622 ELECTRICAL METHODS [Chap. 10 



measurements are given in units of ohm-meters or ohm-feet. Potential 

 ratios are plotted against horizontal distance or equivalent depth. Elec- 

 tromagnetic fields are measured absolutely in microgauss (1.10~^ gauss) 

 or semiabsolutely in units of microgauss per ampere primary (loop) current. 

 The fields are represented by their in-phase and quadrature components 

 or by the value of the total vector and its phase. Ratio measurements 

 likewise furnish the in-phase and quadrature components of the electro- 

 magnetic field or its amplitude and phase by successive multiplication or 

 addition respectively along a continuous traverse. 



In electrical prospecting, as in other geophysical methods, the distinct- 

 ness of surface indications depends on the contrasts in the physical prop- 

 erties of geologic bodies and their surroundings. The following properties 

 are involved: electrochemical activity, conductivity, dielectric constant, 

 and permeability. Comparatively little is known about electrochemical 

 and dielectric rock properties and geologic factors controlling them; more 

 extensive information is available on rock conductivities. While in most 

 other geophysical methods the distinctness of surface indications in- 

 creases in linear proportion with differences (or ratios) of the rock proper- 

 ties involved, this is not true for all electrical prospecting methods. In 

 potential methods, a saturation effect is encountered, so that indications 

 from large differences in conductivity are not proportionately stronger 

 than indications obtained from small differences. Therefore, potential 

 methods are particularly suitable for the detection of small differences in 

 conductivity. The same appears to hold for electromagnetic methods 

 with galvanic power supply. Inductive methods, on the other hand, are 

 controlled by absolute conductivities and are therefore best suited for the 

 detection of very good conductors. 



Continuity of physical properties is an essential characteristic for the 

 usability of any geophysical method. In those electrical methods that are 

 used for the purpose of determining depths of horizontal formations (re- 

 sistivity, potential-drop-ratio, and inductive methods), it is necessary that 

 these physical properties remain continuous in a horizontal direction since 

 the spacing of transmitting and receiving units is changed horizontally to 

 obtain increased depth penetration. In the application of electrical meth- 

 ods to ore location, these requirements are not, and need not be, fulfilled 

 since horizontal discontinuities in conductivity are the object of detection. 

 To obtain distinct results it is, of course, desirable that the physical prop- 

 erties remain fairly continuous vertically and in the strike of an ore body. 



As in other geophysical methods the uniqueness of interpretation of elec- 

 trical prospecting results depends on the ease with which interfering factors 

 can be eliminated. Terrain, for instance, affects the surface potential 

 methods much more than it does the electromagnetic methods. High 



