The effect of palaeoclimate on heat flow data

One common method to determine heat flow in a well is to use a Bullard plot, which graphs thermal resistance (m$^2$K W$^{-1}$) against temperature. The gradient of this line is heat flow, and the uncertainty determined from the error of linear regression. Thermal resistance is calculated by: $$ R = \sum_{i=0}^{n} \left( \frac{\Delta z_i}{k_i} \right) $$ which is the cumulative summation of resistors down the length of a borehole. The uncertainty on $R$ should increase as the individual errors on $k$ accumulate.