Three-Dimensional Controlled-Source Electromagnetic Modeling Using Octree-Based Spectral Element Method

It is well known that no single geophysical method can provide a completepicture of the earth and its properties and recently integrated interpretationand joint inversion of multiple data types has become a much studied topic(Chen et al, 2007; Harris et al, 2009; Jegen et al, 2009; de Stephano et al,2011). The seismic method is in many situations the tool of choice: it isgeneral and widely applicable, and can provide detailed images of sub-surfacestructure and stratigraphy from which complex geological models can beconstructed. If seismic methods can provide the answer to the question ofinterest then this is undoubtedly the tool to use. However although seismicdata are extremely sensitive to the changes in lithology occurring at theboundaries between geological units, they are less sensitive to fluid changeswithin these units. This is because acoustic and elastic properties of theearth show only small changes when the fluid content or saturation is changed. These changes can in some circumstances be detected and used to provideinformation on fluid distribution. In other situations this is difficult orimpossible to do with certainty, and complementary geophysical methods must beemployed to meet the reservoir characterization goal. In many situations electrical resistivity is driven by the properties anddistribution of fluids in the earth. Commercial hydrocarbon deposits may bemany times more resistive than surrounding lithologies. This change inresistivity caused by variations in fluid content and saturation can, inprinciple, be detected using CSEM tools (See Constable & Srnka 2007 for areview of CSEM technology). However when only CSEM data are considered, structural resolution is poor because of the diffusive nature of the EM fields, and the results can be ambiguous because the effect of an increase in porefluid resistivity cannot be distinguished from the effect of a decrease inporosity. The presence of frustrating resistors in the section (for exampletight carbonates, cemented sandstones or volcanics) can also complicate theinterpretation. Well-log data provide a range of measurements, including both resistivity andacoustic/elastic properties as well as a range of further properties. Apetrophysicist analyzing this well log data will take all of these measurementsand integrate them together to provide an interpretation of the lithology andfluid properties. The resistivity measurement in particular provides keyinformation on the fluid content at the well bore. However such informationcannot provide any constraint on the variation of properties away from thewell, across a reservoir. For any given geophysical question, the most robust answer will be obtained byusing the tool, or combination of tools best suited to the task, andintegrating the resulting data within a rock physics framework, to provide ashared earth model that is geologically reasonable, and consistent with each ofthe geophysical data types available. Careful integration of multiple datatypes can allow the strengths in one method to compensate for the weaknesses inanother. Here we consider three data types: seismic, controlled source electromagnetic(CSEM), and well log data. The workflow applied is shown in figure 1. CSEM dataare first inverted to provide a measure of resistivity. Because of the poorstructural resolution of CSEM data taken in isolation, this inversion isconditioned with seismic structural information. From the results, thetransverse resistance (the vertically integrated resistivity), which is wellconstrained by the CSEM method, can be calculated within the intervals ofinterest.

Abstract. The non-hydrostatic (NH) compressible Euler equations for dry atmosphere were solved in a simplified two-dimensional (2-D) slice framework employing a spectral element method (SEM) for the horizontal discretization and a finite difference method (FDM) for the vertical discretization. By using horizontal SEM, which decomposes the physical domain into smaller pieces with a small communication stencil, a high level of scalability can be achieved. By using vertical FDM, an easy method for coupling the dynamics and existing physics packages can be provided. The SEM uses high-order nodal basis functions associated with Lagrange polynomials based on Gauss–Lobatto–Legendre (GLL) quadrature points. The FDM employs a third-order upwind-biased scheme for the vertical flux terms and a centered finite difference scheme for the vertical derivative and integral terms. For temporal integration, a time-split, third-order Runge–Kutta (RK3) integration technique was applied. The Euler equations that were used here are in flux form based on the hydrostatic pressure vertical coordinate. The equations are the same as those used in the Weather Research and Forecasting (WRF) model, but a hybrid sigma–pressure vertical coordinate was implemented in this model. We validated the model by conducting the widely used standard tests: linear hydrostatic mountain wave, tracer advection, and gravity wave over the Schär-type mountain, as well as density current, inertia–gravity wave, and rising thermal bubble. The results from these tests demonstrated that the model using the horizontal SEM and the vertical FDM is accurate and robust provided sufficient diffusion is applied. The results with various horizontal resolutions also showed convergence of second-order accuracy due to the accuracy of the time integration scheme and that of the vertical direction, although high-order basis functions were used in the horizontal. By using the 2-D slice model, we effectively showed that the combined spatial discretization method of the spectral element and finite difference methods in the horizontal and vertical directions, respectively, offers a viable method for development of an NH dynamical core.

This paper discusses spectral and spectral element methods with Legendre-Gauss-Lobatto nodal basis for general 2nd-order elliptic eigenvalue problems. The special work of this paper is as follows. (1) We prove a priori and a posteriori error estimates for spectral and spectral element methods. (2) We compare between spectral methods, spectral element methods, finite element methods and their derivedp-version,h-version, andhp-version methods from accuracy, degree of freedom, and stability and verify that spectral methods and spectral element methods are highly efficient computational methods.

An extensive marine controlled source electromagnetic (CSEM) data set was collected in the Gulf of Mexico targeting gas hydrate deposits at four geologically distinct areas within water depths that varied from 900 to 3000 m. The Joint Industry Project 2 drilled two of these locations, Green Canyon 955 and Walker Ridge 313, providing ground truth for the CSEM data. A third survey location, Mississippi Canyon 118 (MC118), is a designated hydrate observatory. We collected both traditional ocean bottom electromagnetic receiver data as well as continuous profiling data using a novel towed three-axis electric field receiver. The towed receiver measured data at a fixed offset of 300 m behind the transmitter, while the ocean bottom receivers recorded data over a range of offsets as the transmitter passed by each receiver. We inverted the CSEM data using a newly developed parallel goal-oriented adaptive finite element-modeling algorithm for efficient 2.5D imaging of the CSEM data. 2D inversions from MC118 are consistent with 1D apparent resistivity pseudosections and reveal resistive areas associated with the carbonate/hydrate mound. An extensive conductive region exists below this, associated with conductive brines sourced from a deeper more resistive salt body. Away from the mound the resistivity structure is a fairly uniform and homogeneous. We present new 2D inversions of the CSEM data that consider electrical anisotropy and constrained inversions that leverage structural control using key seismic horizons along the profiles.

In August 2008, a marine Controlled Source ElectroMagnetic (CSEM) survey was acquired by WesternGeco EM for the previous operator (Encana) of Lady Franklin and Atammik blocks offshore west Greenland. The objective of the project was to rank possible hydrocarbon prospects in the upper Cretaceous section. Therefore, the survey was designed to optimize CSEM responses based on the previous operator's interpretation within a limited budget, resulting in a pseudo-3D survey, sub-optimal to assess actual interpretation updated according to the results of the three exploration wells drilled by Cairn in the block during 2011. Independent 3D inversions of the CSEM data run before the drilling campaign suggest a consistent background resistivity over the different prospects. Drilling results confirmed the resistivity distribution as recovered by 3D CSEM inversions within the data resolution limit. After the drilling campaign, a sensitivity study was carried out in order to assess the limit at which the geological problem can be tuned by the electromagnetic data response. This showed that the resistive anomalies visible on some of the prospects in the block cannot be assessed for hydrocarbon saturation on the basis of electromagnetic data alone as they are below the resolution of the method given the non-uniqueness of the results. The present paper highlights how non-seismic data, such as electromagnetic, can provide additional information which, integrated with other geophysical and geological data, can help exploration during the different phases. The pre-drilling analysis of the resistivity distribution as recovered by 3D inversion of the data identified resistive anomalies that were interpreted as hydrocarbon indicators. The drilling results identified the geological meaning of these anomalies, which were further analyzed to include and extrapolate the new post-drilling interpretation in order to evaluate the non-drilled prospects. The sensitivity study was critical to understand the EM data limitations in this geological context before investing in a new project. Introduction Electromagnetic data are widely applied in the oil industry in order to de-risk exploration, reducing the uncertainty related to reservoir saturation (Hesthammer et al., 2012). Many success stories increased the confidence in electromagnetic data as hydrocarbon detectors: in fact, electromagnetic data provide information exclusively regarding the resistivity distribution of the underlying earth. Resistive anomalies may be related to fluid saturation, but they may also be produced by different geological features not necessarily related to fluid saturation: volcanics, carbonates, and evaporites are only some examples of lithologies characterized by high resistivity values. In addition, inversion of electromagnetic data is a non-unique problem and therefore constraining the inversion with the available geological and geophysical information is critical in order to produce a geologically meaningful model and subsequent interpretations. The limited data and frequency range (~ 0.1 to 10 Hz), the diffusive physics of CSEM, and the necessity (usually) of using a smooth stabilizer for inversion result in low resolution.

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.

A scaled experiment for the verification of the SeaBed Logging method

Subsurface resistivity mapping based on Controlled Source Electromagnetic (CSEM) measurements is an attractive technology for exploration as it offers the possibility to distinguish between hydrocarbon and brine bearing prospects where conventional seismic methods prove inconclusive. In Shell, we have applied the CSEM method on a worldwide scale since 2003 to both de-risking and portfolio polarization. Early on in the development of the CSEM technique some compelling results were obtained with single 2D profiles over prospects. Unfortunately, the lack of subsurface coverage of this type of acquisition often leads to ambiguous results because the Earth rarely satisfies the 2D assumption at the scale of the CSEM experiment. To reduce such ambiguities, we have focused efforts on the development of 3D processing and inversion capabilities as well as interpretation workflows that take the complexity of the Earth into account. In this paper, we share some of our motivations behind our approach and illustrate its effectiveness with both real and synthetic data examples.

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.

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.

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.

The whole subject of three-dimensional (3-D) electromagnetic (EM) modelling and inversion has experienced a tremendous progress in the last decade. Accordingly there is an increased need for reviewing the recent, and not so recent, achievements in the field. In the first part of this review paper I consider the finite-difference, finite-element and integral equation approaches that are presently applied for the rigorous numerical solution of fully 3-D EM forward problems. I mention the merits and drawbacks of these approaches, and focus on the most essential aspects of numerical implementations, such as preconditioning and solving the resulting systems of linear equations. I refer to some of the most advanced, state-of-the-art, solvers that are today available for such important geophysical applications as induction logging, airborne and controlled-source EM, magnetotellurics, and global induction studies. Then, in the second part of the paper, I review some of the methods that are commonly used to solve 3-D EM inverse problems and analyse current implementations of the methods available. In particular, I also address the important aspects of nonlinear Newton-type optimisation techniques and computation of gradients and sensitivities associated with these problems.

Time/wave domain analysis for axially moving pre-stressed nanobeam by wavelet-based spectral element method

Chapter 14 - Electromagnetic Methods in the Frequency and Time Domains

Spectral element method in time for rapidly actuated systems