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1.
In the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground-based long-offset transient-electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large-scale survey resulted in an extensive dataset acquired with multiple high-power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi-dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step-on transients are well fitted using a conventional one-dimensional inversion. Due to superimposed induced polarization effects, the transient step-off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long-offset transient-electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one-dimensional inversion models of frequency-domain data show strong multi-dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency-domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2-D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.  相似文献   

2.
SeaBed Logging (SBL) is an application of the marine controlled source electromagnetic (CSEM) method that is used to directly detect and characterize possible hydrocarbon-bearing prospects. Although the CSEM method has been used by academia for more than three decades, the application as a direct hydrocarbon indicator was first introduced about five years ago. The central idea of SBL is the guiding of electromagnetic energy in thin resistive layers within conductive sediments. Even if it has been well known for a long time that electromagnetic signals can propagate from a conductive region to another via resistive regions such as air or resistive parts of the lithosphere, the application to hydrocarbon exploration has not been developed until recently. This might be due to the uncertainty of getting any significant response from thin resistive layers such as hydrocarbon reservoirs since electromagnetic energy is highly attenuated in conductive sediments. Thus, during the early development phase of the SBL technique, a scaled laboratory experiment was performed to validate if a thin resistive layer (e.g. hydrocarbons) buried within conductive media (e.g. sediments) could be remotely detected by using electric dipoles as sources and receivers. Data from this experiment were compared to a forward modelling code for layered media, and the comparison showed good agreement between experimental and theoretical results. This suggested that thin resistive layers buried in conductive media are detectable due to the guiding of the electromagnetic field within the resistor. The successful results were vital for realizing the application of marine CSEM as a hydrocarbon exploration technique. We here present the results of the first scaled SBL experiment.  相似文献   

3.
Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt water-bearing formations. Interpretation models for these aquifers typically contain four layers: a highly resistive unsaturated zone; a surficial fresh water aquifer of intermediate resistivity; an underlying conductive, salt water saturated aquifer; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated: direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thicknesses of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently.  相似文献   

4.
Electromagnetic methods are routinely applied to image the subsurface from shallow to regional structures. Individual electromagnetic methods differ in their sensitivities towards resistive and conductive structures and in their exploration depths. If a good balance between different electromagnetic data can be be found, joint 3D inversion of multiple electromagnetic datasets can result in significantly better resolution of subsurface structures than the individual inversions. We present a weighting algorithm to combine magnetotelluric, controlled source electromagnetic, and geoelectric data. Magnetotelluric data are generally more sensitive to regional conductive structures, whereas controlled source electromagnetic and geoelectric data are better suited to recover more shallow and resistive structures. Our new scheme is based on weighting individual components of the total data gradient after each model update. Norms of individual data residuals are used to assess how much of the total data gradient must be assigned to each method to achieve a balanced contribution of all datasets for the joint inverse model. Synthetic inversion tests demonstrate advantages of joint inversion in general and also the influence of the weighting. In our tests, the controlled source electromagnetic data gradients are larger than those of the magnetotelluric and geoelectric datasets. Consequently, direct joint inversion of controlled source electromagnetic, magnetotelluric, and geoelectric data results in models that are mostly dominated by structures required by the controlled source electromagnetic data. Applying the new adaptive weighting scheme results in an inversion model that fits the data better and resembles more the original model. We used the modular system electromagnetic as a framework to implement the new joint inversion and briefly describe the new modules for forward modelling and their interfaces to the modular system electromagnetic package.  相似文献   

5.
李刚  李予国  韩波  段双敏 《地球物理学报》2017,60(12):4887-4900
在海洋可控源电磁法勘探中,接收站常置于海底.在进行海洋电磁场模拟时,由于海水和海底介质存在显著电性差异,这给海底接收点处场值的求取带来困难.本文提出一种新的接收点插值算法,该算法考虑到海底电场法向分量不连续性问题,用法向电流分量进行插值以准确求取海底任意接收点处电磁场值.本文利用交错网格有限差分法实现了二维介质中频率域海洋可控源法(CSEM)正演.对构造走向做傅里叶变换,将三维电磁模拟问题转换为波数域2.5维问题,即三维场源激励下针对二维地电模型的电磁模拟问题.使用交错网格有限差分法,基于一次场/二次场分离方法导出波数域二次电场离散形式,并进一步求得波数域电磁场.采用本文提出的改进的插值算法可求得海底任意接收点处波数域电磁场,采用傅里叶逆变换对波数域电磁场进行积分可得到接收点处空间域电磁场.模型算例表明,与常规的线性插值和严格插值算法相比,本文提出的改进的插值算法具有更高的精度.  相似文献   

6.
Time‐domain marine controlled source electromagnetic methods have been used successfully for the detection of resistive targets such as hydrocarbons, gas hydrate, or marine groundwater aquifers. As the application of time‐domain marine controlled source electromagnetic methods increases, surveys in areas with a strong seabed topography are inevitable. In these cases, an important question is whether bathymetry information should be included in the interpretation of the measured electromagnetic field or not. Since multi‐dimensional inversion is still not common in time‐domain marine controlled source electromagnetic methods, bathymetry effects on the 1D inversion of single‐offset and multi‐offset joint inversions of time‐domain controlled source electromagnetic methods data are investigated. We firstly used an adaptive finite element algorithm to calculate the time‐domain controlled source electromagnetic methods responses of 2D resistivity models with seafloor topography. Then, 1D inversions are applied on the synthetic data derived from marine resistivity models, including the topography in order to study the possible topography effects on the 1D interpretation. To evaluate the effects of topography with various steepness, the slope angle of the seabed topography is varied in the synthetic modelling studies for deep water (air interaction is absent or very weak) and shallow water (air interaction is dominant), respectively. Several different patterns of measuring configurations are considered, such as the systems adopting nodal receivers and the bottom‐towed system. According to the modelling results for deep water when air interaction is absent, the 2D topography can distort the measured electric field. The distortion of the data increases gradually with the enlarging of the topography's slope angle. In our test, depending on the configuration, the seabed topography does not affect the 1D interpretation significantly if the slope angle is less or around 10°. However, if the slope angle increases to 30° or more, it is possible that significant artificial layers occur in inversion results and lead to a wrong interpretation. In a shallow water environment with seabed topography, where the air interaction dominates, it is possible to uncover the true subsurface resistivity structure if the water depth for the 1D inversion is properly chosen. In our synthetic modelling, this scheme can always present a satisfactory data fit in the 1D inversion if only one offset is used in the inversion process. However, the determination of the optimal water depth for a multi‐offset joint inversion is challenging due to the various air interaction for different offsets.  相似文献   

7.
A three-dimensional finite-element time-domain forward-modelling algorithm is developed to simulate transient electromagnetics excited by grounded-wire sources. The main advantage of this finite-element time-domain algorithm is that full transmitting-current waveforms and complex-shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half-space model, a stratified-medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half-space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step-off, step-on and direct current waveforms. For the stratified-medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non-negligible topography, the effects of which on the shapes of the grounded-wire sources must be taken into account when implementing the finite-element time-domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.  相似文献   

8.
We compare selected marine electromagnetic methods for sensitivity to the presence of relatively thin resistive targets (e.g., hydrocarbons, gas hydrates, fresh groundwater, etc.). The study includes the conventional controlled‐source electromagnetic method, the recently introduced transient electromagnetic prospecting with vertical electric lines method, and the novel marine circular electric dipole method, which is still in the stage of theoretical development. The comparison is based on general physical considerations, analytical (mainly asymptotic) analysis, and rigorous one‐dimensional and multidimensional forward modelling. It is shown that transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods represent an alternative to the conventional controlled‐source electromagnetic method at shallow sea, where the latter becomes less efficient due to the air‐wave phenomenon. Since both former methods are essentially short‐offset time‐domain techniques, they exhibit a much better lateral resolution than the controlled‐source electromagnetic method in both shallow sea and deep sea. The greatest shortcoming of the transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods comes from the difficulties in accurately assembling the transmitter antenna within the marine environment. This makes these methods significantly less practical than the controlled‐source electromagnetic method. Consequently, the controlled‐source electromagnetic method remains the leading marine electromagnetic technique in the exploration of large resistive targets in deep sea. However, exploring laterally small targets in deep sea and both small and large targets in shallow sea might require the use of the less practical transient electromagnetic prospecting with vertical electric lines and/or marine circular electric dipole method as a desirable alternative to the controlled‐source electromagnetic method.  相似文献   

9.
A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Fréchet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.  相似文献   

10.
海底电性源频率域CSEM勘探建模及水深影响分析   总被引：4，自引：3，他引：1       下载免费PDF全文
为了探索我国海域油气和水合物等高阻目标体CSEM勘探的可行性和方法技术,本文研究了在海水中水平电性源激励下有限水深海洋地电模型的频率域电磁响应,为进一步的1D和3D仿真计算奠定了理论基础.在推导电磁响应公式时,首先给出了各层介质的Lorentz势,然后根据Coulomb势与Lorentz势的关系,得到了各层介质的Coulomb势.各层介质中的电磁场均可以由Lorentz势或者Coulomb势计算得到,但在有限元计算时Coulomb势具有优势.长导线源的电磁场和势函数可以由电偶源的电磁场和势函数沿导线长度积分得到.文中具体给出了海水中水平电偶源和长导线源在海水层的电磁场公式,并根据该公式计算了不同水深环境下海底表面的电磁场分布,分析了海水深度对海底油气储层电磁异常的影响.结果表明,随着水深减小,异常幅度和形态特征发生明显变化.当水深很浅时(如50 m),只有同线方向的Ex和Ez两个电场分量存在明显异常.最后,以两个已知海底油田为例,计算了不同水深环境下可观测到的电场异常,展示了电性源频率域CSEM在海底勘探中(包括浅海环境)的良好应用前景.对于该方法实用化过程中还需进一步解决的问题,文中结尾部分也进行了初步探讨.  相似文献   

11.
Gas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time‐domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 Ωm, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 Ωm conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter‐receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 Ωm, 36 m thick hydrate layer overlying a 0.7 Ωm sedimentary half‐space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.  相似文献   

12.
Previous studies using commercial airborne electromagnetic equipment that is not optimized for marine surveying have demonstrated the use of airborne electromagnetic methods for measuring water depth and estimating sediment thickness. A new prototype helicopter time-domain airborne electromagnetic system, SeaTEM(0), is now under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, New South Wales, Australia, in shallow water up to ∼30 m in depth. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Water and Pittwater, which join in Broken Bay give rise to paleovalleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ∼200 m. The survey area also included a tombolo with a beach either side, which provided the opportunity to measure water depth through a surf zone. Sediment thickness and water depth is predicted from stitched layered-earth inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ∼±1 m and ∼±0.5 m with known water depths in areas less than 20 and 6 m deep respectively. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleovalleys to depth limits of ∼60 to 80 m below sea level in 20 to 30 m water depth, as well as resolving bedrock ridges and exposed reefs in shallow waters.  相似文献   

13.
海洋可控源电磁法(CSEM)对海底高阻体的反映比较灵敏,可用于天然气水合物探测资料的定性解释和反演研究.海洋CSEM资料的共中心点(CMP)域转换方式,可在横向上较好地分辨高阻储层.本文提出在CMP域实现一维频率域海洋可控源电磁资料高斯-牛顿反演算法.鉴于一维反演是解释地球物理资料的基础,较于二维和三维反演方法有着更高的计算效率和更低的硬件要求,将二维模型的响应在CMP域单元内表达为一维模型的响应,进而运用一维高斯-牛顿反演解释二维海洋CSEM资料.模型数据试算表明,海洋CSEM的CMP域反演速度较快,能够实现二维CSEM资料的反演解释.  相似文献   

14.
The controlled source extremely low frequency (CSELF) electromagnetic method is characterized by extremely long and powerful sources and a huge measurement range. Its electromagnetic field can therefore be affected by the ionosphere and displacement current. Research on 3D forward modeling and inversion of CSELF electromagnetic data is currently in its infancy. This paper makes exploratory attempts to firstly calculate the 1D extremely low frequency electromagnetic field under ionosphere-air-earth coupling circumstances, and secondly analyze the propagation characteristics of the background electromagnetic field. The 3D staggered-grid finite difference scheme for solving for the secondary electric field is adopted and incorporated with the 1D modeling algorithm to complete 3D forward modeling. Considering that surveys can be carried out in the near field and transition zone for lower frequencies, the 3D Limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) inversion of CSELF electromagnetic data is presented (in which the sources, or primary fields, are included), with the aim of directly inverting the impedance data, regardless of where it is acquired. Derivation of the objective functional gradient is the core component in the inversion. Synthetic tests indicate that the well-chosen approximation to the Hessian can significantly speed up the inversion. The model responses corresponding to the coexistence of conductive and resistive blocks show that the off-diagonal components of tensor impedance are much more sensitive to the resistivity variation than the diagonal components. In comparison with conventional scalar inversion, tensor inversion is superior in the recoveries of electric anomalies and background resistivity.  相似文献   

15.
We study a new marine electromagnetic configuration that consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulphide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modelling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly buried conductive material at up to ～100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few tens of metres. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modelling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulphide deposits.  相似文献   

16.
To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.  相似文献   

17.
徐震寰  李予国 《地球物理学报》2019,62(12):4874-4885
海底采集到的电磁数据按照其主要包含的信息及研究目的大致可分为海洋可控源电磁场(CSEM)信号、天然场源大地电磁场(MT)信号、海洋环境电磁场信号以及其他随机干扰信号.常常通过计算功率谱密度、时频分析和极化分析的方法研究海洋电磁场特征.本文介绍一种新方法——时频方向谱分析法及其在实测海洋电磁数据处理中的应用,该方法能够在一定的时间-频率尺度上有效分辨场源信号的运动方向.对于海洋CSEM数据,利用该方法可以估算发射源的运动方向,进而在发射源或采集站方位信息缺失情况下,实现海洋CSEM数据的旋转电性轴处理.对于海洋电磁数据,利用该方法可以详细分析海水运动感应电磁场的信号特征.  相似文献   

