  The inverse problem of reconstructing a source term from boundary
measurements, for the wave equation, is revisited. We propose a novel approach
to recover the unknown source through measuring the wave fields after injecting
small particles, enjoying a high contrast, into the medium. For this purpose,
we first derive the asymptotic expansion of the wave field, based on the
time-domain Lippmann-Schwinger equation. The dominant term in the asymptotic
expansion is expressed as an infinite series in terms of the eigenvalues
$\{\lambda_n\}_{n\in \mathbb{N}}$ of the Newtonian operator (for the pure
Laplacian). Such expansions are useful under a certain scale between the size
of the particles and their contrast. Second, we observe that the relevant
eigenvalues appearing in the expansion have non-zero averaged eigenfunctions.
We prove that the family $\{\sin(\frac{c_1}{\sqrt{\lambda_n}}\, t),\,
\cos(\frac{c_1}{\sqrt{\lambda_n}}\, t)\}$, for those relevant eigenvalues, with
$c_1$ as the contrast of the small particle, defines a Riesz basis (contrary to
the family corresponding to the whole sequence of eigenvalues). Then, using the
Riesz theory, we reconstruct the wave field, generated before injecting the
particles, on the center of the particles. Finally, from these (internal values
of these) last fields, we reconstruct the source term (by numerical
differentiation for instance). A significant advantage of our approach is that
we only need the measurements on $\{x\}\times(0, T)$ for a single point $x$
away from $\Omega$, i.e., the support of the source, and large enough $T$.
