Marine Controlled-source Electromagnetic Research Articles

Gaussian Process Methodology for Multi-Frequency Marine Controlled-Source Electromagnetic Profile Estimation in Isotropic Medium

The marine controlled-source electromagnetic (CSEM) technique is an application of electromagnetic (EM) waves to image the electrical resistivity of the subsurface underneath the seabed. The modeling of marine CSEM is a crucial and time-consuming task due to the complexity of its mathematical equations. Hence, high computational cost is incurred to solve the linear systems, especially for high-dimensional models. Addressing these problems, we propose Gaussian process (GP) calibrated with computer experiment outputs to estimate multi-frequency marine CSEM profiles at various hydrocarbon depths. This methodology utilizes prior information to provide beneficial EM profiles with uncertainty quantification in terms of variance (95% confidence interval). In this paper, prior marine CSEM information was generated through Computer Simulation Technology (CST) software at various observed hydrocarbon depths (250–2750 m with an increment of 250 m each) and different transmission frequencies (0.125, 0.25, and 0.5 Hz). A two-dimensional (2D) forward GP model was developed for every frequency by utilizing the marine CSEM information. From the results, the uncertainty measurement showed that the estimates were close to the mean. For model validation, the calculated root mean square error (RMSE) and coefficient of variation (CV) proved in good agreement between the computer output and the estimated EM profile at unobserved hydrocarbon depths.

Efficient use of sparsity by direct solvers applied to 3D controlled-source EM problems

Controlled-source electromagnetic (CSEM) surveying becomes a widespread method for oil and gas exploration, which requires fast and efficient software for inverting large-scale EM datasets. In this context, one often needs to solve sparse systems of linear equations with a large number of sparse right-hand sides, each corresponding to a given transmitter position. Sparse direct solvers are very attractive for these problems, especially when combined with low-rank approximations which significantly reduce the complexity and the cost of the factorization. In the case of thousands of right-hand sides, the time spent in the sparse triangular solve tends to dominate the total simulation time, and here we propose several approaches to reduce it. A significant reduction is demonstrated for marine CSEM application by utilizing the sparsity of the right-hand sides (RHS) and of the solutions that results from the geometry of the problem. Large gains are achieved by restricting computations at the forward substitution stage to exploit the fact that the RHS matrix might have empty rows (vertical sparsity) and/or empty blocks of columns within a non-empty row (horizontal sparsity). We also adapt the parallel algorithms that were designed for the factorization to solve-oriented algorithms and describe performance optimizations particularly relevant for the very large numbers of right-hand sides of the CSEM application. We show that both the operation count and the elapsed time for the solution phase can be significantly reduced. The total time of CSEM simulation can be divided by approximately a factor of 3 on all the matrices from our set (from 3 to 30 million unknowns, and from 4 to 12 thousands RHSs).

On the physical principles underlying electromagnetic induction

This paper provides a theoretical overview of some of the fundamental concepts underlying electromagnetic (EM) induction exploration methods using marine controlled-source EMs as an exemplar. In particular, it will be shown, from different vantage points, that EM induction operates in the magnetoquasistatic regime in which inductive effects dominate, capacitive effects are ignored, and the displacement current is negligible; hence, charge polarization and dielectric phenomena play no role. We determine some of the major physical consequences of this approximation, and we make a distinction between wave physics and diffusive behavior, which is of particular interest in the special case of time-periodic excitation. We distinguish the fundamentally different roles of mobile charge carriers and bound charges in EM induction. It is emphasized that EM induction cannot be fully understood by comparing and contrasting Maxwell’s equations with governing equations from other disciplines that possess a similar mathematical structure. It is suggested that visualizations of energy flow using the Poynting vector and the Joule heating parameters provide a powerful tool to understand how the geologic medium shapes EM induction responses.

Marine controlled-source electromagnetics with geothermal purposes; central Gulf of California, Mexico

We acquired and interpreted data from two controlled-source electromagnetic profiles, in the Gulf of California off the coast of Santa Rosalía, Baja California Sur, Mexico. This area is interesting from the geothermal point of view due to its proximity to the Guaymas Basin, the La Reforma Caldera, and the Las Tres Vírgenes volcano, the first with intense submarine hydrothermal manifestations and the final one with a geothermal field in production. The survey consisted of the deployment of 10 Ocean Bottom Electromagnetic receivers, distributed along two ~14 km-long profiles. These sensors recorded the two horizontal components of the electric and magnetic fields. The source consisted of a horizontal 200 m electric dipole towed behind the vessel. The processed data were inverted to estimate the subsurface distribution of electrical resistivity using a finite-element two-dimensional multi-frequency program, resulting in adequate fits between observed and calculated data. To estimate how well the models are constrained by the data, sensitivity tests were applied to the models. The tests were based on the Jacobian matrix and a perturbation analysis of selected features of the model. They show that the model in its central part is well constrained by the data down to a depth of about 2.5 km. With the perturbation approach, 16 resistivity features out of the 19 tested turned out to be well constrained by the data. The estimated resistivity distribution was compared with the information obtained from other geophysical methods (bathymetry, gravity, and 2D multi-channel seismic reflection), resulting in reasonably good correlations between them. For the first profile, we find a remarkable similarity with the current conceptual models proposed in the literature for a volcanic geothermal system, where an altered conductive clay cap overlays another with higher electrical resistivity. The second profile shows agreements with anomalies detected with echo-sounder bathymetry suggesting the existence of hydrothermal manifestations.

Fictitious wave domain modelling and analysis of marine CSEM data

SUMMARY Modelling marine controlled-source electromagnetic (CSEM) responses in the fictitious time domain is a novel approach, which facilitates the full exploration of EM diffusive properties in the fictitious wave domain (FWD). Concepts, such as reflections, refractions, diffractions and transmissions, which are used for the analysis of elastic wave propagation can thus be adopted in FWD for interpreting CSEM data. In this paper, we use a high-order finite difference time domain (FDTD) algorithm for modelling marine CSEM responses in both the fictitious time domain and the diffusive frequency domain. A complex frequency shifted perfectly matched layer (CFS–PML) boundary condition is adopted to the FDTD modelling. We demonstrate the performance of the CFS–PML boundary condition and validate the high-order FDTD code in the FWD with the half-space sea water model and in the frequency domain with the 1-D canonical reservoir model. We investigate and analyse the propagation characteristics of electromagnetic fields in the FWD, where we apply wave propagation concepts to interpret marine CSEM data. Similarities between wave and field propagations relevant for marine CSEM data are demonstrated through several 1-D to 3-D numerical examples.

Prediction of hydrocarbon depth for seabed logging (SBL) application using Gaussian process

Seabed Logging (SBL) is an application of marine Controlled-Source Electromagnetic (CSEM) technique to characterize hydrocarbon-filled layers underneath the seabed remotely in deep water regions. This technique maps structure of subsurface electrical resistivity in the offshore environment. Basically, exploration of offshore hydrocarbon is based on the contrast of electrical resistivity between hydrocarbon reservoir and its surrounding sea sediments. Modelling offshore hydrocarbon is a core analysis and time consuming task. Current numerical modelling techniques used in SBL application involve meshes and complicated mathematical equations. Thus, a simple supervised learning method which is Gaussian Process (GP) is proposed to process synthetic SBL data which are generated through Computer Simulation Technology (CST) software to predict the depth of hydrocarbon. This statistical model is able to provide additional hydrocarbon information by utilizing the prior information. 1-dimensional (1-D) forward GP models have successfully been developed to predict the presence of hydrocarbon and as the continuation work, 2-dimensional (2-D) forward GP model is then developed to be used as the standard profile in order to predict the depth of hydrocarbon. This shall give indication that GP can be used as the methodology to predict the depth of hydrocarbon reservoir underneath the seabed.

3-D Marine Controlled-Source Electromagnetic Modeling in Electrically Anisotropic Formations Using Scattered Scalar–Vector Potentials

Marine controlled-source electromagnetic (CSEM) method has become a popular technique for offshore hydrocarbon exploration. It has been well recognized that marine CSEM data are strongly affected by electrical anisotropy of geologic formations in practice. Here, we present a robust and efficient finite volume algorithm for simulating marine CSEM responses in 3-D arbitrarily anisotropic formations. The algorithm is based on scattered scalar–vector potentials which improve the ill-conditioning of the resulting linear system by deflating the null space of the curl operator, and a conservative volume averaging scheme is utilized for the discretization of arbitrary electrical anisotropy. The accuracy of our algorithm is validated against quasi-analytic solutions for a layered vertical transverse isotropic reservoir model. It is then demonstrated by numerical results that the marine CSEM fields are significantly affected by the anisotropic conductivity tensor, and neglect of the full anisotropy of geologic formations may cause misleading data interpretation.

Cloud computing for large-scale controlled-source electromagnetic inversions, Barents Sea, Norway

Since the late 1990s, the continuous development and application of the Controlled-Source Electromagnetic (CSEM) technology have vastly improved its usage in exploration (e.g., Constable, 2010; Loseth et al., 2015; Zach et al., 2009). CSEM inversion results are the main input for interpretation of resistivity data, especially potential fluid anomalies. Complementary to other geophysical methods, e.g. Amplitude versus Offset (AVO), it can therefore be a valuable tool in hydrocarbon exploration. Yet, the unconstrained nature of the subsurface and non-unique¬ness of inversions is a tough predicament. For example, several features in the subsurface could cause high resistivity, such as abundant limestone, salt, basalt, mature source rocks or hydro¬carbon-filled sandstones (Baltar and Barker, 2015). As a result of enhanced data quality, data processing and inversion capabilities, together with improved workflows and accumulated experience, CSEM interpretation has become increasingly more robust (Buland et al., 2011; Karman et al., 2013; Loseth et al., 2014). The 3D Gauss-Newton (GN) inversion algorithm is a more robust method compared to other inversion types. GN thus provides a new, powerful methodology for generating marine CSEM resistivity images. Nonetheless, it has formerly been com¬putationally exhaustive to apply. With the Hoop area, Barents Sea, recently being nominated for the Barents Sea exploration licensing round, a tight deadline was given for the CSEM inversion of an expansive 11,000 km2, over 130 blocks (refer to Figure 1). Adopt¬ing the flexible and agile cloud system into the CSEM workflow, it has improved the 3D GN work efficiency and provided overall consistent results when compared with other CSEM inversion schemes. This paper will illustrate how a systemic workflow of CSEM 3D GN inversions together with efficient cloud computing, could lead to a more consistent interpretation of this data.

Marine dipole–dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry

Seafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings. In 2016, CSEM and TEM experiments were carried out at several locations near the Trans- Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures.We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.

3-D Adaptive Finite-Element Modeling of Marine Controlled-Source Electromagnetics with Seafloor Topography Based on Secondary Potentials

We present a solution of using adaptive nodal finite-element (FE) method to solve the marine controlled-source electromagnetic (CSEM) problem for 3-D earth models in the frequency domain. The forward problem is solved based on the secondary Coulomb-gauged electromagnetic (EM) potentials. We implement an adaptive mesh refinement algorithm according to an a posteriori error estimator based on a gradient-recovery operator of the secondary EM potentials. To increase the quality of the mesh at the receiver locations, the elements containing the receiver locations are constrained by assigning them a maximum volume for the initial mesh. An unstructured tetrahedral mesh used in our approach can provide an accurate description of complex structures such as dipping layers and rough topography that are not accurately fitted using structured meshes. We first validate the adaptive FE approach and demonstrate the convergence of the adaptive grid refinement procedure using a 1-D layered model. The canonical disc model example illustrates the capability of the adaptive FE approach for 3-D CSEM modeling. The bathymetry model shows that the algorithm is well suited to deal with complex seafloor topography, which needs to be simulated exactly to avoid the misinterpretation of marine CSEM data sets.

Induced polarization in a 2.5D marine controlled-source electromagnetic field based on the adaptive finite-element method

The induced polarization (IP) in rocks and minerals is of significance to the marine controlled-source electromagnetic (CSEM) field. We propose an adaptive finite-element algorithm for the 2.5D frequency-domain forward modeling of marine CSEM that considers the induced polarization. The geoelectrical model is discretized using an unstructured triangular elemental grid that accommodates the complex topography and geoelectrical structures. We use the Cole–Cole model to describe the IP and develop a complex resistivity forward modeling algorithm. We compare the simulation results with published 1D model results and subsequently calculate the electromagnetic field for variable azimuth sources, IP parameters, and topography. Finally, we analyze the IP effect on the marine CSEM field and show that IP of oil reservoirs and topography affects the marine CSEM electromagnetic field.

Numerical Analysis of Land-Based Inline-Source Configuration for the Controlled-Source Electromagnetic Method

The geometry of source and receiver is one of the most crucial factors for the success of a controlled-source electromagnetic (CSEM) investigation. The inline electric-source configuration, widely used in the marine CSEM method, is not commonly employed for land CSEM surveys. To examine the feasibility of the inline electric-source time domain electromagnetic method (TEM), we first analyzed the optimal observing area for the land-based electric-dipole source by imaging the distribution of each component and the anomalies caused by a resistive or conductive target layer. Then, we conducted an error-space analysis to evaluate the sensitivity of each inline component based on the result of the optimal observation area. Finally, we collected field data from a typical inline dipole-dipole configuration and inverted the data with an algorithm based on Occam's principle. [Figure: see text]

A marine controlled-source electromagnetic survey to detect gas hydrates in the Qiongdongnan Basin, South China Sea

The Qiongdongnan Basin (QB) is an important prospecting area for gas hydrates in the South China Sea (SCS). However, it is difficult to infer gas hydrates using a bottom simulating reflector (BSR) under the influence of flat seabed, multiples and bubble effects. In the QB, the upper limit of the gas hydrate stability zone (GHSZ) and the distribution of gas hydrates in the shallow seabed are still not well understood. A marine controlled-source electromagnetic (MCSEM) survey was completed in order to collect electric resistivity information along a 4.5-km profile. Electromagnetic data were acquired and processed at nine sites. The seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis (OPRA) method. As a result of isotropic, vertical transverse-isotropic (VTI), and triaxial anisotropic inversion of electric field Pmax data, a 2-D resistivity image of the seabed was presented. Preliminary results show that the shallow seabed can be divided into four electric layers. According to thermodynamic conditions in the QB, the estimated thickness of GHSZ is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area. The BSR and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data. The MCSEM result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the GHSZ.

Application of 3D marine controlled-source electromagnetic finite-element forward modeling to hydrocarbon exploration in the Flemish Pass Basin offshore Newfoundland, Canada

In recent years, marine controlled-source electromagnetic (CSEM) surveying has become an effective supplemental interpretation tool to the seismic reflection method to help mitigate risk in an offshore exploration setting. Interpretation of marine CSEM data is commonly achieved via finite-difference inversions on rectilinear meshes, which has its merits, but the results are typically of very low resolution. The alternative is forward modeling, which requires a model to be known a priori, but the detail of the model can be created to reflect realistic geologic conditions. What is typically seen in the literature are applications of EM forward modeling codes to synthetic, and sometimes complex synthetic, models. However, what the literature is missing is an application that overcomes the challenges of applying a 3D forward modeling method to real models constructed from real information. We have developed an application of a 3D marine CSEM finite-element forward modeling method to the Bay du Nord prospect in the Flemish Pass Basin offshore Newfoundland. The 3D resistivity model, composed of four topographical layers and the Bay du Nord reservoir body, was built using 2D seismic data, one well log, and a marine CSEM inversion. Although other mesh representations have their merits, we chose to discretize our 3D model into an unstructured tetrahedral mesh because its flexibility enabled the accurate representation of complex structures while minimizing the number of unknowns. The availability of measured marine CSEM data allowed for the resistivities of each layer in the 3D model to be refined, and it also allowed for the simulated data to be assessed in the context of the real noise levels. A subsequent sensitivity analysis of the forward modeling results provided insights regarding the detectability of the Bay du Nord reservoir.

Robust data processing of noisy marine controlled-source electromagnetic data using independent component analysis

Data processing techniques are often used to estimate the noise-free response of marine controlled-source electromagnetic (CSEM) data and magnetotelluric transfer functions. We have implemented a new CSEM data processing scheme that uses a robust method based on independent component analysis (ICA) to extract interpretable datasets from noisy marine CSEM data. We applied the data processing scheme to signals from a new CSEM observation system comprising a remotely operated vehicle (ROV) and an ocean bottom electromagnetometer (OBEM). These datasets were obtained around the Iheya North hydrothermal field, Okinawa Trough, Japan. The observation system allows a small-scale CSEM survey to be conducted in areas of steep topography, such as hydrothermal fields, because the ROV can deploy the OBEM at the exact observation site. The results show that the coherent and environment noise that exists in the raw time series is reduced sufficiently by ICA processing. It makes interpretation of the resulting electric field data possible. The results also show that the processed data has a higher signal-to-noise ratio in the middle-to-high-frequency band than the data without ICA. The normalised spectrum, obtained by normalising the observed data from the hydrothermal area, indicates that a conductive anomaly exists in the near-offset area around the OBEM. We apply 2D inversion to the electric field data and find that a low resistivity body exists beneath the OBEM and 50 m offset from the OBEM. This resistivity structure is consistent with images taken by the ROV that show characteristic organisms in hydrothermal seepage around the OBEM site.

2.5D marine CSEM modeling in the frequency-domain based on an improved interpolation scheme at receiver positions

In the marine controlled-source electromagnetic (CSEM) survey, the receivers are usually placed at the seafloor. The resistivity contrast between the seawater and seafloor sediments is large, which can cause difficulties in numerical modeling of CSEM fields at receiver locations. In this paper, we present an improved interpolating method for calculating electric and magnetic fields at the seafloor with a resistivity contrast. This method is applied to the 2. 5 dimensional (2. 5D) frequency-domain CSEM modeling with towed transmitters and receivers located at the seafloor. Considering the discontinuity of the normal electric fields, we use the normal current electric density for interpolation. We simulate the 2. 5D marine CSEM responses by the staggered finite-difference (SFD) method with Fourier transform to the strike direction. The final SFD equations are solved by the direct solver MUMPS (MUltifrontal Massively Parallel Sparse direct Solver). To avoid the source singularities, the secondary-field approach is used and the primary fields excited by the electric dipole source can be calculated quasi-analytically for the one-dimensional (1D) layered background model. We focus on interpolating of electric and magnetic fields in the wavenumber domain to the receiver locations at the seafloor interface between the conductive seawater and resistive seafloor formation. The secondary electric and magnetic fields are used for interpolation instead of the total fields for high numerical accuracy. After performing the inverse Fourier transform to the wavenumbers, the electric and magnetic fields in the space domain are obtained. To check the accuracy of our 2. 5D marine CSEM SFD modeling algorithm with the improved receiver interpolating technique, we compare our results with both the 1D analytical results and the adaptive finite element results. The SFD numerical results are approved to be accurate. We also compare the numerical accuracy between our improved interpolation scheme and others, i.e., the conventional linear interpolation and the rigorous interpolation. The proposed interpolation only utilizes the nodes below/above the seafloor interface, and is proved to be much more accurate than the other two interpolating methods used.

Improved Model Analysis and Current-Loop Design for Marine Controlled Source Electromagnetic Transmitter

The marine controlled source electromagnetic transmitter is a crucial device of marine electromagnetic method, its transient state and steady state is followed with interest. In this paper, an improved model of the phase-shifted full-bridge converter is minutely described on the basis of some specific challenges of electromagnetic transmitter based on full-bridge structure, such as power losses and duty-cycle loss. Then, an efficient current-loop compensator designed with improved model proposed in this paper is mentioned. The comparative analysis and simulation results of the improved model and the traditional model could prove the superiority of the improved to the system response speed and control stability. The electromagnetic transmitter could achieve better transient state and steady state characteristics by the compensator designed with the improved model.

