830 



EXPLORATION GEOPHYSICS 



Figure 512 shows the relative amplitude versus frequency of certain 

 components of the reflections from an area near Houston, Texas. The 

 incoming waves were recorded on 35 mm. film, and then played back 

 through the Mirragraph wave analyzing system whereby the amplitude 

 of each of the various frequency components could be measured. It is 

 interesting to note the greater energy at the lower frequency range, and 

 the gradual decrease in wave energy as the frequency increases. Of special 

 importance are the prominent frequency bands. Some of these peaks may 

 be partially instrumental, but the main bands are believed to be due to 

 structure and are characteristic of the area. 



10 20 30 40 



50 60 70 BO 90 100 110 120 130 140 150 160 170 ISO 190 200 

 FREQUENCY ~ CYCLES/ SECOND 



Fig. 512. — Relative amplitudes for different frequency components in the seismic 

 wave. (Courtesy of International Geophysics Company.) 



Much remains to be learned of the mechanism of energy travel from 

 the shot-point to the seismometers. The explosion itself occurs within a 

 few milliseconds, but the initial steepness of the wave front resulting from 

 the explosion is modified by the characteristics of the material immediately 

 surrounding the shot-point. The initial effect may be classified as a steep- 

 front compressional disturbance. As this disturbance travels through the 

 earth, its energy is partially reflected at various interfaces, some of it being 

 absorbed by creating oscillation of certain beds. The magnitude, frequency, 

 and persistence of these oscillations are governed by the physical properties 

 and size of the bed and its surrounding media. There is good experimental 

 evidence indicating that some beds vibrate at a characteristic frequency 

 which in some cases allows their identification. Advantage may be taken of 

 this phenomenon by frequency analysis of seismic waves recorded with true 

 fidelity equipment. 



