Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 25 



described above. Elevations are considered by a topographic correction 

 to shot datum; by reducing to a regional datum, variations in the geology 

 of surface beds may be allowed for. Sometimes it is necessary to make 

 corrections for horizontal velocity variations. 



In practice, reflection technique is applied as correlation shooting, con- 

 tinuous profiling, or dip shooting. Correlation shooting consists of placing 

 individual reflection locations from one-half to one mile apart and cor- 

 relating reflection depths through that distance. In continuous profiling, 

 there is an overlap of reflection profiles, whereas in dip shooting, profiles 

 are shot in opposite directions. It may be necessary to use dip shooting 

 on horizontal, but discontinuous beds to avoid errors due to miscorrelation. 

 The reflection method is at present the most accurate method of determin- 

 ing depths of formations in oil exploration. 



V. ELECTRICAL METHODS 



General. Mineral deposits and geologic structures may be mapped by 

 their reaction to electrical and electromagnetic fields. These are produced 

 by either direct or alternating current, except where ore bodies spontane- 

 ously furnish their own electrical field (self -potential methods). Elec- 

 trical energy may be supplied to the ground by contact or by induction. 

 The field of the electrical currents so produced may likewise be surveyed 

 by contact or by induction. In respect to surveying procedure and the 

 field measured, three main groups of electrical methods may be distin- 

 guished: (1) self -potential, (2) surface-potential, and (3) electromagnetic 

 methods. Frequently the first two groups are combined into one group 

 of potential methods; the electromagnetic methods are usually subdivided 

 into galvanic-electromagnetic and inductive-electromagnetic in respect to 

 the manner in which the primary field is applied. 



Four frequency bands may be used in connection with alternating cur- 

 rent electrical prospecting: (1) low frequencies of from 5 to about 100 

 cycles; (2) the audio-frequency range of from 200 to 1000 cycles; (3) high 

 frequencies of from 10 to 80 kilocycles; and (4) radio frequency of from 

 100 kilocycles to several megacycles. The low-frequency range is appHed 

 in most potential methods; the audio-frequency range is used in some po- 

 tential and most electromagnetic methods; the high-frequency range in 

 the high-frequency electromagnetic methods; and radio frequency in the 

 radio methods of electromagnetic prospecting. The application of high 

 and radio frequencies is limited owing to their lack of depth penetration; 

 of greatest importance are the audio frequencies and the low frequencies. 

 In a number of respects, electrical methods are similar to seismic methods; 

 comparable to the refraction methods are the resistivity and the potential- 



