Method and system for correcting a natural gamma-ray measurement performed in a wellbore

A system for correcting a downhole natural gamma-ray measurement performed in a wellbore wherein a gamma-ray measurement including at least a gamma-ray count rate is obtained by a gamma-ray detector disposed in a bottom hole assembly having a mud channel inside the assembly, such that mud flows downwards in the mud channel and upwards outside of the assembly, and a neutron source situated above the gamma-ray detector and activating the mud, and wherein the system further comprises a processor configured to:

• Determine from the gamma-ray measurement an interval count rate corresponding to gamma-rays having an energy within a predetermined correction interval, wherein the correction interval comprises energies above 2.615 MeV;
• Based on an outside calibration ratio corresponding to the ratio of a gamma ray count rate in the correction interval to a gamma-ray count rate outside of the correction interval representative of gamma-rays generated by the activation of mud flowing outside of the assembly and on the interval count rate, determine a first correction count rate,
• Based on at least an inside calibration ratio corresponding to the ratio of a gamma ray count rate in the correction interval to a gamma-ray count rate outside of the correction interval representative of the activation of mud flowing inside of the assembly and on the outside calibration ratio, determine a second correction count rate, wherein the sum of the first and second correction account for the count rate from the mud activated inside and outside of the assembly
• Subtract from the total count rate obtained from gamma-ray measurement the sum of first and second correction count rates,
• Based on the corrected gamma ray count, determining at least a property of the formation.