GEOLOGIC AND ECONOMIC BACKGROUND 23 



able geological structure. The basic procedure in all methods, therefore, 

 consists in (a) measuring variations in force fields at the surface of the 

 ground and (b) predicting, on the basis of knowledge of the influencing 

 physical properties, the probable configuration of subsurface materials 

 (geological structure) which might cause the measured effects. 



As will be shown later in this chapter, the relationship of these various 

 physical properties of the rock materials to problems of economic geology 

 is usually the main factor dictating applicability and choice of methods for 

 exploring any given area. 



Technique of Applying a Geophysical Method. — In applying a geo- 

 physical method the mode of attack may be either direct or indirect. In the 

 direct attack, the purpose of the geophysical measurements is to locate a 

 subsurface body or geological condition by means of an anomaly which is 

 directly associated with, and the direct result of, this body or condition. 

 Examples of the direct method are the location of bodies of sulphide ore 

 by means of their high electrical conductivity, the location of bedrock 

 underneath fill material by the differences in electrical conductivity, the 

 detection of a petroleum accumulation by an increased concentration of 

 hydrocarbons in the soil, etc. In one type of the indirect mode of attack, 

 the probable location of the desired subsurface deposits is inferred from 

 the similarity of the geological structure, determined by the geophysical 

 survey, to those structures known commonly to contain such deposits. An 

 example of the indirect attack is the locating of a petroleum deposit by the 

 detection of a subsurface structure or trap suitable for petroleum accumu- 

 lation which, when drilled, may be found to contain petroleum. An example 

 in mining exploration is the indirect location of placer gold associated with 

 black sand concentrations by means of the magnetic anomalies produced 

 by these black sands. 



In mining exploration the direct method is of predominant importance, 

 but there are many applications of the indirect type. A combination of both 

 techniques, where possible, has produced the most satisfactory results in 

 many cases. 



The choice of a particular type of geophysical method is commonly 

 dictated by existing conditions. The mapping of basic igneous dikes that 

 intrude sedimentary formations is an example. For this particular problem, 

 the use of magnetic methods combined with available surface geological 

 data will usually give maximum information at minimum cost. In a major- 

 ity of cases, however, the controlling factors are not so obvious, and 

 judicious exploration may demand the use of more than one geophysical 

 method. 



Initial exploratory studies of a reconnaissance nature commonly are 

 conducted to indicate, in a general way, the major geological features of 

 probable importance. Further investigations may be carried out with a 

 method of greater selectivity to obtain confirmatory and more detailed 

 evidence in certain parts of the area. An example of this procedure in 



