1072 



EXPLORATION GEOPHYSICS 



which at a position on the film corresponding to a given depth of the sonde has an 

 amplitude indicative of the magnitude of the S.P. or apparent resistivities measured 

 at that depth. The film drive is usually connected by a servo-mechanism with the 

 sheave or a measuring wheel frictionably held against the cable. By use of suitable 

 gearing, a proper relationship is obtained between a given movement (in feet) of the 

 cable, and a movement (in inches) of the recording paper, usually 1" to 100', 2" to 100' 

 or 5" to 100'. A sheave wheel feed system is illustrated in Figure 652. 



Modern equipment for the electrical logging of deep wells may employ as many 

 as six conductors, in an armored cable, connected to a nine-galvanometer recorder. This 

 equipment is appropriate for service in the deepest holes, and can record on a single 

 run: the S.P. curve, three resistivity curves, including as well reduced sensitivity scales 

 for each of the resistivity curves where necessary, and an amplified recording of the 

 short normal, if desired. For shallow wells, a four-conductor cable covered with nylon 

 braid is often used with a nine-galvanometer or a five-galvanometer recorder. 



The standard electrical logging sonde can be used in holes as small as 5" in 

 diameter. For smaller holes special core-hole equipment is available. 



Photographic means are usually provided for developing and fixing the film imme- 

 diately upon completion of the survey. The film is then dried. If required, short prints 

 may be made of the section of immediate interest. 



Fig. 668. — Resistivity cell for determining the specific resistance of the 

 drilling mud. A, plastic cylinder; B, end contact plugs; C, ohm-meter; 

 D, thermometer. 



A resistance cell and thermometer should be provided for determining the specific 

 resistance of the mud. One type of cell is illustrated in Figure 668, and comprises a 

 plastic cylinder of known cross-sectional area, fitted with metal end contact plugs, 

 spaced a fixed distance apart. A thermometer is immersed in a well, for measuring 

 the cell temperature at the time the resistivity determination is made. A conventional 

 ohm-meter may be employed for measuring the resistance of the cell. The specific 

 resistivity of the mud may then be calculated, knowing the area of the mud and the 

 distance between the end plugs. (See page 467.) 



METHODS OF LOCATING WATER SOURCES 

 IN A DRILL HOLE 



The location of the source, or point of entrance, of formation water 

 in oil wells is a problem of considerable economic importance in oil 

 production. It is good engineering practice to keep the production of 



