Miscellaneous Subsurface Methods 557 



envelopes before and after use. They may be subjected to weak light for 

 some time before the disc is discolored. 



The type "M" drift indicator is run in the same barrels and in the 

 same manner as was described for the type "W" instrument. Similar 

 rental agreements and service arrangements are made. 



Table 28a shows data on type "M" instruments and barrels. 



A few to 15-degree-range instruments are available for use in high 

 drift-angle wells. 



TABLE 28a 

 Type "M" Drift Indicator 



Instrument Barrels* 



Data table type "M" Standard Small 



to 6° 

 % in. 



131/2 in. 

 12 oz. 



1% in. 



5 ft. 9% in. 



28 lbs. 



1%" 



5 ft. 8 in. 



22 lbs. 



Angle range 



Diameter 



Length 



Weight 



* Barrels are interchangeable with either wire line, or "go devil" connections. Sinker 



bars are also available. 



Drift-indicator readings usually are taken at about 100-foot intervals 

 as a well is drilled. By simple computations, and applying the drift to the 

 correct segments of the hole surveyed, one can obtain a maximum possible 

 deviation. By inspection of the different readings taken as drilling pro- 

 ceeds, remedial measures can be taken to assure that the well will not be 

 kinked or "dog-legged," or that the hole does not assume an angle higher 

 than the contract limit specified. 



All single-shot instruments in general use are the photographic type, 

 the data being recorded upon a sensitized disc by a simple camera which 

 photographs an angle unit and compass. These instruments indicate both 

 the drift and the direction of the bore at any point in a drilling well 

 where the hole is open or uncased. The single-shot machine is the most 

 convenient instrument for surveying when directional-drilling techniques 

 are being used. Its use is essential in vertical-hole drilling near lease 

 lines and where correct bottom-hole spacing is vital. Since, like the drift 

 indicator, it is run as the drilling proceeds, it enables the driller to take 

 steps to change the course of the well if it is not satisfactory. Detailed 

 advantages of the use of a single-shot instrument will be discussed later 

 in this section. 



There are two kinds of Eastman single-shot instruments, basically 

 similar in construction and operation. The regular or open-hole single 

 shot is 2^ inches in diameter while the small type or inside single shot 

 is 1^ inches. 



The Eastman regular single shot (fig. 272) contains a contact-timing 

 watch which controls the time at which a record is taken by turning on 

 the lights. Large flashlight cells supply the current for the light bulbs 

 which illuminate an angle unit. A simple camera lens focuses the image 



