1078 



EXPLORATION GEOPHYSICS 



Curve* racordad on 

 Clactrodci #I,M2,&«3 

 at indicated 



Indicotion of 

 hole inclination 



is operative where there are no geological disturbances between the wells 

 and when three correlatable logs are available. 



Electrical dipmetersf utilize a somewhat similar idea, but require the use 

 of only one well. The principle is illustrated by the electrode arrangement 

 shown schematically in Figure 671, in which the spontaneous potential is 

 logged simultaneously on three sides of an uncased hole. The three inde- 

 pendent electrodes are located 120° apart in a plane perpendicular to the 



axis of the instrument and are 

 mounted at a radial distance 

 such that they are all fairly 

 close to the wall of the hole. 

 (Also see page 506.) 



In Figure 672, curves 1, 2, 

 and 3 represent the S.P. curves 

 recorded simultaneously by 

 each of the three electrodes as 

 the sonde traverses the bore 

 hole. The sharp break in each 

 curve represents the instant 

 when the corresponding elec- 

 trode crosses the formation 

 boundary. It is noted that 

 these breaks are not recorded 

 at exactly the same depth on 

 each electrode, but differ by an 

 amount a for electrodes 1 and 

 2 and an amount h for elec- 

 trodes 1 and 3. These variations in depth correspond to, and are a measure 

 of, the dip of the formation boundary, and consequently of the formations 

 themselves. 



If the hole were assumed to be vertical, it would be easy, knowing the diameter 

 of a circle through the dipmeter electrodes and the differences in depth between 

 correlated points on the recorded curves, to calculate the angle of dip of the formations. 

 In order to determine the direction of the dip, the orientation of the instrument must 

 be known. 



When the drill hole is not vertical, as is the general case, the problem becomes 

 more complex, and for the correct evaluation of the angle of dip a knowledge of 

 the inclination of the hole and of the orientation of the instrument is also necessary. 

 The additional data required to determine completely both the amount and direction of 

 the dip are furnished by a photoclinometer which is included as a part of the dipmeter 

 assembly. The photoclinometer picture (bottom of Figure 673) serves two purposes: 

 (1) a triangular marker (a) on the edge of the picture shows the position of the #1 

 dipmeter electrode with respect to magnetic north, i.e., indicates the orientation of the 

 instrument; and (2) the position of the black dot (b) shows the amount and direction 

 of inclination of the drill hole from the vertical. 



Position of 

 W i Electrode 



Ptiolocllnometer Picture 



Magnetic North 



Fig. 672. — Schematic an-angement of the S.P. dipmeter. 



t H. G. Doll, "The S.P. Dipmeter," A.I.M.E. Tech. Pub. 1547 (1943). 

 H. G. Doll, "Method and Apparatus for Determining the Dip of Strata Traversed by a Bore- 

 hole," U. S. Patent 2,427,950, Sept. 23, 1947. 



