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MEASUREMENT PROCEDURES IN GEOPHYS- 

 ICAL EXPLORATION 



I. SIGNIFICANCE AND MEASUREMENT OF PHYSICAL 

 QUANTITIES INVOLVED 



In geophysical exploration geologic bodies are located by surface 

 observation of their physical fields. What type of physical field is selected 

 as being the most characteristic of a subsurface body depends entirely 

 upon what rock property is most characteristic of tiiat body. The choice 

 of which field parameter is used, in turn, is determined by four factors: 

 (1) distinctiveness of response; (2) ease and rapidity of field determination; 

 (3) accuracy; and (4) freedom from interference (of surface or subsurface 

 origin). 



Notwithstanding the widely varying nature of physical fields observed, 

 there is a definite similarity in the parameters measured by various meth- 

 ods. Broadly interpreted, geophysical methods fall into two groups: 

 (1) methods in which the propagation of a field with time (and location) is 

 observed ; (2) methods in which the characteristics of stationary and quasi- 

 stationary fields are measured. Virtually all seismic and certain electrical 

 methods fall in the first group. Related in principle are those methods in 

 which the variation of a field with time is observed. In this group fall 

 the majority of the "recording" methods, such as the observation of time 

 variations in gravity; of magnetic variations, of fluctuations in electric 

 currents produced by corrosion of pipes, and so forth. There are some 

 methods in which quantities related to time — for example, phase shift and 

 frequency — are measured (A.C. potential and electromagnetic method and 

 dynamic soil testing method); however, as they operate with quasi-sta- 

 tionary fields they are discussed in the following group. 



Most geophysical exploration is concerned with stationary and quasi- 

 stationary fields, such as in the gravitational, magnetic, self-potential, and 

 other electrical prospecting methods. It is here that similarities in the 

 selection of a suitable field parameter and method of observation may be 

 most readily observed. In these methods we- usually work with fields 



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