Electromagnetic propagation resistivity sensors have become common in MWD logging applications. These tools are unique among resistivity sensors in that the depth of investigation of the phase shift and attenuation resistivity measurements varies dramatically with the formation resistivity, measuring deeper into the formation in higher resistivity environments. The fact that the depth of investigation can vary by more than a factor of two across a normal range of formation resistivities requires the log analyst to have an understanding of this phenomenon when performing both qualitative and quantitative interpretations of these logs. Other measurement characteristics such as regions of negative or sharply-rising radial response can produce log responses which may seem peculiar when compared to traditional induction logs or laterologs. However, the radial response in virtually any borehole and formation environment can be quickly determined by computer modeling. This modeling, coupled with comparisons between propagation resistivity and induction logs can provide the log analyst with an understanding of the radial response of these sensors and facilitate both qualitative and quantitative interpretations.

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