The SPAC (Spatial AutoCorrelation) method was proposed by Aki in the 1950s. This method is based on the principle that when microtremors are recorded by two-dimensional arrays (nonlinear as in the ReMi) in which pairs of sensors are scanned at different distances with different azimuthal angles, it is possible to estimate the phase velocity of the surface waves.

From this technique it then evolved into ESAC (Extended Spatial AutoCorrelation) and also into noise tomography.