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The most direct way of finding water resistivity (''R''_w) is to obtain a sample of formation water and measure its resistivity. However, this is seldom possible in practice, as formation water samples, if available, are invariably contaminated by mud filtrate. ''R''_w is therefore usually calculated, and there are three methods available for this purpose:

The most direct way of finding water resistivity (''R''_w) is to obtain a sample of formation water and measure its resistivity. However, this is seldom possible in practice, as formation water samples, if available, are invariably contaminated by mud filtrate. ''R''_w is therefore usually calculated, and there are three methods available for this purpose:

* SP method

* [[Basic open hole tools#Spontaneous potential|SP]] method

* Archie equation

* Archie equation

* Resistivity ratio method

* Resistivity ratio method

[[file:determination-of-water-resistivity_fig2.png|thumb|{{figure number|2}}Example of a reverse SP deflection.<ref name=pt04r3>Dewan, J. T., 1983, Essentials of modern open-hole log interpretation: Tulsa, OK, Penn Well Books.</ref>]]

[[file:determination-of-water-resistivity_fig2.png|thumb|{{figure number|2}}Example of a reverse SP deflection.<ref name=pt04r3>Dewan, J. T., 1983, Essentials of modern open-hole log interpretation: Tulsa, OK, Penn Well Books.</ref>]]

The first step in the interpretation of the SP log is the establishment of “sand” and “shale” lines, as shown in [[:file:determination-of-water-resistivity_fig1.png|Figure 1]]. These are arbitrary limits, with the sand lines normally representing the maximum deflection to the left and shale lines representing the maximum deflection to the right (in shales). Deflections to the left of the shale line are regarded as normal or negative, and correspond to porous and permeable zones containing a more saline interstitial water than the drilling mud (''R''_w R_mf).

The first step in the interpretation of the [[Basic open hole tools#Spontaneous potential|SP log]] is the establishment of “sand” and “shale” lines, as shown in [[:file:determination-of-water-resistivity_fig1.png|Figure 1]]. These are arbitrary limits, with the sand lines normally representing the maximum deflection to the left and shale lines representing the maximum deflection to the right (in shales). Deflections to the left of the shale line are regarded as normal or negative, and correspond to porous and permeable zones containing a more saline interstitial water than the drilling mud (''R''_w R_mf).

If the mud is more saline than the formation water, the SP currents flow in the opposite direction and the corresponding deflection will be to the right of the shale line. Such a deflection is considered to be reversed or positive ([[:file:determination-of-water-resistivity_fig2.png|Figure 2]]). If there is no salinity contrast between the mud and the formation water, no SP currents are generated and no deflection will be observed (SP = 0).

If the mud is more saline than the formation water, the SP currents flow in the opposite direction and the corresponding deflection will be to the right of the shale line. Such a deflection is considered to be reversed or positive ([[:file:determination-of-water-resistivity_fig2.png|Figure 2]]). If there is no salinity contrast between the mud and the formation water, no SP currents are generated and no deflection will be observed (SP = 0).