Ch. 4] EXPLORATION GEOPHYSICS 79 



Small explosions at the surface are recorded by the well detector at 

 various depths so that individual formation velocities can be deter- 

 mined. It will be found that the formation velocities offer valuable 

 guidance to the identification of the beds, becoming, thereby, a veloc- 

 ity signature. 



This property is used in the refraction seismic method which maps 

 only the major velocity horizons. The method is useful in large-scale 

 reconnaissance surveys where fine detail is not required. It is also 

 adaptable to small-scale problems such as the measurement of the 

 thickness of soil and overburden, the depth to bedrock, and the dis- 

 tribution of these quantities over sites for buildings and public works 

 construction, highways, reservoirs and storage basins, and in the 

 search for new supplies of ground water. 



In seismology for petroleum exploration, the general principles of 

 earthquake seismology are employed in a restricted degree. It is 

 sufficient to note that the velocity of sound in rocks is the variable 

 parameter upon which the success of the seismic methods depends. 

 Their use is restricted to the delineation of structure and stratigraphy 

 in rocks. If these factors are systematically related to the occur- 

 rence of oil and gas and of ore deposits, the method may achieve 

 economically useful results. There is no implication that an accurate 

 seismic survey directly indicates the occurrence of valuable treasure. 

 This inference must be based upon other considerations, principally 

 geological. The use of the seismograph is comparable with the use 

 of the transit in surface surveying. Both collect numerical data which 

 may be contoured to show relative relief over the area surveyed. Both 

 methods are precision forms of engineering application, and as the 

 indications of either are established by the presentation of numerical 

 data, other realms of science become involved. It is outside the juris- 

 diction of the map maker to interpret the significance of the data 

 listed thereon. 



Other properties of rocks whose distribution may have bearing 

 upon the occurrence of petroleum and minerals are specific gravity, 

 magnetic susceptibility, electric conductivity, and radioactivity. By 

 gathering data at the surface of the earth, or in bore holes drilled 

 especially for the purpose, it is possible to infer something of the 

 distribution of rocks beneath the surface from a map of the observed 

 and reduced data. 



Variations in subsurface rock densities reveal themselves at the 

 surface by the distribution of contours of measured values of gravity. 

 By making very precise measurements of gravity at a series of closely 

 spaced points it is possible to derive significant indications of the 



