Chap. 10] 



ELECTRICAL METHODS 



621 



SYMBOLS USED IN CHAPTER 10-Concluded 



excitation and consequent reaction of subsurface bodies and are not based 

 on spontaneous effects like the magnetic and gravity methods. It is a 

 common characteristic of these indirect methods that the depth from 

 which reactions are obtained can be controlled by the spacing of the 

 transmitting and receiving points. Most potential methods (resistivity 

 and potential-drop-ratio methods in particular) have distinct depth con- 

 trol and are in many ways comparable to seismic refraction methods. 

 Electromagnetic, most inductive, and equipotential methods have com- 

 paratively little depth control. The spontaneous polarization method 

 lacks depth control completely. 



Physical quantities measured in electrical exploration vary greatly and 

 depend largely on the method applied. In the self-potential, equipoten- 

 tial, resistivity, and potential-drop-ratio methods, results are obtained in 

 the form of electrical potentials or potential differences (sometimes referred 

 to the primary field in regard to amplitude and phase) and in the form of 

 ratios of adjacent potential differences. In electromagnetic methods the 

 magnetic field produced by the currents flowing beneath the surface is 

 determined either semiabsolutely (in reference to the phase and amplitude 

 of the excitation current) or in the form of ratios and phase differences of 

 the fields on successive points. Essentially, therefore, the purpose of po- 

 tential methods is a determination of direction and intensity of the elec- 

 trical field, whereas in electromagnetic methods direction and intensity of 

 the electromagnetic field is measured. Measurements of electrical and elec- 

 tromagnetic fields are made by contact probes and coils. Null methods 

 and bridge arrangements are employed extensively for the determination 

 of the direction of current flow, of voltages, voltage ratios, intensities, and 

 intensity ratios. 



Potentials and potential gradients are expressed in volts or millivolts 

 absolute or in volts or millivolts per unit distance. Results of resistivity 



