Chap. 3] MEASUREMENT PROCEDURES 39 



which have a potential and are characterized by four parameters: (1) the 

 potential itself; (2) the potential gradient, or field intensity; (3) the direc- 

 tion of the field ; and (4) the field gradient and its direction (second deriva- 

 tives of the potential) . Derivatives of the potential higher than the second 

 are rarely, if ever, measured in geophysical exploration. Interpretative 

 experience indicates that the distinctiveness and therefore the resolving 

 power of a method increases with the order of the potential derivative. 

 However, accuracy requirements and the field difficulties also increase. 

 Measurements of absolute potentials are rarely undertaken. They 

 probably would accomplish no useful purpose in gravitational and mag- 

 netic exploration. Potential measurements in electrical prospecting 

 actually involve a determination of potential difference, potential gradient 

 or electrical field strength. 



The first derivative of a potential is measured in most methods em- 

 ploying stationary or quasi-stationary fields. In gravitational exploration, 

 the first derivative of the gravity potential with respect to the vertical — 

 that is, gravity — is determined in a number of ways, particularly by 

 pendulums and gravimeters. The horizontal derivative (horizontal 

 gravity component) , is not measured directly,^ although there is no reason 

 why instruments for this purpose could not be developed and perform a 

 useful function. In magnetic exploration, the derivatives of the magnetic 

 potential in almost any desired direction can and have been measured. 

 Instruments have been developed for the determination of the total inten- 

 sity and its vertical and horizontal components. However, the vertical 

 component has been found to be of the greatest diagnostic value. In 

 electrical prospecting, potential difference and therefore electrical field 

 strength is determined in the self-potential, the potential profile, and the 

 resistivity methods. Intensities of electromagnetic fields are observed in 

 the electromagnetic, the inductive, and the high-frequency methods of 

 electrical prospecting (although these fields do not derive from a potential). 



As far as technique is concerned, simple conditions prevail "when merely 

 the direction of a field is observed. This is done in magnetic exploration 

 by measuring declination and inclination and in electrical prospecting by 

 tracing equipotential lines and by observing the minimum-signal position 

 of a reception coil. However, direction observations are less readily 

 interpreted than intensity measurements; hence, the latter are usually 

 given preference. 



Measurements of second derivatives of a field potential play an impor- 



1 This component may be obtained indirectly from torsion balance observations 

 by graphical integration of curves representing the horizontal variation of curvature 

 values. 



