Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the [[Archie equation|Archie formula]], doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}. | Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the [[Archie equation|Archie formula]], doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}. |