A method having obtaining one or more porosity data of a subsurface of the Earth using a data acquisition tool; computing one or more permeability data from the porosity data using a transformation function; computing a first quantile regression function and a second quantile regression function of the one or more permeability data; producing a combined mean regression function of the one or more permeability data by summing the first quantile regression function and the second quantile regression function, wherein the first quantile regression function is multiplied by a first coefficient and the second quantile regression function is multiplied by a second coefficient; and using the combined mean regression function to predict a mean permeability of the subsurface.