642 EXPLORATION GEOPHYSICS 



The reflected longitudinal waves are recognized as subsequent events on 

 the seismograms. 



As in earthquake seismology, two quantities are usually measured in 

 applied seismology, i.e., time and distance. Amplitude and frequency char- 

 acteristics are used in a qualitative manner in the correlation of events on 

 various seismograms. In natural earthquake studies, the amplitude and 

 frequencies are used to some extent in a quantitative manner as a means 

 of recognition of various wave phases recorded. Velocities, depths of 

 penetration, and space wave paths may be computed from the measured 

 times and distances. The penetration, wave paths, and velocities depend 

 upon the contrast in elastic properties of the adjacent geologic media 

 through which the waves pass. In this manner, inferences may be made 

 as to the distribution and attitude of subterranean geological units. These 

 may be interpreted in terms of geological structure or the presence of 

 specific masses distinguished from the surrounding media by the contrast 

 of their elastic properties. 



Seismology 



Early instruments are illustrated in Figures 398 and 399. These 

 illustrations show typical earthquake seismographs (instruments for 

 detecting and recording the arrival of earthquake waves.)* Figure 398 

 shows a vertical seismograph which is critically damped by electromag- 

 netic means and employs electrical recording. This seismograph, con- 

 structed by Galitzin in 1908, differs only in arrangement and size of parts 

 from the typical seismograph employed in exploration at present.** 



Most modern seismographs are designed to record the seismogram on 

 a moving strip of photographic paper. Oscillatory currents are generated 

 by the relative motion between the seismometer frame and the inertia reactor. 

 Figures 400, 401, 402, and 403 are illustrations of modern stationary 

 earthquake-recording seismograph equipment. This equipment represents 

 an improvement in both mechanical stability and fidelity in recording the 

 actual motion of the ground as compared with earlier models. An illustra- 

 tion of the push-pull principle of the variable reluctance transducer as used 

 in these modern instruments is given by Figure 404. The amplitude of the 

 trace on the photographic record is ordinarily about 200 times greater than 



* Descriptions of seismological instruments and records commonly contain the 

 following technical terms : seismometer, seismograph, and seismogram. A seis- 

 mometer is a device for measuring vibrations due to earthquakes. A seismograph is 

 a seismometer equipped with an indicator (mechanical, optical, or electrical) for 

 recording a quantity which depends on the vibration or relative motion of the instru- 

 ment. The quantity recorded is usually a function of the displacement, velocity, or 

 acceleration. A seismogram is the record, for example photographic record, made by 

 a seismograph. 



** A detailed description of the principal types of seismographs employed in 

 seismological observatories is given in "Selection, Installation, and Operation of 

 Seismographs" by H. E. McComb, U. S. Department of Commerce, Special Pub. 

 No. 206. 



