Tianyao Hao

Regional gravity anomaly separation using wavelet transform and spectrum analysis

In this paper, we propose the separation of the regional gravity anomaly using wavelet multi-scale analysis and choose rational decomposition results based on the spectrum analysis and their depth estimation results. The isotropic, symmetrical and low-pass wavelet filter, Halo wavelet, is used for wavelet transform and regional anomaly separation. The anomaly separation method is verified by the synthetic model of combined cuboids. It has also been used for regional gravity separation and deep structure study in the Dagang area, North China. The regional anomalies are obtained from large-scale wavelet decomposition results. The average depths estimated from the radial power spectrum and the stratum density are used to establish the relationship between the separation anomaly and buried geologic units. The results from seismic data and magnetotellurics inversion are used to assist the regional anomaly interpretation. From the wavelet analysis and spectrum comparisons, the regional anomalies caused by Moho and Cenozoic basement are given for further deep structure analysis.

A global optimizing approach for waveform inversion of receiver functions

A global optimizing approach is developed and implemented to retrieve one-dimensional crustal structure by waveform inversion of teleseismic receiver functions. The global optimization for the inversion is performed using a Differential Evolution (DE) algorithm. This modeling approach allows the user to perturb, within a preset range of reasonable bounds, multiple parameters such as Vp, Vp/Vs, thickness and anisotropy of each layer to fit the receiver function waveforms. Compared with linear modeling methods, the global optimal solution can be achieved with fewer model parameters (e.g., a small number of layers) and hence eliminate potential artifacts in the final model. Receiver function bins with small ray parameter intervals are used in the inversion, which can reduce distortion caused by modeling a single receiver function stacked from many recordings spread over a wide range of epicenter distance. The efficacy of this global optimizing approach is demonstrated with synthetic datasets and real receiver functions from the permanent seismic station BJT.

Joint land-sea seismic survey and research on the deep structures of the Bohai Sea areas

This paper presents the survey and research work of two land-sea profiles in the Bohai Sea, China, carried out in 2010–2011, including the seismic sources on land and in the sea, the ocean bottom seismographs (OBS) and their recovery, the coupling of OBS and the environment noise in sea area, the data quality of OBSs, and the result of data analysis. We focused on the investigation of crustal structures revealed by the two NEEW-trending joint land-sea profiles. In combination with the Pn-velocity distribution and gravitymagnetic inversion results in the North China Craton, we propose that the undulation of the Moho interface in the Bohai and surrounding areas is not strong, and the lithospheric thinning is mainly caused by the thinning of its mantle part. The research result indicates that obvious lateral variations of Moho depth and seismic velocity appear nearby all the large-scale faults in Bohai Sea, and there is evidence of underplating and reforming of the lower crust by mantle material in the Bohai area. However, geophysical evidence does not appear to support the “mantle plume” or “delamination” model for the North China Craton destruction. The crustal structure of the Bohai Sea revealed “a relatively normal crust and obviously thinned mantle lid”, local velocity anomalies and instability phenomena in the crust. These features may represent a combined effect of North China-Yangtze collision at an early stage and the remote action of Pacific plate subduction at a late stage.

Tectonic framework of China and its relation with mineral resources——Cognition from geophysical data

TECTONIC and its evolution are theoretical basis of geology and summation in geosciences from investigation of the earths structure, composition, motion and history. Therefore, they are of great importance to the industrial activities. Various geological maps have been compiled in different scales after long-time observation of rock outcrops in mountains and torrents. These tectonic maps, which are obtained based upon the cognition and judgment of tectonic movements in different ages, are summarization regarding the regularity of geological phenomenon in terms of theory, and provide service of exploration and references for environmental protection and prevention and control of disaster. Using physical methods and technique, such as gravity, magnetism, electromagnetics, seismology, geotherrnics and radioactivity, geophysicists investigated the earth, made some measurements for special purposes, and provided basic data for interior structure and condition inside the earth. For example, the seismic tomography gives three-dimensional velocity image beneath the surface. Geochemistry, starting from the composition of chemical elements, analyzes and measures the stratigraphic rock, thus providing a new method to explore the earth. These studies, which open up and deepen the theory and practice of the earth studies, not only enrich the study greatly, but also provide data in a wide range. Thus, geoscience needs comprehensive studies of geology, geophysics, geochemistry and other disciplines to understand the earth. So does the foundamental scientific investigation and geoscience application investigation. The real specific study should be conducted under the guidance of.macroscopic cognition. Study on the tectonic framework of China has macroscopic and overall characteristics. Based on the work of predecessors, it should be extended and deepened and.include both the latest results of geology and geophysics, providing hypothesis for geosciences and initial conditions for solving the problems, and serving as the basis of further investigation.

