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In most parts of the world, open hole logging with con-ventional gamma density tool remains the only option available, which is, though, not practical under complex well conditions when it is difficult to obtain formation parameters and control the operational risks. Therefore, the application of through-casing density logging is es-sential. However, through-casing gamma density meas-urement could be challenging due to gamma’s weak pen-etrating power. To better overcome these challenges, a new four-detector gamma density tool is designed and presented in this work together with a novel interpreta-tion algorithm for through-casing density measurements. In order to identify the relationship between the four de-tectors, a significant amount of Monte Carlo models is constructed and executed to quantitatively evaluate the response of tool under various environmental conditions such as lithology, fluid, salinity, casing and cement. It is proven through open-hole benchmarking and extensive modelling that the four-detector design is capable to pro-vide sufficient information for removal of diverse envi-ronmental effects via an inversion algorithm developed by obtaining a formula that correlates the four-detector responses. This algorithm is validated against actual log-ging data and show a better though-casing evaluation than when using three or two detectors. In this work a case story using the four-detector tool to obtain through-casing formation density and using the algorithm mentioned above for correction is presented. The final results are analyzed and compared against open hole measurements, a very good consistency is achieved which demonstrates the correctness of the inversion al-gorithm.

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