TRACER DISPERSION IN ROUGH OPEN CRACKS

Tracer dispersion is studied in an open crack where the two rough crac k faces have been translated with respect to each other. The different dispersion regimes encountered in rough-wall Hele-Shaw cell are first introduced, and the geometric dispersion regime in the case of self-a ffine crack surfaces is treated in detail through perturbation analysi s. It is shown that a line of tracer is progressively wrinkled into a self-affine curve with an exponent equal to that of the crack surface. This leads to a global dispersion coefficient which depends on the di stance from the tracer inlet, but which is still proportional to the m ean advection velocity. Besides, the tracer front is subjected to a lo cal dispersion (as could be revealed by point measurements or echo exp eriments) very different from the global one. The expression of this a nomalous local dispersion coefficient is also obtained.