High resolution traveltime imaging

It is well known that traveltime tomography does not provide sufficient image quality of the earth. There are two major reasons: (1) resolution in traveltime inversion critically depends on the completeness of the recording aperture; (2) the number of independent traveltime equations is usually much less than the number of unknowns. The latter situation leads to the existence of different solutions which perfectly suit the same traveltime equations. Correct solutions cannot then be distinguished unless independent conditions (constraints) are imposed.