With the help of a simulation of the perturbation field and a comparison with Mirnov measurements, it becomes possible to investigate coupled MHD modes. The structure and the width of the different coupled islands can be estimated. For a reliable simulation, it is necessary to take into account the induced screening currents in the vessel wall and in some of the in-vessel components. The fields of these screening currents have a large influence on the measurements. This improved analysis method is demonstrated for the development of coupled modes during a density limit disruption. It is clearly shown that an increase of the coupled islands leads to minor disruptions. Owing to mode locking, reliable results are not possible before major disruptions