Controlled-source electromagnetic sounding in shallow water: Principles and applications

Geophysics ◽  
2008 ◽  
Vol 73 (1) ◽  
pp. F21-F32 ◽  
Author(s):  
David Andréis ◽  
Lucy MacGregor
Keyword(s):  
Shallow Water ◽  
Frequency Domain ◽  
Time Domain ◽  
Electromagnetic Sounding ◽  
Analytical Analysis ◽  
Controlled Source ◽  
The Earth ◽  
Higher Dimensional ◽  
Marine Csem ◽  
Water Depths

The marine controlled-source electromagnetic (CSEM) method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently, its use was confined to deepwater (water depths greater than approximately [Formula: see text]) because of the interaction of signals with the atmosphere in shallower water depths. The purpose of this study was to investigate, using a simple 1D analytical analysis, the physics of CSEM in shallow water. This approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency-domain or time-domain methods. Stepping away from wavelike approaches, which if applied without care can be misleading for the diffusive fields of CSEM, we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the EM signal into modes and using only the mode least affected by interaction with the atmosphere. Such decomposition is straightforward in a 1D earth, and we demonstrate that the approach remains valid in higher dimensional structures. We also show that the coupling between signals diffusing through the earth and those that have interacted with air can be used to our advantage in the interpretation of marine CSEM data.

Mating Analysis and a Typical Floatover Design

2012 ◽  
Author(s):  
Alan M. Wang ◽  
Ruhua Yuan ◽  
Shaohua Zhu ◽  
Min He ◽  
Ju Fan ◽  
...  
Keyword(s):  
Diffraction Analysis ◽  
Shallow Water ◽  
Frequency Domain ◽  
Time Domain ◽  
Bohai Bay ◽  
Hydrodynamic Properties ◽  
Nonlinear Time Domain ◽  
Offshore Operations

This paper presents a typical floatover design in the shallow water and benign environment of Bohai Bay, China and the major floatover installation devices, as well as the nonlinear time-domain mating analysis. The nonlinear mating simulations are performed using SIMO based on the hydrodynamic properties of the floatover barge, obtained by WADAM, from the linear diffraction analysis in frequency domain. The mating analysis yields numerical findings in selecting and designing floatover devices critical to the success of the floatover operations, thus minimizing any potential operation risks and enabling the offshore operations as smoothly and efficiently as possible.

Large-scale Gauss-Newton inversion of transient controlled-source electromagnetic measurement data using the model reduction framework

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. E161-E171 ◽  
Author(s):  
M. Zaslavsky ◽  
V. Druskin ◽  
A. Abubakar ◽  
T. Habashy ◽  
V. Simoncini
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Large Scale ◽  
Jacobian Matrix ◽  
Computational Cost ◽  
Measurement Data ◽  
Multiple Sources ◽  
Time Data ◽  
Controlled Source ◽  
The Time Domain

Transient data controlled-source electromagnetic measurements are usually interpreted via extracting few frequencies and solving the corresponding inverse frequency-domain problem. Coarse frequency sampling may result in loss of information and affect the quality of interpretation; however, refined sampling increases computational cost. Fitting data directly in the time domain has similar drawbacks, i.e., its large computational cost, in particular, when the Gauss-Newton (GN) algorithm is used for the misfit minimization. That cost is mainly comprised of the multiple solutions of the forward problem and linear algebraic operations using the Jacobian matrix for calculating the GN step. For large-scale 2.5D and 3D problems with multiple sources and receivers, the corresponding cost grows enormously for inversion algorithms using conventional finite-difference time-domain (FDTD) algorithms. A fast 3D forward solver based on the rational Krylov subspace (RKS) reduction algorithm using an optimal subspace selection was proposed earlier to partially mitigate this problem. We applied the same approach to reduce the size of the time-domain Jacobian matrix. The reduced-order model (ROM) is obtained by projecting a discretized large-scale Maxwell system onto an RKS with optimized poles. The RKS expansion replaces the time discretization for forward and inverse problems; however, for the same or better accuracy, its subspace dimension is much smaller than the number of time steps of the conventional FDTD. The crucial new development of this work is the space-time data compression of the ROM forward operator and decomposition of the ROM’s time-domain Jacobian matrix via chain rule, as a product of time- and space-dependent terms, thus effectively decoupling the discretizations in the time and parameter spaces. The developed technique can be equivalently applied to finely sampled frequency-domain data. We tested our approach using synthetic 2.5D examples of hydrocarbon reservoirs in the marine environment.

