1048 EXPLORATION GEOPHYSICS 



impervious beds of low resistivity are characterized by slope changes, or curvatures, 

 with the convexity toward the positive side of the log. Highly resistive formations 

 correspond to substantially straight parts of the S.P. log. 



Field Examples. — Figure 646 shows two typical field examples of S.P. logs and 

 the corresponding resistivity logs. 



Figure 646(a) illustrates the case of a section comprising mostly shales and sands. 

 In front of bed 1 the greatest S.P. deflection — about 115 millivolts from the base line — 

 is attained. It corresponds to a sand having a low resistivity ; the S.P. curve is steep 

 and the boundaries of the sand are well defined. It is likely that the magnitude of the 

 deflection is not far from the value of the static S.P. 



Although sand body 3 is, of greater total thickness, the S.P. deflection is appre- 

 ciably less— about 95 millivolts. This is due partly to the presence of intercalated shale 

 beds, notably at 6830', 6835', and 6840', and partly to the higher resistivity of the sand, 

 which accounts also for a certain rounding off of the S.P. curve. A similar feature is 

 observed in front of bed 5. Since bed 2 is very thin, its indication on the S.P. log is a 

 sharp peak whose amplitude is definitely lower than the value of the static S.P. 



In front of bed 4 the S.P. log shows an almost constant slope; the S.P. rises pro- 

 gressively from the base line to a maximum deflection of about 105 millivolts. This 

 probably indicate^ a transition zone of shaly sand. 



Figure 646(b) illustrates an S.P. log recorded in a limestone section. The perme- 

 able zones are shown by the S.P. log, but their limits are not well defined. 



The two S.P. logs of Figure 646 are very different in shape, although the E.M.F.'s 

 involved are of the same order of magnitude. The first shows steep variations of great 

 amplitude, and the boundaries of the permeable zones, at least of those of appreciable 

 thickness, can be determined with good accuracy. The second, on the contrary, due to 

 the efl'ect of the high resistivities encountered in the limestone, presents more gradual 

 variations of the S.P., and the boundaries of the different formations, particularly of 

 the permeable zones, are poorly indicated. 



Measurement of S.P. — The recording of S.P. logs is a simple technique, as shown 

 in Figure 647. A measuring electrode M is lowered in the hole at the end of an insu- 

 lated cable. The differences of potential between an electrode N* which, being station- 

 ary, is at a fixed potential, and electrode M, whose potential varies as it is moved along 

 the hole, are observed by means of a recording galvanometer. A curve is recorded on 

 which the ordinates are proportional to the depth of electrode M, and the abscissae 

 represent the potential of electrode M with reference to electrode N.** 



The drill holes in which the S.P. logs are recorded are usually filled with mud 

 having a water base. The density of the mud is ordinarily such that at each depth the 

 hydrostatic pressure in the hole is greater than the formation pressure ; as a result, the 

 fluid contained in the permeable beds cannot contaminate the mud. Also, the mud has 

 been in constant circulation during the drilling operation, prior to the logging, and 

 therefore it is homogeneous. 



From the circuit shown on Figure 647, it can be seen that the recording galvanom- 

 eter measures all the differences of potential appearing between electrodes M and A^. 

 However, provided proper precautions are taken, experience has shown that under usual 

 conditions the deflections on the S.P. log correspond to phenomena occurring at the 

 contacts between the mud and the different beds, and also at the contacts between the 

 beds themselves. These phenomena produce an electric current, called the S.P. currentf 

 which uses the mud as its return path. Potential differences are created by ohmic effect. 



Other sources of potential, which are not related to the formation, do not usually 

 cause any deflection on the S.P. log ; if they are present and bothersome, proper steps 



* Usually placed in the mud pit, for convenience. 

 ** In usual practice, the S.P. curve is recorded simultaneously with the resistivity curve. 

 t The expression S.P. current may seem rather illogical, as S.P. stands for spontaneous poten- 

 tial; however, as it has become common usage, it will be employed here. 



