‘‘Echo’’ tracer dispersion in porous media

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Abstract
The reversibility of tracer dispersion in porous media is investigated experimentally using a ‘‘dispersion echo’’ technique in which tracer is injected into the sample and then pumped back. In homogeneous materials, the ‘‘echo’’ dispersion coefficient is the same as for ‘‘transmission’’ measurements at all Peclet numbers, and the dispersion is fully irreversible. In heterogeneous materials (giving non-Gaussian transmission curves), the dispersion is partly reversible; ‘‘echo’’ dispersion curves are Gaussian with a dispersion coefficient lower than in transmission experiments. It is shown that the dispersion irreversibility corresponds to the combined effects of geometrical disorder in the flow field and of molecular diffusion.