Janniche Iren Nordskag’s research while affiliated with Statoil ASA and other places

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Publications (12)


Comparing large-scale 3D Gauss–Newton and BFGS CSEM inversions
  • Conference Paper

September 2016

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35 Reads

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20 Citations

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Janniche Iren Nordskag

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Electromagnetic signals are exponentially attenuated in conductive media. Thus, marine controlled-source electromagnetic (CSEM) data where the source and the receivers are located in the water column has exponentially low sensitivity towards the deep stratigraphy, compared to the shallow stratigraphy. In addition, CSEM inversions are also highly non-linear and ill-posed. It is therefore often difficult to achieve good inversion results for the deeper part of the subsurface using gradient based inversion methods. In this abstract, we describe a large-scale 3-dimensional anisotropic Gauss-Newton (3DGN) CSEM inversion implementation and discuss its advantages over gradient based algorithms. We also show, by synthetic and real data case studies, the large improvements in the 3DGN inversion results compared to those from the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm. Presentation Date: Tuesday, October 18, 2016 Start Time: 3:45:00 PM Location: 174 Presentation Type: ORAL


Inversion of inline and broadside marine controlled-source electromagnetic data with constraints derived from seismic data

August 2015

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72 Reads

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18 Citations

Geophysical Prospecting

We present a structural smoothing regularization scheme in the context of inversion of marine controlled-source electromagnetic data. The regularizing hypothesis is that the electrical parameters have a structure similar to that of the elastic parameters observed from seismic data. The regularization is split into three steps. First, we ensure that our inversion grid conforms with the geometry derived from seismic. Second, we use a seismic stratigraphic attribute to define a spatially varying regularization strength. Third, we use an indexing strategy on the inversion grid to define smoothing along the seismic geometry. Enforcing such regularization in the inversion will encourage an inversion result that is more intuitive for the interpreter to deal with. However, the interpreter should also be aware of the bias introduced by using seismic data for regularization. We illustrate the method using one synthetic example and one field data example. The results show how the regularization works and that it clearly enforces the structure derived from seismic data. From the field data example we find that the inversion result improves when the structural smoothing regularization is employed. Including the broadside data improves the inversion results even more, due to a better balancing between the sensitivities for the horizontal and vertical resistivities.


Model Compression by Grid Shaking for Nonlinear Electromagnetic Inversions

June 2015

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38 Reads

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1 Citation

CSEM inversion for large 3D models is computationally very intensive, making the use of advanced inversion algorithms like Gauss-Newton particularly challenging. To overcome this problem model compression techniques can be applied. By decreasing the number of parameters they reduce both memory needs and computing time at the cost of a decreased resolution. We propose a new model compression approach where the compression operator is slightly changed at each iteration. Like many compression techniques it uses a coarse grid to reduce the number of parameters, but random lateral and vertical shifts are applied to this grid at the beginning of every iteration. Compared to usual model compression with a static compression operator, the new method presents exactly the same benefits in terms of computational efficiency but it preserves resolution much better. We have illustrated this approach with Gauss-Newton inversion of synthetic 2D CSEM data. It shows that a significantly higher degree of model compression than with static approaches can be used: good inversion results are obtained with 132 times less parameters than the number of nodes in the modeling grid. This approach will help making Gauss-Newton algorithms applicable to 3D inversion of larger 3D CSEM data sets.


Figure 2: Vertical resistivity section through the Skrugard well (left panel) and the 7219/9-1 well (right panel) of the 3D CSEM inversion result overlain on depth migrated seismic. 
Figure 3: Posterior hydrocarbon probability from PCube through the Skrugard well (left panel) and the 7219/9-1 well (right panel). The bright yellow colour shows high probability for hydrocarbon saturated sand. Water surface multiples contaminate the seismic above the solid black line invalidating the PCube results here. 
CSEM exploration in the Barents Sea: Joint CSEM & seismic interpretation
  • Conference Paper
  • Full-text available

September 2013

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223 Reads

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2 Citations

Download



Elimination of the water‐layer response from multi‐component source and receiver marine electromagnetic data

December 2008

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22 Reads

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5 Citations

Geophysical Prospecting

This paper presents the theory to eliminate from the recorded multi-component source, multi-component receiver marine electromagnetic measurements the effect of the physical source radiation pattern and the scattering response of the water-layer. The multi-component sources are assumed to be orthogonally aligned above the receivers at the seabottom. Other than the position of the sources, no source characteristics are required. The integral equation method, which for short is denoted by Lorentz water-layer elimination, follows from Lorentz' reciprocity theorem. It requires information only of the electromagnetic parameters at the receiver level to decompose the electromagnetic measurements into upgoing and downgoing constituents. Lorentz water-layer elimination replaces the water layer with a homogeneous half-space with properties equal to those of the sea-bed. The source is redatumed to the receiver depth. When the subsurface is arbitrary anisotropic but horizontally layered, the Lorentz water-layer elimination scheme greatly simplifies and can be implemented as deterministic multi-component source, multi-component receiver multidimensional deconvolution of common source gathers. The Lorentz deconvolved data can be further decomposed into scattering responses that would be recorded from idealized transverse electric and transverse magnetic mode sources and receivers. This combined electromagnetic field decomposition on the source and receiver side gives data equivalent to data from a hypothetical survey with the water-layer absent, with idealized single component transverse electric and transverse magnetic mode sources and idealized single component transverse electric and transverse magnetic mode receivers. When the subsurface is isotropic or transverse isotropic and horizontally layered, the Lorentz deconvolution decouples into pure transverse electric and transverse magnetic mode data processing problems, where a scalar field formulation of the multidimensional Lorentz deconvolution is sufficient. In this case single-component source data are sufficient to eliminate the water-layer effect. We demonstrate the Lorentz deconvolution by using numerically modeled data over a simple isotropic layered model illustrating controlled-source electromagnetic hydrocarbon exploration. In shallow water there is a decrease in controlled-source electromagnetic sensitivity to thin resistors at depth. The Lorentz deconvolution scheme is designed to overcome this effect by eliminating the water-layer scattering, including the field's interaction with air.


Asymptotic airwave modeling for controlled source electromagnetic surveying

November 2007

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47 Reads

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46 Citations

Geophysics

Marine controlled-source electromagnetic (CSEM) surveying, also called seabed logging, is a technique for finding resistive layers in the subsurface, which can be hydrocarbon-saturated reservoirs. The EM signal from the horizontal electric dipole source induces an airwave along the air/sea surface that interferes with the signal from the subsurface and dominates at larger offsets. The airwave commonly denotes the energy that propagates from the source via the atmosphere to the receiver on the seabed. The airwave component is especially problematic in shallow waters, where it is less attenuated during its up-and-down propagation in the water column than for deeper waters. An asymptotically derived space-domain equation that describes the airwave component in a water half-space bounded by air is well known from the EM literature. We demonstrate that by taking into account the reflections and reverberations of the airwave component in the water column between the seabed and the sea surface at both the source side and the receiver side, this equation can be generalized to a water layer. The new model we present can serve as an integrity check for other modeling schemes. Our airwave model shows how the airwave component depends on water depth, source-receiver range, frequency, and seabed resistivity. The model is verified by extensive numerical testing. Finally, we show how the source-induced airwave may affect the marine CSEM recordings and how the airwave component in principle can be suppressed by a modeling-and-subtraction approach.


Cross-property relations between electrical conductivity and the seismic velocity of rocks

September 2007

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678 Reads

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156 Citations

Geophysics

Cross-property relations are useful when some rock properties can be measured more easily than other properties. Relations between electrical conductivity and seismic velocity, stiffness moduli, and density can be obtained by expressing the porosity in terms of those properties. There are many possible ways to combine the constitutive equations to obtain a relation, each one representing a given type of rock. The relations depend on the assumptions to obtain the constitutive equations. In the electromagnetic case, the equations involve Archie's law and its modifications for a conducting frame, the Hashin-Shtrikman (HS) bounds, and the self-similar and complex refraction-index method (CRIM) models. In the elastic case, the stress-strain relations are mainly based on the time-average equation, the HS bounds, and the Gassmann equation. Also, expressions for dry rocks and for anisotropic media, using Backus averaging, are analyzed. The relations are applied to a shale saturated with brine (overburden) and to a sandstone saturated with oil (reservoir). Tests with sections of a North Sea well log show that the best fit is given by the relation between the Gassmann velocity and the CRIM, selfsimilar, and Archie models for the conductivity.