Automatic lineament extraction from potential-field images using the Radon transform and gradient calculation

Linear anomalies are critical in the interpretation of gravity and magnetic data. Visual identification of lineaments is usually done by experienced interpreters, and identification results then have to undergo a digitization or import procedure. The traditional identification method has unavoidable subjectivity and inefficiency. To overcome these limitations, we fuse the Radon transform (RT) with gradient calculation to process gravity or magnetic data and to realize automatic detection and extraction of lineaments. As part of the detection procedure, we define the RT-based mean gradient (MG), effective mean gradient (EMG), and residual mean gradient (RMG) in order to highlight long linear segments or to enhance short linear ones in the transform domain. The gradient forms are applied self-adaptively and self-selectively to gravity or magnetic images according to specific conditions. Gradient directions are also taken into account in the transformation procedure to emphasize the characteristics of linear anomalies. To extract the position and length of the detected lineaments from the transform domain, a constraint inverse searching method (CISM) is given and used to locate the starting and end points of the lineaments. The method can deal with the condition that there is at least one linear section in a specific direction or that separate linear sections may belong to one lineament. Through tests with synthetic images and with real data from the Haijiao upheaval area in the East China Sea Basin, the detection and extraction methods are shown to be more effective and robust than the conventional RT applications. The results from the real data roughly coincide with major geologic faults that are visually identified. These results show that the methods constitute a useful tool to aid fault interpretation.

Investigation of igneous rocks in Huanghua depression, North China, from magnetic derivative methods

The igneous rock study plays an important role in understanding the tectonic evolution of sedimentary basins. The distribution of igneous rocks is one of the basic aspects in the igneous rock study. Based on the magnetic susceptibility contrast of igneous and other rocks, magnetic methods are usually used in the igneous rock investigation. In this paper, we delineated the distribution of igneous rocks in Huanghua depression based on the reduction to pole (RTP) magnetic anomaly and its derivative processing results. The main methods used to enhance the anomaly character of igneous rocks include total horizontal derivative, analytic signal modules and the tilt angle. Based on the RTP anomaly and the derivative results, three types of igneous rock distribution areas are predicted in Huanghua depression. The large scale igneous rocks are mainly distributed in the north part of Huanghua depression. The string bead-shaped igneous rocks are mainly located in the north and southwest parts. Some craters are located in the middle-south part. The distribution of igneous rocks is controlled by the deep buried fault along the coastal boundary and the associated tertiary faults. Our study verified the validity of the magnetic derivative methods for the planar distribution study of igneous rocks. The study results of Huanghua depression are the basis of the seismic interpretation of igneous rocks and the tectonic study in this area.

Waveform Retrieval and Phase Identification for Seismic Data from the CASS Experiment

The little destruction to the deployment site and high repeatability of the Controlled Accurate Seismic Source (CASS) shows its potential for investigating seismic wave velocities in the Earths crust. However, the difficulty in retrieving impulsive seismic waveforms from the CASS data and identifying the seismic phases substantially prevents its wide applications. For example, identification of the seismic phases and accurate measurement of travel times are essential for resolving the spatial distribution of seismic velocities in the crust. Until now, it still remains a challenging task to estimate the accurate travel times of different seismic phases from the CASS data which features extended wave trains, unlike processing of the waveforms from impulsive events such as earthquakes or explosive sources. In this study, we introduce a time-frequency analysis method to process the CASS data, and try to retrieve the seismic waveforms and identify the major seismic phases traveling through the crust. We adopt the Wigner-Ville Distribution (WVD) approach which has been used in signal detection and parameter estimation for linear frequency modulation (LFM) signals, and proves to feature the best time-frequency convergence capability. The Wigner-Hough transform (WHT) is applied to retrieve the impulsive waveforms from multi-component LFM signals, which comprise seismic phases with different arrival times. We processed the seismic data of the 40-ton CASS in the field experiment around the Xinfengjiang reservoir with the WVD and WHT methods. The results demonstrate that these methods are effective in waveform retrieval and phase identification, especially for high frequency seismic phases such as PmP and SmS with strong amplitudes in large epicenter distance of 80–120xa0km. Further studies are still needed to improve the accuracy on travel time estimation, so as to further promote applicability of the CASS for and imaging the seismic velocity structure.

