مؤشر المسامية الثانوية (SPI)

{{#category_index:S|secondary-porosity index (SPI)}} A measure of the secondary porosity, usually attributed to fractures or vugs, calculated from sonic-log values in conjunction with either density-log or neutron-log values. If φ_density is the porosity calculated from a density (or neutron) log and φ_sonic the porosity calculated from a sonic log, SPI is sometimes defined as (φ_density–φ_sonic), usually as (1–φ_sonic/φ_density).