This is a log of measurements along a well bore (a well-log). The code FXN is a unique identifier to recognize this particular log and can be found in any (ASCII) log files provided by the logging company. The code stands for 'Neutron Formation Exposure Time'. The usage of this log is well established.

This log can be used to evaluate the time delay between the drill bit and a neutron sensor reaching a given point in the wellbore during MWD/LWD logging.

Furthermore, this can be explained by looking at the delay time. This is a measure of the time difference by which one event lags another. In refraction seismic work, the additional time required to traverse a raypath over the time which would be required to traverse the horizontal component at the highest velocity encountered on the raypath.

This log usually has the unit 'Time (hours)'.

Data from this log can be used to investigate the time interval of the formation. Similar logs are listed below:

• Neutron Time After Bit (TAB_NEU)
• Neutron Porosity Time Since Drilled, Transmitted (NPEX)
• Density Time After Bit (TAB_DEN)
• Smoothed Mud Transit Time (SMTT)
• ARC Gamma Ray Time After Bit (AGTM)
• Smoothed Neutron Formation Exposure Time (SFXN)
• ARC Resistivity Time After Bit (ARTM)
• Time Since Drilled from Gamma Ray, Transmitted (GRTX)
• Time Since Drilled from MPR Attenuation 400kHz (ATTL)
• Time Since Drilled from Phase Difference (PDT)