(a) Lg Q tomographic map at a central frequency of 2.0 Hz. (b) The resolution of the model represented by Backus-Gilbert SF in kilometres calculated from the model resolution matrix. (c) The relative uncertainty of the model represented by δ(1/Q)/(1/Q) in percentage, where δ(1/Q) is one standard deviation of 1/Q calculated from the diagonal elements of the model covariance matrix.