(a) The resolution of the model represented by Backus-Gilbert SF in kilometres calculated from the model resolution matrix. (b) 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. (c) Ray path coverage obtained from the RTS (blue), RTE (red) and SC-TS (green) Q measurements. (d) Checkerboard test with alternative Q values of 100 and 900 at 0.8° × 0.8° grids.