This paper discusses some aspects of the use of a priori information, which allows unique determination of parameters of seismic models. If the problem can be formulated in terms of the system of linear equations, then analysis of a priori information can be completely formalized and described in terms of the kernel of the matrix of the corresponding system of linear equations. In this case, term ``pseudo-a priori'' information is defined. Introduction of this term allows representation of the problem of parameter estimation in two steps. First, the most stable and unique solution is build on the basis of the ``pseudo-a priori'' information. Second, this solution is corrected using the minimum amount of the a priori data. For both procedures knowledge of the structure of the ``null-space'' of the corresponding system of linear equations is crucially important. Therefore, the main emphasis of this paper is on analysis of the ``null-space'' structure for selected problems.
Key words: seismics, data processing, a-priori information, systems of linear equations.