SPI. An estimate of the secondary porosity, calculated from sonic log values in conjunction with either density or neutron log values, or porosity resulting from a crossplot of density and neutron porosities. If f_D 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)