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Imaging of 3D electromagnetic data at low-induction numbers (2012)

Pérez-Flores, Marco A. ; Antonio-Carpio, Ricardo G. ; Gómez-Treviño, E. ; Ferguson, Ian ; Méndez-Delgado, S.

Society of Exploration Geophysicists (SEG)

In: Geophysics 77: WB47-WB57.

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Publication Date: 2012-07-01

Description: We expressed electromagnetic measurements at low induction numbers as spatial averages of the subsurface electrical conductivity distribution and developed an algorithm for the recovery of the latter in terms of the former. The basis of our approach is an integral equation whose averaging kernel is independent of the conductivity distribution. That is, the recovery of conductivity from the measurements leads to a linear inverse problem. Previous work in one and two dimensions demonstrated that using a kernel independent of conductivity leads to reasonably good results in quantitative interpretations. This study extended the approach to 3D models and to data taken along several profiles over a given area. The algorithm handles vertical and horizontal magnetic dipoles with multiple separations for appropriate depth discrimination. The approximation also handles issues like negative conductivity measurements, which commonly appear when crossing near-surface conductors. This happens particularly when using vertical magnetic dipoles; whose averaging kernel has significant negative weights in the space between the dipoles, something that does not happen for the horizontal dipoles. In general, the more complex the kernel, the more complicated the signature of any given anomaly. This makes qualitative interpretations of pseudosections somewhat difficult when dealing with more than one conductive or resistive body. The algorithm was validated using synthetic data for imaging data from horizontal or vertical coils or from a combination of them. Imaging of field data from a mine tailings site recovered a shallow 3D conductive anomaly associated with the tailings.

Print ISSN: 0016-8033

Electronic ISSN: 1942-2156

Topics: Geosciences , Physics

Published by Society of Exploration Geophysicists (SEG)

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Splitting marine controlled-source electromagnetic responses into sea and subseafloor contributions: Grounding the air wave (2017)

Calderon-Moctezuma, A., Gomez-Trevino, E., Gallardo, L. A.

Society of Exploration Geophysicists (SEG)

In: Geophysics

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Publication Date: 2017-09-19

Description: We have found how the effects of the air wave in marine controlled-source electromagnetic (CSEM) methods gradually vanish in the sea for shallow waters, and how at the same time they gradually grow below the seafloor, in an effort to comprehend existing detectability definitions. The transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea. We consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs, particularly in shallow-water explorations and, specifically, on how the air wave affects detection. Our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1D profile. Two-dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets. Results include the expected growth of the inhibiting effect of the sea as the water depths decrease. However, we also find that this happens up to a point and that from then on its effect decreases to zero. Regarding the resistive layer at depth, its importance grows to a constant as the water depth decreases to zero. As a function of offsets, there appear first the direct current effects. The induction zone is next and is dominated by contributions from the underlying formations. The third zone, which corresponds to the air wave, is largely dominated by contributions from the sea. The fourth and last zone is the plane-wave asymptote. All four classical zones identified in marine CSEM are also present in land CSEM.

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Electronic ISSN: 1942-2156

Topics: Geosciences , Physics

Published by Society of Exploration Geophysicists (SEG)

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Stable 2D magnetotelluric strikes and impedances via the phase tensor and the quadratic equation (2017)

Muniz, Y., Gomez-Trevino, E., Esparza, F. J., Cuellar, M.

Society of Exploration Geophysicists (SEG)

In: Geophysics

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Publication Date: 2017-05-31

Description: A combination of the magnetotelluric phase tensor and the quadratic algorithm provides a fast and simple solution to the problem of a 2D impedance tensor distorted by 3D electrogalvanic effects. The strike direction is provided by the phase tensor, which is known to provide unstable estimates for noisy data. We obtain stable directions in three steps. First, we use bootstrapping to find the most stable estimate among the different periods. Second, this value is used as the seed to select the neighbor strikes assuming continuity over periods. This second step is repeated several times to compute variances. The third step, which we call prerotating, consists of rotating the original impedance tensor to a most favorable angle for optimal stability and then rotating it back for compensation. The procedure is developed as a progressing algorithm through its application to the gradually more difficult data sets COPROD2S1, COPROD2, far-hi, and BC87, all available for testing new ideas. Alternately, using the Groom-Bailey terminology, the quadratic algorithm provides the amplitudes and phases independently of the strike direction and twist. The amplitudes and phases still need to be tuned up by the correct shear. The correct shear is obtained by contrasting the phases from the phase tensor and from the quadratic equation until they match for all available periods. The results are the undistorted impedances. Uncertainties are computed using formulas derived for the quadratic equation. We use the same data sets as for the strike to illustrate the recovery of impedances and their uncertainties.

Print ISSN: 0016-8033

Electronic ISSN: 1942-2156

Topics: Geosciences , Physics

Published by Society of Exploration Geophysicists (SEG)

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