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induction sonde A variety of electromagnetic (EM) instrument for producing and measuring eddy currents within the rocks surrounding a borehole. The strength of the eddy currents is proportional to the conductivity (1/resistivity) of the surrounding rocks. Penetration is usually greater than by laterolog sondes. The main application of the log in hydrocarbon exploration and production is in the estimation of water saturation. Interpretation is affected by the Delaware effect.