A multi-frequency receiver function inversion approach for crustal velocity structure

In order to constrain the crustal velocity structures better, we developed a new nonlinear inversion approach based on multi-frequency receiver function waveforms. With the global optimizing algorithm of Differential Evolution (DE), low-frequency receiver function waveforms can primarily constrain large-scale velocity structures, while high-frequency receiver function waveforms show the advantages in recovering small-scale velocity structures. Based on the synthetic tests with multi-frequency receiver function waveforms, the proposed approach can constrain both long- and short-wavelength characteristics of the crustal velocity structures simultaneously. Inversions with real data are also conducted for the seismic stations of KMNB in southeast China and HYB in Indian continent, where crustal structures have been well studied by former researchers. Comparisons of inverted velocity models from previous and our studies suggest good consistency, but better waveform fitness with fewer model parameters are achieved by our proposed approach. Comprehensive tests with synthetic and real data suggest that the proposed inversion approach with multi-frequency receiver function is effective and robust in inverting the crustal velocity structures. Multi-frequency receiver functions are used in waveform inversion.Reliable crustal velocity model can be obtained.The code is suitable for both radial and transverse receiver functions.Good constraints can accelerate the convergence in the inversion.

Deep Fault Plane Revealed by High‐Precision Locations of Early Aftershocks Following the 12 September 2016 ML 5.8 Gyeongju, Korea, EarthquakeShort Note

An M-L 5.8 earthquake, which is large for a stable continental region, occurred in southeastern Korea on 12 September 2016. Ten days of data from a temporary seismic network deployed immediately after the mainshock are combined with data from permanent seismic stations to determine high-precision locations of early aftershocks to reveal the geometry of the causative structure at depth. Well-constrained relative earthquake hypocenters and focal mechanisms are used to define the subsurface fault plane with a strike of similar to N28 degrees E and dip of similar to 78 degrees to the east-southeast. This fault plane extends from 12 to 15 km depth and may have been responsible for most of the early earthquakes in the Gyeongju earthquake sequence. A pre-existing weak zone in a strike-slip duplex that formed from subsidiary Riedel shears beneath the Yangsan fault system may have been reactivated to nucleate the mainshock and aftershocks.

Performance of preconditioned iterative and multigrid solvers in solving the three-dimensional magnetotelluric modeling problem using the staggered finite-difference method: a comparative study

An effective solver for the large complex system of linear equations is critical for improving the accuracy of numerical solutions in three-dimensional (3D) magnetotelluric (MT) modeling using the staggered finite-difference (SFD) method. In electromagnetic modeling, the formed system of linear equations is commonly solved using preconditioned iterative relaxation methods. We present 3D MT modeling using the SFD method, based on former work. The multigrid solver and three solvers preconditioned by incomplete Cholesky decomposition—the minimum residual method, the generalized product bi-conjugate gradient method and the bi-conjugate gradient stabilized method—are used to solve the formed system of linear equations. Divergence correction for the magnetic field is applied. We also present a comparison of the stability and convergence of these iterative solvers if divergence correction is used. Model tests show that divergence correction improves the convergence of iterative solvers and the accuracy of numerical results. Divergence correction can also decrease the number of iterations for fast convergence without changing the stability of linear solvers. For consideration of the computation time and memory requirements, the multigrid solver combined with divergence correction is preferred for 3D MT field simulation.