GEOLOGIC AND ECONOMIC BACKGROUND 25 



namely, the determination of local subsurface conditions. In certain cases, 

 the use of aerial photographs will be of much assistance and will be justified 

 economically.! Careful and systematic inspection of air photos, particularly 

 with a stereoscope, provides valuable clues to the dip of strata, the position 

 of faults and critical rock ledges, and the location of native vegetation and 

 crops which may impede geophysical work, as well as roads and trails. The 

 use of aerial photography, however, is more readily justified for relatively 

 extensive exploration in undeveloped areas and under difficult terrain 

 conditions. Considerable discrimination is necessary at all times to insure 

 that the economic value of the technical data secured justifies the expense. 

 All preliminary maps should be prepared with due regard to their proposed 

 use in conjunction with the planning of a survey and the plotting and inter- 

 pretation of the geophysical results. 



The fourth step in a general exploration program is the determination 

 of the applicability of the various geophysical methods to the problem at 

 hand, and the selection of one or more methods. (This step may, of course, 

 be contemporaneous with preceding steps.) If the previously acquired 

 geological information is comprehensive and requires for completion of the 

 study only an extension of known surface features to greater depths, it is 

 probable that the need of a detailed survey by a particular geophysical 

 method in certain limited parts of the area will be clearly indicated. The 

 choice of method commonly is controlled by the geological setting, the infor- 

 mation desired, and the cost. Where the area is large and available geolog- 

 ical information is meager, economic considerations may dictate a prelimi- 

 nary reconnaissance by one of the more rapid and less costly geophysical 

 methods before certain parts of the area are selected for detailed survey 

 by a more costly method. 



The fifth step is the performance of the geophysical field work. This is 

 an engineering as well as a scientific task. The engineering work of geo- 

 physical surveys has much in common with that of other land surveys 

 requiring the compilation of a series of observations at designated locations. 

 The taking of notes, the proper recording of station locations, and the 

 plotting of stations and recorded data on maps are similar in all types of 

 land surveys. 



The tabulated geophysical measurements are next subjected to compu- 

 tations by means of appropriate formulas and corrections. The computed 

 results should show the variations, if present, in some physical property of 

 the subsurface such as magnetic permeability, electrical conductivity, or 

 density; or they may show the depth and altitude of reflecting beds, elastic 

 wave velocity, etc. Usually the computed results are plotted in a manner 

 which lends itself readily to subsequent analysis and correlation with known 

 geology and surface features. 



The sixth step is the analysis and interpretation of the observed and 

 recorded data. The interpretation consists primarily of translating the 



t Report of symposium on "Aerial Geologizing." (Tech. Pub. No. 756, A.I.M.E., 1936). 



