STATISTICAL INVERSION OF CONTROLLED-SOURCE SEISMIC DATA USING PARABOLIC WAVE SCATTERING-THEORY

Statistical parameters describing heterogeneity in the Proterozoic bas ement of the Baltic Shield were estimated from controlled-source seism ic data, using a statistical inversion based on the theory of wave pro pagation through random media (WPRM), derived from the parabolic wave approximation. Synthetic plane-wave seismograms generated from models of random media show consistency with WPRM theory for forward propagat ion in the weak-scattering regime, whilst for two-way propagation a di screpancy exists that is due to contamination of the primary wave by b ackscattered energy. Inverse modelling of the real seismic data sugges ts that the upper crust to depths of similar to 15 km can be character ized, subject to the range of spatial resolution of the method, by a m edium with an exponential spatial autocorrelation function, an rms vel ocity fluctuation of 1.5 +/- 0.5 per cent and a correlation length of 150 +/- 50 m. Further inversions show that scattering is predominantly occurring in the uppermost similar to 2 km of crust, where rms veloci ty fluctuation is 3-6 per cent. Although values of correlation distanc e are well constrained by these inversions, there is a trade-off betwe en thickness of scattering layer and rms velocity perturbation estimat es, with both being relatively poorly resolved. The higher near-surfac e heterogeneity is interpreted to arise from fractures in the basement rocks that close under lithostatic pressure for depths greater than 2 -3 km.