secondary-porosity index

1. n. [Formation Evaluation]
An indicator of the porosity that does not contribute to a sonic measurement of interval transit time. The transit time is little affected by vugs, fractures and other irregular events because the sonic wave finds a faster path around them. Spherical pores such as oomolds also have less effect on traveltime than oblate pores. Thus, when the sonic porosity is less than some measurement of the total porosity, the difference can be attributed to the presence of post-depositional, or secondary, porosity. The sonic porosity is usually derived from the Wyllie time-average equation, or some other suitable transform, and the total porosity taken as the density-neutron crossplot porosity.
See: sonic log