The Earth can be viewed as a spherically layered plane.
Chemical stratification occurs between the crust, mantle, and core.
A one dimensional model can be accurately applied to this level of stratification. Geophysicists have been using earthquake travel times to one dimensionally map this stratification for years. However, some lateral variations also exists at any and all given depths within the earth. This feature is recognized by looking at scatter of seismic wave arrival times at all distances. Consistent scatter of seismic waves implies geological or geochemical features existing laterally in the earth and show that there are dynamic processes occuring in the earth's interior.