Changkai Qiu

• Doctor of Philosophy
• Development and Research Center, China Geological Survey

Geomagnetic depth sounding (GDS) is a geophysical electromagnetic (EM) method that studies the deep structure and composition of the earth by using long-period EM signals from geomagnetic observatories and satellites. In this paper, a three-dimensional (3D) anisotropic GDS modeling algorithm is developed. The curl-curl equation is discretized using...

We propose a novel smoothing regularization scheme for three-dimensional (3D) magnetotelluric (MT) inversion based on unstructured tetrahedral discretization. Different from conventional methods that explicitly add smoothing constraints to model parameters, we choose to do the gradient filtering to smooth the model updates in an implicit way. By tr...

Bayesian inversion of electromagnetic data can obtain key information on the uncertainty of subsurface resistivity. However, due to its high computational cost, Bayesian inversion is largely limited to 1D resistivity models. In this study, a fast Bayesian inversion method is implemented by introducing the spatial correlation as prior information. T...

We propose a new three‐dimensional anisotropic inversion scheme for magnetotelluric (MT) data. In this method, the earth is discretized into unstructured tetrahedral grids that can fit complex structures well, such as the earth topography and coastline. We use a 3 × 3 tensor to describe the anisotropic conductivity in the governing equation for MT...

With geophysical surveys evolving from traditional 2D to 3D models, the large volume of data adds challenges to inversion, especially when aiming to resolve complex 3D structures. An iterative forward solver for a controlled-source electromagnetic method (CSEM) requires less memory than that for a direct solver; however, it is not easy to iterative...

The conventional, L2-norm-based, regularization term in electromagnetic (EM) inversions implements smooth constraints on model complexity in the space domain, which can smoothen the boundaries of complex underground structures. To improve the resolution of 3-D frequency-domain airborne EM (AEM) inversions, we propose a new algorithm for sparse-regu...

S U M M A R Y In this paper, we propose a spectral element method (SEM) based on unstructured tetrahedral grids for direct current (dc) resistivity modelling. Unlike the tensor-product of 1-D Gauss-Lobatto-Legendre (GLL) quadrature in conventional SEM, we use Proriol-Koornwinder-Dubiner (PKD) polynomials to form the high-order basis polynomials on...

The spectral element method (SEM) based on high-order complete orthogonal polynomials is an accurate and efficient numerical method for electromagnetic modelling due to its spectral accuracy and exponential convergence. The SEM combines the flexibility of the finite-element method and the high accuracy of the spectral method. In this paper, we intr...

S U M M A R Y We introduce a novel block rational Krylov method to accelerate 3-D time-domain marine controlled-source electromagnetic modelling with multiple sources. This method approximates the time-varying electric solutions explicitly in terms of matrix exponential functions. A main attraction is that no time stepping is required, while most o...

S U M M A R Y The transient electromagnetic (TEM) data acquired in mountainous area are seriously affected by topography, implying that consideration of topographic effect in the interpretation is in urgent need. To deal with this problem, we propose a new method to invert TEM data by simulating the topography. The unstructured tetrahedral grid wit...

Controlled-source audio-frequency magnetotellurics (CSAMT) is playing an important role in shallow subsurface exploration. Most current three-dimensional (3D) forward modeling for CSAMT are based on an isotropic earth model which neglects the electrical anisotropy in the earth. Here we implement a 3D vector finite-element modeling algorithm to solv...

Electrical anisotropy exists everywhere in nature that has become an important issue non-ignorable in geophysical exploration. Especially in areas of sedimentary rocks where strata in the earth result in variation of resistivity varying with the direction of current flow. In this paper, we review the research on electrical anisotropy in geo-electro...

Marine controlled-source electromagnetic (MCSEM) method is an important predrill reservoir appraisal method to reduce exploration risk in detecting sub-seafloor hydrocarbon reservoirs. Most 3D forward modelings for MCSEM are based on conventional numerical methods like finite-difference and finite-element method. In this paper, we introduce spectra...

This paper presents a 3D forward modelling algorithm with an adaptive finite-element method based on unstructured grids that when used with a circular scanning DC measurement technique can provide an indication of anisotropy in a layered earth. The accuracy of this algorithm is checked against 1D semi-analytical solutions for an arbitrarily anisotr...

Airborne transient electromagnetic (ATEM) data frequently shows sign reversal in the late-time channels duo to IP effect. Traditional inversion algorithms based on real resistivity cannot deliver reasonable results for such kind of data. They are usually deleted in the data processing. For in-depth understanding the characteristics of EM diffusion...

With airborne electromagnetic (AEM) systems being widely used in the mineral, oil and gas, ground water, environmental and engineering exploration, AEM data interpretation and imaging become more and more important. However, there are imaging techniques in AEM community in which parameters like imaging depth can only be determined empirically but c...

The resistivity and thickness inverted by traditional single-site 1D methods for time-domain airborne electromagnetic (AEM) data frequently demonstrate sharp variations along a profile. This phenomenon occurs even between neighbor stations, resulting in non-smooth layer interfaces, which can seriously complicate the interpretation. Considering that...

As an effective and efficient geophysical tool, airborne EM (AEM) is specifically suitable for the exploration in areas of high mountains, desert, swamp, and forest. With the development of national economy of China, the demand for mineral resources increases sharply, geophysical explorations in areas with favorable geological conditions have been...