Abstract
SUMMARY Here we assess the potential of the visco-acoustic frequency domain full-waveform inversion (FWI) to reconstruct P-wave velocity (VP) and P-wave attenuation factor (Q) from surface onshore seismic data. First, we perform a sensitivity analysis of the FWI based upon a grid search analysis of the misfit function and several synthetic FWI examples using velocity and Q models of increasing complexity. Subsequently, we applied both the acoustic and viscoacousticFWItorealsurfacewide-apertureonshoreseismicdatafromthePolishBasin,wherea strongattenuationoftheseismicdataisobserved.Thesensitivityanalysisofthevisco-acoustic FWI suggests that the FWI can jointly reconstruct the velocity and the attenuation factor if the signatureoftheattenuationissufficientlystronginthedata.Asyntheticexamplecorresponding to a homogeneous background model with an inclusion shows a reliable reconstruction of VP and Q in the inclusion, when Q is as small as 90 and 50 in the background model and in the inclusion, respectively. A first application of acoustic FWI to real data shows that a heuristic normalization of the data with offset allows us to compensate for the effect of the attenuation in the data and reconstruct a reliable velocity model. Alternatively, we show that visco-acoustic FWI allows us to reconstruct jointly both a reliable velocity model and a Q model from the true-amplitude data. We propose a pragmatical approach based upon seismic modelling and source wavelet estimation to infer the best starting homogeneous Q model for visco-acoustic FWI. We find the source wavelet estimation quite sensitive to the quality of the velocity and attenuation models used for the estimation. For example, source-to-source wavelets are significantly more consistent when computed in the final FWI model than in the initial one. A good kinematic and amplitude match between the early-arriving phases of the real and time-domain synthetic seismograms computed in the final FWI model provides an additional evidence of the reliability of the final FWI model. We find the recovered velocity and attenuation models consistent with the expected lithology and stratigraphy in the study area. We link high-attenuation zones with the increased clay content and the presence of the mineralized fluids.
Highlights
In recent years there has been an increasing interest in the use of full waveform inversion (FWI) for imaging nuclear waste deposit sites, monitoring CO2 sequestration, assessing natural hazards and characterising geothermal and oil and gas reservoirs
We present one of the first application of viscoacoustic frequency-domain FWI to real onshore surface seismic data that were recorded in the Polish Basin
In order to estimate by trial-and-error the best homogeneous Q starting model for FWI, we compared the AVO responses of the real data with the response of the synthetic data calculated in the starting velocity model using different constant Q models (Fig. 10; Pratt 1999; Gao et al 2006)
Summary
In recent years there has been an increasing interest in the use of full waveform inversion (FWI) for imaging nuclear waste deposit sites, monitoring CO2 sequestration, assessing natural hazards and characterising geothermal and oil and gas reservoirs. Ribodetti et al (2000) develop visco-acoustic ray+Born migration/inversion and concluded that velocity and attenuation can be reliably reconstructed only when a reflector is illuminated by reflected waves from above and beneath. An application of visco-acoustic time-domain FWI to a realistic synthetic data set at a crustal scale has been presented by Askan et al (2007), while Liao & McMechan (1995, 1996) have presented the first attempts to reconstruct velocity and attenuation by frequency-domain FWI of synthetic data. Application of visco-acoustic FWI to synthetic models of increasing complexities supports a view that the velocity and attenuation can be reliably reconstructed from surface wideaperture seismic data by non-linear inversion. The relevance of the velocity and attenuation models are discussed based upon checkerboard tests, source wavelet estimation, seismic modelling and geological and petrophysical interpretations
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