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 most appropriate method for characterizing the depth of investigation, using the results of Monte Carlo calculations by the MCNP code. A common measure of the depth of investigation called S₁(h) characterizes the detector response to a limited thickness of formation with a vacuum beyond. Alternative parameters which characterize average or median values of depth have been calculated. 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. An alternative function S₂(h), based on the effect of a small 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. An alternative function S₂(h) curve, which is directly relevant to cases of invasion by a low-Z mudcake, is very different from S₁(h) and predicts substantially lower penetration depths