778 EXPLORATION GEOPHYSICS 



A large number of comparatively shallow salt domes has been located 

 by fan shooting. At the present time, it is believed that the numerous 

 fan surveys carried out in the Gulf Coast have probably disclosed prac- 

 tically all the shallow salt domes in that region. 



In addition to discovering new salt domes, fan shooting in some 

 cases has outlined successfully regions underlain by large structural 

 features. * 



Profiling Salt Dome Boundaries 



Bore Hole Method. — A method for use in bore holes has been pro- 

 posed by McCollum. § The principles of the method will be evident from 

 Figure 474 wherein represents the shot-point, 7 the recorder, and 5 the 

 seismometer. The seismometer 5" is adjusted to various positions in the bore 

 hole as, for example, c?2, c?3 or de, by means of a hoisting winch and cable. 

 Consider that the seismometer is stationed at d^. The wave traversing 

 the path Okf^de will reach the point c?g in the shortest possible time. This 

 wave will actuate the seismometer 6" and its exact time of arrival will be 

 recorded by 7. Let the time of travel over the path Okf^d^ be T. 



Evidently, 



r = fi + fo + ^3 



where ti is the time to traverse the path Ok; to is the time to traverse 

 kfe, and ^3 is the time to traverse fedf^. The last equation may also be 

 written in the form 



where Vg is the velocity along the path kfe and Ve is the velocity along the 

 path fade. 



Here ti and Va may be determined from reflection travel-time data 

 using the normal arrangement wherein both seismometer and shot-point 

 are located at the surface of the earth.ff The velocity Vg will depend on 

 the path kde ; that is, the velocities along the various paths /s^/s, fodo, etc. 

 will, in general, be different. However, a mean value of V5 can be deter- 

 mined by obtaining the travel-times T5, Tq and T^ between a shot-point 

 located at a point 0' on the surface near the bore hole and a seismometer 

 positioned at ^5, de and d-r respectively.** 



The position of the point k can be determined from reflection travel- 

 time data. The position of the point do is, of course, known. 



* The theoretical basis of this type of refraction shooting has been discussed by 

 Barton t and by Roman, t. 



t D. C. Barton, "The Seismic Method of Mapping Geologic Structure," A.I.M.E. Geophysical 

 Prospecting, 1929, pp. 690-598. 



t Irwin Roman, "Analysis of Seismic Profiles," A.I.M.E. Geophysical Prospecting, 1934, 

 pp. 493-527. 



§ Burton McCollum, "Seismic Method of Profiling Geologic Formations," U. S. Patent 

 1,923.107. Issued August 22. 1933. 



tt Burton McCollum, U. S. Patents 1,724,495 and 1,724,720. 



** The velocity thus determined is an average or equivalent velocity. 



