Marine Controlled-source Electromagnetic Research Articles

Marine magnetotelluric and controlled source electromagnetics signal simulative generator

Ocean bottom electromagnetic receivers (OBEMs) are invaluable tools for observing seafloor electromagnetic signals, primarily used in magnetotelluric (MT) sounding and marine controlled source electromagnetics (MCSEM). However, the high operational costs of deploying OBEMs require thorough pre-deployment testing to enhance the data reliability and quality. Commercial signal generators used for MT and controlled source electromagnetic (CSEM) testing often fall short of the restricted terms of the frequency range, signal-to-noise ratio, storage depth, time synchronization, and amplitude dynamics. To address these limitations, we developed a specialized marine MT and CSEM signal simulative generator. The generator employs pseudorandom number code-based white noise to simulate MT signals and uses multi-frequency synthesis and amplitude modulation (AM) to generate CSEM signals. The system is developed using a low-power microcontroller unit and a field programmable gate array platform, integrating various circuits such as power, clock, encoding, control, data storage, chopper, and amplification circuits. The generator simultaneously outputs four-channel white noise and AM signals. The white noise feature provides a wide and adjustable frequency band and amplitude, while the AM feature provides a large dynamic amplitude range and multi-frequency synthesis at various carrier frequency points. Comprehensive testing verified the performance of the proposed simulative generator. The test results demonstrate that the MT simulated signals cover a frequency range from 1 E4s to 100 Hz, providing adjustable amplitudes between 10 μVpp and 10 mVpp. The CSEM simulated signals include standard square waves, third harmonic waves, and seventh harmonic waves, with amplitudes adjustable from 10 mVpp to 10 Vpp. The proposed simulative generator enhances the efficiency of OBEM functional testing and provides essential equipment support for maintaining the quality of marine data.

Three-dimensional modeling of Marine Controlled Source Electromagnetic field using a space-wavenumber domain method

Under complex conditions, the three-dimensional modeling of Marine Controlled Source Electromagnetic field requires a significant amount of computation, resulting in slow calculation speed and high storage requirements. To solve these problems, we propose a 3D numerical simulation method of electromagnetic field in the space-wavenumber domain under the Lorenz gauge. Firstly, the new method utilizes the two-dimensional Fourier transform in the horizontal direction to transform the 3D partial differential equations of the Lorenz vector potentials into multiple independent ordinary differential equations. Secondly, the finite element method is used to solve the ordinary differential equations. The fields in the spatial domain are then obtained by using the inverse Fourier transform, and finally, an iterative method with a compact operator is applied to approximate the true solution. The approach requires less computation and storage, and the algorithm is highly parallel, which effectively improves the efficiency of 3D forward modeling of electromagnetic fields. The correctness, effectiveness, and computational efficiency of the method are verified by the design model.

Marine CSEM synthetic study to assess the detection of CO2 escape and saturation changes within a submarine chimney connected to a CO2 storage site

SUMMARY Carbon capture and storage (CCS) within sealed geological formations is an essential strategy to reduce global greenhouse gas emissions, the primary goal of the 2015 United Nations Paris Agreement. Large-scale commercial development of geological CO2 storage requires high-resolution remote sensing methods to monitor CO2 migration during/after injection. A geological formation containing a CO2 phase in its pore space commonly exhibits higher electrical resistivity than brine-saturated (background) sediments. Here, we explore the added value of the marine controlled-source electromagnetic (CSEM) method as an additional and relevant geophysical tool to monitor moderate to significant changes in CO2 saturation within a fluid conduit breaking through the seal of a CCS injection reservoir, using a suite of synthetic studies. Our 2-D CSEM synthetic models simulate various geological scenarios incorporating the main structural features and stratigraphy of two North Sea sites, the Scanner Pockmark and the Sleipner CCS site. Our results show significant differentiation of leakage through the seal with CO2 saturation ($S_{{\rm CO}_2}$) ranging between 20 and 50 per cent, while our rock physics model predicts that detection below 20 per cent would be challenging for CSEM alone. However, we are able to detect with our 2-D inversion models the effects of saturation with 10 and 20 per cent CO2 within a chimney with 10 per cent porosity. We demonstrate that simultaneous inversion of Ey and Ez synthetic electric field data facilitates a sharper delineation of a CO2 saturated chimney structure within the seal, whereas Ez synthetic data present higher sensitivity than Ey to $S_{{\rm CO}_2}$ variation, demonstrating the importance of acquiring the full 3-D electric field. This study illustrates the value of incorporating CSEM into measurement, monitoring and verification strategies for optimal operation of marine CCS sites.

Research on online data transmission technology in a marine controlled-source electromagnetic transmitter

Abstract. This paper proposes a method for acquiring complete status information and data from the marine controlled-source electromagnetic (MCSEM) transmitter during offshore experiments. The subordinate machine system is constructed on the STM32 platform and incorporates a real-time operating system. It utilizes the internet of things (IoT) concept to interconnect various modules within the transmitter, enabling intelligent control and management. At the same time, data are uploaded to the control room on the deck through photoelectric composite cables, and the host computer's software, designed with Python language, will process and store all the data. This allows workers on the deck to control the subordinate computer and obtain high-precision, complete data in real time. The joint tests between the subordinate and host computers have demonstrated the stability and reliability of the online transmitter system, which provides significant convenience for offshore exploration.

Rapid imaging approach for marine controlled-source electromagnetic with deployed transmission system

The marine controlled-source electromagnetic (MCSEM) method is widely used in gas hydrate exploration. For the deployed transmission system of MCSEM, it is equipped with two groups of orthogonal transmission antennas to obtain more comprehensive information. The mainstream processing method involves using a single rotated field component for conventional 2D inversion to obtain the resistivity model. However, data preprocessing and inversion speed are relatively slow, making it difficult to quickly or nearly real-time identify gas hydrate anomalous areas. In order to leverage the benefits of multi-component data and obtain a potential real-time imaging method for the future, we have developed a rapid imaging technique for the deployed transmission system to obtain apparent resistivity pseudosections. The current and electromagnetic field data in different directions are replaced by rotational invariants, and an approximate formula is derived based on the half-space formula and test corrections. This formula maps the apparent resistivity and field data. Results from model tests demonstrate that the apparent resistivity pseudosections can qualitatively identify reservoirs, particularly in the lateral range. In 2015, the South China Sea gas hydrate exploration project acquired data using the deployed transmission system. The pseudosections reveal that the rapid imaging method can directly detect resistivity anomalies and serve as an approximate and effective display method for deployed transmission system of MCSEM, thanks to its simplicity, efficiency, and low computational cost.

Time-Lapse 3D CSEM for Reservoir Monitoring Based on Rock Physics Simulation of the Wisting Oil Field Offshore Norway.

The marine controlled-source electromagnetic (CSEM) method has been used in different applications, such as oil and gas reservoir exploration, groundwater investigation, seawater intrusion studies and deep-sea mineral exploration. Recently, the utilization of the marine CSEM method has shifted from petroleum exploration to active monitoring due to increased environmental concerns related to hydrocarbon production. In this study, we utilize the various dynamic reservoir properties available through reservoir simulation of the Wisting field in the Norwegian part of the Barents Sea. In detail, we first developed geologically consistent rock physics models corresponding to reservoirs at different production phases, and then transformed them into resistivity models. The constructed resistivity models pertaining to different production phases can be used as input models for a finite difference time domain (FDTD) forward modeling workflow to simulate EM responses. This synthetic CSEM data can be studied and analyzed in the light of production-induced changes in the reservoir at different production phases. Our results demonstrate the ability of CSEM data to detect and capture production-induced changes in the fluid content of a producing hydrocarbon reservoir. The anomalous CSEM responses correlating to the reservoir resistivity change increase with the advance of the production phase, and a similar result is shown in anomalous transverse resistance (ATR) maps derived from the constructed resistivity models. Moreover, the responses at 30 Hz with a 3000 m offset resulted in the most pronounced anomalies at the Wisting reservoir. Hence, the method can effectively be used for production-monitoring purposes.

Marine Controlled-Source Electromagnetic Data Denoising Method Using Symplectic Geometry Mode Decomposition

The marine controlled-source electromagnetic (CSEM) method is an efficient tool for hydrocarbon exploration. The amplitudes of signals decay rapidly with the increasing offset, so signals are easily contaminated by various kinds of noise. A denoising method is critical to improve the data quality, but the diversity of noise makes denoising difficult. Specific frequency signals are transmitted for exploration requirements, and thus traditional filtering methods are not suitable. Symplectic geometry mode decomposition (SGMD), a new method to decompose signals, has an outstanding decomposition performance and noise robustness. Furthermore, it can reduce multiple types of noise by reconstructing the single components. In this study, we introduced SGMD to reduce the noise of marine CSEM data and improved the data quality significantly. The experiments show that SGMD is better than variational mode decomposition and the sym4 wavelet method.

Marine controlled source electromagnetic data denoising while weak signal preserving based on jointly sparse model and dictionary learning

The amplitude of the marine controlled source electromagnetic (CSEM) signal decays dramatically with the augment of offset. The signal is easily contaminated by surrounding noise when the transmitter-receiver offset is large. Denoising is crucial for marine CSEM data processing and interpretation. Nowadays, most denoising methods focus on suppressing the noise influences for a single component. The inherent relations among different electromagnetic (EM) components are neglected. Besides, some weak signals are removed mistakenly to get a better denoising effect. In this paper, a new denoising method is proposed based on a jointly sparse model and dictionary learning, which utilizes the correlations among multi-components of marine CSEM data. Synthetic experiments prove that it can not only effectively remove noise, but also successfully protect weak signals. Field data application further validates the effectiveness of the proposed method.

Controlled-source electromagnetic modeling using a versatile secondary electric-field formulation and efficient multigrid-based preconditioner

Frequency-domain controlled-source electromagnetic (CSEM) surveying is a widely used geophysical electromagnetic exploration method. To avoid the singularity near the transmitting sources and improve the accuracy of three-dimensional (3D) CSEM forward modeling, we adopt the vector finite element method and secondary electric field formulation combined with hexahedral grids to solve the frequency-domain Maxwell equations. Primary field computation applies the fast Hankel transform using digital linear filters to quickly calculate the electromagnetic field generated by different electromagnetic dipoles or loops in the layered medium. Therefore, our forward modeling algorithm is highly versatile and compatible with various electromagnetic sources and scenarios of CSEM application (e.g., airborne, offshore, or marine). To improve the convergence rate, we use an algebraic multigrid solver based on the established Hiptmair–Xu decomposition. Using direct solvers requires a vast amount of memory, while conventional iterative solvers usually fail or converge only very slowly. Our new iterative scheme does not require the divergence corrections commonly applied in finite difference or finite volume electromagnetic modeling to accelerate the convergence. We implement complete parallelism in our finite element programming to efficiently solve the CSEM forward problems on massively parallel clusters. We use different numerical methods to obtain high-precision comparison results for one-dimensional and typical 3D models to demonstrate our algorithm's effectiveness and correctness. Finally, a public model of 3 million cells is used to simulate the responses of marine CSEM surveying. The proposed new solver shows robust scalability when using nearly 1000 cores on the cluster, demonstrating that our algorithm is particularly suitable for large-scale 3D forward modeling.

New Concept Ocean-Bottom Multiphysics (OBMP) Nodes for Reservoir Monitoring

Marine-controlled source electromagnetics (CSEM) have been extensively applied to various exploration scenarios worldwide. However, its perceived value and cost relative to seismic and the scarcity of realistic case studies have limited the industry’s interest in time-lapse reservoir-monitoring (4D) applications. A feasible way to make demand for CSEM for 4D-monitoring programs would be to increase the value of information and reduce survey costs by performing joint operations where seismic and CSEM data are acquired during the same survey and at equivalent spatial densities. To this end, we propose a new multiphysics ocean-bottom nodes (OBN) concept and show the industry that CSEM can be a cost efficient and effective integrators to 4D seismic projects. To this end, we conducted a feasibility study demonstrating that horizontal magnetic field components have the required sensitivities and can be used instead of horizontal electric field components in mapping the 3D resistivity distribution and 4D fluid change responses in a given reservoir. This makes engineering a new OBN class simpler and cheaper, as various miniaturized magnetic field sensors are available off-the-shelf or readily working along with packaging and coupling solutions.

Analyzing the effect of dipping anisotropy on 3D marine controlled-source electromagnetic response based on a goal-oriented adaptive finite element method

This paper presents a goal-oriented adaptive edge-based finite element (FE) algorithm for modelling marine controlled-source electromagnetic (CSEM) responses in three-dimensional (3-D) dipping anisotropic conductive media. We adopt the secondary field formulation for quasi-static Maxwell's equations to avoid the source singularity. The algorithm is implemented on unstructured tetrahedral grids, which allow for the simulation of complex model geometries, including tilted electrical interfaces and bathymetry. We validate the adaptive finite element code against a one-dimensional (1-D) layered model with vertical anisotropy. Further, we compare the 3-D FE numerical solutions with those obtained by a 2-D FE code for a two-dimensional dipping anisotropic model. The modelling results from the new 3-D FE algorithm are in good agreement with both the analytical solutions derived from the 1-D code and the numerical results from the anisotropic 2-D FE algorithm. We then use the algorithm to simulate the marine CSEM responses over different anisotropic 3-D models, including an anisotropic reservoir, anisotropic surrounding sediments, and an anisotropic overburden, and performed a detailed analysis of the effect of VTI, HTI, and TTI anisotropy on 3-D marine CSEM response. The modelling results illustrate that marine CSEM responses are considerably influenced by various anisotropic parameters, but to different extents. The research on different types of anisotropy within different media can help us avoid the misinterpretation of marine CSEM data when processing and interpreting of the measured geophysical electromagnetic data.

High-Resolution Resistivity Imaging of a Transversely Uneven Gas Hydrate Reservoir: A Case in the Qiongdongnan Basin, South China Sea

Marine controlled-source electromagnetics (MCSEM) is an effective method to map the spatial distribution of gas hydrate and calculate gas hydrate saturation. An MCSEM survey is conducted in the Lingnan low uplift (LNLU), Qiongdongnan Basin (QDNB), South China Sea (SCS), and then the measured data are processed to obtain the geoelectric structure. The estimated gas hydrate stability zone (GHSZ) ranges from 0 to 320 mbsf, and shallow high-conductive sediments serving as gas hydrate caps are at depths ranging from 0 to 100 mbsf (meters below the seafloor). The 2D resistivity model reveals multiple high-resistivity bodies at depths ranging from 100 to 320 mbsf, and BSRs are at depths of 240 mbsf to 280 mbsf, indicating a transversely uneven gas hydrate reservoir in the study area. Moreover, two high-resistivity bodies are detected beneath the GHSZ, implying the presence of potential gas transport pathways. The gas hydrate saturation with a variation of 0–68.4% is calculated using the MCSEM resistivity and Archie’s law. According to the resistivity model and geological data, the transversely uneven gas hydrate reservoir may be associated with multiple gas sources, including shallow biogenic gas and deep pyrolytic gas. The shallow biogenic gas is transported to the GHSZ via short-distance migration and free diffusion, and the deep pyrolytic gas is transported to the GHSZ via two microcracks. In addition, this case emphasizes that the dynamic accumulation of gas hydrate is an important factor causing reservoir heterogeneity.

Enhanced detectability using differential fields for marine controlled-source EM data in shallow waters

ABSTRACT In shallow waters, the interpretation of the frequency-domain marine controlled-source electromagnetic (CSEM) data is challenging due to the airwave. The airwave dominates and will lead to misinterpretation of the data set. A differential-field approach is presented to attenuate the airwave for the shallow-water marine CSEM data. The difference of the fields between two consecutive receivers or source points is calculated and weighted by the geometric spreading related factor. By using differential fields, the detectability given by the field ratio between the models with and without the target is enhanced, which indicates that the impact of airwaves is suppressed.

Three-Dimensional Modeling of Marine Controlled Source Electromagnetic Using a New High-Order Finite Element Method With an Absorption Boundary Condition

The marine controlled source electromagnetic method (MCSEM) is widely used in marine oil, gas exploration and deep structures investigation because of its low cost and high efficiency. In this paper, a high-precision forward algorithm for 3D modeling for MCSEM is proposed. Unstructured meshes are used to discretize the model domain, and local mesh refinement techniques are applied, which is conducive to simulating complex terrains and targets. The application of electric dipole discrete technology can load wire excitation sources with arbitrarily complex spatial shapes, which can simulate more realistic electromagnetic field distribution of marine controllable sources electromagnetic in actual exploration. Absorption boundary conditions based on real number and exponential stretching techniques are introduced to improve the numerical solution accuracy. To improve the efficiency of solving, the open-source massively parallel solver (MUMPS) based on the multifrontal algorithm is used to solve the finite element equations. Finally, some typical geoelectric models are designed to verify the correctness and effectiveness of the proposed algorithm. The calculation results show that higher order finite elements bring about higher accuracy. In addition, the loading of absorption boundary conditions is simple and effective than conventional Dirichlet boundary conditions.

Monitoring Offshore CO2 Sequestration Using Marine CSEM Methods; Constraints Inferred from Field- and Laboratory-Based Gas Hydrate Studies

Offshore geological sequestration of CO2 offers a viable approach for reducing greenhouse gas emissions into the atmosphere. Strategies include injection of CO2 into the deep-ocean or ocean-floor sediments, whereby depending on pressure–temperature conditions, CO2 can be trapped physically, gravitationally, or converted to CO2 hydrate. Energy-driven research continues to also advance CO2-for-CH4 replacement strategies in the gas hydrate stability zone (GHSZ), producing methane for natural gas needs while sequestering CO2. In all cases, safe storage of CO2 requires reliable monitoring of the targeted CO2 injection sites and the integrity of the repository over time, including possible leakage. Electromagnetic technologies used for oil and gas exploration, sensitive to electrical conductivity, have long been considered an optimal monitoring method, as CO2, similar to hydrocarbons, typically exhibits lower conductivity than the surrounding medium. We apply 3D controlled-source electromagnetic (CSEM) forward modeling code to simulate an evolving CO2 reservoir in deep-ocean sediments, demonstrating sufficient sensitivity and resolution of CSEM data to detect reservoir changes even before sophisticated inversion of data. Laboratory measurements place further constraints on evaluating certain systems within the GHSZ; notably, CO2 hydrate is measurably weaker than methane hydrate, and >1 order of magnitude more conductive, properties that may affect site selection, stability, and modeling considerations.

A review of marine controlled-source electromagnetic data preprocessing technology

The marine controlled-source electromagnetic (MCSEM) method is an important geophysical technique for seabed resource exploration. During the MCSEM exploration process, distortion and noise will inevitably appear in the original data collected by the receiver due to various interferences and geological factors. The data preprocessing technology, therefore, is essential to improve the data quality for further inversion explanation of the seabed’s geological structure. Through tracing and analyzing the relevant literature published over two decades on MCSEM data preprocessing technology, including data correction and noise suppression methods, this Review first introduces the basic principle of MCSEM exploration and then analyzes the advances, limitations, and challenges of its current application. Ultimately, this Review discusses the prospects of this technology, providing a reference to relevant industries and researchers.

Joint focusing inversion of marine controlled-source electromagnetic and full tensor gravity gradiometry data

We have developed an approach for integrating the interdependent information of different geophysical methods to obtain a self-consistent geophysical model in offshore exploration. Electromagnetic (EM) and gravity data complement seismic data by providing information about the physical properties of prospective reservoir rocks. We have combined marine EM and gravity gradiometry geophysical data in joint inversion using a joint focusing stabilizer. The method enforces coupling between different physical models and simultaneously promotes focused images of the targets. We also consider the importance of using robust norms to analyze data collected by marine controlled-source EM surveys. The practical effectiveness of the developed method is illustrated by a case study of integrating and imaging marine multiphysics data collected in the Nordkapp Basin, Barents Sea, Norway.