Citations (7)


... The study by Shelkovnikov and Budimir (2004) [5] theoretically defined the phase corresponding of directed transverse magnetic (TM) and transverse electric (TE) modes with multi-layer waveguide completed from isotropic nonconductor materials. The sensitivity of TM mode to hydrocarbon saturated reservoirs has been reported in [6]; the study also proved the sensitivity of TM mode to the geological features that relate to local bigger scale resistive constructions. However, the study failed to prove the sensitivity of TE mode to hydrocarbon saturated reservoirs despite its sensitivity to any local larger scale resistive structures. ...

Reference:

Generate high data rate of optical carries by using nanomaterial graphene in slab waveguide
Decomposition of Marine Electromagnetic Fields Into TE and TM Modes for Enhanced Interpretation
  • Citing Conference Paper
  • January 2007

... We also evaluate two optimization algorithms: the steepest descent (using a simple line search strategy) and a momentum method [20], parameterized according to the Adam algorithm [21], known as a low memory demand method. We also compared the results with those obtained using L-BFGS-B [22,23], which is traditionally used in minimization problems. Our numerical experiments show that both algorithms successfully fit the observations and correctly localize the inversion targets, but our momentum approach has low storage demand. ...

Comparing large-scale 3D Gauss–Newton and BFGS CSEM inversions
  • Citing Conference Paper
  • September 2016

... For more detailed imaging, petrophysical constraints using Gaussian mixture models (Giraud et al., 2019c), as well as structural constraints (Giraud et al., 2019d;Martin et al., 2020) and multiple interval bound constraints (Ogarko et al., 2021a), can be used depending on the requirements of the study and the information available. In the case of single physics inversion with structural constraints, structural similarity between a selected reference model and the inverted models can be maximised using structural constraints based on cross-gradients (Gallardo and Meju, 2003) and locally weighted gradients in the same philosophy as Brown et al. (2012), Wiik et al. (2015), Yan et al. (2017), and Giraud et al. (2019d). Generally speaking, in the joint inversion case, the two models inverted for are linked using the structural constraints just mentioned or petrophysical clustering constraints in the same spirit as Carter-McAuslan et al. (2015), Kamm et al. (2015), Li (2015, 2017), Zhang and Revil (2015), and Bijani et al. (2017). ...

Inversion of inline and broadside marine controlled-source electromagnetic data with constraints derived from seismic data
  • Citing Article
  • August 2015

Geophysical Prospecting

... Alexander [5] described theoretically the perfect phase matching of guided transverse electric (TE) and transverse magnetic (TM) modes with multi-layer waveguide that was made from isotropic dielectric materials. In 2007, [6] proved that TM mode is sensitive to hydrocarbon saturated reservoirs and to geological features relating to local larger scale resistive structures; while TE mode is not sensitive to hydrocarbon saturated reservoirs, but it is sensitive to any local larger scale resistive structures. Moreover, the airwave predominantly is a TE mode. ...

Decomposition of Marine Electromagnetic Fields Into TE and TM Modes for Enhanced Interpretation

... The airwaves traveling through the air are mainly refracted energy from the air-seawater interface (e.g., Hunziker et al. 2011;Mittet and Morten 2013). There are some solutions to the airwave elimination, mostly based on 1D analysis (Nordskag and Amundsen 2007;Løseth et al. 2010;Chen and Alumbaugh 2011;Mittet and Morten 2013;Chave et al. 2017). ...

Asymptotic airwave modeling for controlled source electromagnetic surveying
  • Citing Article
  • November 2007

Geophysics

... Late 2000s, coworkers (2009, 2010a-d) made the understanding that U/D decon works well when the seabed is flat. Holvik and Amundsen (2005) extended U/D decon to Betti decon for elastic data and Nordskag et al. (2009) similarly to Lorentz decon for electromagnetic data. Other significant work was published by Ten Kroode (2007) (1) OBN data with sea-surface present into those data that would be recorded with sea-surface absent and constant-velocity water layer speed, (2a) source array (air gun and/or vibrator) to point source (e.g., monopole), and (2b) physical source wavelet to desired source wavelet. ...

Elimination of the water‐layer response from multi‐component source and receiver marine electromagnetic data
  • Citing Article
  • December 2008

Geophysical Prospecting

... Electrical resistivity (or conductivity) exhibits distinct sensitivity to other petrophysical parameters. When combined with seismic data, it becomes possible to accurately estimate petrophysical parameters together with geophysical parameters such as velocity and resistivity (Carcione et al. 2007;Jones et al. 2009;Guerra et al. 2013;Miotti et al. 2014). ...

Cross-property relations between electrical conductivity and the seismic velocity of rocks

Geophysics