Patent ID: 8000899

Claim:
A method for determining a corrected porosity of earth formation penetrated by a borehole, the method comprising: conveying a tool along said borehole, wherein said tool comprises a source of neutron radiation and at least a near and a far detector axially spaced from said source at different spacings; generating detector responses for each said detectors indicative of neutron radiation from said source interacting with said formation; transforming said near detector response and said far detector response into at least a first and a second apparent porosity; computing a correction factor that is a function of a difference between said first and the second apparent porosity and a known tool standoff from the borehole wall; and determining the corrected porosity of the earth formation by applying the correction factor to at least one of the first and the second apparent porosity, wherein the computed correction factor is obtained from a function of the form: ΔΦ computed =d 1 ×(Φ far −Φ near )×(Φ far −d 2 ) 2 where ΔΦ computed is the computed correction factor, Φ far and Φ near are respectively the far and near apparent porosities, and d 1 and d 2 are coefficients that have been determined by response of the tool for a known formation porosity and a known tool standoff from the borehole wall, wherein the transforming, computing and determining steps are performed by a processor.