This type of inversion uses not only the M0 curve but also higher-mode curve(s) for the inversion. The underlying motivation is to increase the accuracy (resolution) of the final
1D Vs profile by narrowing the range of solutions with 1D Vs profiles otherwise equally well suited if only the M0 curve is used (Fig. 1). This, therefore, tries to alleviate the
intrinsic problem with the inversion method of non-uniqueness in general. Some investigators, however, raise a warning on this approach by stating that it can make the
problem even worse by introducing more degrees of freedom with one or more variables added into the system of inversion.