This is the name of a log measured along a well bore (a well-log). The code
PO5B is a unique identifier to recognize this particular log and can be found in any (ASCII) log files provided by the logging company.
Partial porosity from MRIL (single element) is the full name for this log. This log is often used in the industry. This log displays readings of 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 is a measurement of the apparent porosity. In other words, 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.
Concluding, this is a measure of the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs. At a more general level these values are measures of 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: