The surface subsidence at a horizontal distance \( X \) from the centerline of a cylindrical tunnel is given by \( S = S_{\text{max}} \times \exp\left(\frac{-X^2}{0.5H^2}\right) \), where \( S_{\text{max}} \) is the maximum subsidence above the tunnel and \( H = 50 \) m is the depth of the tunnel centerline. The ratio of surface subsidence at \( X = 10 \) m to that at \( X = 20 \) m is .............
