An electrical sonde for measuring the electrical resistivity of rocks within a borehole (resistivity logging). Short sondes (40 cm or less) normally measure the resistivity of the mud cake and invaded (flushed) zone. Longer laterologs measure the resistivity of the outer edges of the invaded zone and of the uninvaded rocks. Major differences in the resistivity values measured by shallow and deep sondes will occur in reservoir rocks containing appreciable quantities of hydrocarbons, so permitting the estimation of the thickness of hydrocarbon reservoirs. Interpretation of long laterologs is affected by the Delaware effect. See also microlog.