Influence of surface cover-layer and basalt thickness on the effective density spectrum. Panel (a) shows the changes in the effective density spectrum with surface cover-layer thicknesses (﹩{T}_{s}﹩) of 0, 0.01, 0.1, and 1 km, while the basalt thickness ﹩{T}_{b}﹩ is fixed at 0.5 km. Panel (b) sisplays the changes in the effective density spectrum with basalt thicknesses (﹩{T}_{b}﹩) of 0, 0.01, 0.1, 1, and 10 km, while the surface cover-layer thickness ﹩{T}_{s}﹩ is fixed at 0.1 km.