This is a well log used in the oil and gas industry. You could have found the code
PO11D in your log evaluation software or your corporate datastores. The official name for this log is
'Partial porosity from MRIL (single element)'. The business value of this log is often deamed high. This log helps to evaluate the contribution of porosity from a given "bin" (with certain characteristics) to the total porosity; e.g., in nuclear magnetic resonance data processing, the "bins" typically correspond to different pore size ranges. In general this log therefore displays units of the apparent porosity. This can be explained as being the initial estimate of porosity, normally from logs, not corrected for the (often large) effects of clays, gas, etc. Such estimates should only be used for qualitative interpretation of clean, gas-free intervals.
So basically this is a measure of the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs.
Furthermore, this can be explained by looking at the volume fraction. This is a measure of the ratio of the volumes occupied by two components, or by one component to the total volume of the system.
Data for this log is produced by a tool that uses a nmr logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: