Abstract
Abstract. Detailed reconstruction of deep crustal targets by seismic methods remains a long-standing challenge. One key to address this challenge is the joint development of new seismic acquisition systems and leading-edge processing techniques. In marine environments, controlled-source seismic surveys at a regional scale are typically carried out with sparse arrays of ocean bottom seismometers (OBSs), which provide incomplete and down-sampled subsurface illumination. To assess and minimize the acquisition footprint in high-resolution imaging process such as full waveform inversion, realistic crustal-scale benchmark models are clearly required. The deficiency of such models prompts us to build one and release it freely to the geophysical community. Here, we introduce GO_3D_OBS – a 3D high-resolution geomodel representing a subduction zone, inspired by the geology of the Nankai Trough. The 175km×100km×30km model integrates complex geological structures with a viscoelastic isotropic parameterization. It is defined in the form of a uniform Cartesian grid containing ∼33.6e9 degrees of freedom for a grid interval of 25 m. The size of the model raises significant high-performance computing challenges to tackle large-scale forward propagation simulations and related inverse problems. We describe the workflow designed to implement all the model ingredients including 2D structural segments, their projection into the third dimension, stochastic components, and physical parameterization. Various wavefield simulations that we present clearly reflect in the seismograms the structural complexity of the model and the footprint of different physical approximations. This benchmark model is intended to help to optimize the design of next-generation 3D academic surveys – in particular, but not only, long-offset OBS experiments – to mitigate the acquisition footprint during high-resolution imaging of the deep crust.
Highlights
To make a step change in our understanding of the geodynamical processes that shape the earth’s crust, we need to improve our ability to build 3D high-resolution multiparameter models of geological targets at the regional scale
3D sparse ocean bottom seismometers (OBSs) deployments are conducted with a similar number of stations with an aim to apply full waveform inversion (FWI). Which of those two different acquisitions – dense 2D or sparse 3D – are closer to the optimal setting? Was the 2D Seize France Japan (SFJ) profile oversampled or are today’s 3D experiments maybe undersampled? Is it possible to solve the sparse OBS acquisition issues with more robust sparsity-promoting regularization techniques? Or maybe this undersampling can be solved by acquiring the coincident long-streamer data? If so, what would be the most efficient shooting strategy comprising both types of experiments? We believe that for a new FWI-optimized OBS surveys we should look for a justification of a proposed acquisition geometry rather than use an ad hoc configuration
We developed the GO_3D_OBS reference geomodel for the purpose of assessing different crustal-scale seismic imaging methods, as well as for guiding various seismic acquisition designs
Summary
To make a step change in our understanding of the geodynamical processes that shape the earth’s crust, we need to improve our ability to build 3D high-resolution multiparameter models of geological targets at the regional scale. Long-offset OBS data recorded by stationary-receiver surveys contain a wider aperture content and richer information about the deep crust and upper mantle This information can be processed by travel time tomography and waveform inversion techniques – the former providing a kinematically accurate starting model for the latter (Kamei et al, 2012; Górszczyk et al, 2017). The proposed geomodel is intended to serve as an experimental setting for various imaging approaches with a special emphasize on multi-parameter waveform inversion techniques For this purpose, the size of geological features we introduce shall be detectable by seismic waves and span tens of kilometres (major structural units building mantle, crust, volcanic ridges, etc.) to tens of metres – namely to the order of the smallest seismic wavelengths (sedimentary cover, subducting channel, thrusts, and faults, etc.). We present some simulations of OBS data in the 3D model with the finite-difference and spectralelement methods, and we summarize the article with a discussion and conclusion
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