, Nachr. Ges. Wiss. Goettingen, loc. cit. 



