This is an abbreviation for a well log used in the oil and gas industry. You could find this abbreviation
PO3 in your log evaluation software or any (ASCII) log files provided by the logging company. The full name for this log used in the industry is
'Partial porosity from MRIL (single element)'. 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 log therefore displays units 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.
Simply said, this is a log that describes 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. Better said, 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: