Bathymetric effects and corrections in marine CSEM data

This paper presents an anisotropic 3D inversion algorithm for marine controlled-source electromagnetic (CSEM) data with the assumption that electrical anisotropy is represented by the vertical and horizontal resistivities. For such anisotropic media, the horizontal and vertical electric fields are related to the horizontal and vertical resistivities, respectively. In addition, the horizontal electric field is insensitive to thin, horizontal resistors associated with hydrocarbon reservoirs. Thus, the horizontal electric field contributes mainly to defining the horizontal resistivity of the background media, but not to resolving thin horizontal resistors. As a result, the resolution of marine CSEM data to reservoirs is lower in anisotropic media than for the isotropic case, unless some possible relation between the two resistivities is accounted for in anisotropic inversion. Synthetic examples show that imposing the equality of the two resistivities is important in stabilizing the inversion process and improving the resolution of the reservoir. It is also shown that the reservoir at a depth of 3 km below the seafloor can be well delineated from three-frequencies (0.1, 0.3, and 0.5 Hz) data in deep water (1 km deep), while additional lower frequency (0.05 Hz) data are required to detect the same reservoir in shallow water (100 m deep).

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.

We present a new method to estimate receiver orientation angle from electric and magnetic data measured during a controlled source electromagnetic (CSEM) survey. By combining field responses at a receiver for pairs of equidistant source-receiver offsets,

We consider a simple one dimensional marine gas sand model: a reservoir sand layer is buried in an encasing shale host. Using well established petrophysical relationships such as Archie’s law and Gassman’s equations, we create corresponding geophysical models of resistivity, density, P-wave velocity and S-wave velocity. From these models we calculate synthetic frequency domain marine controlled-source electromagnetic (CSEM) data and amplitude vs. angle (AVA) traces to examine a petrophysically coupled method for joint inversion. We then analyze the resolution properties of a joint inversion problem where not only the thickness of the gas layer, the gas saturation and the sandstone porosity are unknown, but several rock physics parameters are as well. Results show that in certain cases, gas saturation and thickness can be independently resolved with a joint inversion. However, the sandstone porosity may be intercorrelated with the bulk and shear moduli of the sand. This result is one of many different outcomes and shall examine a variety of rock physics and reservoir models in our talk.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 16879, “Mapping Hydrocarbons Integrating Seismic With Electromagnetic and Gravity Attributes: Application in the Barents Sea,” by Paolo Dell’Aversana, Stefano Colombo, Barbara Ciurlo, Johan Leutscher, and Jan Seldal, Eni, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Electromagnetic attributes have been used extensively for interpreting marine controlled-source electromagnetic (CSEM) data. One of the basic attributes is called NAR (normalized-anomalous-amplitude response). Combining NAR and CSEM asymmetry represents a relatively simple way to use the benefits of both attributes jointly. This integrated approach, based on seismic data combined with electromagnetic and gravity attributes, has been applied in a complex exploration area in the Barents Sea. Introduction Inversion of geophysical data measured at surface represents a quantitative approach for estimating physical properties of hydrocarbon reservoirs and obtaining information about background geology (faults, structural trends, and lateral geological variations). This approach generally is based on the minimization of an objective function including the misfit between observed and predicted responses plus a regularization function. Inversion methods generally require long computation times and extensive use of parallel-computing resources. Moreover, they can produce nonunique solutions. A different but complementary approach for interpreting geophysical observations is based on the analysis of quantities, often called attributes, extracted from measurements by simple mathematical manipulation. Examples of electromagnetic attributes recently applied in hydrocarbon exploration are the gradient of CSEM magnitude vs. offset and the symmetry properties of the CSEM response itself. It has been demonstrated that CSEM attributes allow, at least in relatively simple scenarios, accurate and fast identification of reservoir boundaries. Also, in the gravity domain, attributes can work efficiently for rapid identification of important general features, such as basement trends, presence of fault systems, depth range of density anomalies, and lateral extension of salt domes and basalts. Attribute analysis cannot be considered a substitute of inversion, especially in complex geological settings, but instead should be considered as a complementary approach. In fact, in complicated scenarios, attributes can be used as a quick and efficient tool for preliminary identification of the main features of the model (resistivity and density boundaries, presence of faults, and approximate depth range of the target). This preliminary model can be used as a starting point for optimized inversion. In this paper, the authors show how several types of electromagnetic and gravity attributes can cooperate with seismic information for rapid delineation of the main geological trends in complex exploration areas. Moreover, the authors show how these attributes also can be complementary for identification of hydrocarbon reservoirs and for restricting the range of possible interpretive models. The authors use both synthetic tests and real examples for supporting their integrated method, which is based on combined seismic, electromagnetic, and gravity information.

