Environmental corrections in nuclear spectroscopy using variable element ratio

A method for estimating an aspect of a formation using a nuclear spectroscopy tool, comprising:

• placing a nuclear spectroscopy tool including a neutron source and a gamma ray detector into a borehole;
• performing a plurality of environmental measurements;
• emitting neutrons from the nuclear spectroscopy tool such that some of the neutrons can generate gamma rays from a formation adjacent the nuclear spectroscopy tool and some of the neutrons can generate gamma rays from elements within the nuclear spectroscopy tool;
• detecting an energy spectrum of gamma rays induced by the emitted neutrons, the energy spectrum including a tool background comprising the gamma rays from elements within the nuclear spectroscopy tool having a plurality of measured spectral components including an iron spectral component;
• estimating a background ratio between the iron spectral component of the tool background and at least one other spectral component of the tool background in accordance with the plurality of environmental measurements;
• analyzing the detected gamma ray spectrum using a combination of standard spectra and subtracting the iron spectral component of the tool background in accordance with the background ratio;
• and
• estimating an aspect of the formation based on the analyzed spectrum.