The term applied to a variety of techniques of measurement of physical properties (by lowering suitable instruments) in a borehole to provide information about the geological strata traversed, or the directional attitude of the borehole itself. Logs may be run to give continuous and detailed information through-out the full depth of a borehole, or they may be restricted to selected zones of particular interest for potential production of oil, gas, or water. In zones of poor core recovery, well-logging has great advantages in providing information of use in solving problems of correlation between adjacent wells, because of the completeness of the well log record. Techniques employed include:


The measurement of variations in potential due to the natural currents which flow in the circuit formed by less permeable strata such as shale, the drilling fluid in the borehole, and more permeable strata such as sandstone.


The measurement of the resistivity of strata by means of electric currents applied via a multi-electrode sender. The correlated interpretation of S.P. and resistivity logs can provide information about the porosity and permeability of strata, and about the nature of the pore fluid.


The measurement of the natural gamma ray activity of the strata by means of a Geiger counter or, preferably, a scintilla meter.


The measurement of induced gamma ray activity due to the capture of neutrons emitted from a suitable source. Gamma ray and neutron logs provide information about the nature of the strata penetrated and give measure of the hydrogen-bearing fluid content.


The variation of temperature in a bore-hole due to differential rates of heat exchange between drilling fluid and formations can yield information about the nature of the strata. A temperature log can also locate the source of gas flow, zones of lost circulation of drilling fluid, and the position of cement set between casing.

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