Zhikai Wang

Zhikai Wang

Newton International Research Fellow

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Pages

Posts

Future Blog Post

less than 1 minute read

Published:

This post will show up by default. To disable scheduling of future posts, edit config.yml and set future: false.

Blog Post number 4

less than 1 minute read

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This is a sample blog post. Lorem ipsum I can’t remember the rest of lorem ipsum and don’t have an internet connection right now. Testing testing testing this blog post. Blog posts are cool.

Blog Post number 3

less than 1 minute read

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Blog Post number 2

less than 1 minute read

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Blog Post number 1

less than 1 minute read

Published:

This is a sample blog post. Lorem ipsum I can’t remember the rest of lorem ipsum and don’t have an internet connection right now. Testing testing testing this blog post. Blog posts are cool.

portfolio

publications

talks

Event registration of converted-wave images using the inverted PS-wave section and dynamic time warping

Published:

Summary: Converted wave seismic data processing and analysis can preserve and exploit the information carried by PP- and PS-wave. PP- and PS- reflection registration is a crucial step in joint PP and PS analysis and can provide the same depth/time domain seismic data for further interpretation. In this abstract, we solve the registration by combining pre-stack AVO inversion and dynamic time warping. Because of the waveform similarity between PS-wave section and pseudo-PS-wave section is between than that between PP- and PS-wave section, firstly a pre-stack inversion method is used to obtain a pseudo-PS-wave section based on pre- stack PP-wave gathers. Then dynamic time warping method based on waveform similarity is used to calculate the time shift and map the PS-wave section to PP-wave time domain. The method is used to synthetic data and field data and the results demonstrate shows a very good improvement in the event registration and this method is faster than cross-correlation method.

An Explicit Finite Difference Method Based on the Mixed Domain Function Approximation and Adaptive Spatial Operator Length Scheme

Published:

Summary: Explicit finite-difference (FD) methods with high accuracy and efficiency are preferred in full-waveform inversion and re- verse time migration. The Taylor-series expansion (TE)-based FD methods can only obtain high accuracy on a small wave- number zone. We have developed a new explicit FD method with spatial arbitrary even-order accuracy based on the mixed k (wavenumber)-space domain function approximation for the acoustic wave equation, and we derived the FD coefficients by minimizing the approximation error in a least-squares (LS) sense. The weighted pseudoinverse of mixed k-space matrix is introduced into the LS optimization problem to improve the accuracy. The new method has an exact temporal derivatives discretization in homogeneous media and also has higher tem- poral and spatial accuracy in heterogeneous media. Approxima- tion errors and numerical dispersion analysis demonstrate that the new FD method has a higher numerical accuracy than conventional TE-based FD and TE-based time-space domain dispersion-relation FD methods. Stability analysis reveals that our proposed method requires a slightly stricter stability condi- tion than the TE-based FD and TE-based time-space domain dispersion-relation FD methods. Numerical tests in the homo- geneous model, horizontally layered model, and 2D modified Sigsbee2 model demonstrate the accuracy, efficiency, and flex- ibility of the proposed new FD method.

A New Class of Central Compact Finite-difference Scheme with High Spectral Resolution for Acoustic Wave Equation

Published:

Summary: Based on the existing cell-node and cell-centered compact finite difference schemes, we developed a new central compact scheme with a high spectral resolution for the acoustic wave equation. In the new scheme, both the function values on the cell-nodes and cell-centers are used to compute the second-order spatial derivatives on the cell-nodes. The cell-centered values are stored and updated as independent variables in the modeling. The spatial derivatives on the cell-centers are evaluated by half shifting the indices in the formula designed for the cell-nodes. Compared to the conventional compact interpolation scheme, the proposed approach can avoid introducing transfer errors. Either Taylor-series expansion-based or optimized least-squares-based methods are used to calculate the finite difference coefficients. This new scheme can promise higher accuracy considering the same formal truncation errors and model parameters. Thus, it can maintain superior precision while using a shorter spatial finite difference stencil and yield equally as accurate results with less time consuming.

True-amplitude versus trace-normalized full waveform inversion

Published:

Summary: We studied the performance of true-amplitude FWI and trace-normalized-residual-based FWI in the time domain. The misfit function of trace-normalized-residual-based FWI is defined such that the adjoint source used in gradient calculation is the trace-normalized seismic residual. We compare the two inversion schemes with synthetic seismic data simulated on laterally invariant models and the more complex 2-D Marmousi model. In order to simulate realistic scenarios, we perform the elastic FWI ignoring attenuation to noisy seismic data and to the synthetic data modelled using a viscoelastic modelling scheme. Comparisons of seismic data and adjoint sources show that trace-by- trace normalization increases the magnitude of seismic data at far offsets, which are usually more cycle-skipped than those at near offsets. The inverted results on linear-gradient models demonstrate that trace-by-trace normalization increases the non-linearity of FWI, so an initial model with sufficient accuracy is required to guarantee the convergence to the global minimum. The inverted results and the final seismic residuals computed using seismic data without trace- by-trace normalization demonstrate that true-amplitude FWI provides inverted models with higher accuracy than trace-normalized-residual-based FWI, even when the unknown density is updated using density–velocity relationship in inversion or in the presence of noise and complex physics, such as attenuation.

Evidence for uniform crustal accretion and 2D passive mantle upwelling in the equatorial Atlantic Ocean from wide-angle seismic tomography

Published:

Summary: The crustal accretion along mid-ocean ridges is known to be spreading-rate dependent. Along fast-spreading ridges, two-dimensional sheet-like mantle upwelling creates relatively uniform crust. In contrast, the crust formed along slow-spreading ridges shows large along-axis thickness variations with thicker crust at segment centres, which is hypothesised to be due a three-dimensional plume-like mantle upwelling or due to focused melt migration to segment centres. Using wide-angle seismic data acquired from the equatorial Atlantic Ocean, here we show that the crustal thickness is nearly uniform (~5.5 km) across five crustal segments for crust formed at the slow-spreading Mid- Atlantic Ridge with age varying from 8 to 70 Ma. The crustal velocities indicate that this crust is predominantly of magmatic origin. We suggest that this uniform magmatic crustal accretion is due to a two-dimensional sheet-like mantle upwelling facilitated by the long-offset transform faults in the equa- torial Atlantic region and the presence of a high concentration of volatiles in the primitive melt in the mantle.

Deep hydration and lithospheric thinning at oceanic transform plate boundary

Published:

Summary: Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending to ~60 km below sea level. We performed three-dimensional thermal modelling that suggests the anomaly is probably due to extensive serpentini- zation down to ~16 km, overlying a hydrated, shear mylonite zone down to 32 km. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere–asthenosphere boundary, causing substantial thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high-temperature melt may occur beneath the ridge axis.

Crustal thickness and Moho transition zone variations along a young ridge segment at 9°N East Pacific Rise

Published:

Summary: Oceanic crust is formed from basaltic melt produced by decompression melting due to mantle upwelling at mid-ocean ridges. This crust is separated from the underlying mantle either by a sharp Mohorovičić (Moho) discontinuity or a thick Moho transition zone (MTZ). Determining the relationship between the oceanic crustal structure and the MTZ is critical for understanding the crustal accretion processes at mid-ocean ridges. However, this relationship remains elusive due to the lack of high-resolution velocity model of the oceanic crust and MTZ. Here, we present result from the application of full waveform inversion to wide-angle seismic data acquired over a young oceanic crust near the 9oN East Pacific Rise, allowing us to obtain the crustal and MTZ thicknesses along a ~70 km-long segment. We find that the crustal thickness and the MTZ thickness vary along the segment and they are inversely correlated, although the total cumulative thickness does not vary much along the profile. These variations could be attributed to the different melt migration efficiency or the variations in mantle thermal/chemical structure, indicating mantle heterogeneity along the ridge.

Fine-scale crustal velocity structure at the Lucky Strike segment from full waveform inversion of OBS data

Published:

Summary: The Lucky Strike segment at 37°N on the Mid‐Atlantic ridge, characterized by a well‐defined median valley with a central volcano, is an archetypical slow‐spreading ridge segment. By applying full waveform inversion to ocean bottom seismometer refraction data from this segment, we show that although the crustal thickness varies from 8.4 km at the segment centre to 3.9±0.2 km at segment ends, the upper crustal thickness remains nearly constant (3.0±0.3 km). The large variation in crustal thickness is dominantly due to the thinning of gabbroic lower crust, which accounts for ~2/3 of crustal thickness at the segment centre but only 10% at segment ends. We suggest that most of the upper crust is formed by lateral dyke propagation from the melt-rich segment centre to melt-poor segment ends. This indicates that the magmatic accretion along slow-spreading ridges is highly three-dimensional.

teaching

Teaching experience 1

Undergraduate course, University 1, Department, 2014

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Teaching experience 2

Workshop, University 1, Department, 2015

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