AUXILIARY REGISTRATIONS IN BOREHOLE SURVEYS 29 



These difficulties may occur when 



1. Casing operations are obstructed. 



2. Cutting bits jam on extraction. 



3. Abrasion develops at localized places. 



4. Cushioning occurs on the percussion stroke, 



5. Water circulation is affected. 



6. Sludging, pumping, bailing and such operations are 

 hindered. 



The action of these gages need not be intermittent, i.e., 

 a continuous reading can be made for only one insertion 

 of the apparatus. Former methods of laborious multiple 

 readings are thus avoided. A borehole becomes restricted 

 chiefly owing to the following causes: 



1. Inexpert tiller work on hand-turned drilling with a 

 straight bit; cruciform or horseshoe bits are less likely to 

 cause diameter restriction. 



2. Buckled casing due to joint or sheet rupture under 

 internal pressure or external strata movement on weak 

 casings. 



3. Earthquakes. 



4. Time charges at hole base. 



5. Curvature of the borehole and its causes. 



6. Uneven wear on the cutting tool not attended to in 

 time.^ 



Rumpf and Kleinhenn's Apparatus. — This apparatus 

 can also . be used for tubes and flues. The chief part 



Fig. 8. 



of the device (Fig. 8) consists of a system of calipers 

 arranged to follow the inner walls of the borehole or casing, 

 its movement being obtained as a magnified image either 

 optically or mechanically inside the borehole, 



iWoTZASEK, F., Z. I.V.B., p. 178, June 20, 1928. 



