Recently a measure of structural similarity (SSIM) that compares local patterns between two images has been developed by \citet{Wang2004}. The SSIM index is a decimal value between 0 and 1, where value 1 is only reachable in the case of two identical sets of data. Figure \ref{fig:SSIM} shows the SSIM between the reference model and one random SGS realization and between the reference model and the inversion result for $P$-wave and $S$-wave velocity, density and porosity. 
  