Method and apparatus for determining elemental concentrations for .gamma. r a

A method for determining the concentrations of the elements Si, Ca, S, Ti, Gd and Fe in a formation surrounding a borehole, comprising:

• a) positioning a tool in the borehole adjacent a formation to be investigated
• b) irradiating the formation with neutrons from the tool;
• c) detecting .gamma. rays resulting from neutron capture by said elements in the formation so as to obtain a spectrum;
• d) determining the contribution of each element in the formation to the spectrum;
• e) determining a factor for modifying the contribution of iron to the spectrum .gamma. ray yields to compensate for a lack of aluminum measurement;
• and
• f) using the spectrum and the factor to determine the concentrations of said elements in the formation.

Apparatus for determining the elemental concentrations in a formation surrounding a borehole, comprising:

• a) a tool body for positioning in the borehole adjacent a formation to be investigated
• b) a neutron source in said tool body for irradiating the formation;
• c) at least one .gamma. ray detector in said tool body for detecting .gamma. rays resulting from neutron capture by the elements in the formation so as to obtain a spectrum;
• and
• d) means for using the spectrum and a predetermined factor modifying the contribution of iron to the spectrum .gamma. ray yields to compensate for a lack of aluminum measurement to determine elemental concentrations in the formation.