  There are a finite number of distinct mechanically stable (MS) packings in
model granular systems composed of frictionless spherical grains. For typical
packing-generation protocols employed in experimental and numerical studies,
the probabilities with which the MS packings occur are highly nonuniform and
depend strongly on parameters in the protocol. Despite intense work, it is
extremely difficult to predict {\it a priori} the MS packing probabilities, or
even which MS packings will be the most versus the least probable. We describe
a novel computational method for calculating the MS packing probabilities by
directly measuring the volume of the MS packing `basin of attraction', which we
define as the collection of initial points in configuration space at {\it zero
packing fraction} that map to a given MS packing by following a particular
dynamics in the density landscape. We show that there is a small core region
with volume $V^c_n$ surrounding each MS packing $n$ in configuration space in
which all initial conditions map to a given MS packing. However, we find that
the MS packing probabilities are very weakly correlated with core volumes.
Instead, MS packing probabilities obtained using initially dilute
configurations are determined by complex geometric features of the basin of
attraction that are distant from the MS packing.
