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2-mhz Propagation Resistivity Modeling In Invaded Thin Beds

Modeling of resistivity devices requires some knowledge of the interaction of one-dimensional effects such as radial effects (borehole and invasion) and axial effects (thin layers). This is particularly true for the 2-MHz propagation resistivity devices because their high frequency of operation results in more interaction between the one-dimensional effects. In this article, a new integral equation computer program is used to model a 2-MHz propagation resistivity tool in formations with both radial and axial boundaries. Results of this modeling show that the amplitude ratio measurement is deeper that the phase difference for almost all cases of practical interest. However, when a thin bed is invaded, the poorer vertical resolution of the amplitude ratio measure can result in a reversal of the normal order of the two curves. (This is an effect that can also be seen in the medium and deep induction pair.) In addition, the depth of investigation increases as the nearby resistivity increases. The negative-response zones observed in the modeling of radial responses are also seen when thin beds are also present. Comparison of a case of a ramp-invasion profile with one using a step-invasion profile shows that the two different models produce virtually the same results if the step is placed at the center of the ramp.
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