Browsing by Author "Guan, Zhe"
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Item Improving upper-mantle receiver function imaging with slowness weighted stacking and fast-marching based 3D Pds traveltime(2018-04-19) Guan, Zhe; Niu, FenglinCommon-conversion-point (CCP) stacking of receiver functions is a widely used technique to image velocity discontinuities in the mantle. The CCP imaging technique assumes that receiver functions are composed solely of P to S conversions at velocity boundaries, whose depths can be mapped out through their arrival times. The multiple reflections at shallow boundaries with large velocity contrasts, such as the base of unconsolidated sediments and the Moho, can lead to artificial structures in the CCP images. We develop a refined CCP stacking method that uses relative slowness as a weighting factor to suppress the multiples (slowness weighted CCP stacking; SWCCP). We conduct extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images. We apply this technique to receiver function data of NECESSArray in China and the transportable array in western US, and find most of the events in the depth range of 200-400 km shown in the regular CCP images are eliminated. The SWCCP images, on the other hand, reveal a clear negative event under some parts of the two arrays, indicating the presence of low velocity layer above the 410-km discontinuity, which was reported by previous studies. In CCP stacking of receiver functions, most of the current studies computed 3D relative Pds traveltime corrections by integrating traveltime anomalies along 1D ray paths. This ray-tracing approach is generally time consuming and less accurate when prominent velocity anomalies exist and effects of the 3D ray paths become significant. In this study we introduce a new scheme that utilizes the fast-marching method eikonal solver to improve both the efficiency and accuracy of 3D Pds traveltime computation. We first employ a 1D ray tracing method and the iasp91 model to calibrate the accuracy of the new scheme and optimize the parameters of the numerical solver. We then apply the new scheme to compute a massive number of Pds traveltimes using two 3D synthetic models, one with a high-velocity slab and another one with a low-velocity plume, and compare these 3D traveltimes with those computed with the ray-tracing approach. We found 3% ray paths in the slab model and 12% ray paths in the plume model show a 3D traveltime difference of more than 0.5s. We apply the proposed scheme to the TA receiver functions that sample the transition zone structure beneath the Yellowstone hotspot and find that CCP stack using 3D Pds traveltimes computed by eikonal solver has better focused P660s than that by the ray-tracing method. Finally we illustrate that the computational times can be reduced by 1 to 2 orders of magnitude with the new scheme to compute the 3D Pds traveltimes of 20,000-200,000 receiver functions.Item Slant stacking based common conversion point stacking technique in suppressing multiples(2016-04-22) Guan, Zhe; Niu, FenglinOn a large extent global seismology is working as a data driving subject. To process real data and then image the deep earth, we need to develop various imaging techniques. Among them, common conversion point (CCP) stacking is a powerful migration method in imaging the deep structures of the earth interior by using receiver function (RF) data. One critical assumption in CCP stacking is that the P to S conversion signals in the RF data are all coming from the direct P wave conversion phases at the velocity discontinuities. However, in real case the RFs computed contain other phases such as the Moho reverberations and other lithosphere discontinuity multiples, along with numerical noises due to the deconvolution instability and the approximation that the vertical component serving as the source function of the earthquake. Thus, in the imaging results there inevitably exist unfavorable fake structures and artifacts. Realizing this potential problem, we combine the widely used slant stacking technique in the CCP stacking and develop a robust algorithm to suppress these fake structures while conserving the real existing ones. We test this technique by using synthetic data. In the final migration result, at the depth range of 200 km to 1000 km there only exist signals due to the P to S conversion at 410 and 660 discontinuities, which are the only real structures in our earth model, compared with obvious fake signals between $200-300$ km from the traditional CCP imaging. We then apply it in the real data and compare to the traditional CCP imaging results, and the comparison indicates that our technique could dramatically suppress the imaging artifacts and highlight the globally existed discontinuities, along with other real structures induced by local effects.Item Using Fast Marching Eikonal Solver to Compute 3‐D Pds Traveltime for Deep Receiver‐Function Imaging(Wiley, 2018) Guan, Zhe; Niu, FenglinIn common‐conversion‐point stacking of receiver functions, most current studies compute 3‐D relative Pds traveltime corrections by integrating traveltime anomalies along 1‐D raypaths. This ray tracing approach is generally time‐consuming and less accurate when prominent velocity anomalies exist and effects of 3‐D raypaths become significant. In this study we introduce a new scheme that utilizes a fast‐marching method eikonal solver to improve both the efficiency and accuracy of 3‐D Pds traveltime computation. We first employ a 1‐D raytracing method and the iasp91 model to calibrate the accuracy of the new scheme and optimize the parameters of the numerical solver. We then apply the new scheme to compute a massive number of Pds traveltimes using two sets of 3‐D synthetic models, one set with a high‐velocity slab and another set with a low‐velocity plume, and compare these 3‐D traveltimes with those computed with the raypath integrating approach. We find 2.7% and 11.8% raypaths in the two slab models and 7.8% and 12.0% raypaths in the two plume models show a 3‐D traveltime difference of ≥0.5 s. We apply the proposed scheme to a subset of transportable array receiver functions that sample the transition zone beneath the Yellowstone hotspot and find that a common‐conversion‐point stack using 3‐D Pds traveltimes computed by the eikonal solver method has the best focused P660s. Finally, we illustrate that computational times can be reduced by 1 to 2 orders of magnitude with the new scheme to compute 3‐D Pds traveltimes of 20,000–200,000 receiver functions.