Wavelet optimized finite difference simulation for marine CSEM geophysical survey

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.

Marine controlled source electromagnetic (CSEM) data inversion for hydrocarbon exploration is often challenging due to high computational cost, physical memory requirement and low resolution of the obtained resistivity map. This paper aims to enhance both the speed and resolution of CSEM inversion by introducing structural geological information in the inversion algorithm. A coarse mesh is generated for Occam’s inversion, where the parameters are fewer than in the fine regular mesh. This sparse mesh is defined as a coherence-based irregular (IC) sparse mesh, which is based on vertices extracted from available geological information. Inversion results on synthetic data illustrate that the IC sparse mesh has a smaller inversion computational cost compared to the regular dense (RD) mesh. It also has a higher resolution than with a regular sparse (RS) mesh for the same number of estimated parameters. In order to study how the IC sparse mesh reduces the computational time, four different meshes are generated for Occam’s inversion. As a result, an IC sparse mesh can reduce the computational cost while it keeps the resolution as good as a fine regular mesh. The IC sparse mesh reduces the computational cost of the matrix operation for model updates. When the number of estimated parameters reduces to a limited value, the computational cost is independent of the number of parameters. For a testing model with two resistive layers, the inversion result using an IC sparse mesh has higher resolution in both horizontal and vertical directions. Overall, the model representing significant geological information in the IC mesh can improve the resolution of the resistivity models obtained from inversion of CSEM data.

Controlled source electromagnetic sounding (CSEM) is now becoming a mainstream exploration tool in a wide range of geological environments; and to date the technique has been primarily used on a standalone basis. It is recognised that by accurately integrating CSEM data with other geophysical data types, such as rock physics and seismic, that significant improvements in subsurface understanding can be gained throughout the E&P process.

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,

The Controlled-Source Electromagnetic (CSEM) method is one of the electromagnetic methods utilized in geophysical exploration. This method provides a subsurface image through the resistivity anomalies of materials encountered by electromagnetic waves. The research area is located near a major city, resulting in high electromagnetic noise. Electromagnetic noise can be categorized into two types of the noise namely periodic noise and sporadic noise. Eliminating noise is a crucial objective to enhance data quality, as it can introduce uncertainty into interpretations. Three noise removal techniques are employed: pre-stack to filter the harmonic noise, stacking to remove the sporadic noise, and post-stack for smoothing. The CSEM data used consists of signals in the time domain with a 10-second period and a 50% duty cycle. The results of applying these noise removal techniques indicate that all three methods are highly effective in noise reduction. The pre-stack technique can remove periodic noise, while sporadic noise is addressed by the stacking technique, and signal smoothing can be achieved using the poststack technique.

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.

Over the past years, Rocksource has studied and tested the application of Controlled Source Electromagnetic (CSEM) surveys for hydrocarbon detection in numerous shallow and deep water settings, mainly in the Norwegian continental shelf, the UK continental shelf, offshore West Africa, and offshore East India.

Summary Landslides are one of the most commonly occurring natural disasters. They claim hundreds of human lives and cost billions of dollars every year. In order to provide geophysical tools and techniques to better characterize sites prone to slide, we have been carrying out and evaluating potential utility of several geophysical surveys over a quick clay landslide site in southwest Sweden since 2011. The measurements include 2D and 3D P- and S-wave high resolution surface seismics, radio- and controlled-source electromagnetics, geoelectrics, ground gravity and magnetic surveys. A particular focus here is given to the seismic studies in the site. Combined with downhole geophysical and geotechnical measurements, we show that majority of reflections correlate well with sandy-silty formations in the site. Quick clays often occur above these formations, which may be an indication of the role of coarse-grained formations to not only partly form quick clays but also triggering them when pore-water pressure is significantly increased.

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.

Chapter 14 - Electromagnetic Methods in the Frequency and Time Domains

<p>This study introduces the remapping process of the state-of-the-art Numerical Weather Prediction (NWP) model, the Korean Integrated Model (KIM) system, developed by the Korea Institute of Atmospheric Prediction Systems (KIAPS). The KIM employs the quasi-uniform and pole singularity-free spherical geometry, a cubed-sphere grid, essentially requiring the grid transformation in the pre- and post-process. The remapping process of the KIM consists of two parts: (1) figuring out reference points from source grid points and weights at each reference point and (2) calculating the values at destination points by multiplying the values at reference points and appropriate weights. The KIM uses the linked-cell mapping algorithm to search the reference points around the destination point. The KIM provides various interpolation methods including the alternative-bilinear algorithm (Kim et al., 2019), which can obtain the weights from irregular grid points without assuming a rectangular shape. We are developing an advanced pre- and post-process that downscales cubed-sphere grids to a higher resolution using neural network techniques as an ongoing project. We also plan to share our results that improved the efficiency of the remapping process, which requires a massive resource compared to the simplicity of its calculation. For example, only the address and weights of reference grid points required for the interpolation are stored in a remap matrix (pruning) as an efficient storage managing strategy. Moreover, the advantage of parallelization, evaluated in the Korean 5th super-computer, will also be presented. Through parallelizing the remapping process, the speed improves up to about twelve times, and a more stable calculation speed is expected.</p>

The finite difference time domain (FDTD) method is considered a prominent approach in computational electromagnetics for the wave propagation analysis of closed geometries in time domain. However, since this uses a structured mesh, the staircase approximation at the curved boundaries is a computational challenge in FDTD. Recently, meshless methods based on radial point interpolation method (RPIM) on unstructured mesh have gained importance in electromagnetic simulations due to their capability of modeling arbitrary shaped geometries. In the RPIM, the field variable at a node is interpolated by the values at the surrounding nodes which fall in the support domain. After assembly, this results in a banded matrix of size N χ N, where N is the total number of mesh cells. The computational cost of solving this is high for large structures. On the other hand, FDTD is a simple method in which the field variables are directly calculated using update equations. So, the FDTD and Meshless RPIM methods can be combined to model structures with partial curved geometries, with meshless RPIM method at the curved regions and the Cartesian FDTD for the rest of the domain. To validate the method numerically, a WR-229 90 degree waveguide bend is modelled using meshless method at the curved region and FDTD at the straight waveguide regions. Gaussian radial basis functions are used for the interpolation and to generate the shape function matrices in the meshless region. Interface matching conditions for the electric and magnetic fields are matched across the interface boundaries. The external domains are truncated with 2nd order MUR absorbing boundary conditions. The results of the hybrid approach are compared with those from the regular FDTD, with the Meshless approaches and the improvement in the computational cost is reported.

Controlled-source electromagnetic (CSEM) surveys are moving increasingly toward realistic and complicated scenarios where the survey region may contain undulated topography, complex geometries, and electrical anisotropy media. In this paper, the adaptive finite element numerical method is employed to discrete the total electric field equation, which can provide precise electromagnetic (EM) responses even with a coarse initial mesh. The unstructured tetrahedral grid is employed to effectively address arbitrary irregular geometry and mountain terrain. Then, the flexible generalized minimum residual solver (FGMRES), auxiliary-space Maxwell pre-conditioner, and grid division technique were used to solve the large-scale linear system of equations, which can stably solve ill-conditioned problems using fewer computing resources. Finally, we conducted a numerical experiment via our newly proposed forward modeling scheme on a synthetic model with multi-contrast electrical anisotropy. It validated that accurate EM fields could be obtained against the semi-analytic solutions, and this iterative solver has good robustness for various anisotropic media. As a result, we have developed a state-of-the-art 3D CSEM anisotropic forward modeling engine, which can quickly and accurately deal with large-scale and complex geo-models.

Deterministic inversion of electromagnetic (EM) data yields a single best fitting resistivity model of the subsurface, which can be used to interpret the geological subsurface. Such a model fails to capture the uncertainty in both resistivity and geology. This limitation is critical, as multiple, geologically dissimilar subsurface configurations can yield equivalent EM responses, meaning a single model representation can be inaccurate or even misleading. Probabilistic inversion of the EM data provides a principled solution by characterizing the range of subsurface models consistent with data, and thereby explicitly quantifying uncertainty in both the geophysical and geological models.Here we invert by rejection sampling of pre-computed geophysical and geological 1D prior models. This allows for fast and efficient probabilistic inversion of large-scale EM surveys containing thousands of soundings. An added benefit of using pre-computed geological models is the possibility to encode geological expert knowledge into the models as direct information. In this context expert knowledge can be many things, for example the resistivity-lithology relationship, the chronological sequence of geological units, or the relative occurrence of various lithologies to name a few.In the presentation, we demonstrate how such a probabilistic inversion workflow can be set up and applied on towed transient EM data from geophysical surveys in varying geological settings. The required inputs are (i) a geophysical dataset consisting of EM soundings, and (ii) an expert-based assessment of the plausible geological subsurface architectures in the survey area. Optionally, geophysical and lithological well logging can be used to further constrain the inversion. We will highlight the tool/software (GeoPrior1D) we have developed to construct prior ensembles with encoded geological knowledge, especially suited for such a workflow. GeoPrior1D is an open-source tool for generating ensembles of one-dimensional geological and geophysical models that explicitly represent prior models for probabilistic inversion problems.Finally, we present key outcomes of the probabilistic modelling. This includes resistivity models with uncertainty, lithological models with uncertainty (entropy), class probabilities, and various themed maps. The produced models and maps, always accompanied by rigorously quantified uncertainties, enable better and more reliable decision-making across applications such as geohazard risk assessment, resource volume estimates, groundwater modelling, and much more.

A time domain (TD) solution for the analysis of multiple diffraction of spherical waves after an array of nonperfectly conducting wedges with different interior angles is presented.The proposed TD solution is based on the representation of the inverse Laplace transform of the corresponding frequency domain solution in closed form, as it is given by a hybrid of the uniform theory of diffraction (UTD)‐physic optics (PO) solution. The new formulation, validated with the results from technical literature, does not need to incorporate the TD version of the higher‐order diffraction coefficients and allows for the calculation of large number of wedges. It has the major advantage in terms of the mathematical complexity and computation efficiency when compared with other TD solutions. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1006–1008, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25094