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New Zealand Listener, Volume 32, Issue 823, 6 May 1955, Page 21

galvanometer (G in Fig: 1) set up on the surface. The voltage dif- crence between S and S' 1S read on the galvanometer and shows the resistanceof the stratum between these two circles (in actual fact we should speak of two concentric spheres, but in the drawing these are shown as circles). The measurements are repeated at SCHLUM BE various depths, and a graph is made Krc * of the course of the resistances Since salt water, fresh water and oil all conduct electricity to a greater or less extent, as the graph shows, the presence of these substances in the layers investigated can be presumed from studying such a line. And as an exact record is kept of the depth; a clear picture is thus obtained of the nature of the layers drilled: Eyes 40 20,30,40 2 300 that see 350 RESISTANCE through the Ground iNOzMS M2M 3 20 30.40 Put a matchbox on a high cupboard amination of drill cuttings or 400 and tie a length of string on 1t, and samples of cores will tell him quite you have 4 scale model" of an oil a lot: But this can involve consider- 1 well: The matchbox is the derrick; able waste of time, or be very inade- the string the drill- ~pipes or well quate. The ability to see under- 450 'PLANE Of FRACTURE casing: Various things can happen ground would be of great assistance B in So deep a shaft ~collapse of the to him. hole, penetration of masses of In course of time technical science 500 water from strata which have been has provided him with 4 number of { 6� instruments which can see under 2 the ground for him. One of these is the Schlumberger apparatus, noW 6 550 BETDVSEANCELLS widely adopted in the development 20om Fig. 2. of oilfields. This instrument consists in principle of an electrode (A If measurements are carried out in in Fig: 1), which is lowered into several wells located close together, the uncased borehole at the end it is often possible, by comparison of a long electric cable A of the graphs, to draw conclusions certain electric voltage is applied to as to the subsurface structures (see this electrode and transferred Fig: 2) through the water-filled borehole A great deal of other data can be into the surrounding layers. This got by applying the Schlumberger voltage decreases the greater the method. What has been told here S distance from the electrode, or as is only one example of the way in the character of the strata changes: which: technical science is continu- 6 ally inventing new methods to aid It should now bepossible to connect the search for and production of 2 points of equal voltage by a line, in petroleum. 2 Fig. I which we can imagine circles arising The Shell Group such as those which intersect points of oil companies consistently tries drilled through, and SO on: The S and S' in Fig. I. At these circles * apply the best methods for pro- driller must rely on his professional two more electrodes,M and N;hang ducing oil in order to satisfy the knowledge and intuition to put in the well and are connected to a growing demand for petroleum and things right again. Top: The measuring cat the products derived from it: Often he wants to sce what sort of Left : Diagram of the Schlumberger apbaratus. No. 4 of (L series of articles published by THE Right : Comparison of two graphs shows 4 SHELL COMPANY OF NEW ZEALAND strata he has bored through: Ex- fault plane. LIMITED (Incorborated in England)