  We present in this paper an experimental study of the invasion activity
during unstable drainage in a 2D random porous medium, when the (wetting)
displaced fluid has a high viscosity with respect to that of the (non-wetting)
displacing fluid, and for a range of almost two decades in capillary numbers
corresponding to the transition between capillary and viscous fingering. We
show that the invasion process takes place in an active zone within a
characteristic screening length from the tip of the most advanced finger. The
invasion probability density is found to only depend on the distance to the
latter tip, and to be independent of the value for the capillary number Ca. The
mass density along the flow direction is related analytically to the invasion
probability density, and the scaling with respect to the capillary number is
consistent with a power law. Other quantities characteristic of the
displacement process, such as the speed of the most advanced finger tip or the
characteristic finger width, are also consistent with power laws of the
capillary number. The link between the growth probability and the pressure
field is studied analytically and an expression for the pressure in the
defending fluid along the cluster is derived. The measured pressure are then
compared with the corresponding simulated pressure field using this expression
for the boundary condition on the cluster.
