Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 ADVANCES IN OIL PROSPECTING 103 of the bridge; the fixed comparison coils are in the other arm of the bridge; both connect to the variable slide wire and the secondary of the buzzer coil. The comparison coils are made up of three fixed re- sistances of 10, 100, and 1,000 ohms. An additional resistance of 100 ohms may be used in series with the cup when its resistance is low. The balance is adjusted on the slide wire, with proper setting of the fixed resistance
Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 ADVANCES IN OIL PROSPECTING 103 of the bridge; the fixed comparison coils are in the other arm of the bridge; both connect to the variable slide wire and the secondary of the buzzer coil. The comparison coils are made up of three fixed re- sistances of 10, 100, and 1,000 ohms. An additional resistance of 100 ohms may be used in series with the cup when its resistance is low. The balance is adjusted on the slide wire, with proper setting of the fixed resistances, and the reading on the slide wire is multiplied by the resistance of the comparison coil used. The use of the cup is very convenient as not only soils, but also electrolytic solutions may be tested in it. It may be standardized by using solutions of known concentrations. The results must be re- Fig. 5.—Shepard earth-resistivity meter. ferred to a normal temperature. The publication by R. O. E. Davis (ref. list No. Ii) gives a number of tables on resistivities of soils and solutions which will be found valuable when using this bridge. b. DETERMINATION OF RESISTIVITIES ON OUTCROPS On account of the difficulties which are inherent in the accurate determination of the resistivity of samples and which have been dis- cussed before, determinations of the resistivities on outcrops, or, generally speaking, on the ground surface are ordinarily much more satisfactory. Quite a few methods may be used for this purpose. For approxi- mate tests, the Shepard earth-resistivity meter may be employed (Fig. 5). It is an instrument working on ; the ohmmeter and the battery are mounted on one probe; the probes are made of iron, and 433
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