This efficient multiscale method for time-domain waveform tomography incorporates filters that are more efficient than Hamming-window filters. A strategy for choosing optimal frequency bands is proposed to achieve computational efficiency in the time domain. A staggered-grid, explicit finite-difference method with fourth-order accuracy in space and second-order accuracy in time is used for forward modeling and the adjoint calculation. The adjoint method is utilized in inverting for an efficient computation of the gradient directions. In the multiscale approach, multifrequency data and multiple grid sizes are used to overcome somewhat the severe local minima problem of waveform tomography. The method is applied successfully to 1D and 2D heterogeneous models; it can accurately recover low-and high-wavenumber components of the velocity models. The inversion result for the 2D model demonstrates that the multiscale method is computationally efficient and converges faster than a conventional, single-scale method.

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SUMMARY Recent deep‐crustal seismic reflection profiling across the central Alps of eastern and southern Switzerland has provided a detailed image of this continental collision zone. As part of the Swiss National Science project NFP 20, a series of reflection profiles was recorded during two phases along N‐S transects crossing the Alpine edifice. The eastern and southern profiles coincide with the Alpine segment of the European Geotraverse (EGT) and in part with the earlier Swiss Geotraverse. These data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the Alps. On a larger scale, the new reflection data also add to a growing set of profiles crossing the Alpine fold belt which includes an additional traverse carried out by NFP 20 across the western Swiss Alps and the ECORS‐CROP profile across the western Alps. The initial results of the reflection data across the central Alps outline a crustal framework involving northward indentation of the Adriatic hinterland into the subducting European foreland. This occurs beneath the collapsed oceanic basins of the Penninic allochthon which is defined by its highly reflective crystalline nappes. The present crustal thickness within the Alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the Adriatic wedge. The south‐plunging European foreland implies that portions of the thickened crust have been subducted into the upper mantle. This general framework of flake tectonics is laterally consistent with the ECORS‐CROP results, from the western Alps. However, the western Alps show a higher degree of crustal imbrication which may be related to the presence of the rigid Ivrea body and an overall greater amount of crustal shortening compared to the central Alps.

A recently developed time-domain multiscale waveform tomography (MWT) method is applied to synthetic and field marine data. Although the MWT method was already applied to synthetic data, the synthetic data application leads to a development of a hybrid method between waveform tomography and the salt flooding technique commonly use in subsalt imaging. This hybrid method can overcome a convergence problem encountered by inversion with a traveltime velocity tomogram and successfully provides an accurate and highly resolved velocity tomogram for the 2D SEG/EAGE salt model. In the application of MWT to the field data, the inversion process is carried out using a multiscale method with a dynamic early-arrival muting window to mitigate the local minima problem of waveform tomography and elastic effects. With the modified MWT method, reasonably accurate results as verified by comparison of migration images and common image gathers were obtained. The hybrid method with the salt flooding technique is not used in this field data example because there is no salt in the subsurface according to our interpretation. However, we believe it is applicable to field data applications.