60 APPLICATIONS OF GEOPHYSICS IN ENGINEERING [Chap. 6 



overburden is usually lower than the resistivity of the more consolidated 

 bedrock. Both seismic velocities and electrical conductivities furnish 

 valuable information on the type of rock encountered in dam, tunnel, or 

 similar sites. Additional information may be obtained by dynamic vibra- 

 tion tests which will give the natural frequency of a building site as well 

 as the elastic moduli and bearing strengths of overburden and bedrock. 



B. Location of Construction Materials 



This application of geophysics is important in (1) foundation and hy- 

 draulic engineering in connection with the construction of dams, canals, 

 tunnels, bridges, and the hke; (2) sanitary engineering; (3) transportation 

 engineering (railway, highway, tramway, and subway construction); (4) 

 structural engineering ; and (5) military engineering. Electrical-resistivity, 

 potential-drop-ratio and seismic-refraction methods are applicable ; details 

 were given previously in the section on nonmetallics in Chapter 5. 



C. Location of Water 



The location of water plays an important part in sanitary engineering 

 in connection with water supply, sewage disposal, irrigation, and drainage 

 problems. In transportation engineering, determination of water levels 

 and water-bearing fissures is essential for subway and tunnel construction. 

 The same applies to military engineering. 



Prospecting for water is one of the most difficult tasks in geophysical 

 exploration. It requires exceptional geologic ability on the part of the 

 geophysicist. In the ideal case, the geophysicist should be able to (1) 

 locate the water, (2) determine its salinity, and (3) estimate yields. At 

 the present status of technique, he can rarely hope to predict yields, and 

 he can make only approximate calculations of salinity when working away 

 from wells of known water composition. Hence, the geophysicist's task 

 is narrowed down primarily to the location of water itself. 



Underground waters may be divided as follows^ in decreasing order of 

 geophysical importance: (1) ground water proper, which includes waters 

 derived from precipitation and "connate" water; (2) fissure water; (3) 

 cavern water; (4) spring water; and (5) water issuing from leaks in water 

 pipes. Applications of geophysics to waters in the first group are con- 

 trolled largely by the geometric disposition of the reservoir. The following 

 types are of importance: (a) horizontal (or stratigraphic) boundaries; 

 (6) lateral confinement by vertical boundaries such as faults, dikes, or 

 fracture zones; and (c) erosional boundaries of impervious rocks. 



1 See tabulation in C. F. Tolman, Ground Water, p. 265, McGraw-Hill (1937) and in 

 Trans. Am. Geophys. Union (1937), Hydrology section, p. 575. 



