Dictionary:Secondary-porosity index (SPI)

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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 φdensity is the porosity calculated from a density (or neutron) log and φsonic the porosity calculated from a sonic log, SPI is sometimes defined as (φdensity–φsonic), usually as (1–φsonicdensity).