466 EXPLORATION GEOPHYSICS 



methods,* because the field work can be conducted so as to introduce a 

 depth variable which relates the electrical data obtained at the surface with 

 the variation of effective resistivity with depth. 



Operating Principles 



The field procedure employed in resistivity determinations consists 

 in passing a measured current through a selected portion of the earth 

 and measuring the potential drop, or some other electrical quantity 

 associated with this flow of current. This is usually accomplished by 

 passing the current between energizing electrodes placed at two selected 

 points and measuring the potential difference between two or more 

 auxiliary electrodes placed at other points in the area under investigation. 

 For convenience in the interpretative analysis, all the electrodes are usually 

 placed in a straight line. From the observed values of the current and 

 potential, the apparent resistivity of the material included within 

 the zone of measurement can be calculated for any given electrode 

 configuration. 



The effective depth of measurement is governed, to a considerable 

 extent, by the spacings of the various electrodes involved in the measure- 

 ments and by the relative resistivities of the various geologic strata included 

 in the measurement. In a particular area it is necessary to evaluate the 

 effective depth of measurement empirically, using whatever is known of 

 the local geology as a control. 



Generally, the subsurface is heterogeneous and the potential distribu- 

 tion at the surface of the ground is affected by the size, shape, compo- 

 sition, and relative positions of the subsurface rock masses. Because these 

 are the elements of geologic study, the geoelectrical problem is to infer 

 the subsurface geology from the surface measurements of electrical quan- 

 tities. Interfering factors are always present. These include natural ground 

 currents, polarization phenomena, uneven topography, and non-structural 

 near-surface resistivity variations. The effect of such factors must be 

 evaluated or minimized by proper technique in the field work or in the 

 interpretation. 



Resistivity investigations as usually conducted may be divided into 

 two general classifications : ( 1 ) studies wherein the specific subsurface 

 conditions are to be deduced; (2) studies wherein relative subsurface 

 conditions at one locality (station) are to be compared with the conditions 

 existing at another locality. 



In (1), the field work and the interpretative technique must be so 

 conducted that deductions may be made regarding the configuration, mass, 

 distribution, and depths of the materials which constitute the subsurface. 

 Analysis of the data is based on relationships derived for certain relatively 

 simple conditions. The interpretative technique includes the following 



* Potential surface methods include all summation potential methods, such as the 

 magnetic, gravitational, and the self-potential and equipotential line methods. 



