Chap. 6] APPLICATIONS OF GEOPHYSICS IN ENGINEERING 63 



zones of weakness may be found and failures predicted. In connection 

 with subsidence investigations, mine workings have been thus tested to 

 predict wall or roof failure; shafts and tunnels have been examined in 

 areas of active faults and earthquakes; strain gauges have been installed 

 in important structures, such as Boulder Dam; and like methods have 

 been used on models of dams and other structures. 



C. Corrosion Surveys 



The purpose of these surveys is to follow the process of corrosion, chiefly 

 on buried pipes, and to determine progress made by preventive measures, 

 such as cathodic protection and coating. Two procedures of geophysical 

 exploration are applicable: (1) self-potential surveys and recordings, and 

 (2) resistivity measurements. By self-potential measurements along a 

 pipe, the areas of ingress and egress of current and therefore the zones of 

 greatest destruction of pipe material may be located. Recording fluctua- 

 tions of spontaneous potentials issuing from buried pipes, gives information 

 about occurrence and time variations of vagabondary currents producing 

 corrosion. Resistivity measurements serve to find areas of high ground 

 conductivity in which, according to experience, corrosion is greatest. 



D. Pipe and Metal Location 



Occasionally it is necessary to locate pipes of which the record has been 

 lost. If the lost pipe is part of a network accessible elsewhere, it may be 

 energized by contact and located by following the electromagnetic field 

 surrounding it. Isolated pipes can be found by so-called "treasure 

 finders," consisting of combined radio transmitters and receivers, described 

 in more detail in Chapter 10. Magnetic prospecting can be applied in 

 favorable cases. Methods for the location of buried ammunition and 

 other war machinery, and procedures for the detection of metal and 

 weapons on workers and visitors of mints and penal institutions belong 

 in the same category and require the same or similar procedures. 



E. Sound Ranging and Other Acoustic Detection Methods 



As outlined at the end of Chapter 2, (page 37), acoustic detection 

 methods are used for communication, direction finding and noise analysis, 

 position finding, sound ranging, and echo sounding. Applications of these 

 methods in the fields of civil and military engineering are numerous. 

 Acoustic means of communication, particularly at supersonic frequencies, 

 are widely used between surface vessels and submarines, and between 

 submarines and shore stations. Radio-acoustic position finding is a valu- 



