Miaoyue Wang

Comparison of source-independent methods of elastic waveform inversion

In this paper, we investigate several source-independent methods of nonlinear full-waveform inversion of multicomponent elastic-wave data. This includes iterative estimation of source signature (IES), standard trace normalization (STN), and average trace normalization (ATN) inversion methods. All are based on the finite-element method in the frequency domain. One synthetic elastic crosshole model is used to compare the recovered images with all these methods as well as the known source signature (KSS) inversion method. The numerical experiments show that the IES method is superior to both STN and ATN methods in two-component, elastic-wave inversion in the frequency domain when the source signature is unknown. The STN and ATN methods have limitations associated with near-zero amplitudes (or polarity reversals) in traces from one of the components, which destroy the energy balance in the normalized traces and cause a loss of frequency information. But the ATN method is somewhat superior to the STN method in suppressing random noise and improving stability, as the developed formulas and the numerical experiments show. We suggest the IES method as a practical procedure for multicomponent seismic inversion.

Multi-geophysical Investigation of Geological Structures in a Pre-selected High-level Radioactive Waste Disposal Area in Northwestern China

A potential high-level radioactive waste (HLRW) disposal site in northwestern China was investigated to determine its suitability for such a use. The site is primarily covered with welldeveloped metamorphic granite rocks. The primary targets for geological repositories are three granite rock masses (I, II, and III). Only surface geological data were available from previous studies. The objective of this study was to evaluate the quality of the rock mass and identify any weak geological structures that could jeopardize this future underground repository. We used gravity and aeromagnetic data on a large scale to study the regional geological structures within and around the three rock masses. Subsequently, in 2009, a controlled source audio-frequency magnetotelluric (CSAMT) survey was conducted to study rock mass I in more detail. This paper introduces three-dimensional (3-D) tomography imaging of gravity and aeromagnetic data. 3-D tomography imaging was carried out on previously collected gravity and aeromagnetic data and, using the results of different depth slices, we evaluated the rock mass quality and interpreted the geology. The aeromagnetic depth slices show that at about 1-km deep in rock masses I and III there is a high magnetic susceptibility body, possibly caused

Migration of ground‐penetrating radar data with a finite‐element method that considers attenuation and dispersion

We use a numerical model to study the effects of attenuation and dispersion upon the migration of ground-penetrating radar (GPR) profiles. A finite-element method (FEM) is developed that incorporates attenuation and is used to generate synthetic GPR profiles with random noise. These profiles are then migrated with and without the attenuation term using our FEM codes. The misfit between the position of interfaces in the model and the position of corresponding interfaces in the migrated profile is greatly decreased when the attenuation term is considered in the migration process. The improvement in resolution results from the use of the group velocity rather than any phase velocity. Consequently, for dispersive media the attenuation term of high-frequency GPR waves cannot be ignored in migration.

Finite-element implementation of reverse-time migration for anisotropic media

Reverse-time migration for post-stack seismic data in anisotropic media is implemented using the finite-element method. As an accurate digital method, the finite-element method is flexible for dealing with complicated geological structures, inner and man-made boundaries despite its intensive computation. Applying it in reverse-time migration may produce accurate images for anisotropic media. To eliminate man-made boundary reflections, the absorbing boundary condition for anisotropic elastic waves is also studied. An efficient and stable absorbing boundary scheme is presented combining a discrete transparent boundary condition with an attenuation boundary condition.

Pre-stack full wavefield inversion for elastic parameters of TI media

Pre-stack full wavefield inversion for the elastic parameters of transversely isotropical media is implemented. The Jacobian matrix is derived directly with the finite element method, just like the full wavefield forward modelling. An absorbing boundary scheme combining Liaos transparent boundary condition with Sarmas attenuation boundary condition is applied to the forward modelling and Jacobian calculation. The input data are the complete ground-recorded wavefields containing full kinematic and dynamic information for the seismic waves. Inversion with such data is desirable as it should improve the accuracy of the estimated parameters and also reduce data pre-processing, such as wavefield identification and separation. A scheme called energy grading inversion is presented to deal with the instability caused by the large energy difference between different arrivals in the input data. With this method, parameters in the shallow areas, which mainly affect wave patterns with strong energy, converge before those of deeper media. Thus, the number of unknowns in each inversion step is reduced, and the stability and reliability of the inversion process is greatly improved. As a result, the scheme is helpful to reduce the non-uniqueness in the inversion. Two synthetic examples show that the inversion system is reliable and accurate even though initial models deviate significantly from the actual models. Also, the system can accurately invert for transversely isotropic model parameters even with the introduction of strong random noise.

2-D finite element modeling for seismic wave response in media with sand bodies

Abstract Forward modeling plays an important role in structural trap exploration because of its usefulness in identifying seismic wave characteristics. Structural traps have been discovered in the Dongpu depression of Zhongyuan Oil Field of China. However, its proved reserves still fall far short of total estimated reserves, suggesting that there are undiscovered lithologic traps. According to the stratum properties revealed by borehole surveying and exploration seismograms, we have constructed the models of deposit sand body traps and conducted a complex seismic wave modeling study using the finite element method. Our models contain scattering bodies. The trap media is considered viscous–elastic and finite element equations are derived with artificial boundary terms. The absorbing effect of the artificial boundary terms we obtained is fairly good. The source is simulated by concentrated forces or explosive sources. We obtain the common source/receiver record profiles by putting secondary sources directly on the interface nodes. Direct waves and surface waves of single-shot seismograms can be removed with Huygens principle. We also put the filter of instrument response into the theoretical seismograms. In the paper, we have designed several models of sand bodies, and obtained their realistic seismic profiles with the finite element method. The corresponding records have been analyzed and the characteristics have been extracted. The method will provide an analytical tool for exploring lithologic traps effectively.

“Earth‐ionosphere” mode electromagnetic forward modeling

GEM Beijing 2011: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Beijing, China. October 10-13, 2011. “Earth-Ionosphere” mode electromagnetic forward modeling Diquan Li*, School of Geosciences and Info-physics, Central South University, Changsha, China Qingyun Di and Miaoyue Wang, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The contrast of response characteristics between large power long bipole and circle current source

For electromagnetic (EM) fields exited by dozens of kilometers long bipole or large diameter circle current sources, the effect on EM field by ionosphere cannot be ignored. In this case, we call it “earth-ionosphere” model which includes ionosphere, atmosphere and earth media. In order to discuss the EM field with these two sources, we modeled the frequency responses and attenuation characteristics of EM fields for three layers earthionosphere and half-space model. The results show that for the long bipole source the ionosphere has an enhancement effect on the fields, and the fields in the earth-ionosphere model decay slower than half-space with given frequency. However, we draw an inverse conclusion for the circle current source.

TIME-DOMAIN FINITE-ELEMENT WAVE FORM INVERSION OF ACOUSTIC WAVE EQUATION

The paper derives the finite element equation for acoustic wave in time domain and presents a transparent-plus-attenuation boundary condition. Forward modeling demonstrates that the boundary condition absorbs boundary reflection wave very well. On these bases, we derive the equation satisfied by elements of Jacobi matrix used in the inversion of the physical property parameters of acoustic media. In fact, the equation is the same as that of forward modeling in form. Only the right force item is different. So with the same method of forward modeling, we can get the elements of Jacobi matrix. Because the elements are variable with time and the present inversion does not permit too many unknowns. We integrate the finite elements with the same physical property as one unknown structure unit (for example, a horizontal layer or an oblique layer, etc.) and inverse the physical property parameters of these unknown structure units instead all elements unknown parameters. The method greatly reduces calculation time and saves computer memory. Also, it improves the accuracy of the inversion results and improves the stability of the solving process. The inversion equations are solved with QR decomposition method. Model results prove that the full wave equation inversion method in time domain is effective.