SEISMIC METHODS 917 



The engineering and geological data needed to solve such problems 

 may, in many cases, be secured by geophysical investigations at less expense 

 and more quickly than by other means, such as drilling, test pits, trenching, 

 hand auger holes, etc. 



Geophysical work in connection with geological engineering problems 

 has been carried on for some years by several federal and state govern- 

 mental agencies. These include the U.S. Bureau of Reclamation, the Corps 

 of Engineers, the U.S. Bureau of Mines, the U.S. Geological Survey, and 

 the U.S. Bureau of Roads, as well as a number of state geological surveys 

 and highway departments. Similar use of refraction profiles was made in 

 the investigations conducted in connection with the atomic bomb tests of 

 1946, to obtain information on the subsurface structure of Bikini Atoll. 

 For this work, naval depth charges were fired on the lagoon bottom along 

 four lines across the atoll, and the refractions were picked up by water- 

 coupled seismometers. t Similar exploration has been done by many con- 

 sulting geophysicists and by institutions of higher education having courses 

 in exploration geophysics. It is pertinent that this field of engineering 

 geophysics is in its infancy. 



Instruments and Methods. — The geophysical instruments employed 

 in shallow prospecting are essentially those used in oil and mining explora- 

 tion. With but minor exceptions, no instrumental changes are required, 

 since the same physical quantities and variations are measured. The tech- 

 niques of application are practically identical, except that the station 

 spacing and the areas covered are smaller in engineering geophysics than 

 in petroleum geophysics. 



The equipment generally comprises a 6- to 12-trace camera, for shallow 

 refraction shooting, with film speeds of about 15 to 20 inches per second. 

 Automatic gain control and filtering are seldom advantageous. Standard 

 geophones are satisfactory, but the natural periods should be about 10 cps. 

 Portable equipment is desirable, with daylight loading and developing 

 facilities. 



Drills are not required, as the shot holes are dug with a shovel or hand 

 auger. Powder charges are small, usually from V^ to 2 pounds of dynamite. 

 In some cases, the blasting cap alone or a hammer blow will furnish suffi- 

 cient energy to produce a good useable record. 



The spread lengths are generally from 200 to 1500 feet, with seismom- 

 eters spaced 25 to 150 feet apart. The stations are spaced as required, 

 usually about equal to the depth of investigation. Refraction profiles are 

 generally run across the area, and interpretation is based upon breaks in 

 the travel-time curves. 



The accuracy of this type of refraction work is generally ±: 5 per cent 

 of the depths obtained by drilling. The crew personnel usually comprises 



t M. B. Dorbin, B. Perkins, Jr., and B. L. Snavely, "Subsurface Constitution of Bikini Atoll 

 as Indicated by a Seismic-Refraction Survey," Bulletin of the Geological Society of America, Vol. 

 60, May, 1949. 



