650 EXPLORATION GEOPHYSICS 



instrument and the instrument which receives the seismic energy and 

 transforms it into some other type of energy that can be recorded.* 



Two methods, described as reflection and refraction shooting, are 

 employed in seismic prospecting.** In both methods a sudden, artificial 

 disturbance is produced at the shot-point station — for example, by explod- 

 ing a charge of dynamite. When the dynamite is exploded, the earth 

 is set in motion like a miniature earthquake and the motion of the sur- 

 face of the earth is detected and recorded at several seismometer stations 

 located at known distances from the shot-point station. In addition to 

 the observations of the times of arrival at various seismometer stations, 

 an accurate record is obtained of the instant of explosion of the shots. 



The basic difference between the two methods lies in the character 

 (refracted or reflected) of the waves whose times of arrival are utilized. 

 In the refraction method, travel-time data are obtained for those artifi- 

 cially produced elastic waves which have been refracted at boundaries 

 separating media of different elastic constants or density in such manner 

 that portions of the wave paths in the subsurface are approximately par- 

 allel to the refracting boundaries. In the reflection method, observations 

 are made of the times of arrival of the artificially produced elastic waves 

 which have been reflected from subsurface horizons. 



In addition to measuring travel-times of different types of waves, the 

 reflection and refraction methods differ in numerous practical and theo- 

 retical details. In the refraction method, only the times of the first motions 

 at the seismometers generally are used; in the reflection method, on the 

 other hand, the times of later motions are of chief importance. 



By knowing the travel-times (intervals between occurrence of explo- 

 sion and arrivals of the elastic waves at various seismometer stations) and 

 the distances between the source of explosion and the seismometers, it 

 is possible, in many cases, to calculate the depths and dips of the re- 

 fracting boundaries (refraction shooting) or reflecting horizons (reflection 

 shooting). 



A modification of the refraction method, known as fan shooting, is 

 used to find structures, e.g., salt domes having an elastic wave velocity 

 markedly different from that of the surrounding materials. In this 

 method, the travel-times for paths of equal lengths are compared. 



Physical Principles. — A disturbance of equilibrium conditions pro- 

 duced by a sudden stress at any point in an unbounded elastic solid will 

 cause two types of elastic waves to be propagated: (a) longitudinal or 

 compressional waves, i.e., waves in which the particles vibrate in a direc- 



* There is no consistent terminology in seismic prospecting for the device which 

 converts seismic energy into some other type of energy that can be recorded. The 

 term seismometer will be employed throughout this chapter. This term is commonly 

 used even though it is not strictly correct, because, as previously stated, a seismometer 

 is a device for measuring seismic motion. 



** The reflection method, however, is at present much more extensively employed 

 than the refraction method. 



