Effects of galvanic distortions on magnetotelluric data: Interpretation and its correction using deep electrical data

The non-inductive galvanic disturbances due to surficial bodies, lying smaller than high frequency skin depth, cause serious interpretational errors in magnetotelluric data. These frequency independent distortions result in a quasi-static shift between the apparent resistivity curves known as static shift. Two-dimensional modelling studies, for the effects of surficial bodies on magnetotelluric interpretation, show that the transverse electric (TE) mode apparent resistivity curves are hardly affected compared to the transverse magnetic (TM) mode curves, facilitating the correction by using a curve shifting method to match low frequency asymptotes. But in the case of field data the problem is rather complicated because of the random distribution of geometry and conductivity of near surface inhomogeneities. Here we present the use of deep resistivity sounding (DRS) data to constrain MT static shift. Direct current sensitivity studies show that the behaviour of MT static shift can be estimated using DC resistivity measurements close to the MT sounding station to appreciable depths. The distorted data set is corrected using the MT response for DRS model and further subject to joint inversion with DRS data. Joint inversion leads to better estimation of MT parameters compared to the separate inversion of data sets.     

Accounting for near-surface inhomogeneities over a horizontally layered section in magnetotelluric sounding

This paper presents a numerical model for the effect of near-surface inhomogeneities over a one-dimensional horizontally layered geoelectric section and the distortions they cause during magnetotelluric sounding (MTS). The electromagnetic field within the layer of near-surface inhomogeneities is calculated using the Trefftz method. Expressions are derived for the boundary conditions on the day surface and on the roof of the underlying inhomogeneity of a horizontally layered medium. These boundary conditions allow for the excitation of TM-mode fields by subsurface inhomogeneities and their penetration into the atmosphere and the underlying medium. The spatial distribution and characteristics of galvanic and inductive distortions over different time periods during MTS have been studied. Experimental data show that accounting for galvanic distortions is possible with synchronous recording of the distribution of components of the electric and magnetic fields in a limited area of the Earth’s surface.     

Migration of Magnetotelluric Data in Two——Dimensional Model

Since the wave equation of magnetotelluric(MT)field is similar to the one of seis-mic,the migration techniques used in seismic can be applied to MT data.In thispaper we make use of the principle of reflector mapping(i.e.U/D imaging principle)to image MT data.That is,the MT wavefield observed on the surface of the earth canbe resolved into upgoing and downgoing waves,the waves are extrapolated downward bythe phase-shift method or the phase-shift plus interpolation(PSPI)method.Conductivi-ty interfaces of the medium could be found by using the time coincidence of the upgoingand downgoing waves.Theoretical calculations show that the migration technique of MTdata presented here is very effective.It can not only enhance the lateral resolution of MTdata,but also obtain the visual image of subsurface interfaces.As compared with theconventional 2-D inversion,this procedure is more simple in calculation and can be easi-ly put into practice on a personal computer and is able to obtain the MT depth section,which is     

Petrophysical features of the tectonic structures of the Okhotsk continent–ocean transition zone

Petrophysical characteristics are determined for the rock complexes of the Okhotsk margin within the continent–ocean transition zone. Petrophysical maps showing the major inhomogeneities of the main tectonic elements of the studied area are constructed. Petrophysical inhomogeneities and the corresponding geophysical field anomalies are compared. A relationship between the magnetic field anomalies and subsurface rock complexes is revealed. The gravity field anomalies are related to deep inhomogeneities and are almost independent of the subsurface rock complexes of relatively low thickness.     

Results of inversion of distorted magnetotelluric sounding curves (numerical experiment)

The paper presents the results of a joint inversion of magnetotelluric sounding (MTS) curves taken at several stations without preliminary selection and normalization and distorted by the presence of lateral electrical-conductivity inhomogeneities in the medium. In the calculations, we used synthetic MTS data for a three-dimensional model. Preparation and interpretation of data are carried out by the Trefftz method using a numerical model of the field and MTS curve distortions. To solve the inverse problem and optimize the subsurface model, we used a nonlinear least-squares method and an iterative process with calculation of the sensitivity matrix and its singular decomposition. The target functional is determined by the discrepancies between the model and synthetic experimental apparent-resistivity curves corresponding to the elements of the impedance tensors on the lateral diagonal. The reliability of the reconstructed subsurface model is characterized by the dispersion of the deviations of its parameters from the parameters of the known model used for the preparation of synthetic experimental data. The joint consideration of distorted apparent-resistivity curves at several stations increases the reliability of interpretation results. The obtained solution to the inverse problem is approximate and can be used as a starting model for more complex algorithms and programs.     

Geostatistical integration of MT and borehole data for RMR evaluation

The geostatistical approach was applied to integrate MT (Magneto-telluric) resistivity data and borehole information for the spatial RMR (Rock Mass Rating) evaluation. Generally, resistivity of the subsurface is believed to be positively related to the RMR, thus the resistivity and borehole RMR information was combined in a geostatistical approach. To relate the two different sets of data, the MT resistivity data were used as secondary information and the RMR mean values were estimated at unsampled points by identification of the resistivity to the borehole data. Two types of approach are performed for the estimation of RMR mean values. Then the residuals of the RMR values around the borehole sites are geostatistically modeled to infer the spatial structure of difference between real RMR values and estimated mean values. Finally, this geostatistical estimation is added to the previous means. The result applied to a real situation shows prominent improvements to reflect the subsurface structure and spatial resolution of RMR information.     

Inversion of GDS data of northwest Himalaya using EM2INV

This paper reviews the validity of earlier models obtained after quantitative interpretation of GDS data and presents a fresh model using the inversion scheme EM2INV. The 2-D inversion of data is more objective than the earlier interpretation performed by using trial and error method. The inversion results indicate that the present model differs from the earlier ones. The reason could be that available GDS data are sufficient only for deriving the horizontal variation of subsurface resistivity. In order to study the vertical resistivity variation additional MT sounding data would be required. It would therefore be desirable to carry out MT survey in the specified area. A more comprehensive/appropriate model could be derived from joint inversion of GDS and MT data.     

Investigating 2-D MT inversion codes using real field data

There are currently a significant number of two-dimensional (2-D) and three-dimensional (3-D) inversion codes available for magnetotelluric (MT) data. Through various 2-D inversion algorithms suggested so far, the classical Occam's inversion, the data space Occam's inversion, the nonlinear conjugate gradient (NLCG) method, and the Gauss–Newton (GN) method are fundamental driving methods to find optimum earth models, and OCCAM, DASOCC, NLCG, and MT2DInvMatlab are possible candidates one can find in the public domain that implement these algorithms for 2-D MT inversions, respectively. In this study, we investigate the pros and cons (strength and weakness) of these codes to help one use them efficiently in practical works and, as an introductory guide, further develop (sophisticate or extend) them, especially for the 3-D case. To achieve this goal, we applied each one of the four aforementioned codes on a profile of real MT field dataset. Then, further investigations have been done by performing several inversion tests to see how each code can find the appropriate model to reconstruct the subsurface resistivity structure. Numerical experiments show that the two parameters, regularization and target misfit, in addition to the main criteria of inversion (such as the forward and the sensitivities calculation method, and the type of inversion algorithm), are very important to produce the expected model in inversion. The regularization parameter that acts to trade off between model norm and data misfit can affect the inversion process in terms of both the computational efficiency and the accuracy of the obtained model. Also, lack of insufficient precision to choose the target misfit can lead the inversion to produce and reach an incorrect model.     