Forward calculation of 3D controlled-source electromagnetic responses based on joint application of secondary field and coupled potential formulations

In the traditional framework of 3D controlled-source electromagnetic (CSEM) modeling, either the field formulation or the coupled potential formulation is implemented. We have developed an improved algorithm that can calculate the 3D CSEM responses based on a joint application of the secondary field and secondary coupled potential formulations. The vector finite-element method is implemented for the discretization of the curl-curl equation of the secondary field. The vector and nodal elements are implemented simultaneously for the discretization of the coupled potential formulation. We use the stiffness matrix resulting from the coupled potential formulation to construct an efficient preconditioner, which is used for the solution of the linear system of equations obtained based on the secondary field formulation. The linear system of the equations could be solved efficiently by the quasiminimum residual method within several hundred iterations with the constructed preconditioner. We compare different modeling schemes to calculate the CSEM responses for a horizontally layered earth model excited by an electric dipole and find that the iteration process of the improved algorithm converges faster on average than other modeling schemes for the frequency band considered. Thus, the least computation time can be obtained over the frequency band for the improved algorithm compared with other modeling schemes, with the same level of accuracy reached. We further verify the efficiency of the algorithm through a small-scale 3D earth model excited by a magnetic dipole. Finally, we illustrate the performance of the algorithm with two 3D subsurface models, including a land-based model and a marine disk model. By comparison with published work, it is illustrated that our algorithm can efficiently calculate accurate land-based and marine CSEM responses with the new constructed preconditioner implemented.

Denoising Marine Controlled Source Electromagnetic Data Based on Dictionary Learning

Marine controlled source electromagnetic (CSEM) is an efficient method to explore ocean resources. The amplitudes of marine CSEM signals decay rapidly with the measuring offsets. The signal is easily contaminated by various kinds of noise when the offset is large. These noise include instrument internal noise, dipole vibration noise, seawater motion noise and environmental noise Suppressing noise is the key to improve data quality and interpretation accuracy. Sparse representation based denoising method has been used for denoising for a long time. provides a new way to remove noise. Under the framework of sparse representation, the denoising effect is closely related to the chosen transform matrix. This matrix is called dictionary and its column named atom. In general, the stronger the correlation between signal and dictionary is, the sparser representation will be, and further the better the denoising effect will be. In this article, a new method based on dictionary learning is proposed for marine CSEM denoising. Firstly, the signal segments suffering little from noise are captured to compose the training set. Then the learned dictionary is trained from the training set via K-singular value decomposition (K-SVD) algorithm. Finally, the learned dictionary is used to sparsely represent the contaminated signal and reconstruct the filtered one. The effectiveness of the proposed approach is verified by a synthetic data denoising experiment, in which windowed-Fourier-transform (WFT) and wavelet-transform (WT) denoising methods and three dictionaries (discrete-sine-transform (DST) dictionary, DST-wavelet merged dictionary and the learned dictionary) under a sparse representation framework are tested. The results demonstrate the superiority of the proposed dictionary-learning-based denoising method. Finally, the proposed approach is applied to field data denoising process, coupled with DST and DST-wavelet dictionaries based denoising methods. The outcomes further proves that the propsoed approach is effective and superior for marine CSEM data denoising.

A cost-effective three-dimensional marine controlled-source electromagnetic survey: Exploring seafloor massive sulfides

A 3D marine controlled-source electromagnetic (CSEM) survey for mapping hydrocarbons uses dozens of ocean-bottom electric (OBE) receivers deployed in a grid pattern and several transmitter towlines. This study considers seafloor massive sulfides (SMS) exploration, and the horizontal survey scale of SMS is a few kilometers, which is small compared with hydrocarbon surveys of tens of kilometers. If we apply a 3D CSEM survey using a receiver deployment on grids to map SMS, high survey costs will be incurred despite the small survey size. We have developed a cost-effective 3D marine CSEM survey that uses fewer receivers than the survey with a receiver deployment on grids to reduce survey costs for SMS. This CSEM survey uses a line of OBE receivers in the center of the survey area and several transmitter towlines. Numerical tests demonstrate that our survey (seven OBE receivers) using 80% fewer receivers than the survey with a receiver deployment on grids (35 OBE receivers) is able to accurately map SMS, obtaining a performance similar to that of the receiver deployment on grids. Then, we explore SMS in the Ieyama hydrothermal area off Okinawa, southwest Japan, using our 3D CSEM survey with a line of six OBE receivers and three transmitter towlines. The resulting 3D resistivity distribution from the observed data highlights three potential SMS zones consisting of 0.2 ohm-m low resistivity embedded into 1 ohm-m sediment.