Ultra-low frequency AVA inversion of plane-wave primary reflection seismograms
An amplitude versus angle (AVA) inversion method is presented for estimating density and velocities of a stratified elastic medium from reflection seismograms in the intercept time-horizontal slowness domain. The elastic medium parameters are assumed to vary continuously with depth. The seismograms are Greens function pre-critical incidence primary P-wave reflections of time length T assumed to obey differential equations of a model for elastic primary P-wave back-scattering, similar to seismograms representing the first term in the well-known Bremmer series/WKBJ iterative solution model. A relation is found between plane-wave Greens function seismograms at each horizontal slowness and the medium properties in time. The Greens function seismograms after NMO-correction are directly inverted for the medium parameters as function of zero-offset traveltime. It is documented theoretically and verified numerically that the signal at the fundamental frequency f=1/ T must be present in the seismograms for the AVA method to provide the parameter trends of the elastic medium, implying that ultra-low frequencies <1 Hz for T >1 s must be generated and recorded. Noise in the seismograms at ultra-low frequencies is not considered since the theoretical AVA model does not handle microseisms that would be measured in real data. The main mathematical findings are illustrated by using simple model seismograms.