This is the name of a log measured along a well bore (a well-log). You might have encountered this acronym
TNPU in your log evaluation software or any (ASCII) log files provided by the logging company. The official name for this log is
'Thermal Neutron Porosity, Up'. Usage of this log is not high and encountering it is a rare occasion. Use this log to investigate the porosity as measured with a thermal neutron logging device.
So basically this is a measure of the porosity as measured with an neutron logging device. At a more general level these values are measures of the apparent porosity. This is a measure of 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.
Furthermore, this can be explained by looking at the volume fraction. In other words, the ratio of the volumes occupied by two components, or by one component to the total volume of the system.This log usually has the unit
'Porosity'. This is the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs.
Data for this log is produced by a tool that uses a neutron logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: