2 

 METHODS OF GEOPHYSICAL EXPLORATION 



I. CLASSIFICATION 



Geophysical methods may be broadly classified under two headings: 

 major and minor (see Tables 1 and 2). There are four major geophysical 

 methods: gravitational, magnetic, seismic, and electrical. 



In the gravitational methods, measurements are made of anomalies in 

 gravity attraction produced by differences in densities of formations and 

 structures. In the magnetic method, measurements are made of anomalies 

 in the earth's magnetic field due to geologic bodies of different degrees of 

 para- or dia-magnetism. In either case, the reactions of geologic bodies 

 are permanent, spontaneous, and unchangeable; the operator cannot con- 

 trol the depth from which they are received.^ 



In the other two major methods, energy is applied to the ground for 

 the purpose of producing a measurable reaction of geologic bodies. This 

 gives the possibility of spacing transmission and reception points in such 

 a manner that the depth range can be controlled. In the seismic method, 

 energy is supplied by dynamite explosions, and the travel times (time 

 interval between firing of the shot and reception of elastic impulses) of 

 refracted and reflected waves are measured. 



In one group of the electrical methods energy is applied galvanically, 

 and the distribution of the potential or the electromagnetic field resulting 

 from conductive bodies is measured. These are known as potential and 

 electromagnetic (electromagnetic-galvanic) methods. In another group, 

 known as electromagnetic-inductive methods, the primary energy is ap- 

 plied inductively to the ground and the distortions of the electromagnetic 

 field are determined. 



^ If only one geologic body is present, this limitation may be overcome by varying 

 the position of the receiving units in a horizontal direction away from the axis of the 

 geologic body. Thereby, a variation of the anomaly with distance is introduced 

 and, for one geologic body at least, enough equations may be established so that 

 direct depth determinations may be made. Horizontal changes of distance may be 

 supplemented by vertical changes in distance, with observations from scaffolds and 

 aircraft. In practice, these methods are of limited value when a plurality of geologic 

 bodies exists. 



