  We experimentally study a two-dimensional (2D) granular pack of photoelastic
disks subject to vertical manual tapping. Using bright and dark field images,
we employ gradient-based image analysis methods to analyse various structural
quantities. These include the packing fraction ($\phi$), force per disk ($F_d$)
and force chain segment length ($l$) as a function of the tapping number
($\tau$). The increase in packing fraction with tapping number is found to
exponentially approach an asymptotic value. An exponential distribution is
observed for both the cumulative numbers of force per disk $F_d$ :
$N_{cum}(F_d) = A_F \exp (-\frac{F_d}{F_0})$, and force chain segment length
$l$ : $N_{cum}(l) = A_l \exp (-\frac{l}{l_0})$. Whereas the coefficient ${A_F}$
varies with $\tau$ for force per disk, the force chain segment length shows no
dependence on $\tau$. The $\tau$ dependence of $F_d$ and $\phi$ allows us to
posit a linear relation between the total force of the granular pack
($F_{tot}^*(\tau)$) and $\phi(\tau)$.
