Pre-stack Full Waveform Inversion Research Articles

Pre-stack full waveform inversion of ultra-high-frequency marine seismic reflection data

The full waveform inversion (FWI) of seismic reflection data aims to reconstruct a detailed physical properties model of the subsurface, fitting both the amplitude and the traveltime of the reflections generated at physical discontinuities in the propagation medium. Unlike reservoir-scale seismic exploration, where seismic inversion is a widely adopted remote characterization tool, ultrahigh-frequency (UHF, 0.2–4.0 kHz) multichannel marine reflection seismology is still most often limited to a qualitative interpretation of the reflections’ architecture. Here we propose an elastic FWI methodology, custom-tailored for pre-stack UHF marine data in vertically heterogeneous media to obtain a decimetric-scale distribution of P-impedance, density and Poisson’s ratio within the shallow subseabed sediments. We address the deterministic multiparameter inversion in a sequential fashion. The complex trace instantaneous phase is first inverted for the P-wave velocity to make up for the lack of low frequency in the data and reduce the nonlinearity of the problem. This is followed by a short-offset P-impedance optimization and a further step of full offset range Poisson’s ratio inversion. Provided that the seismogram contains wide reflection angles (>40°), we show that it is possible to invert for density and decompose a posteriori the relative contribution of P-wave velocity and density to the P-impedance. A broad range of synthetic tests is used to prove the potential of the methodology and highlights sensitivity issues specific to UHF seismic. An example application to real data is also presented. In the real case, trace normalization is applied to minimize the systematic error deriving from an inaccurate source wavelet estimation. The inverted model for the top 15 m of the subseabed agrees with the local lithological information and core-log data. Thus, we can obtain a detailed remote characterization of the shallow sediments using a multichannel sub-bottom profiler within a reasonable computing cost and with minimal pre-processing. This has the potential to reduce the need of extensive geotechnical coring campaigns.

Prestack full waveform inversion for estimating elastic properties of marine sediments

Prestack seismic waveform inversion is a highly challenging task. The nonlinearity and nonuniqueness together with computed intensive forward modeling make the problem intractable. Here we report on a new approach to seismic waveform inversion based on a gradient descent scheme that makes use of innovative gradient calculation, adaptive regularization, and an efficient conjugate gradient algorithm. We exploit sparsity of the gradient matrix based on physics in designing an algorithm that is computationally very efficient. The results from our approach are highly encouraging in that we are able to invert large 3‐D dataset with nearly 1,000 model parameters fairly rapidly. Not only that, because of the use of adaptive regularization, we are able to track smooth as well as sharp variations in the impedance, Vp and Vs profiles when realistic starting solutions are used. We have applied the technique successfully to numerous 3‐D exploration and academic datasets. Of particular interest is our ability to estimate density from seismic data that can be used to map porosity in an effective manner. Application to seismic data collected over offshore Oregon resulted in estimates of Vp and Vs that could be used in estimating hydrate concentration in the shallow sediment.