Obtaining inelastic spectra free of detector born gamma rays induced by neutrons

A computing device for measuring properties of a formation or well structure, comprising:

• a microprocessor;
• and
• memory including instructions that, when executed by the microprocessor, cause the computing device to:
• receive a first measurement of gamma rays from the formation or well structure via a detector during a first period of time;
• receive a second measurement of gamma rays from the formation or well structure via the detector during a second period of time;
• remove the second measurement from the first measurement to produce an inelastic spectrum;
• determine a spectral slope from the inelastic spectrum;
• determine a scaling factor based on the spectral slope;
• determine a spectral shape associated with the detector;
• determine a detector-induced spectrum by applying the scaling factor to the spectral shape;
• and
• remove the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum for use in evaluating properties of the formation or well structure.

A system for conducting measurement operations for measuring properties of a formation or well structure, the system comprising:

• a neutron measurement device forming at least a portion of a downhole tool string, the neutron measurement device operable to generate measurement data for detecting a wellbore characteristic, wherein the neutron measurement device comprises:
• a neutron generation unit operable to emit neutrons toward a target;
• a detector unit fixed relative to the neutron generator and operable to detect gamma rays from the target;
• a microprocessor;
• and
• memory including instructions that, when executed by the microprocessor, cause the system to:
• receive a first measurement of gamma rays from the formation or well structure via a detector during a first period of time;
• receive a second measurement of gamma rays from the formation or well structure via the detector during a second period of time;
• remove the second measurement from the first measurement to produce an inelastic spectrum;
• determine a spectral slope from the inelastic spectrum;
• determine a scaling factor based on the spectral slope;
• determine a spectral shape associated with the detector;
• determine a detector-induced spectrum by applying the scaling factor to the spectral shape;
• and
• remove the detector-induced spectrum from the inelastic spectrum to produce a clean inelastic spectrum for use in evaluating properties of the formation or well structure.