CuAP. 12J MISCELLANEOUS GEOPHYSICAL METHODS 911 



damage should be considered not in the Hght of static, but of dynamic, 

 phenomena. Specifically, the damage depends on the frequency response 

 of a structure and thei'efore on its interaction with the ground, whose re- 

 sponse to earthquakes and industrial vibrations, in turn, is likewise a func- 

 tion of its fi'equency characteristics. Damage is therefore primarily a 

 local phenomenon, depending individually on the design of the structure 

 and the surface geologic conditions. It is the function of vibration- 

 recording and dynamic-testing methods to determine the frequency re- 

 sponse of both structure and groiuid and to devise means by which reso- 

 nance between the two can be avoided. 



In both cases, the significant characteristics of ground and structure are 

 natural frequency and damping. They may be determined by observa- 

 tions of (1) free vibrations, and (2) forced vibrations. In the first case, 

 the ground or building is subjected to a static deflection and is released. 

 From the free vibration that follows, natural frequency and damping may 

 be calculated. Deflections leading to free vibrations may be due to 

 natural causes, such as wind, or they may be produced at will by blasts, 

 mechanical shocks, and so on. To obtain the vibration characteristics of 

 buildings before construction, it is convenient to use models. Methods of 

 measuiing free vibrations are briefly referred to here as vibration recording. 



In dynamic investigations, the building or ground under test is set into 

 forced oscillation by vibrators (also called oscillators or agitators) whose 

 frequency is varied during the experiment. Thus the frequency (or 

 dynamic) response of the structure is obtained, from which natural fre- 

 quency and damping may be calculated. For proposed structures, similar 

 tests are made on models before construction. Dynamic ground tests 

 furnish the frequency response of the foundation and thus, indirectly, its 

 bearing capacity and vibration absorption characteristics. Since the 

 speeds of the sustained waves produced by a vibrator may be determined 

 from measurements in various distances, it is possible to arrive at depths 

 of formation members and their elastic moduli. For complex geologic 

 sections whose geometric dimensions and elastic characteristics have 

 been determined previously by other geophysical methods, it may be con- 

 venient to supplement the in situ work by model experiments, particularly 

 if the natural setup is likely to be disturbed by later excavations. 



Strain gauging, discussed at the end of this section, is concerned with 

 the measurement of displacements in structures and rocks under the in- 

 fluence of natural or artificial loads, static or transient. Structural tests 

 are made on bridges, road beds, dams, and the like, after completion, or 

 on models. Strains in rocks are measured in connection with luider- 

 ground operations in transportation, hydraulic, and mining enghieering. 

 Closely related to this application is the observation of rock bursts and 

 fault activity. 



