162 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



epoch in the evolution of borehole deflection apparatus; 

 it was the impetus to many later designs and constructions. 

 It conquered the continuous record problem, if however 

 crudely, successfully. We may mark from its inception 

 the rapid evolution of new methods which began in the 

 first decade of this century. Its chief drawbacks are: 



1. It is costly and complicated to make. 



2. It is heavy though easy to manipulate. 



3. Its mechanism and tubes limit the diameters for which 

 it can be adopted. 



Firsi- Measuremenfwifh Apparatus 



Second " 



Normal ]nsirumeniail Survey 



Fig. 101. — Checked survey by Erlinghagen's method. 



4. Pin-pricking devices are crude and likely to cause 

 confusion in reading. 



5. Moisture is likely to injure the apparatus and cable. 

 Thurmann's Apparatus. — This apparatus is built on the 



proportionality principle, the basis of the lead-basket 

 plumbing method, but it greatly extends the limits of 

 applicabihty of that principle. 



H. Thurmann, Sr., of Halle obtained reliable results 

 with his apparatus, which is a double plumb bob and linked- 

 tube device, at fair depths. The invention^ (Figs. 1 to 9, 

 Plate XI) consists of straight tubes joined by special cruci- 

 form joints movable in all side directions but not rotatable. 



1 Organ des Verein der Bohrtechniker, No. 17, p. 190, 1909. 



