Subsurface Methods as Applied in Geophysics 



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potential of either natural or artifically created force fields. For example, 

 in the electrical-resistivity methods, the depth of penetration is dependent 

 upon the electrode spacing at the surface, whereas the depth of penetra- 

 tion of the reflection seismograph is in no way related to the interval 

 between the shot point and the detectors. In the magnetic and gravity 

 methods, the magnitude and distribution of the anomaly at the surface 

 are dependent on the size, the contrast of physical properties, and the 

 depth. Reflections may be obtained from great depths with about the same 

 ease that they are obtained from shallow depths. 



The seismic methods are similar to optics in regard to the physical 

 phenomenon involved, in that they both deal with a type of energy prop- 

 agated in the form of waves. Such physical quantities as velocity, fre- 



FiGURE 575. Schematic diagram of seismic-wave paths showing direct, refracted, and 

 reflected waves and recorded arrivals. In order to record direct waves distance 

 between shot point and detectors would be much less than depth to base of 

 weathered layer. 



