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Offset-variable density improves acoustic full-waveform inversion: a shallow marine case study

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Title: Offset-variable density improves acoustic full-waveform inversion: a shallow marine case study
Authors: Silverton, A
Warner, M
Morgan, J
Umpleby, A
Item Type: Journal Article
Abstract: We have previously applied three-dimensional acoustic, anisotropic, full-waveform inversion to a shallow-water, wide-angle, ocean-bottom-cable dataset to obtain a high-resolution velocity model. This velocity model produced: an improved match between synthetic and field data, better flattening of common-image gathers, a closer fit to well logs, and an improvement in the pre-stack depth- migrated image. Nevertheless, close examination reveals that there is a systematic mismatch between the observed and predicted data from this full-waveform inversion model, with the predicted data being consistently delayed in time. We demonstrate that this mismatch cannot be produced by systematic errors in the starting model, by errors in the assumed source wavelet, by incomplete convergence, or by the use of an insufficiently fine finite-difference mesh. Throughout these tests, the mismatch is remarkably robust with the significant exception that we do not see an analogous mismatch when inverting synthetic acoustic data. We suspect therefore that the mismatch arises because of inadequacies in the physics that are used during inversion. For ocean-bottom-cable data in shallow water at low frequency, apparent observed arrival times, in wide-angle turning-ray data, result from the characteristics of the detailed interference pattern between primary refractions, surface ghosts, and a large suite of wide-angle multiple reflected and/or multiple refracted arrivals. In these circumstances, the dynamics of individual arrivals can strongly influence the apparent arrival times of the resultant compound waveforms. In acoustic full-waveform inversion, we do not normally know the density of the seabed, and we do not properly account for finite shear velocity, finite attenuation, and fine-scale anisotropy variation, all of which can influence the relative amplitudes of different interfering arrivals, which in their turn influence the apparent kinematics. Here, we demonstrate that the introduction of a non-physical, offset-variable, water density during acoustic full-waveform inversion of this ocean- bottom-cable field dataset, can compensate efficiently and heuristically for these inaccuracies. This approach improves the travel-time match, and consequently increases both the accuracy and resolution of the final velocity model that is obtained using purely acoustic full-waveform inversion at minimal additional cost.
Issue Date: 25-Oct-2015
Date of Acceptance: 29-Jun-2015
URI: http://hdl.handle.net/10044/1/26123
DOI: https://dx.doi.org/10.1111/1365-2478.12336
ISSN: 0016-8025
Publisher: Wiley
Start Page: 1201
End Page: 1214
Journal / Book Title: Geophysical Prospecting
Volume: 64
Issue: 5
Copyright Statement: © 2015 European Association of Geoscientists & Engineers. This is the accepted version of the following article: Silverton, A., Warner, M., Morgan, J. and Umpleby, A. (2015), Offset-variable density improves acoustic full-waveform inversion: a shallow marine case study. Geophysical Prospecting, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1111/1365-2478.12336/full
Sponsor/Funder: BG International Limited
Maersk Olie OG Gas As
Nexen Energy ULC
Hess Corporation
Dong E&P A/S
Tullow Oil Ltd
TGS Geophysical Company (UK) Limited
CGG Services (UK) Limited
Woodside Energy Ltd
Chevron North Sea Ltd
Woodside Energy Ltd
Statoil Petroleum AS
Funder's Grant Number: 4100005341
Service Order: 3500114397
NAV 000607
Keywords: Geochemistry & Geophysics
0404 Geophysics
Publication Status: Published
Appears in Collections:Earth Science and Engineering
Faculty of Engineering