基于空气地球分离策略的大地电磁三维正演

大地电磁正演常采用电场双旋度方程,由于工作频率低、空气电导率为零,电场双旋度方程含有空解空间,导致离散化的线性方程组严重病态、具有高的条件数.为降低大地电磁场双旋度正演方程的病态性,本文提出了一种基于空气与地球分离策略的大地电磁混合正演公式.首先,在空气层中将双旋度算子转换为Laplace算子,在地下利用电导率大于零的特征保留双旋度算子,建立空气与地下分离的混合公式系统,以便实现混合公式系统对应的离散化线性方程组条件数的大幅度降低.然后,在空气与地下分界面上,施加电磁场的切向与法向连续性条件实现空气与地下区域的耦合与信息交换,以保证电磁场分布的唯一性.再则,利用非结构化四面体网格离散技术来高精度模拟任意起伏地形和复杂地下三维地电结构,并结合节点有限元与棱边有限元法,高精度求解该混合公式系统.最后,利用国际标准大地电磁测试模型来验证该空气与地球分离算法的正确性,并展示其有效降低原有电场双旋度方程的病态性的能力.本文提出的空气与地球分离思想为寻求大地电磁最优化控制方程开启了新的求解思路,预计会产生更多、性能更佳的大地电磁混合公式,以便服务于大地电磁正反演技术的快速发展.

     

Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

用Occam法及广义逆法对兰州地区MT资料的一维反演解释

分别用Occam法和广义逆法对兰州地区MT资料进行了一维反演，两种反演结果可相互印证，使得解释结果更为可靠。并和相应资料的二维反演结果相一致。     

腾冲火山多地球物理参数模型

腾冲火山区是中国最年轻的板内火山之一,岩浆活动频繁以及高温地热异常使其受到广泛关注.以往的地球物理探测结果显示腾冲火山区下方存在地壳岩浆囊,但其深部几何形态和分布一直存在争议.本文基于腾冲火山地区的大地电磁测深结果,并结合相关地质与地球物理资料,提出腾冲火山岩石圈尺度的多地球物理参数模型.我们的模型显示在腾冲火山区中下地壳内存在三个岩浆囊;腾冲火山区岩石圈上地幔可能存在岩浆囊,但需要长周期大地电磁测深数据进一步的验证.

     

Forward plane-wave electromagnetic model in three dimensions using hybrid finite volume–integral equation scheme

We present a concept of the hybrid finite volume–integral equation technique for solving Maxwell's equation in a quasi-static form. The divergence correction was incorporated to improve the convergence and stability of the governing linear system equations which pose a challenge on the discretization of the curl–curl Helmholtz equation. A staggered finite volume approach is applied for discretizing the system of equations on a structured mesh and solved in a secondary field technique. The bi-conjugate gradient stabilizer was utilized with block incomplete lower-upper factorization preconditioner to solve the system of equation. To obtain the electric and magnetic fields at the receivers, we use the integral Green tensor scheme. We verify the strength of our hybrid technique with benchmark models relative to other numerical algorithms. Importantly, from the tested models, our scheme was in close agreement with the semi-analytical solution. It also revealed that the use of a quasi-analytical boundary condition helps to minimize the runtime for the linear system equation. Furthermore, the integral Green tensor approach to compute at the receivers demonstrates better accuracy compared with the conventional interpolation method. This adopted technique can be applied efficiently to the inversion procedure.     

Hydrogeological investigation in Santiago Island (Cabo Verde) using magnetotellurics and VLF methods

A geoelectromagnetic research was carried out in the Santa Cruz region (Santiago Island, Cabo Verde) during June 2004. The survey comprised MT soundings and VLF profiles. The main purpose of the MT profile, carried out across three important valleys associated with freshwater aquifers, was to study the tectonic structures correlated to seawater infiltration. The VLF method was used inside of the valleys for investigating shallow structures related to the aquifer contamination by seawater.Numerical modelling shows that the ocean effect is not important for MT data collected at periods shorter than 1 s. The MT data were inverted using a two-dimensional approach, to obtain the sub-superficial electrical conductivity distribution. The VLF data were processed applying the Karous–Hjelt filters to obtain the equivalent current distribution and inverted using 2-D approach. The results obtained in one of the most important valleys show anomalous current concentration/low resistivity (<20 Ω m) areas at depths greater than 40 m that may correspond to an increase in seawater content.The MT data modelling show that the deep zones beneath the valley are strongly fractured representing good pathways for seawater circulation. The depth of the conductive zones increases from south to north, suggesting a northward decreasing of the seawater infiltration effect. This observation correlates very well with in situ geochemical observations.     

A magnetotelluric study of the Damara Belt in Namibia: 1. Regional scale conductivity anomalies

The Namibian margin is dominated by the late Proterozoic to early Cambrian fold belts of the Damara Orogen, which wrap around and separate the Congo and Kalahari Cratons. This mosaic of relatively ‘soft’ fold belts and ‘hard’ cratons apparently controlled the path for the opening of the South Atlantic in the early Cretaceous. The continents split along the coast-parallel fold belts of the Damara Orogen while the inland fold belt (Damara Belt) was effected by extension and widespread igneous intrusion but never developed to the rift stage. This paper is concerned with the interpretation of magnetotelluric (MT) data along a 200 km NW-SE profile across the Damara Belt in NW Namibia. The regional, two-dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones: (i) a generally very resistive upper crust which is typical for the granites and metasediments of the Damara Belt, (ii) two subvertical conductors in upper to mid-crustal levels which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust. We interpret the high electrical conductivity in terms of graphite (or other forms of mineralization) enrichment along the shear planes. This zone of crustal weakness may have originated in Pre-Damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic.     

On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 10² Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >10³ Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry.     

Space and time spectra of stationary stochastic waves,with special reference to microtremors

It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau's alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation. This study examines the Deda ancient landslide, situated in the Chalong-ranbu fault zone, where creep deformation suggests a complex underground structure. By integrating remote sensing, field surveys, Audio-frequency Magnetotellurics (AMT), and Microtremor Survey Method (MSM) techniques, along with engineering geological drilling for validation, to uncover the landslide’s spatial features. The research indicates that a fault is developed in the upper part of the Deda ancient landslide, and the gully divides it into Deda landslide accumulation zone I and Deda landslide accumulation zone II in space. The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers. Our findings include the identification of two sliding zones in the Deda I landslide, the shallow sliding zone (DD-I-S1) depth is approximately 20 m, and the deep sliding zone (DD-I-S2) depth is 36.2‒49.9 m. The sliding zone (DD-II-S1) depth of the Deda II landslide is 37.6‒43.1 m. A novel MSM-based method for sliding zone identification is proposed, achieving less than 5% discrepancy in depth determination when compared with drilling data. These results provide a valuable reference for the spatial structural analysis of large-deep-seated landslides in geologically complex regions like the eastern Tibetan Plateau.     

阿尔金走滑断裂带昌马段的电性结构样式及构造意义

阿尔金断裂带东段走滑速率沿断裂走向方向存在明显的流失现象,有关阿尔金断裂带的影响范围及走滑速率变化的机制需要有更多的深部结构证据来提供支撑.本文以阿尔金断裂带昌马段为窗口,获取了4条横穿阿尔金断裂带及相邻地区的大地电磁测深剖面.二维电性剖面显示在阿尔金断裂带北侧中上地壳以连续的高阻体为主,而南侧祁连山内部的深部电性结构在横向上有较为复杂的变化.这一点与区域构造背景相对应,即北侧的塔里木盆地东缘依然具有较好的整体性,南侧的祁连山是青藏高原北缘生长的最前端,变形强烈.在断裂带的结构特征上,阿尔金断裂带沿走向方向的切割深度在昌马盆地西侧发生了显著的降低,与阿尔金断裂带相对应的电性边界在这里向南偏移了约15 km,对应F18断裂,并与昌马盆地相接.祁连山北部的断裂带,包括昌马断裂、旱峡—大黄沟断裂总体呈现出低角度南倾的样式,切过高阻异常体的顶部.虽然昌马盆地可以起到连接断裂带的阶区的作用,将部分阿尔金断裂的走滑分量转移到盆地南侧的昌马断裂上,但是昌马断裂的走滑速率从西向东是增加的,东侧的走滑速率甚至大于阿尔金断裂沿走向方向的流失分量.我们认为在青藏高原北部主要断裂带的活动还是受印度—欧亚板块碰撞引起的远程挤压效应的影响,包括阿尔金断裂以及祁连山内部系列断层都处于斜向挤压应力环境.在这种基本构造模式下,阿尔金断裂、断裂F18、昌马盆地、昌马断裂构成了一个局部的走滑速率分解-转换-吸收体系,对局部应力状态产生影响.