82 beers. GEOPHYSICAL PROBLEMS [Ch. 4 



place, and we now find ourselves practically in the same position as in 

 1925. 



In Canada more success has attended the applications of geophysics. 

 Magnetic and electric methods are in general use. Much new ore has 

 been found from these applications. The airborne magnetometer of- 

 fers considerable promise in the development of large virgin ter- 

 ritories which are otherwise inaccessible. The advances to be ex- 

 pected in this field in the immediate future are those in transpor- 

 tation and operational facilities. Probably no revolutionary discov- 

 eries in geophysical principles should be expected. 



ENGINEERING GEOPHYSICS (Including Soil Mechanics) 



In the preceding section it was intimated that some exploration 

 techniques might be applied with advantage to problems in engineering. 

 There are many problems, closely related to each other, which would 

 benefit by a modest effort in research and development. These prob- 

 lems are recognized by many, but there seems to be no widespread 

 understanding that geophysics might be of aid in their solution. Un- 

 like the petroleum industry, there has been no sponsoring benefactor 

 to pay for development costs. It is possible that state and federal 

 agencies or other institutions may attack these problems to advantage. 

 The general principles outlined here are intended to guide those wish- 

 ing to pursue future courses of action. These principles will be il- 

 lustrated by a few examples. 



In the selection of a site for the foundation of a dam, bridge, aque- 

 duct, viaduct, highway, or a large building construction, the con- 

 struction engineer and the contractor seldom have adequate data on 

 the subsurface underlying the site. Current practice is to subcon- 

 tract for a series of boreholes to bedrock, or to such depths that piles 

 can be driven, to support foundations. In cases where bedrock is 

 found it may happen that the rock encountered is a first layer, under 

 which other unconsolidated material may lie, hidden from sight. 

 The location of the boreholes is usually such that the profile of bed- 

 rock surveys is assumed to be smooth between boreholes. It is rare 

 that enough boreholes are drilled because their cost is very high. 



Where piles are driven, the bearing capacity is calculated from em- 

 pirical formulas based upon assumptions and experiences which may 

 be quite removed from those of the site under consideration. Sample 

 cuttings of boreholes in unconsolidated materials display properties 

 quite unlike those encountered in situ. 



Foundation sites for many large structures have often been selected 



