1. . 
A distance that characterizes how far a logging tool measures into the formation from the face of the tool or the borehole wall. The depth of investigation summarizes the radial response of the measurement in one or more directions. For nuclear and resistivity measurements, the depth of investigation should be associated with the percentage of signal received from within that depth, typically either 50% or 90%. Most quoted depths of investigation assume a homogeneous formation with certain properties, such as a given resistivity or fluid content. The depths of investigation can vary considerably in inhomogeneous conditions, and at different values of the properties concerned. They should be considered only a qualitative guide to tool response. For other measurements, the depth of investigation is either well-defined by the tool physics (in the case of nuclear magnetic resonance), or else can be given only approximately, an accurate value being too dependent on formation properties (in the case of acoustic and electromagnetic propagation). The term is used for all measurements but is most appropriate for azimuthally focused devices such as nuclear logs. For azimuthally symmetric devices such as resistivity logs, the term radius of investigation is more appropriate.