Depth of investigation of density tools

The depth of investigation is an important parameter for all tools used in well logging. The authors examine the issue in detail for the dual detector gamma-gamma sonde, using Monte Carlo simulations. A common measure of the depth of investigation is based on the detector response S(h) to a limited thickness of formation with a vacuum beyond. The 90% point depth, which produces 90% of the full detector response, is often quoted in the literature. However, it is considered by the authors that this parameter would be better described as a measure of maximum penetration than average penetration, and can therefore be misleading if quoted in isolation. Alternative parameters which give an indication of average or median penetration have been derived from the results of Monte Carlo calculations in idealized geometries. It is shown that these values are substantially lower than the 90% point depth. An alternative to S(h) is also considered. The variation of penetration depth as a function of the formation density, source collimation angle, and short spacing detector collimation angle has been characterized. It is shown that all these factors have a significant effect on the penetration depth of a tool.<>