CORE ORIENTATION 57 



1. There must be no mud or cavings between the core 

 and borehole walls. 



2. The core must be sufficiently rigid so as not to fracture 

 on extraction and to preserve the markings. 



The changing of the rods, etc., make condition 1 very- 

 difficult, since we then interfere with the rinsing. In 

 very hard rocks condition 2 might be impossible, owing 

 to lack of clarity in the marking. In soft rocks this latter 

 condition is impossible. These methods, it will be seen, 

 take up much time and are not now in operation. 



Vivian's Method. — The method of the American diamond 

 driller, Vivian,^ marked a new departure and significant 

 advance in core orientation. He drilled a small pilot hole 

 of a few inches diameter and lowered a small instrument 

 case into it, so that a part of it was fixed in the pilot hole. 

 This case held a compass needle clamped by a weight used 

 in setting the case. When the core was recovered the case 

 was also recovered attached to its upper end. Figure 30 

 shows the compass c and its arresting apparatus a and the 

 tap neck 5 in the pilot hole d. The needle, free at first, 

 is fixed by letting down the weight. This was all retrieved 

 later in the normal method of core catching. Above- 

 ground the needle is freed and the core turned to give the 

 position before arrest. The core now is in the same posi- 

 tion as in the hole, and so its dip and strike can be obtained. 

 The demerits are 



1. The apparatus is almost, if not quite, impossible of 

 use under a big head of water pressure. 



2. Cavings filling the pilot hole as when concussion occurs 

 during coring, rupture of the core and mud. 



3. In small holes the pilot hole thins the core itself to a 

 too fragile degree, the wall thickness in diamond boring 

 needing to be at least 12 to 18 mm. and in addition we must 

 consider the play on both sides. 



4. A compass can not be set vertically true in a small 

 core. 



1 Trans. N. E. Inst. Min. Eng., p. 45, 1881-1882. 



