Chap. 6] APPLICATIONS OF GEOPHYSICS IN ENGINEERING 61 



There are three general possibilities for the geophysical location of water : 

 (1) direct, (2) structural, and (3) stratigraphic location. 



1. Direct location. Water occurring in the form of thermal, saline, or 

 radioactive springs may be located by temperature, electrical, or radio- 

 activity measurements, or by its noise in escaping from pipe leaks. Brine 

 accumulations in salt mines may be found by electrical-resistivity, induc- 

 tive, or radio-transmission observations. Water occurring in caves and 

 fissures is on the borderline between direct and stratigraphic location, as 

 it is often difficult to decide whether the indication comes from the water 

 as such or from an impregnated medium. Water filling large cavities in 

 limestones or dolomites may be located directly by resistivity measure- 

 ments or, if it is sufficiently conductive, by inductive or radio methods. 

 Water which occupies fissures is often heavily mineralized and may thus 

 be detected by inductive, resistivity, radioactivity, or radio measurements. 

 Direct application of geophysics to the location of the ground-water table 

 is limited to such special problems as the determination of the vertical 

 moisture gradient by hygrometric observations in different depths and 

 the calculation of the rate of motion of a ground-water stream in wells 

 by salting the water and measuring the rate of motion of the surface 

 potential peak due to the lateral motion of the salt-water front. 



2. Structural water location involves the attempt to find locations favor- 

 able for its occurrence. It entails the mapping of certain formations which 

 may or may not be aquifers and which may occur in synclines, troughs, 

 or areas of general depression. Hence, virtually all major geophysical 

 methods are applicable, depending upon whether differences in density, 

 magnetism, elasticity, or conductivity occur on a stratigraphic or erosional 

 boundary. For instance, water-bearing gravel channels and valley fills 

 may be mapped by torsion-balance, seismic, or resistivity methods. The 

 magnetometer may be applied for tracing channels in igneous or meta- 

 morphic rocks. If a reservoir is confined laterally by faults, gravitational, 

 seismic, resistivity, or magnetic methods are applicable (if igneous dikes 

 cut through the water-bearing strata). Key beds in large artesian basins 

 have been surveyed by reflection-seismic methods. 



■ In the location of fissure water the function of structural geophysical 

 work is the mapping of fissures or faults. Gravitational, magnetic, seismic, 

 or electrical methods apply, depending upon whether strata with differ- 

 ences in density, magnetic, elastic, or electric properties have been placed 

 in juxtaposition by the faults. 



3. Stratigraphic water location has for its objective a determination of 

 the condition and depth of the aquifer itself. The choice of geophysical 

 methods is here much more limited. Seismic methods are applicable 

 under favorable circumstances as the elastic wave velocity is greater in 

 moist than in dry, unconsolidated formations. More important and inex- 