Identification and Correction for MT Static Shift Using TEM Inversion Technique

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初至波地震层析技术及其在四川复杂地区的应用

近地表速度场的重建是获得准确静校正量的关键。根据实际地震数据的初至波时间建立初始速度模型;采用改进的最短路径射线追踪算法进行正演;利用改进的约束带阻尼联合迭代重建技术(CDSIRT)求解大型、稀疏的方程组来反演近地表的速度场。理论模型和实际资料的反演结果表明,该方法能稳定、快速、准确地重建复杂地区的近地表速度场,建立准确的近地表速度场,从而可计算准确的静校正量,保证复杂地区的构造成像。     

Magnetotelluric and audiomagnetotelluric measurements in Finland

Joint Finnish—Hungarian MT (magnetotelluric) and AMT (audiomagnetotelluric) measurements were carried out in Finland in the framework of the international ELAS project. The conditions for MT measurements are favorable at these latitudes. Five MT and 150 AMT stations gave information on the electrical conductivity distribution in the area: AMT results guided the choice of MT sites with minimal near-surface distortion effects and helped the interpretation of the MT soundings; the MT measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near-surface dyke structures. This result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area.Any information about the upper mantle would require the use of S_q harmonics because of the crustal conductivity anomalies detected by the MT measurements.     

基于小波分析的MT静态效应识别及校正

大地电磁测深中,观测结果常受静态效应的影响而发生畸变,增加了资料解释工作的难度。基于小波分析理论,提出使用Daubechies小波分解对有静态效应测点进行有效识别。建立含有静态体并镶嵌有高阻异常体的地垒模型,分别采用中值滤波方法、相位法和小波分析方法进行了静态效应的校正。结果表明:相位法校正结果不稳定,相对误差随频点起伏较大;中值滤波方法在高频段误差较大;基于多尺度小波分析校正方法比较稳定,误差基本不超过5%,能够有效的压制静态效应。采用小波分析方法对某区大地电磁(MT)实测数据进行了静态效应识别和校正,取得比较理想的效果。      

The resistivity structure of the Penola Trough,Otway Basin from magnetotelluric data

We present inversion results for a 100 site, broadband magnetotelluric (MT) survey in the Penola Trough, Otway Basin, South Australia. The Penola Trough is host to several petroleum reservoirs and has more recently been a target for unconventional geothermal exploration. We present two interpretations of the MT data. A 1D anisotropic interpretation, where anisotropy is determined within the Otway Basin sequence and basement in the northeastern Penola Trough, fits the impedance tensor well. However, the anisotropy strike is inconsistent with the known orientation of electrically conductive fractures in the Penola Trough. On the other hand, a 3D interpretation, which incorporates lateral variations in resistivity, requires no anisotropy yet it matches the data equally well. Both the 1D and 3D inversions resolve several layers within the Otway Basin sequence, which correspond to stratigraphic units defined in wells and in the coincident Haselgrove–Balnaves 3D seismic survey. These include the Eumeralla and Dilwyn formations, which are poorly resolved in the seismic data. The basin architecture, defined in the 3D inversion, in particular the depth to basement, is consistent with previous interpretations based on seismic reflection data that show that the Otway Basin thins in the northeastern Penola Trough. This does not occur in the anisotropic model. We therefore conclude that the subsurface resistivity appears to be isotropic in the Penola Trough. This contrasts with the anisotropic resistivity structure determined in a previous study in the Koroit region, eastern Otway Basin. The difference in the MT responses between the two regions is supported by resistivity and permeability information from well logs and may reflect differences in the orientation of subsurface fractures, or differences in the present-day stress field, between the two regions.     

2D inversion of the magnetotelluric data from Mahallat geothermal field in Iran using finite element approach

The natural-field magnetotelluric (MT) method has proven very useful for mapping the geothermal fields as resistivity sections. The depth of investigation of the MT method is sufficiently large to penetrate deep into the upper crust. MT soundings along two transects across Mahallat geothermal field in Iran were carried out to determine the crustal structure in the region. The selected MT profiles in the region cross over the hydrothermally altered zones and different geological structures. Data were acquired along two profiles crossing the Mahallat hot springs with a total of 28 MT stations in a frequency range of 8,000 to 0.008 Hz. Spacing between stations was kept 500 m for a good resolution. We have used the code MT2DInvMATLAB for inversion using the method of finite elements for forward modeling. Apparent resistivity and phase data of transverse electric (TE), transverse magnetic (TM), and TE + TM modes along each profile were modeled. The geothermal fluid reservoir is resolved at 1,000 to 3,000 m depth and the geothermal resource is estimated to be located at 7,000 m or deeper.     

起伏地形下可控源音频大地电磁三维数值模拟

起伏地形对可控源音频大地电磁(CSAMT)响应具有强烈的影响,因此在CSAMT数据处理解释时需要考虑地形。同时,实际的地下地质情况和地表的地形情况通常比较复杂,地质结构和地形大部分情况下都是三维的。在水平地表三维有限差分CSAMT数值模拟算法的基础上,推导了利用地下交错网格采样点处的总磁场计算起伏地形下空气-地下介质分界面处的总电场和总磁场的表达式,从而实现了起伏地形下三维CSAMT数值模拟算法。在算法实现过程中,采用伪δ函数代替麦克斯韦方程中的场源项和直接计算总场的策略,避免了原有的将总场分离成背景场和二次场的策略在复杂地质条件下难以选择合适背景电阻率的问题。为了直接模拟总场,起伏地形下三维CSAMT数值模拟算法给出了新的三维正演方程的边界条件。将模拟水平地表三维异常体和三维山峰地形两个理论模型得到的响应结果与前人算法的计算结果进行对比,验证了所实现算法的正确性和有效性。     

Transient electromagnetic profiles across gold-mineralisation zones in central Victoria

High-density transient electromagnetic (TEM) profiles have been obtained along a 170 km long transect in central Victoria north of Bendigo. The transect lies within two of the fault-bounded subdivisions of the western Lachlan Fold Belt (the Bendigo and Melbourne Zones) and coincides with part of a 2006 seismic-reflection traverse and a 2007 magnetotelluric (MT) survey commissioned by Geoscience Victoria as part of the Victorian Government's Gold Undercover initiative. In addition to providing near-surface information that will assist with geological mapping, the TEM survey also presents a viable method for the removal of static offsets present in the MT data. The new dataset also provides an immediate comparison and verification of data acquired along the same line by the helicopter-borne Versatile Transient Electromagnetic (VTEM) system. Ground-based data have been obtained for a total of 55 sites, and high-quality decay curves have revealed a variety of conductivity characteristics with typical penetration to depths of 100 m. Major geological structures can clearly be identified in both the ground and airborne sections, with very good lateral correlation present between the two. The fault zones are easily distinguished as regions characterised by bands of high conductivity along with significant variability in the near-surface structure.     

Potentials and constraints of different types of soil moisture observations for flood simulations in headwater catchments

