62 APPLICATIONS OF GEOPHYSICS IN ENGINEERING [Chap. 6 



pensive are electrical methods. Their application rests on the fact that 

 water in the pores of a rock changes its conductivity to such an extent 

 that the conductivity of the mineral substance is virtually without effect. 

 Hence, the following factors are effective: (1) porosity, (2) percentage of 

 pores filled, and (3) electrolytic conductivity of the water. The latter 

 depends very much on the degree of stagnation in a rock and complicates 

 the geophysical picture, as the groundwater may be either a good or a 

 poor conductor. Very pure waters are more difficult to locate than 

 waters of fair conductivity. The former requires sensitive electrical- 

 potential methods, while waters of high salinity, particularly connate 

 waters, may also be found by inductive-electrical procedures. 



It is further impossible to recognize water by a specific value of rock 

 conductivity. The change brought about by the presence of water is not 

 great enough to produce outstanding values; therefore, it would be diffi- 

 cult to select an arbitrary location in virgin territory and to determine the 

 presence and depth of water from the geophysical response. However, 

 where the presence of an aquifer has been established by wells, it is possible 

 to correlate conductivities with water-bearing formations, their depth, 

 type, and thickness, and to follow this type of indication into unknown 

 territory until a complete change in character occurs. With resistivity 

 methods, frequently a "t3T)ical" ground-water curve is obtained, which is 

 discussed in greater detail in Chapter 10. 



II. NONGEOLOGICAL APPLICATIONS 



Though not directly geophysical but closely related to geophysics, 

 since they involve similar techniques, these applications include: (a) dy- 

 namic vibration tests of structures, (6) strain gauging, (c) corrosion surveys, 

 (d) pipe and metal location, (e) sound ranging and other acoustic detection 

 methods, (/) gas detection, and (g) thermal detection. 



A. Dynamic Vibration Tests of Structures 



These tests involve a determination of natural frequency and damping 

 characteristics of completed structures and of models of proposed struc- 

 tures to determine their seismic resistance to earthquakes and artificial 

 vibrations by the free and forced vibration methods. Details are given 

 in Chapter 12. 



B. Strain Gauging 



The purpose of these measurements is to determine the variations of 

 elastic strains with time in structures and underground workings so that 



