CHAPTER VI 

 COMPASS AND PLUMB-BOB METHODS 



Introductory Note. — The magnetic compass as a direc- 

 tional agent has been known in the East since time imme- 

 morial, coming to Europe in the beginning of this millenium ; 

 consequently it is not surprising to find it one of the chief 

 deviation measurers in borehole surveying. 



It is a simple and reliable tool even in places of rapidly 

 altering declination, since its readings in the borehole 

 are more or less directly under the surface station where 

 they are being deciphered. In regions of great magnetic 

 disturbance, such as the great iron fields of Sweden and 

 the United States and also in certain iron-bearing basic 

 igneous rocks, it is untrustworthy and misleading. These 

 remarks apply also to boreholes which are steel lined to 

 spots near the place of its application and to apparatuses 

 incorporating any other than non-magnetic material in 

 their construction. It is also conceivable that periods of 

 magnetic storms and the phases of diurnal and secular 

 variation might sensibly affect the accuracy of the magnetic 

 needle. 



The plumb bob, on the other hand, is a far more constant 

 servant obeying ever the line of gravitational pull which 

 does not vary to any measurable degree for our present 

 purposes anywhere on the earth. In the short length 

 of the longest plummet used in borehole survey work, 

 masses of great altitude like the great mountain chains or 

 lack of them like the great depths of the sea, in its proximity, 

 can not alter its suspension line to any measurable extent. 



One of the earliest uses of the compass in borehole surveys 

 in Britain was that adopted by Mr. Haddow at Younger's 

 Holywood Brewery, Edinburgh, in 1884. It did much to 

 stimulate interest in the compass as a borehole deflection 



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