Cross-correlation of independent, equipartitioned wavefields is a well-established method to estimate the elastic Green's function, commonly termed seismic interferometry. In this article, the sum of a wavefield recorded at two locations in a single channel is used to estimate the Green's function via the autocorrelation; the result contains some predicted artefacts. The underlying theory and hardware required to estimate the Green's function is presented and compared to traditional seismic interferometry. This technique is used to estimate the elastic Green's function between two locations on an aluminum block with surface scatterers. Wavefields excited via rapid thermoelastic expansion of the surface using a pulsed laser are detected by a dual-beam heterodyne interferometer. The detector is capable of directly recording the sum of a wavefield measured at two locations in a single channel. This method could be an effective, low cost, and non-contacting technique for structural monitoring, particularly where ambient noise has established equipartitioned wavefields in the structure.