18.
The problem of equivalence in direct current (DC) resistivity and electromagnetic methods for a thin resistive and conducting layer is well‐known. Attempts have been made in the past to resolve this problem through joint inversion. However, equivalence still remains an unresolved problem. In the present study, an effort is made to reduce non‐uniqueness due to equivalence using global optimization and joint inversion by successive refinement of the model space. A number of solutions derived for DC resistivity data using very fast simulated annealing global inversion that fits the observations equally well, follow the equivalence principle and show a definite trend. For a thin conductive layer, the quotient between resistivity and thickness is constant, while for a resistive one, the product between these magnitudes is constant. Three approaches to obtain very fast simulated annealing solutions are tested. In the first one, layer resistivities and thicknesses are optimized in a linear domain. In the second, layer resistivities are optimized in the logarithmic domain and thicknesses in the linear domain. Lastly, both layer resistivities and thicknesses are optimized in the logarithmic domain. Only model data from the mean models, corresponding to very fast simulated annealing solutions obtained for approach three, always fit the observations. The mean model defined by multiple very fast simulated annealing solutions shows extremely large uncertainty (almost 100%) in the final solution after inversion of individual DC resistivity or electromagnetic (EM) data sets. Uncertainty associated with the intermediate resistive and conducting layers after global optimization and joint inversion is still large. In order to reduce the large uncertainty associated with the intermediate layer, global optimization is performed over several iterations by reducing and redefining the search limits of model parameters according to the uncertainty in the solution. The new minimum and maximum limits are obtained from the uncertainty in the previous iteration. Though the misfit error reduces in the solution after successive refinement of the model space in individual inversion, it is observed that the mean model drifts away from the actual model. However, successive refinement of the model space using global optimization and joint inversion reduces uncertainty to a very low level in 4–5 iterations. This approach works very well in resolving the problem of equivalence for resistive as well as for conducting layers. The efficacy of the approach has been demonstrated using DC resistivity and EM data, however, it can be applied to any geophysical data to solve the inherent ambiguities in the interpretations.  相似文献   

19.
We investigate the seismoelectric/electroseismic wavefields excited by a point source in an air/seawater/three-layered porous medium configuration containing a hydrocarbon layer. The results show that if an explosive source for excitation is used, receivers at seafloor can record the coseismic electromagnetic fields accompanying the P, S, fluid acoustic waves and the interface responses converted from the acoustic waves at seafloor interface and from the seismic waves at the interfaces beneath the seafloor. Employing a vertical electric dipole source shows that, with the exception of the interface responses converted from electromagnetic waves at seafloor, the interface responses converted from transmitted electromagnetic waves at the interfaces beneath the seafloor can also be identified. Given that the strength of the explosive source is within excitation capability of industry air guns, the generated interface responses from the hydrocarbon layer can be detected by current electromagnetic sensors considering the low ambient noise at the seafloor. Our results demonstrate the feasibility of the seismoelectric method applied to marine hydrocarbon exploration. Electroseismic modelling results suggest that it is not practical to employ this method to prospect marine hydrocarbon layer due to the weak interface response signal, unless a much larger current is injected into seafloor.  相似文献   

20.
Geophysical investigations using conventional techniques applied to groundwater exploration can often present strong limitations involving high financial costs, complex acquisition logistics and high ambiguity in results. Dispersion of the electric current flow, induced polarization) effects, cultural noises and shallow lateral heterogeneities represent the main problems faced by geoelectric methods in these types of surveys. Moreover, elements such as intrusions and mineralization at different depths may be responsible for signal attenuation as well as high resistivity in unsaturated zones and complex three-dimensional formations or clayey zones cause variations in the electric current. The focused source electromagnetic and differentially normalized method approaches can help to solve some these issues. Aiming at a higher signal-to-noise ratio, the focused source electromagnetic method and approaches of the differentially normalized method, first applied to petroleum exploration, are tested on a groundwater target, in a karst environment sectioned by a diabase dyke. We performed the processing and analysis on real IP resistivity profiling data acquired with two-way dipole-dipole array, guided by magnetic data acquired on the same profile, mapping a diabase dyke. The inversion of focused source electromagnetic method/differentially normalized method was not performed, instead that we converted the induced polarization–resistivity data to a differential signal to qualitatively prove the presence of aquifer. Joint interpretation of focused source electromagnetic method curves and inverted two-dimensional induced polarization–resistivity sections allowed for precise delineation of a conductive zone associated with the karst aquifer, le magnetics allowed for the definition of a neighbour dyke. The techniques have great potential in the aid of groundwater exploration, contributing substantially to the reduction of interpretation ambiguity. Focused source electromagnetic method/differentially normalized method/ approaches show that a simple linear combination of the conventional geoelectric data is able to remove the geological noise and provide the vertical focusing of the electric current.  相似文献   

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