  Natural open joints in rocks commonly present multi-scale self-affine
apertures. This geometrical complexity affects fluid transport and heat
exchange between the flow- ing fluid and the surrounding rock. In particular,
long range correlations of self-affine apertures induce strong channeling of
the flow which influences both mass and heat advection. A key question is to
find a geometrical model of the complex aperture that describes at best the
macroscopic properties (hydraulic conductivity, heat exchange) with the
smallest number of parameters. Solving numerically the Stokes and heat equa-
tions with a lubrication approximation, we show that a low pass filtering of
the aperture geometry provides efficient estimates of the effective hydraulic
and thermal properties (apertures). A detailed study of the influence of the
bandwidth of the lowpass filtering on these transport properties is also
performed. For instance, keeping the information of amplitude only of the
largest Fourier length scales allows us to reach already an accuracy of 9% on
the hydraulic and the thermal apertures.
