Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 ADVANCES IN OIL PROSPECTING in a. general: power sources As a source of power, one Radio B battery, or two, may be suffi- cient, depending on the length of the current basis, of course. If very great depth penetration is required, a generator is used, driven by a gasoline engine. When using a Megger, a hand-driven generator is used, which is incorporated in the meter box. Due to the fact that polarization must be eliminated, if non-polarized electrod
Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 ADVANCES IN OIL PROSPECTING in a. general: power sources As a source of power, one Radio B battery, or two, may be suffi- cient, depending on the length of the current basis, of course. If very great depth penetration is required, a generator is used, driven by a gasoline engine. When using a Megger, a hand-driven generator is used, which is incorporated in the meter box. Due to the fact that polarization must be eliminated, if non-polarized electrodes are not used, a commutator is provided in the measuring circuit, which brings it about that in reality the current used is not strictly Potentiometer Mi///'ammeter Electrodes making contact with Earthy C,fe/'— a i|L/ a — -^41 \r7Wr? j^wwav >2 Earth Fig. 8.—Schematic diagram of Gish-Roonejr circuit. , but an alternating current of low frequency. Induction effects are, therefore, sometimes observed between the current and the potential leads; besides, not the ohmic resistance of the ground is obtained, but the impedance for the particular frequency employed, which for the commutators in Gish-Rooney outfits is approximately 16, and nearly 50 in Meggers. The only way to avoid polarization and induction effects is by using non-polarized electrodes, which consist of cups filled with copper sulphate and provided with a permeable bottom, made either of porous clay or of wood. The induction between leads is minimized when using the po- tential measuring line at right angles to the current basis, as already described. 441
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