Megasource time‐domain electromagnetic sounding methods

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 993-1009 ◽  
Author(s):  
George V. Keller ◽  
James I. Pritchard ◽  
Jimmy Joe Jacobson ◽  
Norman Harthill
Keyword(s):  
Time Domain ◽  
Vertical Component ◽  
High Amplitude ◽  
Colorado School ◽  
Electromagnetic Sounding ◽  
The Earth ◽  
Field Versus ◽  
Time Domain Electromagnetic ◽  
Em Field ◽  
Depth Of Investigation

The Colorado School of Mines time‐domain electromagnetic (EM) sounding system makes use of a grounded length of cable powered with high‐amplitude current square waves to generate an EM field for probing the earth. The vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation. With a source moment of a million ampere meters or greater, offset distances of several tens of kilometers can be achieved easily, providing depths of investigation of up to 10 km. The recorded induction field versus time curves are routinely interpreted by comparison with computer‐generated theoretical curves for a layered earth. Megasource EM surveys have been carried out at The Geysers in northern California and near Yakima in central Washington, providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km.

Fictitious wave domain modelling and analysis of marine CSEM data

2019 ◽  
Vol 219 (1) ◽  
pp. 223-238
Author(s):  
Jie Lu ◽  
Yuguo Li ◽  
Zhijun Du
Keyword(s):  
Boundary Condition ◽  
Wave Propagation ◽  
Frequency Domain ◽  
Time Domain ◽  
Sea Water ◽  
High Order ◽  
Water Model ◽  
Complex Frequency ◽  
Novel Approach ◽  
Marine Csem

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.

Improved interpolation scheme at receiver positions for 2.5D frequency-domain marine CSEM forward modelling

2020 ◽  
Author(s):  
Gang Li ◽  
Shuangmin Duan ◽  
Hongzhu Cai ◽  
Bo Han ◽  
Yixin Ye
Keyword(s):  
Frequency Domain ◽  
Forward Modeling ◽  
Computational Domain ◽  
Forward Modelling ◽  
Interpolation Scheme ◽  
Controlled Source ◽  
Numerical Tests ◽  
Marine Csem ◽  
Sediment Layers ◽  
Seafloor Sediment

<p>We present an improved interpolation scheme for 2.5D marine controlled-source electromagnetic (CSEM) forward modeling problem. As the resistivity contrast between the seawater and seafloor sediment layers is significant, it is usually difficult to compute the EM fields accurately at receivers which are usually located at the seafloor. In this study, a new interpolation scheme at receivers is proposed, in which the interpolation of EM fields at the cell nodes for the whole computational domain to arbitrary receiver locations is discussed in detail. Numerical tests indicate that, our improved interpolation is more accurate for simulating the EM responses at receivers located on the seafloor, compared with the linear or rigorous interpolation.</p>

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

2016 ◽  
Vol 208 (2) ◽  
pp. 1026-1042 ◽  
Author(s):  
Alan D. Chave ◽  
Mark E. Everett ◽  
Johan Mattsson ◽  
James Boon ◽  
Jonathan Midgley
Keyword(s):  
Shallow Water ◽  
Frequency Domain ◽  
Controlled Source ◽  
Controlled Source Electromagnetics

Solution of large-scale 3D controlled-source electromagnetic modeling problem using efficient iterative solvers

Geophysics ◽  
2021 ◽  
pp. 1-64
Author(s):  
Changkai Qiu ◽  
Changchun Yin ◽  
Yunhe Liu ◽  
Xiuyan Ren ◽  
Hui Chen ◽  
...  
Keyword(s):  
Linear System ◽  
Linear Equation ◽  
Frequency Domain ◽  
Time Domain ◽  
Large Scale ◽  
Equation System ◽  
Iterative Solvers ◽  
Controlled Source ◽  
Linear Equation System ◽  
The Time Domain

With geophysical surveys evolving from traditional 2D to 3D models, the large volume of data adds challenges to inversion, especially when aiming to resolve complex 3D structures. An iterative forward solver for a controlled-source electromagnetic method (CSEM) requires less memory than that for a direct solver; however, it is not easy to iteratively solve an ill-conditioned linear system of equations arising from finite-element discretization of Maxwell’s equations. To solve this problem, we have developed efficient and robust iterative solvers for frequency- and time-domain CSEM modeling problems. For the frequency-domain problem, we first transform the linear system into its equivalent real-number format, and then introduce an optimal block-diagonal preconditioner. Because the condition number of the preconditioned linear equation system has an upper bound of √2, we can achieve fast solution convergence when applying a flexible generalized minimum residual solver. Applying the block preconditioner further results in solving two smaller linear systems with the same coefficient matrix. For the time-domain problem, we first discretize the partial differential equation for the electric field in time using an unconditionally stable backward Euler scheme. We then solve the resulting linear equation system iteratively at each time step. After the spatial discretization in the frequency domain, or space-time discretization in the time domain, we exploit the conjugate-gradient solver with auxiliary-space preconditioners derived from the Hiptmair-Xu decomposition to solve these real linear systems. Finally, we check the efficiency and effectiveness of our iterative methods by simulating complex CSEM models. The most significant advantage of our approach is that the iterative solvers that we adopt have almost the same accuracy and robustness as direct solvers but require much less memory, rendering them more suitable for large-scale 3D CSEM forward modeling and inversion.

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