28 METHODS OF GEOPHYSICAL EXPLORATION [Chap. 2 



use of so-called "compensators," which measure the voltage between the 

 search electrodes in terms of generator voltage and phase, or split it up 

 into one component which is in phase and another which is 90° out of 

 phase with the generator voltage. Interpretation of equipotential-line 

 methods is largely empirical and makes use of the displacement of the 

 lines from their normal position. More quantitative interpretation is 

 possible by comparing the field results with laboratory experiments made 

 on small scale models. Because of the fact that in stratified ground the 

 conductivity is generally better in the direction of the bedding planes than 

 at right angles thereto, it is possible to use equipotential-line methods for 

 structural and stratigraphic investigations. Since an equipotential line 

 near one electrode will be elliptical instead of circular, the direction of the 

 major axis indicates the direction of strike. 



Resistivity methods. Equipotential-line methods, while useful for the 

 mapping of vertical or steeply dipping geologic bodies, are not suited to 

 the investigation of horizontally stratified ground. Conversely, resistivity 

 methods are applicable to depth determinations of horizontal strata and 

 the mapping of dipping formations. 



In resistivity procedures not only the potential difference between two 

 points but also the current in the primary circuit is observed. The ratio 

 of potential difference and current, multiplied by a factor depending on 

 electrode spacing, gives the resistivity of the ground. True resistivities 

 are observed in homogeneous ground only; the presence of horizontal or 

 vertical boundaries in the range of the instrument gives what is known 

 as "apparent" resistivity. The arrangement in most frequent use is the 

 four-terminal Wenner-Gish-Rooney method (see Fig. 2-13). Resistivity 

 methods may be applied in two ways: (1) with constant electrode separa- 

 tion (that is, constant depth penetration), called resistivity "mapping"; 

 (2) with fixed center point and progressively increasing electrode separa- 

 tion, called resistivity "sounding," whereby the apparent resistivity is 

 observed as a function of electrode separation and therefore of depth. A 

 modification of the resistivity mapping method is used in electrical logging. 



Interpretation of resistivity data may be qualitative and quantitative. 

 The qualitative method uses the appearance of the curves and is applied 

 primarily in resistivity mapping, a drop in apparent resistivity indicating 

 the approach of bodies of better conductivity, arid yice versa. In resistiv- 

 ity sounding, the horizontal variation of apparent resistivity is interpreted 

 in terms of the equivalent vertical variation of resistivity; however, the 

 curves do not have sharp breaks at formation boundaries. Structural 

 correlations are sometimes possible by comparing curves through a series 

 of locations. When only one or two formation boundaries are effective, 

 direct depth determinations are possible by comparing the field data with 



