Which chart is used to correct deep log resistivity to true formation resistivity?

• A. Scatter Plot
• B. Time Series Chart
• C. Logarithmic Plot
• D. Tornado Chart

Answer: (C)

When correcting deep log resistivity to true formation resistivity, you’re often dealing with a relationship that is multiplicative and spans large ranges of values. Using a logarithmic (log-log) plot helps reveal and quantify that relationship. If the true resistivity Rt and the deep reading Rd follow a power-law form Rt ≈ a × (Rd)b, taking logarithms gives log Rt = log a + b log Rd, which plots as a straight line. The slope and intercept of that line let you compute Rt from Rd: Rt = a × (Rd)b. This makes the calibration straightforward and robust across the wide resistivity values you encounter.

Other chart types don’t directly support deriving that multiplicative correction. A scatter plot shows the relationship but doesn’t linearize it for easy parameter extraction; a time-series chart isn’t relevant to resistivity correction, and a tornado chart is used for sensitivity analysis, not calibration.