radius of investigation

1. n. [Formation Evaluation]
A distance that characterizes how far a logging tool measures into the formation from the axis of the tool or borehole. The term is similar to depth of investigation, but is more appropriate for certain azimuthally symmetric measurements, mainly resistivity. The radius of investigation summarizes the radial response of the measurement in one or more distances, and 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 radius of investigation can vary considerably in inhomogeneous conditions, and at different values of the properties concerned. It should be considered only a qualitative guide to tool response.
2. n. [Well Testing]
The calculated maximum radius in a formation in which pressure has been affected during the flow period of a transient well test. While not absolutely accurate, the value has meaning in relation to the total volume of reservoir that is represented by calculated reservoir parameters, such as kh, the permeability thickness. This may also be termed transient drainage radius.