40 MEASUREMENT PROCEDURES [Chap. 3 



tant part because of their greater resolving power. In practice this 

 advantage is sometimes offset by increased difficulties in technique. The 

 best-known example of this method is the torsion balance. Observations 

 of second derivatives have been made in magnetic exploration by an 

 adaptation of the earth inductor. In electrical prospecting it has been 

 found advantageous to measure the ratio of fields in successive intervals 

 rather than to measure their gradients. This advantage is due to the ease 

 with which "ratio" bridges can be designed. Examples are the potential- 

 drop-ratio method and the determination of the ratio of electromagnetic 

 fields with dual coil arrangements. 



As stated at the outset, preference is given to field parameters which 

 can be measured with the greatest accuracy as well as ease and rapidity; 

 hence, the tendency to measure physical fields relatively and not abso- 

 lutely whenever possible. The terms "absolute" and "relative" are 

 somewhat difl&cult to define in geophysical exploration, since most quanti- 

 ties, which by their nature can be determined absolutely, may with greater 

 ease be observed relatively by refererce to a fixed point or base station. 



Absolute procedures include measurements of direction (magnetic 

 inclination and declination, equipotential lines, dip and strike of induction 

 coils), time and distance (seismic refraction and reflection measurements), 

 and electric voltage (as in electrical potential methods). 



Semiabsolute procedures are those which measure a physical parameter 

 by comparison with another which is assumed to remain constant for a 

 reasonable interval of time. Examples are: relative gravity measure- 

 ments, made with the assumption that the pendulum length remains 

 constant; magnetometer measurements, made by comparing the vertical 

 intensity with the presumably constant gravity; the gravimeter, whose 

 reliable functioning depends on the constancy of the elastic comparison 

 force; and many others. 



Relative procedures involve physical measurements by actual con- 

 nection of the detecting unit with a similar unit located at a reference 

 station or an adjacent ground interval. Examples are: the transmission 

 of the oscillation of a reference gravity pendulum from a base station to a 

 field station; the magnetic earth-inductor gradiometer; the potential- 

 drop-ratio method; the comparison of electromagnetic fields with the 

 amplitude and phase of the current in the generator circuit by compen- 

 sator arrangements; and two-coil ratiometer bridges for a comparison of 

 electromagnetic fields with respect to amplitude and phase at successive 

 points. It is seen that by such procedures time variations of comparison 

 parameters are virtually eliminated. 