Flood generation in mountainous headwater catchments is governed by rainfall intensities, by the spatial distribution of rainfall and by the state of the catchment prior to the rainfall, e.g. by the spatial pattern of the soil moisture, groundwater conditions and possibly snow. The work presented here explores the limits and potentials of measuring soil moisture with different methods and in different scales and their potential use for flood simulation. These measurements were obtained in 2007 and 2008 within a comprehensive multi-scale experiment in the Weisseritz headwater catchment in the Ore-Mountains, Germany. The following technologies have been applied jointly thermogravimetric method, frequency domain reflectometry (FDR) sensors, spatial time domain reflectometry (STDR) cluster, ground-penetrating radar (GPR), airborne polarimetric synthetic aperture radar (polarimetric SAR) and advanced synthetic aperture radar (ASAR) based on the satellite Envisat. We present exemplary soil measurement results, with spatial scales ranging from point scale, via hillslope and field scale, to the catchment scale. Only the spatial TDR cluster was able to record continuous data. The other methods are limited to the date of over-flights (airplane and satellite) or measurement campaigns on the ground. For possible use in flood simulation, the observation of soil moisture at multiple scales has to be combined with suitable hydrological modelling, using the hydrological model WaSiM-ETH. Therefore, several simulation experiments have been conducted in order to test both the usability of the recorded soil moisture data and the suitability of a distributed hydrological model to make use of this information. The measurement results show that airborne-based and satellite-based systems in particular provide information on the near-surface spatial distribution. However, there are still a variety of limitations, such as the need for parallel ground measurements (Envisat ASAR), uncertainties in polarimetric decomposition techniques (polarimetric SAR), very limited information from remote sensing methods about vegetated surfaces and the non-availability of continuous measurements. The model experiments showed the importance of soil moisture as an initial condition for physically based flood modelling. However, the observed moisture data reflect the surface or near-surface soil moisture only. Hence, only saturated overland flow might be related to these data. Other flood generation processes influenced by catchment wetness in the subsurface such as subsurface storm flow or quick groundwater drainage cannot be assessed by these data. One has to acknowledge that, in spite of innovative measuring techniques on all spatial scales, soil moisture data for entire vegetated catchments are still today not operationally available. Therefore, observations of soil moisture should primarily be used to improve the quality of continuous, distributed hydrological catchment models that simulate the spatial distribution of moisture internally. Thus, when and where soil moisture data are available, they should be compared with their simulated equivalents in order to improve the parameter estimates and possibly the structure of the hydrological model.     

Requirements for the performance of a ground-penetrating radar system in searching for cavities

The requirements for the performance of a ground-penetrating radar (GPR) system for detecting subsurface cavities are analyzed by numerical modeling of the GPR problem. The algorithm used to solve the forward GPR problem is approximated to a real experiment with regard to the design of the GPR system, the parameters of the source and receiver, and their position relative to the medium under study and its inhomogeneity. We calculated the spatiotemporal distribution of the field of the detected signal from a pulse source located at the interface between the medium and a cavity anomaly of a given geometry. The results were used to estimate the dynamic range of the GPR system necessary for determining the anomaly. We also performed GPR surveys of low-contrast inhomogeneities (cavities in mines) using GROT 12 GPR systems and analyzed the survey results by numerical modeling. It is shown that the GPR performance required to detect and locate inhomogeneities of interest at a certain sounding depth can be estimated in the experiment design phase.     

Hydrogeological framework and estimation of aquifer hydraulic parameters using geoelectrical data: a case study from West Iran

Detailed local geological, geophysical, and hydrogeological investigations were carried out for the alluvial aquifer in the Kangavar basin, West Iran to delineate the architecture of different subsurface geological horizons using lithologs and generated vertical electrical sounding (VES) data. An attempt has also been made to estimate aquifer transmissivity from resistivity data. Forty VESs were recorded with the Schlumberger electrode configuration in the study area; 28 of these were selected for evaluation. The maximum current electrode spacing was 400–500 m. The data obtained were interpreted by computer iterative modeling with curve matching for calibration purposes. In order to ascertain the subsurface geological framework, the general distribution of resistivity responses of the geological formations was obtained and geoelectrical sections along a number of lines were prepared. Probable aquifer horizons from these sections were identified. The transmissivity of the unconfined aquifer was computed by determining the Dar-Zarrouk parameters (longitudinal unit conductance and transverse unit resistance) and were compared with the actual field transmissivity. The results showed a direct relation between aquifer transmissivity and modified transverse resistance.     

用TEM反演法进行MT静位移的识别和校正

对瞬变电磁(TEM)资料进行反演时, 采用观测磁场而不用视电阻率, 避免了由于TEM视电阻率定义而引起的误差, 并且由源象磁场和观测磁场之间的拟合进行反演使反演结果受非均匀性影响较小; 将反演结果构置出地电模型, 计算出它的MT曲线作为静位移校正的参考曲线, 使它用于MT静位移校正更为可靠, 同时也解决了两种电磁资料从时间域到频率域的转换问题.由此实现用TEM反演的方法进行MT静位移的识别和校正.实例表明, 这种方法对MT静位移的识别和校正十分有效.