Cross correlation misfit without observed data.
\\mathbf{f}^\\dagger = \\frac{1}{\\| \\partial_t \\varphi \\|_2^2}
\\partial_t \\varphi \\, \\delta (\\mathbf{x} - \\mathbf{x}_\\mathrm{r}),
where
\\mathbf{f}^\\dagger is the adjoint source,
varphi is the
synthetic waveform, and
\\delta (\\mathbf{x} - \\mathbf{x}_\\mathrm{r})
is a sampling function that extracts the waveforms at the receiver location
\\mathbf{x}_\\mathrm{r}.
Note that this function is only intended for computing the adjoint source
for the given synthetic waveforms. The returned misfit value is
meaningless, as no quantity is computed for it.