Application of the perfectly matched layer in 3-D marine controlled-source electromagnetic modelling

In this study, the complex frequency-shifted perfectly matched layer (CFS-PML) in stretching Cartesian coordinates, is successfully applied to three-dimensional (3D) frequency-domain marine controlled-source electromagnetic (CSEM) field modelling. The Dirichlet boundary, which is usually used within the traditional framework of EM modeling algorithms, assumes the electric or magnetic field values are zero at the boundaries. This requires the boundaries be sufficiently far away from the sources in the area of interest. To mitigate the boundary artifacts, a large modelling area may be necessary even though cell sizes are allowed to grow toward the boundaries due to the diffusion of the electromagnetic wave propagation. Compared with the conventional Dirichlet boundary, the PML boundary is preferred as the modelling area of interest could be restricted to the target region and only a few absorbing layers surrounding can effectively depress the artificial boundary effect without losing the numerical accuracy. Furthermore, for joint inversion of seismic and marine CSEM data, if we used the PML for CSEM field simulation instead of the conventional Dirichlet, the modeling area for these two different geophysical data collected from the same survey area could the same, which is convenient for joint inversion grid matching. We apply the CFS-PML boundary to 3D marine CSEM modelling by using the staggered finite-difference (SFD) discretization. Numerical test indicates that the modeling algorithm using the CFS-PML also shows good accuracy compared to the Dirichlet. Furthermore, the modeling algorithm using the CFS-PML shows advantages in computational time and memory saving than that using the Dirichlet boundary. For the 3D example in this study, the memory saving using the PML is nearly 42 % and the time saving is around 48% compared to using the Dirichlet.

Mapping the resistivity structure of Walker Ridge 313 in the Gulf of Mexico using the marine CSEM method

A marine controlled source electromagnetic (CSEM) campaign was carried out in the Gulf of Mexico to further develop marine electromagnetic techniques in order to aid the detection and mapping of gas hydrate deposits. Marine CSEM methods are used to obtain an electrical resistivity structure of the subsurface which can indicate the type of substance filling the pore space, such as gas hydrates which are more resistive. Results from the Walker Ridge 313 study (WR 313) are presented in this paper and compared with the Gulf of Mexico Gas Hydrate Joint Industry Project II (JIP2) logging while drilling (LWD) results and available seismic data. The hydrate, known to exist within sheeted sand deposits, is mapped as a resistive region in the two dimensional (2D) CSEM inversion models. This is consistent with the JIP2 LWD resistivity results. CSEM inversions that use seismic horizons provide more realistic results compared to the unconstrained inversions by providing sharp boundaries and architectural control on the location of the resistive and conductive regions in the CSEM model. The seismic horizons include: 1) the base of the gas hydrate stability zone (BGHSZ), 2) the top of salt, and 3) the top and bottom of a fine grained marine mud interval with near vertical hydrate filled fractures, to constrain the CSEM inversion model. The top of salt provides improved location for brines, water saturated salt, and resistive salt. Inversions of the CSEM data map the occurrence of a ‘halo’ of conductive brines above salt. The use of the BGHSZ as a constraint on the inversion helps distinguish between free gas and gas hydrate as well as gas hydrate and water saturated sediments.

Sensitivity study of three-dimensional marine controlled-source electromagnetic method

The marine controlled-source electromagnetic (CSEM) method is of increasing importance as a powerful geophysical tool in hydrocarbon exploration. Related to this, three-dimensional (3-D) forward modeling provides an effective method for marine CSEM survey planning and sensitivity analysis (Fréchet derivative). To further evaluate the feasibility of this measurement system as well as its resolution capabilities, we carried out a 3-D sensitivity distribution experiment. The sensitivity calculation used in this work is based on reciprocity theorem and directly affects the accuracy and efficiency of the inversion computation process. This is also a key problem in Gauss-Newton type inversion algorithms. Thus, we analyze the sensitivity distribution of the electric field for both a homogeneous half-space and resistors embedded in conductive sediments. To illustrate the potential of this approach in a 3-D survey, we then adopt a series of simple numerical models to simulate hydrocarbon reservoirs in real situations across a typical suite of configurations. We show that the sensitivity simulation can enhance our understanding of sensitivity characteristics. Indeed, via comparisons and the analysis of sensitivity distributions, we demonstrate the feasibility of this approach for the detection of hydrocarbon reservoirs, and provide additional information for planning and designing marine CSEM surveys.