Subsurface Methods as Applied in Geophysics 1091 



wave fronts. This factor is known as "geometric divergence" or "spread- 

 ing." Scattering of energy occurs as the seismic wave front advances 

 because of reflection and refraction of energy on prominent irregularities 

 in the medium. It has been noted that scattering accentuates the low- 

 frequency content of waves and attentuates the high frequencies as the 

 wave propagates. The effects of dispersion of seismic energy in prospect- 

 ing seismology are not very well known. It has been observed that in 

 stratified ground there is a change of velocity with frequency, which is 

 not observed in homogeneous or nearly homogeneous ground. 



Energy absorption is one of the losses accompanying the decrease of 

 seismic intensity with distance. The damping properties of the materials 

 through which the waves are propagating largely determine the rate of 

 attenuation of the energy. Because the attenuation factor increases with 

 frequency increase, low frequencies are accentuated and high frequencies 

 attenuated. Born **^ has shown that an increase in the moisture content of 

 consolidated rocks also increases the attenuation at high frequencies, in 

 that the damping properties of the rocks are increased. The propagation 

 of waves may also be affected by the dissipation factor, which determines 

 the ability of a material to sustain vibrations and depends upon the in- 

 ternal resistance to elongations and contractions. From this brief dis- 

 cussion it may be seen that a number of factors are acting to attenuate 

 seismic energy, several of which are discriminatory to the higher fre- 

 quencies. Several of the factors affecting the propagation of seismic 

 waves are discussed by Clewell and Simon.'*^ They show that although 

 the earth acts as a low-pass filter for refracted energy, it acts as a band- 

 pass filter for reflected energy. The low-frequency "cut-off point" is in 

 large part determined by the thickness of the reflection horizon. 



Methods of Prospecting 



Prospecting with the seismograph may be divided into the refraction 

 technique, in which travel-time data are obtained for the elastic waves 

 that have been refracted at boundaries separating material of different 

 elastic properties, and the reflection method, in which time data are ob- 

 tained for those elastic waves reflected from subsurface boundaries. 

 Several arrangements of shot points and detectors are employed in the re- 

 fraction technique, depending upon the purpose of the survey and local 

 conditions. The fan-shooting method resulted in the discovery of a large 

 number of the shallower Gulf Coast salt domes in the 1920's and has 

 been employed recently in the search for favorable structure off-shore. 

 The method consists in comparing the travel times from a single shot 

 point to a number of detectors placed at approximately equal distances 

 from the shot point and arranged in the form of a fan. Rays traveling 



^* Bom, W.. T., The Attenuation Constant of Earth Materials: Geophysics, vol. 6 no 2 p 138 

 Apr. 1941. .•.»■• 



^'Clewell, D. H., and Simon, R. F., Seismic Wave Propagation: Geophysics, vol. 15, no 1 pp 50-60 

 Jan. 1950. * 



