Geophysical Journal International, 179, 1617-1629, 2009,doi: 10.1111/j.1365-246X.2009.04350.x
N. Maercklin, A .Zollo
We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to-PP reflection amplitude ratios. The velocity structure above the reflector is constrained by travel time modelling, and the amplitude ratios are determined using using the same source-receiver pair for the measured PP and PS amplitudes (commonoffset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray-theoretical forward modelling of theoretical PS-to-PP ratios. A minimisation of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest (P-velocity, P-to-S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 km/s to 3.5 km/s and a decrease of the P-to-S velocity ratio from 3.6 to 1.75. The proposed PS-to-PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known.
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