A method for analyzing a formation includes entering into a computer a number of detected gamma rays resulting from imparting neutrons into the formation. The detected gamma rays are characterized by energy levels thereof. A number of detected gamma rays in each energy level comprises a measured spectrum. In the computer, a non-Gaussian filter is applied to a reference spectrum to match the measured spectrum in shape. The filtered reference spectrum and measured spectrum are used to determine a fractional volume of at least one component of the formation.