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Table T3. Measurement performance specifications of the geoVISION tool. (See table notes.)

Measurement Vertical
resolution
(inches)		 Depth of
investigation
(inches)		 Diameter of investigation (inch) Horizontal resolution Midpoints
of extremes
Rt/Rxo = 10 Rt/Rxo = 0.1 Rt/Rxo = (10 × π)/56 Rt/Rxo = (0.1 × π)/56
Resistivity at the bit 12–24* 12 32 32      
Ring resistivity 2–3 7 22 25      
Button resistivity
Shallow-focused resistivity 2–3 1 19 24 1.07 1.35 1.2
Medium-focused resistivity 2–3 3 15 21 0.84 1.18 1
Deep-focused resistivity 2–3 5 11 16 0.62 0.9 0.75

Notes: * = depends on bit subs. Vertical penetration rate = 30 m/h (= 1371.6 inches/h = 22.86 inches/min). At typical rotation rate (50 rpm), this penetration rate oversamples vertically. Rt = true resistivity of formation, Rxo = resistivity of zone invaded by drilling fluid. When Rt and Rxo are approximately equal (Rt/Rxo ~1), drilling fluid invasion may be minimal due to measurement of resistivity soon after hole cutting. Rt below frontal thrust = ~0.4–0.6 Ωm, Rxo of drilling fluid (seawater) = 0.19 Ωm; therefore, Rt/Rxo may be >1, depending on invasion. Horizontal resolution for the geoVISION tool is intermediate between the extremes of Rt/Rxo shown in the table. The geoVISION tool records 56 points with each tool revolution. The accuracy of the segment location is ±1°.

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