Abstract：The formula for layer-by-layer downward ltd inversion is derived from the system function model H(jω).The method is based on the assumptions that the system function value for a given frequency involves only the structure informations above a definite depth H,and that the deeper structure informations only influence on the system function values corresponding to the lower frequencies.The inversion starts with H(jωmax) at the highest frequency and the initial two-layer structure to determine the parazxieters of the second layer,then continues withH(jωmax)at lower frequency and the structure added in a deeper discrete layer to produce the parameters of the third layer.The inversion goes on in lower and lower frequencies along with deeper and deeper discrete layers until we achieve the expected object.Computer's modeling result shows that the inversion method is effective,insensitive to the initial choice of ρ1,and noise resistant.