We present a 3D forward modeling algorithm of marine controlled-source electromagnetic (CSEM) fields using finite element method based on unstructured meshes. As the unstructured mesh is incorporated, the algorithm can easily accommodate complex structures, such as dipping interfaces and rough topography. Secondary coupled-potential formulation of Maxwell's equations allows to avoid the singularities introduced by the sources. The primary electromagnetic fields excited by electric dipole source in layered isotropic media are derived from the semi-analytical formulas based on Schelkunoff potentials. To improve the accuracy and efficiency of the algorithm, we propose an effective local mesh refinement strategy, where the elements near the receiver locations and in the abnormal areas are refined using the volume constraints. We validate the finite element code against a 1D and 2D reservoir model and study the performance of the local mesh refinement strategy. The Canonical disc model illustrates the utility of the finite element method for 3D CSEM modeling. The bathymetry model study shows that the seafloor topography gives an important response which needs to be reproduced by numerical modeling to avoid the misinterpretation of measurements.

Michael S. Zhdanov, Evgeny P. Velikhov, Martin Čuma, Glenn Wilson, Noel Black and Alexander Gribenko argue that the key to deriving a reliable quantitative interpretationfrom marine controlled-source electromagnetic (CSEM) data is through the integration of shared earth modelling and robust 3D CSEM inversion. An iterative migration method for CSEM data is presented that is equivalent to rigorous inversion, illustrated with a case study of the Shtokman gas field in the Barents Sea.

Summary In this paper, we present a joint inversion approach of the transmitter navigation and the seafloor resistivity for the marine controlled-source electromagnetic (CSEM) data. The approach utilizes the modified BFGS method and the minimum gradient support penalty. An eigenparameter statistical analysis shows that the seafloor resistivity structure and the transmitter navigation parameters (including transmitter azimuth, dip, and horizontal positions) can be independently resolved. The synthetic inversion tests demonstrate that the joint inversion method is able to reconstruct the seafloor resistivity and the orientation and horizontal positions of transmitter antennas.

Offshore energy-transition activities typically require the construction of new subsurface facilities where overburden heterogeneity and deformation patterns may not be fully understood. Anisotropic marine controlled-source electromagnetic (CSEM) and seismic refraction imaging can be adapted to geohazard assessments of the overburden to mitigate safety and cost risks in such areas. This is investigated using an area in deepwater northwest Borneo as the case study. For this proof-of-concept study, a well-established multiphysics joint inversion code is extended to handle marine seismic refraction data from conventional towed streamers. Realistic field-scale synthetic models with anomalous resistivities and velocities in the shallow section (approximately 400 m below mudline) are built and their CSEM and seismic refraction responses are first inverted separately and then jointly in three dimensions using the cross-gradient method. The joint inversion recovers the anomalous bodies better than the separate inversions. The velocity model from the joint inversion indicates improvement from the separate inversion but requires a good starting model. As a further test of the methodology, first-break traveltimes are picked from legacy 3D seismic data and inverted with the available sparse CSEM data. For model verification, resistivity and velocity profiles from separate and joint inversion models are extracted at a well location in the survey area and compared with actual well logs. The joint inversion models match the well logs better than the separate inversion models. Moreover, known geohazards (complex fault sets and lateral inflows) from independent geomatics study are imaged in the joint inversion models. In addition, the resistivity anisotropy ratio tracks anomalous zones that correspond to areas of recent complex faulting and potential lateral inflows in the overburden mass transport systems. Thus, the joint inversion of CSEM and seismic refraction data has the potential to improve the mapping of overburden hazards offshore, with the associated resistivity anisotropy serving as a proxy for active ground deformation.

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.

In order to suppress the airwave noise in marine controlled-source electromagnetic (CSEM) data, we propose a 3D deconvolution (3DD) interferometry method with a synthetic aperture source and obtain the relative anomaly coefficient (RAC) of the EM field reflection responses to show the degree for suppressing the airwave. We analyze the potential of the proposed method for suppressing the airwave, and compare the proposed method with traditional methods in their effectiveness. A method to select synthetic source length is derived and the effect of the water depth on RAC is examined via numerical simulations. The results suggest that 3DD interferometry method with a synthetic source can effectively suppress the airwave and enhance the potential of marine CSEM to hydrocarbon exploration.

ABSTRACT: The marine controlled-source electromagnetic (CSEM) method provides complementary information to seismic imaging in the exploration of sedimentary basins. The CSEM is mainly used for reservoir scanning and appraisal. The CSEM interpretation workflow is heavily based on inversion and forward - modeling for hypothesis testing. Until the recent past, the effectiveness of a given workflow was achieved after the drilling results, as there wasn’t any geological complex model available to serve as a benchmark. In the present paper, we describe the workflow to build up Marlim R3D, a realistic and complex geoelectric model. Marlim R3D aims to be a reference model of turbidite reservoirs of the Brazilian continental margin. Our model is based on seismic interpretation and constrained by the input of available well-log information. The workflow used is composed of seven steps: seismic and well-log dataset loading, well-tie, Vp cube construction, Vp resistivity calibration, time-depth conversion, resistivity cube construction, and quality-control check. As a result, we obtained an interpreted dataset composed by main stratigraphic horizons, pseudo-well logs, and the resistivity cubes. These elements were made freely available for research or commercial use, under the Creative Common License, at the Zenodo platform.

The complex composition and structure of the submarine reservoir and its secondary pyrite, which are multiphase composite media, can provoke the induced polarization (IP) effect, resulting in the change of the marine controlled-source electromagnetic (CSEM) induction-polarization response, which directly affects geological interpretation results. In this paper, the generalized effective-medium theory of induced polarization (GEMTIP) model is introduced to study the influence of composition, structure and geometric characteristics of submarine reservoirs and secondary pyrite on the 3D marine CSEM induction-polarization response. We first construct the 3D finite-difference frequency-domain (FDFD) electromagnetic field governing equation based on the GEMTIP model, apply the emission source on the non-uniform grid, and solve the linear equations by using the difference coefficient matrix. Then we perform forward calculation on a typical 1D reservoir model to verify the correctness and effectiveness of the proposed algorithm. Finally, we design the reservoir and secondary pyrite models, and analyze the influence laws of IP parameters and polarization layer geometry parameters on the 3D marine CSEM induction-polarization response. These studies are of great value for understanding the relationship between submarine multiphase composite medium and electromagnetic wavepropagation.

Marine controlled-source electromagnetic (CSEM) method has proved to be a powerful tool and found its increasing applications for offshore resource exploration and tectonic studies. However, bathymetric variations and anisotropic structures encountered in practical measurements have posed challenges for reliable interpretation of marine CSEM data collected in increasingly complex geologic settings. In this study, we present a Finite element (FE) based forward algorithm for simulating 3-D marine CSEM responses in geologic settings characterized with rough bathymetry and electrical anisotropy. Unstructured tetrahedral meshes are employed to permit precise descriptions of arbitrary seafloor topography. After validating the accuracy of the algorithm, we investigate the effects of bathymetric variations and complicated electric anisotropy separately and jointly on marine CSEM responses. Numerical results have demonstrated that both the bathymetry and electrical anisotropy can cause significant distortions on marine CSEM responses, but to different extent. The effects of full electrical anisotropy and bathymetry have to be taken into consideration for reliable interpretations of marine CSEM data in practice.

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.