essentially independently for targets with relatively low wind terminal velocities (i.e. $v_\infty$ less than the doublet separation). The key advantage of exploiting radiatively uncoupled doublets is that these transitions arise from the same level of the same ion. Consequently, they sample exactly the same material. Therefore regardless of the total optical depth, and provided the basic assumptions of the Sobolev approximation are satisfied, the ratio of the optical depths of the doublets [*must*]{} be the ratio of their oscillator strengths ($\sim$ 2 for the Si IV 1400 doublet) unless the covering factor of the stellar disk is less than unity (see also Ganguly et al. 1999). As explained by Prinja [&]{} Massa (2010) if the ratio of the Si[iv]{} blue-to-red doublet component optical depth is less than 2 at a given velocity, the covering factor of the stellar surface by the intervening wind material at that velocity must be less than one. This applies regardless of how the column density is accumulated, i.e., whether by encountering a velocity plateau or a density enhancement.

We acknowledge however that there are two scenarios where in this context the Sobolev approximation may not be appropriate: Firstly, the density (or velocity) structure of the 