This is an abbreviation for a well log acquired after or during drilling. You could find this abbreviation
TNP1 in your log evaluation software or any (ASCII) log files provided by the logging company. The code stands for
'Thermal Neutron Porosity, Bank 1'. This log is not used too often in the industry. The values of this log represent the porosity as measured with a thermal neutron logging device.
Concluding, this is a measure of the porosity as measured with an neutron logging device.
In general this log thus reflects 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. In general this log therefore displays units 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.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: