In SP (spontaneous potential) log interpretation of shaly sandstones, the ratio of pseudo-static SP (PSP) to static SP (SSP) is reduced by the presence of shale within the reservoir, since shale's own SP response partially offsets/dampens the clean-sand SP deflection. A commonly used approximate relation is that shale volume Vsh is proportional to (1 − PSP/SSP), meaning a lower PSP/SSP ratio corresponds to a higher shale volume.