48 DEEP BOREHOLE SURVEYS AND PROBLEMS 



6. Gyrostatic methods where the principle of the gyro- 

 scopic compass is employed. 



7. Plastic cast methods in which set models of the hole 

 and its core stump are provided. 



8. Pricker methods operated by electromagnet plungers, 

 levers, plumb bobs or in any other way, on paper strips, 

 soft discs or plates. 



9. Inertia methods wherein the inertia of a heavy rotating 

 body is employed. 



10. Seismographic or geophonic methods in which 

 vibrations caused on the surface by explosions or the vibra- 

 tions caused by drilling, particularly cable-tool drilling, are 

 recorded. 



The general subject of borehole investigation can thus, by 

 the above methods, be broadly divided into two main issues : 



a. The actual survey of the course of the borehole in 

 azimuthal deviation and inclination from the line of its 

 intended course. 



b. Core orientation in which the original underground 

 position of the core is established. It is, of course, limited 

 in its field of application by being only applicable to holes 

 yielding cores. 



The two main branches a and h of our subject necessarily 

 merge one into the other by reason of their close relation 

 and the instruments employed being often of dual utility. 

 Core orientation provides useful information as to the 

 direction and amount of stratigraphic dip; information 

 very difficult to obtain when boreholes incline through 

 inclined beds. This will be seen by Fig. 21, where we will 

 often meet the difficulty of having unreliable data as to 

 whether a or ai is the truthful vertical thickness of the seam. 

 The great value of core orientation surveys in fields insuffi- 

 ciently mapped geologically, as in wild-cat ventures, is 

 obvious; also where evidence is misleading or misinter- 

 preted, as often in unconformities, asymmetric conditions, 

 hidden dislocations, alluvial deposits and where we get 

 change of facies.^ The retention or rejection of accumu- 



^ Macready, G. a., Bull. Amer. Assoc. Petroleum Geol., May, 1930. 



