GEOLOGIC AND ECONOMIC BACKGROUND 59 



Location of Reservoir Sites. — Several years ago the U. S. Federal Govern- 

 ment constructed the Hondo reservoir near Roswell, New Mexico, in connection with 

 an irrigation project at a cost of about $500,000. The rocks underlying the reservoir 

 area consist of alternating beds of gypsum and limestone and the structure of the 

 formation is that of a faulted and collapsed anticline, the fault traversing the center 

 of the reservoir. Underground cavities caused by the dissolving out of the relatively 

 soluble gypsum apparently caused the collapse of the structure and rendered the 

 reservoir useless for water storage. Proper geophysical and geological examinations 

 would probably have saved the wasted expenditures. 



Selection of proper reservoir sites is, of course, closely allied to that of dam 

 sites. The main geological factor to be considered in the selection of reservoir sites 

 is the perviousness of the reservoir rims. Leakage hazards may consist of (1) faults 

 or solution channels in the rim formation and (2) insufficient elevation or breadth 

 of bedrock along the rims relative to the pool level of the proposed reservoir. In many 

 cases, these problems may be readily solved by electrical, seismic, and magnetic 

 methods.* 



Highway Engineering.- — Many of the subsurface geological prob- 

 lems which are encountered in the building of highways, railroads, aqueduct 

 and pipe line routes, etc., may be solved by geophysical methods. Usually, 

 the required depth of study is relatively shallow (less than 100 feet) and 

 geophysical surveys can be conducted rapidly and economically. As in 

 other phases of civil engineering, electrical methods are proving most 

 applicable to these general problems. 



Several fields of investigation may be listed in this general class of 

 engineering: (1) quarries, (2) pipe line and aqueduct routes, (3) tunnel 

 sites, (4) bridge foundations, (5) cut and fill determinations. 



Application of geophysical prospecting to quarry deposits was dis- 

 cussed briefly in the earlier section devoted to non-metallic mining. 

 Quarrying operations are carried on extensively as a part of most civil 

 engineering and construction projects. Selection of suitable quarry sites 

 is based upon the accessibility of the quarry deposit, cost of quarrying 

 and crushing the rock, quality of the deposit, and extent of the deposit. 

 The last three factors are partially dependent upon geological conditions 

 which may be evaluated by geophysical methods. A problem common to 

 all quarrying operations is the determination of the thickness of the 

 overburden, and this is readily accomplished by electrical methods. 

 The electrical resistivity method has been successfully used to determine 

 the extent of quartzite deposits and the yardage of such rock present in 

 the search for riprap for a number of dams in the Missouri River Basin. 

 This work as supplemental to a limited amount of core drilling has reduced 

 the cost of exploring these deposits. Electrical methods can be used also 

 to locate certain kinds of quarry deposits such as gravel, igneous rocks, 

 limestone, etc., and to afford qualitative information as to degree of 

 weathering. The presence of solution channels and sink holes is sometimes 



* Examples of electrical, seismic and magnetic work in examination of reservoir 

 rims are given in later chapters. 



