Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 917 



For testing models of buildings, bridges, dams, and the like, miniature 

 vibrators or electromagnetic drivers are applied. If size permits, an en- 

 tire model may be tested on a shaking table. 



Vibrations may be recorded with a variety of instruments. For testing 

 larger structures and buildings, mechanical vibrographs are generally 

 quite satisfactory. Their magnifications range from 200 in the simpler 

 types to 1500 in the Wood- Anderson seismograph. For model experi- 

 ments, miniature microphones of the electromagnetic or piezoelectric 

 variety are required. 



In dynamic soil testing, the sensitivity of the mechanical vibrograph is 

 not sufl&cient, and more delicate seismographs must be used, equipped 

 with mechanical, optical, or electrical magnification, ranging from 10,000 

 to 50,000 times.^^ The ground vibrations may be recorded in three direc- 

 tions, one being the vertical and the others the two horizontal directions 

 through the station and at right angles to the base line. Although a 

 vibrator may produce only vertical forces, the horizontal components are 

 quite noticeable. Since, however, the maximum response in the horizontal 

 components appears to occur at the same frequency as in the vertical com- 

 ponent, vertical seismographs usually suffice. If a multiplicity of seis- 

 mographs is on hand, it is better to use them at different distances rather 

 than for different components. Vertical seismographs or detectors with 

 electrical transducers are available in any event where seismic refraction 

 work is combined with the dynamic tests. Lastly, it is easier to evaluate 

 the results when vibrations from several stations are recorded on one film 

 (see Fig. 12-19). In order to convert record amplitude to ground ampli- 

 tude in terms of impressed force, the recording channels must be calibrated. 

 With the larger vibrators, the regular phones used in seismic refraction or 

 reflection equipment work satisfactorily without ampUfiers. Where a 

 large number of soil tests have to be made, it may be more convenient to 

 record with (calibrated) amplifiers and to cut down the weight of the 

 vibrator to make it more portable. 



When the variation of vibration amplitude with depth is measured, a 

 hole is dug with a hand auger, and a vibration pickup is placed at its 

 bottom at the desired depth or depths. In some cases, steel rods are 

 driven into the bottom of the hole, to which the detectors are fastened. 



1. Dynamic building tests. To test buildings or structures for earth- 

 quake or other vibration damage, a vibrator is set up on, or clamped to, 

 the actual structure or to a model thereof (model structures require a very 

 light vibrator, see above). Usually, the vibrator is so oriented that hori- 

 zontal forces are produced. The frequency of the vibrator is then varied 



R. Koehler and A. Ramspeck, Zeit. Tech. Phys., 14(11) » 512-514 (1933). 



