56 EXPLORATION GEOPHYSICS 



as they may affect engineering construction such as dams and drainage projects; (3) 

 to determine the distribution of ground water insofar as it may affect conservation of 

 crops or encroachment of sea water, etc. The geological and geophysical problems 

 involved in these determinations are generally resolved into: (1) location of per- 

 manent water table or perched bodies of water; (2) location of subsurface structure 

 favorable to accumulation, impounding, and storage of water; (3) location of boun- 

 daries between fresh and saline waters. 



Geophysical methods as applied to the various problems of water supply are 

 usually of the indirect type wherein structural conditions influencing ground water 

 distribution are determined. In certain relatively simple problems, the contour of the 

 water-table may be mapped directly and isolated accumulations of water may be 

 located.* Structural conditions that most commonly influence accumulation and 

 impounding of subsurface water are: (1) subsurface basins and channels in bed- 

 rock underlying unconsolidated porous materials ; (2) subsurface structural barriers 

 such as erosional relief in the bedrock, igneous dikes, etc.; (3) faults. 



Choice of Methods. — Electrical, magnetic, and seismic methods 

 have been used to secure structural information. Electrical methods are 

 being used more extensively than the other methods and are generally 

 applicable to most phases of the structural problem : viz., determination 

 of depth of unconsolidated alluvium, etc., mapping bedrock contour, 

 and location of faults. Magnetic methods are useful in cases in which 

 impounding of subsurface water is caused by igneous dikes. Seismic 

 work has been employed to map bedrock boundaries. Improvements 

 are being made in the adaptation of seismic methods to shallow work and 

 it is likely, therefore, that the use of these methods in problems of water 

 supply will increase, t 



The actual location of the ground-water table is important in de- 

 termining favorable locations for wells and in evaluating conditions 

 prior to construction of dams, tunnels, etc.** Electrical methods have 

 been used successfully to locate water impounded above bedrock in 

 quaternary gravels and alluvium and to map the upper boundary of 

 ground water at greater depths in the more consolidated sedimentary 

 rocks, t Various investigators have employed the seismic method in the 

 direct location of water tables. The method depends upon increase in 

 velocity of the elastic wave with increase in water content. § 



GEOPHYSICAL METHODS IN CIVIL ENGINEERING 



The application of geophysical methods in civil engineering and 

 construction work has increased rapidly in recent years because of the 

 recognition of the importance of geological structure in all phases of 



* See Chapter V. 



t F. L. Partio and Jerry H. Service, "Seismic Refraction Methods as Applied to Shallow Over- 

 burdens," A.I.M.E. Geophysical Prospecting, 1934. 



** See following section on Geophysical Methods in Civil Engineering. 

 t J. J. Jakosky, C. H. Wilson and J. W. Daly, "Geophysical Examination of Meteor Crater, 

 Arizona," A.I.M.E. Geophysical Prospecting, 1932. 



J. J. Jakosky and C. H. Wilson, "Geophysical Study of Fort Peck Dam Site and Reservoir," 

 technical report. 



§ C. M. Tattam, "Application of Electrical Resistivity Prospecting to Ground Water Problems," 

 Colo. School of Mines Quarterly, Vol. 32, No. 1, January, 1937. 



