Abstract: A system includes a wellbore tool disposed in a wellbore and a surface apparatus disposed at a surface of the wellbore. Prior to operating the tool to take an initial measurement, the wellbore tool is first calibrated to take into account magnetic junk which is magnetically attracted to and accumulated on a magnet housing of the tool. To calibrate the tool, a new tuning frequency, called the Larmor frequency, is determined, and the tool is tuned to the new Larmor frequency before taking an initial measurement. The wellbore tool and the surface apparatus each include a processing system. Each processing system includes a memory. A first part of a tuning frequency determination software is stored in the memory of the tool, and a second part of the frequency determination software is stored in the memory of the surface apparatus. To determine the tuning frequency, the processing system in the wellbore tool initiates execution of the first part of the tuning frequency determination software.