Abstract: A method used for determining true in-situ resistivity of a thin layer within a formation from dual induction log data. The method calculates true resistivity without the existing need: 1) for an accurate estimate of the layer thickness, or 2) to extrapolate correction factors for thin layers. The method first corrects resistivity data read from the dual induction log. A representative shoulder bed resistivity is first determined from average deep resistivity data near the shoulder bed. A paired estimate of layer resistivity and thickness is then computed based upon the determined shoulder bed resistivity, dip angle, and both deep and medium resistivity data related to the thin bed. The method does not require thickness data to be input, and in fact outputs this infomation, if needed.

Abstract: Borehole NMR logging apparatus and methods, and methods for the interpretation thereof. A logging tool is provided which produces a strong, static and homogeneous magnetic field B.sub.0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focussed oscillating magnetic fields B.sub.1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B.sub.1. Extremely rapid damping of the antenna between the transmitting and receiving modes of operation is accomplished by a Q-switch disclosed herein. The invention provides for the direct measurement of NMR decay having transverse relaxation time T.sub.