1080 EXPLORATION GEOPHYSICS 



Field Procedure. — Common practice is to run the dipmeter immediately following 

 the electrical log. The levels to be examined for dip are chosen, as explained later, 

 from the electrical log. Meanwhile, the dipmeter assembly — electrode unit, photo- 

 clinometer, and spring centering guides — has been made up to the cable and lowered 

 into the hole. 



The assembly is lowered to a point in the hole just below the first level or zone. 

 After a short interval to allow the compass needle and the indicating ball in the photo- 

 clinometer to come to rest, a photoclinometer picture is taken. This picture is used to 

 determine the orientation of the electrodes and the inclination of the drill hole at the 

 base of the level. The assembly is then pulled slowly up the hole, and the three dipmeter 

 curves are recorded to the top of the level. At that point, the dipmeter assembly is 

 again stopped long enough to take another photoclinometer picture. If the level is 

 unusually long, an intermediate photoclinometer picture may be taken in the middle 

 of the level. These recordings will permit the interpreter to ascertain if the assembly 

 has rotated excessively while being pulled up-hole. 



The recording of one run over a typical S.P. dipmeter zone is shown in Figure 673. 

 The three curves are recorded on an enlarged depth scale, and are coded as solid curve, 

 dashed curve, or long-dash, short-dash curve, according to which pair of electrodes 

 was used to make the recording. The photoclinometer pictures at the bottom and top 

 of the level are also indicated. 



Interpretation and Accuracy.— The accuracy of the measurements will vary ac- 

 cording to the value of the dip angle, the sharpness of the breaks or other electrical 

 markers in the recorded curves, the angle of drift of the bore hole, and the diameter of 

 the dipmeter electrodes, which depends on the hole diameter. Under normal conditions, 

 it has been found that for dip angles of 10° or greater, the accuracy is very good; 

 between 5° and 10° reliable determinations can be made when the correlations between 

 the curves are good and when the inclination of the hole is not too great; below 5° it 

 is possible to give only the general direction of the dip. 



Selection of the Zones of Measurement. — Experience indicates that the useful- 

 ness of a dipmeter survey depends principally upon a careful selection of the depth 

 zones over which the measurements are made. It is important that the sections chosen 

 include geologic or stratigraphic features which are consistent laterally for some dis- 

 tance and which are not greatly affected by local lensing, cross-bedding, or faulting. 

 A good way to determine this, when possible, is to examine electrical logs of nearby 

 wells and to select zones containing formations which repeat from well to well. 



It has been found that the most satisfactory zones for dip measurements are those 

 consisting of relatively thin beds, 2 to 10 feet thick, having sharp contacts with adjacent 

 formations. Such zones for a resistivity dipmeter level are thin limestones or resistive 

 sandstones interbedded with shale, or for an S.P. dipmeter level, thin sands or sand- 

 stones interbedded in shale. 



On the other hand, thin shale or sandy shale beds within thick sand sections, and 

 thin shale beds or minor resistivity variations within a massive limestone, frequently 

 give erratic dip determinations. 



Areas of Application.t — Those regions where the geologic section is primarily 

 sand and shale, i.e., California or the Gulf Coast, are best suited for the use of the S.P. 

 dipmeter. The spontaneous potential here in general shows sharp, well-defined anomalies 

 at formation boundaries which give definite dip determinations. Likewise, in these 

 areas, a series of bedding surfaces between sand and shale can usually be found at any 

 depth in a well where a dip determination is needed. 



The resistivity dipmeter, on the other hand, has proven of utility in such areas 

 as West Texas, the Mid-Continent, and the Rocky Mountains, where the features of 



1 



t E. F. Stratton and R. G. Hamilton, "Application of Dipmeter Surveys," A.I.M.E. meeting, 

 Tulsa, October, 1947. 



