SPI. An estimate of the [[secondary porosity]], calculated from [[sonic log]] values in conjunction with either [[density log|density]] or [[neutron log]] values, or porosity resulting from a [[crossplot]] of density and neutron porosities. If fD is the porosity calculated from a density (or neutron) log and **f**S is the [[porosity]] calculated from a sonic log, SPI is sometimes defined as (**f**D – **f**S) or (**f**crossplot – **f**S)