  We have studied the dispersive behaviour of imbibition fronts in a porous
medium by X-ray tomography. Injection velocities were varied and the porous
medium was initially prewetted or not. At low velocity in the prewetted medium,
the imbibition profiles are found to be distinctly hyperdispersive. The
profiles are anomalously extended when compared to tracer fronts exhibiting
conventional (Gaussian) dispersion. We observe a strong velocity dependence of
the exponent characterizing the divergence of the dispersion coefficient for
low wetting-fluid saturation. Hyperdispersion is absent at high imbibition
velocities or when the medium is not prewetted.
