916 EXPLORATION GEOPHYSICS 



5. When additional velocity surveys are available which differ appreciably from 

 the original values used in interpretation, request the geophysicist to review 

 previous data to determine effects upon structure. 



6. Remember that the past history of exploration work proves conclusively that 

 many undiscovered oil fields repose peacefully in old files, awaiting discovery. 

 Such files offer a fertile field for good prospects. 



This review process cannot be overemphasized. The basic data are usually on 

 the seismic records but cannot always be correctly interpreted until additional 

 geologic information is available. The geologist-geophysicist team must con- 

 stantly review the files of completed seismic work if maximum advantage is 

 to be secured from money spent for exploration. 



SPECIAL APPLICATIONS OF SEISMIC METHODS 

 Shallow Exploration 



The preceding text has treated seismic exploration as appHed to the 

 search for oil structures. There are also many problems in the fields of 

 civil engineering and engineering geology which are amenable to the 

 geophysical approach. These problems may be classified under the heading 

 of shallov^r geophysical prospecting, because the sought-for deposits or the 

 geological conditions involved are near-surface features. The distinguishing 

 condition of this type of geophysical exploration is the very small depth 

 involved — from tens to a few hundreds of feet only, in contrast to the 

 usual thousands of feet in seismic exploration for oil structures. 



Finding the depth to bed rock at a dam site is, for all practical purposes, 

 merely a solution of the "weathering" problem with a different application. 

 Standard geophysical field equipment can be used with little or no modifica- 

 tion and with only minor variations in the field procedure. The major 

 change required is in the point of view of the geophysicist. He must 

 become accustomed to thinking in terms of localized near-surface geologic 

 problems rather than deeper basin-wide structures. 



In many cases a surprisingly small amount of geophysical field work 

 will solve completely or aid materially in the solution of various problems 

 which confront the engineering geologist in the examination and selection 

 of dam sites and in other phases of his work. Examples are the location of 

 an intrusive dike not visible on the surface or the comparison of depth to 

 bed rock at various sites. Likewise a knowledge of the extent of a strategi- 

 cally-located gravel deposit, which can often be obtained by geophysics, 

 is of great value to the materials engineer who is searching for a source 

 of concrete aggregate. 



Questions of a geologic nature which can be answered by geophysical 

 studies may be quite critical in determining the feasibility of a project. 

 In one such case, the depth to bed rock and the course of a buried glacial 

 gorge were determined by seismic refraction work. It was found that the 

 bed rock was relatively shallow and that water impounded in the reservoir 

 behind a proposed dam would not be bypassed through the gorge and 

 around the dam, as had been feared. 



