  In this paper we refine a previously developed acoustic-source filter
(Bahauddin & Rast 2021), improving its reliability and extending its
capabilities. We demonstrate how to fine-tune the filter to meet observational
constraints and to focus on specific wavefront speeds. This refinement enables
discrimination of acoustic-source depths and tracking of local-source
wavefronts, thereby facilitating ultra-local helioseismology on very small
scales. By utilizing the photospheric Doppler signal from a subsurface source
in a MURaM simulation, we demonstrate that robust ultra-local three-dimensional
helioseismic inversions for the granular flows and sound speed to depths of at
least 80 km below the photosphere are possible. The capabilities of the
National Science Foundation's new Daniel K. Inouye Solar Telescope (DKIST) will
enable such measurements of the real Sun.
