1136 Subsurface Geologic Methods 



consideration in engineering geology are rather small, each geophysical- 

 exploration program must be tailor made to fit the conditions that are 

 expected to be encountered. 



The most widely used methods of geophysical exploration in the en- 

 gineering field are the seismic-refraction and the electrical-resistivity 

 techniques. These two methods depend on artificial or man-made fields of 

 force. The two other forms of geophysical exploration have been used to 

 a minor extent: namely, the magnetic and gravimetric methods. In these 

 last two methods, the force field, in which measurements are made, is set 

 up by nature: i.e., the earth's magnetic and gravitational fields. 



The matter of force fields is brought out at this time because it is 

 basic to the functioning of geophysical methods and their ability to give 

 results interpretable in geologic terms. In field problems amenable to 

 geophysical solution, there must exist contrasts in the physical proper- 

 ties of rocks or formations that give rise to detectable differences in the 

 particular field of force involved. For example, in seismic-refraction 

 measurements for bedrock depth, the bedrock must transmit seismic waves 

 at a higher velocity than the overburden. In like manner, to be detectable 

 with a magnetometer, a dike must be more magnetic than its surrounding 

 rocks. Figures 596 through 599 depict general and ideal application for 

 the various types of geophysical methods named in the preceding para- 

 graphs.^ 



Sequence of Engineering Geologic Investigations 



Most engineering geologic investigations are carried out in successive 

 stages: namely, reconnaissance, site selection, preliminary design and es- 

 timate, final design, and specifications. The amount and type of investi- 

 gation required in each stage cannot always be determined beforehand. 

 Exploration should be limited to the essentials necessary to obtain the 

 information required for any particular stage. It is important that data 

 be assembled during or immediately after the work is finished on each 

 stage, while it is fresh in the geologist's mind and before the material, 

 such as cores, has been affected by exposure to air or moisture. The 

 stages of exploration will be discussed in the following paragraphs. 



Stage A: Reconnaissance 



Stage A or the reconnaissance is what the name implies. It will, in 

 most cases, determine whether it is possible to build economically the 

 desired structure in the area chosen. Usually, the reconnaissance exami- 

 nation for investigation is conducted on a number of possible sites. 

 Reconnaissance examination will involve the utilization of all data, de- 

 tailed or general, already existing for the area, such as topographic and 

 geologic maps and reports by state or federal agencies or private indus- 

 tries. A field examination will be made by experienced geologists and 



* Wantland, Dart, U. S. Bur, Reclamation, Denver, Colorado, personal communication. 



