Geophysical well logging

Abstract

Geophysical logs are made by instruments in sondes, or tools, suspended mostly on a wire, with readings usually taken as they are pulled to the surface. Sondes often contain several instruments that can operate without mutual interference. The records of the variation of various quantities down boreholes are called logs, and are of both geophysical and nongeophysical quantities. Geophysical logs are often more valuable when combined than when used singly. Geophysical logging is not only cheaper than continuous coring but it provides information that cannot be obtained on cores, partly because of the alterations produced by the dribbling, and reduces the need for sampling. Measurements of geophysical parameters in boreholes present the follows characteristics: – Instruments have to be adapted for the dimensions of a borehole and usually to operate submerged in drilling mud. – Measurements have to take into account the conditions (pressure, temperature, and technical conditions of the borehole) resulting from the drilling. The most important application of well logging in the exploration, evaluation, and production of oil and gas, as well as other uses in different geological and engineering branches has been discussed.