Gamma Ray and Spontaneous Potential Schlumberger 2-1 GR Gamma Ray Corrections for Hole Size and Mud Weight For 3 3 / 8-in. and 1 11 /16-in. SGT wireline gamma ray tools GR-1 0 5 10 15 20 25 30 35 40 10.0 7.0 5.0 3.0 2.0 1.0 0.7 0.5 0.3 t (g/cm 2 ) Correction factor 3 3 /8-in. tool, centered 1 11 / 16-in. tool, centered 1 11 / 16-in. tool, eccentered 3 3 /8-in. tool, eccentered © Schlumberger Log interpretation Charts GR-1 and GR-2, replacing Chart Por-7, are based on laboratory work and Monte Carlo calculations to provide improved corrections for 3 3 /8- and 1 11 /16-in. SGT gamma ray tools. The corrections normalize the response of both tools to eccentered positions in an 8-in. borehole with 10-lbm mud. Chart GR-2 provides a correction for barite mud in small boreholes. Although these charts are more difficult to use than the ones they replaced, the results are more exact since they are normal- ized to current tools, no interpolation is required, and the ranges are extended. The input parameter, t, in g/cm 2 , is calculated as follows: The correction for standoff is CF ′ m is the correction factor for centered tools, while CF ′ o is the correction factor for eccentered tools. Both are corrected for barite if it is present in the borehole. S is the actual standoff, and S m is the standoff with the tool centered. Example: GR reads 36 API units, d h is 12 in., and mud weight is 12 lbm/gal. The tool is 3 3 ∕8 in. and centered. Therefore, t = 15.8 g/cm 2 , resulting in a correction factor of 1.6. The corrected GR = 58 API units. CF CF CF CF ′= ′+ - ′ - m o m m m S S S ( ) . 2 t W d d mud hole sonde = - 8 345 2 54 2 2 54 2 . . ( ) . ( ) .