If a clean, water-bearing zone is present or can be assumed, ''R''<sub>w</sub> can be calculated by using the Archie equation. The zone or permeable bed in which water resistivity is determined is selected on the log. This zone must be 100% water saturated and must not contain any clay or shale. The bed must be thick so that the deep investigation resistivity device is not affected by shoulder beds.
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If a clean, water-bearing zone is present or can be assumed, ''R''<sub>w</sub> can be calculated by using the [[Archie equation]]. The zone or permeable bed in which water resistivity is determined is selected on the log. This zone must be 100% water saturated and must not contain any clay or shale. The bed must be thick so that the deep investigation resistivity device is not affected by shoulder beds.