Time‐domain versus frequency‐domain CSEM in shallow water

Andréis, David; MacGregor, Lucy

doi:10.1190/1.2792500

Cited by 6 publications

(6 citation statements)

References 7 publications

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“…By contrast, the phase and magnitude of H s zr (not shown here), corresponding to a pure Transverse-Electric to z mode (TE z ) [28,30], fails to yield significant responsiveness to the resistive formation even when the water depth is increased to 500m. These results qualitatively corroborate prior studies indicating that the sea-air interface can significantly dampen instrument sensitivity to deeply buried hydrocarbon reservoirs [6,36]. However, the sensitivity reduction effect is strongly dependent on the field type (electric versus magnetic) and component (x, y, z), with the dampening effect much more pronounced in measurements derived from the TE z modes as compared to the TM z modes [6,36].…”

Section: Case Study: Marine Hydrocarbon Explorationsupporting

confidence: 90%

Spectral-domain-based scattering analysis of fields radiated by distributed sources in planar-stratified environments with arbitrarily anisotropic layers

Sainath

Teixeira

2014

Phys. Rev. E

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We discuss the numerically stable, spectral-domain computation and extraction of the scattered electromagnetic field excited by distributed sources embedded in planar-layered environments, where each layer may exhibit arbitrary and independent electrical and magnetic anisotropic response and loss profiles. This stands in contrast to many standard spectral-domain algorithms that are restricted to computing the fields radiated by Hertzian dipole sources in planar-layered environments where the media possess azimuthal-symmetric material tensors (i.e., isotropic, and certain classes of uniaxial, media). Although computing the scattered field, particularly when due to distributed sources, appears (from the analytical perspective, at least) relatively straightforward, different procedures within the computation chain, if not treated carefully, are inherently susceptible to numerical instabilities and (or) accuracy limitations due to the potential manifestation of numerically overflown and (or) numerically unbalanced terms entering the chain. Therefore, primary emphasis herein is given to effecting these tasks in a numerically stable and robust manner for all ranges of physical parameters. After discussing the causes behind, and means to mitigate, these sources of numerical instability, we validate the algorithm's performance against closed-form solutions. Finally, we validate and illustrate the applicability of the proposed algorithm in case studies concerning active remote sensing of marine hydrocarbon reserves embedded deep within lossy, planar-layered media.

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Section: Case Study: Marine Hydrocarbon Explorationsupporting

confidence: 90%

Spectral-domain-based scattering analysis of fields radiated by distributed sources in planar-stratified environments with arbitrarily anisotropic layers

Sainath

Teixeira

2014

Phys. Rev. E

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“…While current sources with a few frequencies are used in the deep marine environment, transient measurements are more common in the shallow marine environment and on land (e.g. Andréis & MacGregor 2007;Avdeeva et al 2007;Ziolkowski et al 2007). One of the main reasons is the dominance of the airwave in shallow marine and terrestrial measurements, which can be better separated in the time domain.…”

Section: Introductionmentioning

confidence: 99%

Fast Fourier transform of electromagnetic data for computationally expensive kernels

Werthmüller

Mulder

Slob

2021

Geophysical Journal International

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SUMMARY 3-D controlled-source electromagnetic data are often computed directly in the domain of interest, either in the frequency domain or in the time domain. Computing it in one domain and transforming it via a Fourier transform to the other domain is a viable alternative. It requires the evaluation of many responses in the computational domain if standard Fourier transforms are used. This can make it prohibitively expensive if the kernel is time-consuming as is the case in 3-D electromagnetic modelling. The speed of modelling obtained through such a transform is defined by three key points: solver, method and implementation of the Fourier transform, and gridding. The faster the solver, the faster modelling will be. It is important that the solver is robust over a wide range of values (frequencies or times). The method should require as few kernel evaluations as possible while remaining robust. As the frequency and time ranges span many orders of magnitude, the required values are ideally equally spaced on a logarithmic scale. The proposed fast method uses either the digital linear filter method or the logarithmic fast Fourier transform together with a careful selection of evaluation points and interpolation. In frequency-to-time domain tests this methodology requires typically 15–20 frequencies to cover a wide range of offsets. The gridding should be frequency- or time-dependent, which is accomplished by making it a function of skin depth. Optimizing for the least number of required cells should be combined with optimizing for computational speed. Looking carefully at these points resulted in much smaller computation times with speedup factors of ten or more over previous methods. A computation in one domain followed by transformation can therefore be an alternative to computation in the other domain domain if the required evaluation points and the corresponding grids are carefully chosen.

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“…However, we note that the comparison between FD‐CSEM and TD‐CSEM is included here only for completeness but should not be over‐interpreted due to the reasons explained in Appendix . For an in‐depth comparison between TD‐CSEM and FD‐CSEM, we refer to, for example Andreis and MacGregor (2007), Connell (2011), Connell and Key (2013), and Mörbe (2020).…”

Section: Introductionmentioning

confidence: 99%

Step‐on versus step‐off signals in time‐domain controlled source electromagnetic methods using a grounded electric dipole

Haroon

Swidinsky

Hölz

et al. 2020

Geophysical Prospecting

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The time-domain controlled source electromagnetic method is a geophysical prospecting tool applied to image the subsurface resistivity distribution on land and in the marine environment. In its most general setup , a square-wave current is fed into a grounded horizontal electric dipole, and several electric and magnetic field receivers at defined offsets to the imposed current measure the electromagnetic response of the Earth. In the marine environment, the application often uses only inline electric field receivers that, for a 50% duty-cycle current waveform, include both step-on and step-off signals. Here, forward and inverse 1D modelling is used to demonstrate limited sensitivity towards shallow resistive layers in the step-off electric field when transmitter and receivers are surrounded by conductive seawater. This observation is explained by a masking effect of the direct current signal that flows through the seawater and primarily affects step-off data. During a step-off measurement, this direct current is orders of magnitude larger than the inductive response at early and intermediate times, limiting the step-off sensitivity towards shallow resistive layers in the seafloor. Step-on data measure the resistive layer at times preceding the arrival of the direct current signal leading to higher sensitivity compared to step-off data. Such dichotomous behaviour between step-on and step-off data is less obvious in onshore experiments due to the lack of a strong overlying conductive zone and corresponding masking effect from direct current flow. Supported by synthetic 1D inversion studies, we conclude that time-domain controlled source electromagnetic measurements on land should apply both step-on and step-off data in a combined inversion approach to maximize signal-to-noise ratios and utilize the sensitivity characteristics of each signal. In an isotropic marine environment, step-off electric fields have inferior sensitivity towards shallow resistive layers compared to step-on data, resulting in an increase of non-uniqueness when interpreting step-off data in a single or combined inversion.

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Time‐domain versus frequency‐domain CSEM in shallow water

Cited by 6 publications

References 7 publications

Spectral-domain-based scattering analysis of fields radiated by distributed sources in planar-stratified environments with arbitrarily anisotropic layers

Spectral-domain-based scattering analysis of fields radiated by distributed sources in planar-stratified environments with arbitrarily anisotropic layers

Fast Fourier transform of electromagnetic data for computationally expensive kernels

Step‐on versus step‐off signals in time‐domain controlled source electromagnetic methods using a grounded electric dipole

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