A glance at the correction chart of Figure 3 will give a appreciation of the effect of standoff on the generic thermal neutron tool (although this is for one specific model). The fact that standoff is such an important perturbation to the measurement is not surprising since the borehole fluid in which the tool makes its measurements is generally very hydrogen-rich compared to the formation being analyzed.

For a conventional thermal tool, the amount of standoff, perhaps caused by the combination of hole irregularity and a long tool string, is not known. It is usually assumed to be some small, constant value. Sometimes the standoff is used to shift the log reading to the desired value. However, there is no reason to assume that standoff would be a constant value