Marine Controlled-source Electromagnetic Research Articles

On the physics of frequency domain controlled source electromagnetics in shallow water, 2: transverse anisotropy

In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically-polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the sub-seafloor conductivity that is assumed to be transversely anisotropic, with a vertical-to-horizontal resistivity ratio of 3:1. For an ocean whose electrical thickness is comparable to that of the overburden, the seafloor electromagnetic response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a halfspace, or a stronger and faster response, and displays little to no evidence for the air interaction. For an ocean whose electrical thickness is much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets, and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. By comparison to the isotropic case with the same horizontal conductivity, transverse anisotropy stretches the Poynting vector and the electric field response from a thin resistive layer to much longer offsets. These phenomena can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire sub-seafloor resistivity structure with the sea surface. The Fréchet derivatives are dominated by preferential sensitivity to the vertical conductivity in the reservoir layer and overburden at short offsets. The horizontal conductivity Fréchet derivatives are weaker than to comparable to the vertical derivatives at long offsets in the substrate. This means that the sensitivity to the horizontal conductivity is present in the shallow parts of the subsurface. In the presence of transverse anisotropy, it is necessary to go to higher frequencies to sense the horizontal conductivity in the overburden as compared to an isotropic model with the same horizontal conductivity. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.

Splitting marine controlled-source electromagnetic responses into sea and subseafloor contributions: Grounding the air wave

We have found how the effects of the air wave in marine controlled-source electromagnetic (CSEM) methods gradually vanish in the sea for shallow waters, and how at the same time they gradually grow below the seafloor, in an effort to comprehend existing detectability definitions. The transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea. We consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs, particularly in shallow-water explorations and, specifically, on how the air wave affects detection. Our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1D profile. Two-dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets. Results include the expected growth of the inhibiting effect of the sea as the water depths decrease. However, we also find that this happens up to a point and that from then on its effect decreases to zero. Regarding the resistive layer at depth, its importance grows to a constant as the water depth decreases to zero. As a function of offsets, there appear first the direct current effects. The induction zone is next and is dominated by contributions from the underlying formations. The third zone, which corresponds to the air wave, is largely dominated by contributions from the sea. The fourth and last zone is the plane-wave asymptote. All four classical zones identified in marine CSEM are also present in land CSEM.

Application of the perfectly matched layer in 2.5D marine controlled-source electromagnetic modeling

For the traditional framework of EM modeling algorithms, the Dirichlet boundary is usually used which assumes the field values are zero at the boundaries. This crude condition requires that the boundaries should be sufficiently far away from the area of interest. Although cell sizes could become larger toward the boundaries as electromagnetic wave is propagated diffusively, a large modeling area may still be necessary to mitigate the boundary artifacts. In this paper, the complex frequency-shifted perfectly matched layer (CFS-PML) in stretching Cartesian coordinates is successfully applied to 2.5D frequency-domain marine controlled-source electromagnetic (CSEM) field modeling. By using this PML boundary, one can restrict the modeling area of interest to the target region. Only a few absorbing layers surrounding the computational area can effectively depress the artificial boundary effect without losing the numerical accuracy. A 2.5D marine CSEM modeling scheme with the CFS-PML is developed by using the staggered finite-difference discretization. This modeling algorithm using the CFS-PML is of high accuracy, and shows advantages in computational time and memory saving than that using the Dirichlet boundary. For 3D problem, this computation time and memory saving should be more significant.

2.5D Inversion and Joint Interpretation of CSEM Data at Sleipner CO2 Storage

In this paper, we revisit the marine controlled-source electromagnetic (CSEM) data, acquired above the Sleipner CO2 storage, in order to further study the dataset and conclude the feasibility of marine CSEM for offshore CCS monitoring. There are some challenges with respect to CSEM in this particular area: 1) strong airwave influence (due to regional shallow water depth); 2) potential of weak resistivity anomaly; 3) seabed pipeline network; and 4) rather shallow target depth. We are yet able to extract useful information; interpret further the CSEM inversion results by combining seismic data; and to extract the in situ resistivity and saturation of CO2 in the Utsira formation by applying a rock physics model. In addition, to minimize the influence of the seabed pipeline on the CSEM data, we have muted some of data and receivers near the seabed pipeline network. The results show a good agreement with seismic data, and the estimated total mass of CO2 agrees well with the injection data. This current study confirms that the marine CSEM can be an important and essential tool for offshore CO2 storage monitoring, yet not alone but when combined with both seismic and gravity. Finally, near-future large-scale CCS projects in the North Sea would require extensive infra-structures such as seabed pipeline, etc. This study demonstrates that CSEM may work even with such infra-structures in place.

Accurate Interpolation at Receiver Positions: A Novel Method for Frequency-Domain Marine CSEM Finite-Difference Modelling

We present an accurate interpolating method for calculating electric and magnetic fields at the seafloor with a resistivity contrast. This method is applied to the three-dimensional (3D) frequency-domain marine controlled-source electromagnetic (CSEM) modeling with the towed transmitters and receivers located at the seafloor. We simulate the 3D marine CSEM responses by the staggered finite-difference method with a direct solver. The secondary-field approach is used for avoiding the source singularities and the primary fields excited by the electric dipole source could be calculated quasi-analytically for the one-dimensional layered background. Therefore, in this study, we focus on interpolating of electric and magnetic fields to the receiver locations at the seafloor interface between the conductive seawater and resistive seafloor formation. Considering the discontinuity of the normal electric fields, we use the normal current electric density for interpolation. The secondary electric and magnetic fields are also used for interpolation instead of the total fields for high numerical accuracy. The proposed interpolation only utilizes the nodes below/above the seafloor interface and is approved to be much more accurate than other tested interpolating methods, i.e., the conventional linear interpolation and the rigorous interpolation.

Large-scale 3-D EM modelling with a Block Low-Rank multifrontal direct solver

We put forward the idea of using a Block Low-Rank (BLR) multifrontal direct solver to efficiently solve the linear systems of equations arising from a finite-difference discretization of the frequency-domain Maxwell equations for 3-D electromagnetic (EM) problems. The solver uses a low-rank representation for the off-diagonal blocks of the intermediate dense matrices arising in the multifrontal method to reduce the computational load. A numerical threshold, the so-called BLR threshold, controlling the accuracy of low-rank representations was optimized by balancing errors in the computed EM fields against savings in floating point operations (flops). Simulations were carried out over large-scale 3-D resistivity models representing typical scenarios for marine controlled-source EM surveys, and in particular the SEG SEAM model which contains an irregular salt body. The flop count, size of factor matrices and elapsed run time for matrix factorization are reduced dramatically by using BLR representations and can go down to, respectively, 10, 30 and 40 per cent of their full-rank values for our largest system with N = 20.6 million unknowns. The reductions are almost independent of the number of MPI tasks and threads at least up to 90 × 10 = 900 cores. The BLR savings increase for larger systems, which reduces the factorization flop complexity from O(N2) for the full-rank solver to O(Nm) with m = 1.4–1.6. The BLR savings are significantly larger for deep-water environments that exclude the highly resistive air layer from the computational domain. A study in a scenario where simulations are required at multiple source locations shows that the BLR solver can become competitive in comparison to iterative solvers as an engine for 3-D controlled-source electromagnetic Gauss–Newton inversion that requires forward modelling for a few thousand right-hand sides.

Efficient simulation of marine controlled source electromagnetic responses for axisymmetric scatter by using numerical mode matching approach

Horizontal disk, sphere, and spherical crown are a very important type of scatter in geophysics research. In the marine environment, a disk-like scatter can be used to describe several resistive targets, e.g., basaltic sills and stratigraphic hydrocarbon reservoirs while spherical crown can be used to approximately depict the topography of interface for basement rock. This type of scatter has characteristics of axisymmetrical distribution of the conductivity. If some approaches can be established to efficiently simulate the marine controlled source electromagnetic (MCSEM) response to this scatter, it will be meaningful to investigate the nature of MCSEM responses in complex formation and to build appropriate method of processing and explaining MCSEM data. In this paper, the resistive scatters are approximated by one or several horizontal concentric disks with different radii and thickness values, based on the axially symmetrical spatial distribution of conductivity. Then, a combination of these concentric disks with air, sea water and surrounding beds will construct a horizontally stratified inhomogeneous formation with common axis-center, whose spatial distribution of conductivity is layered in the vertical direction and axisymmetric in the horizontal direction. Based on the approximations mentioned above, the computation of MCSEM response excited by horizontal electrical dipole (HED) located at the z-axis is entirely transformed into two axially symmetrical problems for the Fourier harmonic components of the electromagnetic (EM) fields. The differential operators about the horizontal magnetic components and transformation of horizontal electrical components and other EM components from horizontal magnetic components are derived. Then, the numerical mode matching approach is extended to the simulation of the EM field and three-dimensional (3D) MCSEM responses excited by the HED in the formation. The procedure for solving the EM field is presented. The semi-analytic solution of EM field in the whole space is obtained to efficiently and numerically model MCSEM response in the complex formation. Finally, the efficiency and accuracy of the present method are demonstrated numerically. The characteristics of 3D MCSEM responses in three different cases are further investigated.

The deep-tow marine controlled-source electromagnetic transmitter system for gas hydrate exploration

The Marine Controlled-Source Electromagnetic (MCSEM) method has been recognized as an important and effective tool to detect electrically resistive structures, such as oil, gas, and gas hydrate. The MCSEM performance is strongly influenced by the transmitter system design. We have developed a deep-tow MCSEM transmitter system. In this paper, some new technical details will be present. A 10,000 m optical–electrical composite cable is used to support high power transmission and fast data transfer; a new clock unit is designed to keep the synchronization between transmitter and receivers, and mark the time stamp into the transmission current full waveform; a data link is established to monitor the real-time altitude of the tail unit; an online insulation measuring instrument is adopted to monitor current leakage from high voltage transformer; a neutrally buoyant dipole antenna of copper cable and flexible electrodes are created to transmit the large power current into seawater; a new design method for the transmitter, which is called “real-time control technology of hardware parallelism”, is described to achieve inverting and recording high-power current waveform, controlling functions, and collecting auxiliary information. We use a gas hydrate exploration test to verify the performance of the transmitter system, focusing on more technical details, rather than applications. The test shows that the transmitter can be used for gas hydrate exploration as an effective source.

On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity

In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the subseafloor conductivity that is assumed to be isotropic. The deep water (ocean layer electrically much thicker than the overburden) seafloor EM response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a half-space, or a stronger and faster response. For an ocean whose electrical thickness is comparable to or much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. These transitions can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire subseafloor resistivity structure with the sea surface. A stronger and faster response occurs when guided energy flow is dominant, while a weaker and slower response occurs when the air interaction is dominant. However, at intermediate offsets for some models, the air interaction can partially or fully reverse the direction of energy flux in the reservoir layer toward rather than away from the source, resulting in a stronger and slower response. The Fréchet derivatives are dominated by preferential sensitivity to the reservoir layer conductivity for all water depths except at high frequencies, but also display a shift with offset from the galvanic to the inductive mode in the underburden and overburden due to the interplay of guided energy flow and the air interaction. This means that the sensitivity to the horizontal conductivity is almost as strong as to the vertical component in the shallow parts of the subsurface, and in fact is stronger than the vertical sensitivity deeper down. However, the sensitivity to horizontal conductivity is still weak compared to the vertical component within thin resistive regions. The horizontal sensitivity is gradually decreased when the water becomes deep. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.

Joint inversion for transmitter navigation and seafloor resistivity for frequency-domain marine CSEM data

We present a joint inversion method for the transmitter navigation and the seafloor resistivity for frequency domain marine controlled-source electromagnetic (CSEM) data. The inversion approach is based on the modified BFGS scheme, which has an advantage that one can update the Hessian matrix by using the BFGS scheme rather than computing the Hessian matrix itself during the inversion process. The partial derivatives of the electromagnetic field responses with respect to both the seafloor resistivity and the transmitter navigation parameters including the azimuth, dip and horizontal positions of the transmitter antenna are analytically calculated. We invert for both the navigation parameters of the towed dipole source (including antenna azimuth, dip, and horizontal positions) and seafloor resistivity by using the whole range of data instead of the near-field data (usually source-receiver offset <1km). An eigenparameter analysis shows that seafloor resistivities and transmitter navigation parameters can be independently resolved, and a better reconstruction can be obtained with multiple frequency data. The inversions of both the synthetical and field data sets indicate that our inversion method can simultaneously reconstruct seafloor resistivity structures and transmitter navigation parameters.

Comparison of marine controlled-source electromagnetic data acquisition systems by a reservoir sensitivity index: analyzing the effect of water depths

During the past ten years, a marine controlled source electromagnetic (CSEM) method has been developed rapidly as a technology for hydrocarbon exploration. For shallow water environments, two CSEM data acquisition systems: Seabed Logging (SBL) and towed streamer electromagnetics (TSEM) have been developed in recent years. The purpose is to compare the performance of the SBL and TSEM systems at different water depths. Three different methods for the comparison are presented. The first method is a quick one dimensional sensitivity modelling. As a result, the sensitivity of marine CSEM data increases with water depth for the SBL system. Further, the sensitivity decreases with the increasing water depth for the TSEM system. The two other methods use two dimensional synthetic data from a simple 2-D isotropic model. The second method is a reservoir sensitivity index (RSI) method which has been developed to provide a quick comparison of the two systems. The RSI is calculated as the amplitude of the scattered field dividing by data uncertainty. From the calculations, it is found that with the increasing water depth RSI increases for the SBL system, while it decreases for the TSEM system. The third method uses Occam’s inversion, and applies an anomaly transverse resistance (ATR) ratio for evaluating the resulting resistivity image. In shallow water environments, the resolution of the CSEM inversion results is good for both the SBL and TSEM systems. In deep water environments, the resolution of the CSEM inversion is better for the SBL system than for the TSEM system. The ATR ratios of the resistivity images show the similar conclusion. The SBL data acquisition system has an advantage in deep water environments. The TSEM system, on the other hand, is preferable for the shallow water environments.

Enhancing the detectability of a high-resistivity target by using a synthetic aperture source for 3D marine CSEM modelling of a rugged seafloor

When processing marine controlled-source electromagnetic (CSEM) data from a rugged seafloor, enhancing the reservoir response and suppressing the topographic effect and other interference are significant issues, especially in shallow water. We simulated the CSEM responses specific to these issues using an efficient finite-difference (FD) code. The synthetic aperture technique was applied to steer the EM field toward a high-resistivity target on the seafloor. A weighted 2D synthetic aperture source was constructed by imposing a real weighting factor on each source point. Numerical experiments showed that using the weighted 2D synthetic aperture source significantly enhanced the effective CSEM signals. Because of the destructive interference between bathymetric distortion and the airwave effect in shallow water, the synthetic aperture technique is useful for dealing with seafloor topography. Better results can be obtained before steering the distorted response with a bathymetric correction. However, the detectability results may exhibit a huge difference in numerical values if the background resistivity of the bathymetric model is estimated incorrectly.

A hybrid finite-difference and integral-equation method for modeling and inversion of marine controlled-source electromagnetic data

One of the major problems in the modeling and inversion of marine controlled-source electromagnetic (CSEM) data is related to the need for accurate representation of very complex geoelectrical models typical for marine environment. At the same time, the corresponding forward-modeling algorithms should be powerful and fast enough to be suitable for repeated use in hundreds of iterations of the inversion and for multiple transmitter/receiver positions. To this end, we have developed a novel 3D modeling and inversion approach, which combines the advantages of the finite-difference (FD) and integral-equation (IE) methods. In the framework of this approach, we have solved Maxwell’s equations for anomalous electric fields using the FD approximation on a staggered grid. Once the unknown electric fields in the computation domain of the FD method are computed, the electric and magnetic fields at the receivers are calculated using the IE method with the corresponding Green’s tensor for the background conductivity model. This approach makes it possible to compute the fields at the receivers accurately without the need of very fine FD discretization in the vicinity of the receivers and sources and without the need for numerical differentiation and interpolation. We have also developed an algorithm for 3D inversion based on the hybrid FD-IE method. In the case of the marine CSEM problem with multiple transmitters and receivers, the forward modeling and the Fréchet derivative calculations are very time consuming and require using large memory to store the intermediate results. To overcome those problems, we have applied the moving sensitivity domain approach to our inversion. A case study for the 3D inversion of towed streamer EM data collected by PGS over the Troll field in the North Sea demonstrated the effectiveness of the developed hybrid method.

3‐D INVERSION OF FREQUENCY‐DOMAIN CSEM DATA BASED ON GAUSS‐NEWTON OPTIMIZATION

AbstractQuantitative interpretation of large‐scale controlled‐source electromagnetic (CSEM) data in frequency domain requires efficient and stable 3D forward modeling and inversion codes. In this work, we present an efficient approach to 3D inversion of CSEM data, which is based on Gauss‐Newton (GN) optimization in combination with a direct solver for the forward modeling. In order to avoid computing and storing sensitivity matrix explicitly, a preconditioned conjugate gradient solver (PCG) is used to solve the system of the normal equations resulted from linearization at each GN iteration. This scheme only requires matrix‐vector products of Jocabian and its transpose with vectors, which are equivalent to one forward and one adjoint problem. Therefore the matrix factorization obtained when solving forward problem can be used in subsequent PCG process, which dramatically speeds up PCG iterations and reduces overall computational cost. Numerical experiments on synthetic data from land and marine CSEM surveying configurations show that our inversion scheme exhibits excellent convergence rate and only ten‐odd to tens of iterations are needed to reach desired data misfit, demonstrating its efficiency and stability.

Use of wavelets in marine controlled source electromagnetic method for geophysical modeling

Wavelets gained significant attention in science and engineering disciplines due to their remarkable data analyzing capabilities. Wavelets are also used for finding numerical solution of various engineering and geophysical problems. Marine controlled source electromagnetic (CSEM) method is a relatively new geophysical technique used to explore petroleum reserves from shallow and deep water. This work attempts to use periodic and non-periodic wavelets in CSEM for modeling of geophysical problem. This is results in a solution with multi-scale approximations using multiresolution analysis (MRA) algorithm. In this work, Daubechies wavelets are used, as the basis functions for solving the wave equation of marine CSEM, because of their orthonormality and compact support. Accuracy of the proposed method for marine CSEM is determined by comparing the results with the analytical solution for isotropic problem as well as those obtained using finite element (FE) method. Numerical results show that the governing equations for aforementioned geophysical method yield less obscure results than that of finite element method, for both bounded and unbounded domains. Furthermore, the proposed wavelet based numerical method produces multi-scale approximation that would prove useful for further analysis and data processing. © 2016 - IOS Press and the authors.

MARE2DEM: a 2-D inversion code for controlled-source electromagnetic and magnetotelluric data

This work presents MARE2DEM, a freely available code for 2-D anisotropic inversion of magnetotelluric (MT) data and frequency-domain controlled-source electromagnetic (CSEM) data from onshore and offshore surveys. MARE2DEM parametrizes the inverse model using a grid of arbitrarily shaped polygons, where unstructured triangular or quadrilateral grids are typically used due to their ease of construction. Unstructured grids provide significantly more geometric flexibility and parameter efficiency than the structured rectangular grids commonly used by most other inversion codes. Transmitter and receiver components located on topographic slopes can be tilted parallel to the boundary so that the simulated electromagnetic fields accurately reproduce the real survey geometry. The forward solution is implemented with a goal-oriented adaptive finite-element method that automatically generates and refines unstructured triangular element grids that conform to the inversion parameter grid, ensuring accurate responses as the model conductivity changes. This dual-grid approach is significantly more efficient than the conventional use of a single grid for both the forward and inverse meshes since the more detailed finite-element meshes required for accurate responses do not increase the memory requirements of the inverse problem. Forward solutions are computed in parallel with a highly efficient scaling by partitioning the data into smaller independent modeling tasks consisting of subsets of the input frequencies, transmitters and receivers. Non-linear inversion is carried out with a new Occam inversion approach that requires fewer forward calls. Dense matrix operations are optimized for memory and parallel scalability using the ScaLAPACK parallel library. Free parameters can be bounded using a new non-linear transformation that leaves the transformed parameters nearly the same as the original parameters within the bounds, thereby reducing non-linear smoothing effects. Data balancing normalization weights for the joint inversion of two or more data sets encourages the inversion to fit each data type equally well. A synthetic joint inversion of marine CSEM and MT data illustrates the algorithm's performance and parallel scaling on up to 480 processing cores. CSEM inversion of data from the Middle America Trench offshore Nicaragua demonstrates a real world application. The source code and MATLAB interface tools are freely available at http://mare2dem.ucsd.edu.

A comparative study of Hankel transform filters for marine controlled-source electromagnetic surveys

Using a closed-form solution of the Hankel transform in transverse magnetic mode for a homogeneous half-space model, we can assess the accuracy of digital linear filters for evaluating the Hankel transform. In this paper, we conduct comparative performance tests on the linear filter with known integral transforms. We examine three kinds of filters developed by W. L. Anderson, F. N. Kong and H. Mizunaga, which are known to be suitable for marine controlled-source electromagnetic (CSEM) applications. Kong’s filters perform best in the three kinds of filters over a practical range of offset distances in marine CSEM surveys. While the relative error versus distance appears as a V-shaped curve in semi-log scale, Mizunaga’s filters are the shortest in length and have a performance comparable to Kong’s filters. Anderson’s filters have a quite similar performance between the J0 and J1 filters, although these are somewhat inferior to Kong’s and Mizunaga’s filters when computing marine CSEM fields.

Effects of uncertainties in the position and orientation of both the transmitter and receivers on marine controlled-source electromagnetic data

Simulation and interpretation of marine controlled-source electromagnetic (CSEM) data often approximate the transmitter source as an ideal horizontal electric dipole (HED) and assume that the receivers are located on a flat seabed. Actually, however, the transmitter dipole source will be rotated, tilted and deviated from the survey profile due to ocean currents. And free-fall receivers may be also rotated to some arbitrary horizontal orientation and located on sloping seafloor. In this paper, we investigate the effects of uncertainties in the transmitter tilt, transmitter rotation and transmitter deviation from the survey profile as well as in the receiver’s location and orientation on marine CSEM data. The model study shows that the uncertainties of all position and orientation parameters of both the transmitter and receivers can propagate into observed data uncertainties, but to a different extent. In interpreting marine data, field data uncertainties caused by the position and orientation uncertainties of both the transmitter and receivers need to be taken into account.

Frequency-domain inversion of marine CSEM data in one-dimensional vertically anisotropic structures

The marine controlled-source electromagnetic (CSEM) method has proven to be an important addition to seismic imaging techniques in exploration of offshore hydrocarbon reservoirs and near-surface investigations. In inversion and interpretation of industrial CSEM data sets, this method can be used in a number of situations by assuming isotropy. However, the presence of electric anisotropy in the Earth's crust, due to thin layer interbedding or grain alignments in the sediments, can significantly alter the response measured by the EM receivers. Ignoring anisotropy in interpreting marine CSEM data may lead to distorted images of seabed conductivity structures, even misinterpretation.#br#In this paper, we present an inversion method for frequency domain marine controlled-source electromagnetic data generated by a titled dipole source in vertically anisotropic stratified media. This approach is based on the Gauss-Newton scheme. We extend and generalize the formulation of marine CSEM fields to calculation of the electromagnetic fields excited by arbitrarily oriented dipole sources. The partial derivatives of the electromagnetic fields with respect to both the horizontal and vertical resistivity are analytically calculated. Based on the relationship between the horizontal resistivity (ρh) and the vertical resistivity (ρv) of the inversion model, an adaptive selection method for regularization factors is proposed to balance the effects of the data misfit and the structural constraint.#br#The synthetic and real data inversion tests indicate that our inversion method can reconstruct the anisotropic resistivity of the overburden layer and the basement, and the burial depth, the thickness and the vertical resistivity of the reservoir layer can be well recovered. A better reconstruction can be obtained with multiple frequency and multiple component data sets. Combing inline and broadside geometry data sets can provide high resolution in reconstructing the burial depth of the reservoir layer and resistivity of the anisotropic basement.

Integrated Interpretation and Simultaneous Joint Inversion of 3D Marine CSEM and Seismic Datasets

One of the most complex seismic challenges is the imaging of thick salt bodies, the detection of their base and flanks, and imaging underlying units. To achieve good seismic imaging, the complementary use of non-seismic methods is one of the recommended solutions. Electromagnetic (EM) methods, such as magnetotellurics (MT) and controlled source electromagnetics (CSEM) are sensitive to the presence of salt bodies thanks to the high resistivity contrast with respect to sedimentary units. We present an integrated workflow applied to re-image wide azimuth (WAZ) seismic data acquired by Schlumberger using EM data acquired by EMGS over 35 blocks in the Keathley Canyon, in the Gulf of Mexico to reduce risk in exploration decisions and improve seismic deliverables. Seismic and EM data are utilized first in a cooperative workflow through localized seismic imaging reverse time migration (LSI RTM) to validate new salt structures highlighted by the single domain 3D anisotropic CSEM and MT inversions. They are then fed into a simultaneous joint inversion (SJI) to update a multi-property earth model (velocity and resistivity) by jointly minimizing the CSEM data misfit, the seismic residual move-outs and a relationship between the two properties.