Alan D. Chave

Woods Hole Oceanographic Institution

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R. L. Evans

Woods Hole Oceanographic Institution

Jean H. Filloux

Woods Hole Oceanographic Institution

Alan G. Jones

Heart of England NHS Foundation Trust

Douglas S. Luther

University of Hawaii System

Pascal Tarits

Centre National de la Recherche Scientifique

Charles S. Cox

The University of Texas Health Science Center at Houston

John A. Orcutt

University of California, San Diego

Kiyoshi Baba

The University of Tokyo

J. R. Booker

University of Washington

F. L. Vernon

University of California, San Diego

Cooperative Institutions

Scripps Institution of Oceanography

University of California, San Diego

Woods Hole Oceanographic Institution

University of Cambridge

Geological Survey of Canada

Los Alamos National Laboratory

The University of Tokyo

University of Washington

Electromagnetic Geoservices (Norway)

University of Hawaiʻi at Mānoa

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Energy flow in terrestrial controlled-source electromagnetic geophysics

European Journal of Physics (2019)

Mark E. Everett Alan D. Chave

Visualisations of the flow of electromagnetic energy based on the time-averaged Poynting vector have yielded important and sometimes counter-intuitive physical insights in the case of electric circuits containing resistors and inductors. Less well-understood is the flow of electromagnetic energy in spatially contiguous media excited by grounded sources. In geophysics, for example, it is important for readers to recognise how geological structures help shape controlled-source electromagnetic responses. It is demonstrated herein using energy flow visualisations how a resistive layer impeding vertical electric current flow will produce a larger anomalous response to grounded-source excitation at the Earth's surface than an equivalent conductive layer.

Energy Flow

10.1088/1361-6404/ab3de5

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Citations (2)

Toward a Cyberinfrastructure for the Ocean Observatories Initiative: Enabling Interactive Observation Within the Oceans

AGUFM (2008)

Alan D. Chave M. Arrott Michael Meisinger John A. Orcutt Ingolf Krueger

Cyberinfrastructure

Source

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On the electromagnetic fields induced by oceanic internal waves

Journal of Geophysical Research Atmospheres (1984)

Alan D. Chave

Model spectra for the electric and magnetic fields induced by oceanic internal waves are obtained by combining Green function solutions to the two electromagnetic modal equations with the Garrett‐Munk kinematic description of the internal wave field. The poloidal magnetic mode is dominant at frequencies above 3 f , where f is the local Coriolis frequency, and self and mutual induction are not important over this range. The toroidal magnetic mode is increasingly important at frequencies below 3 f and is sensitive to the conductivity structure below the seafloor for nearinertial frequencies. The moored electric field is shown to be largely a measure of the local velocity field at high frequencies. The vertical electric field is sensitive to the horizontal velocity field, while the horizontal electric field primarily reflects the vertical velocity field and is quite small at the seaearth and sea‐air interfaces. The magnetic field is a measure of the spatially averaged velocity field and is dominated by the poloidal magnetic mode. Electromagnetic boundary effects reduce the horizontal magnetic spectrum by decades at the seafloor and sea surface. At the seafloor and sea surface, internal wave‐induced magnetic fields are within an order of magnitude of their externally induced counterparts, while in the ocean's interior, internal waves are probably the largest source of magnetic signals in the period range one day to one hour. The internal wave‐induced electric field is not measurable except in the vertical component.

10.1029/jc089ic06p10519

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Citations (35)

Mantle dynamics beneath the East Pacific Rise at 17°S: Insights from the Mantle Electromagnetic and Tomography (MELT) experiment

Journal of Geophysical Research Atmospheres (2006)

Kiyoshi Baba Alan D. Chave R. L. Evans Greg Hirth Randall L. Mackie

The electromagnetic data from the Mantle Electromagnetic and Tomography (MELT) experiment are inverted for a two‐dimensional transversely anisotropic conductivity structure that incorporates a correction for three‐dimensional topographic effects on the magnetotelluric responses. The model space allows for different conductivity values in the along‐strike, cross‐strike, and vertical directions, along with imposed constraints of model smoothness and closeness among the three directions. Anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data. The preferred anisotropic model displays a resistive uppermost 60‐km‐thick mantle independent of plate age, except in the vicinity of the ridge crest. In most inversions, a vertically aligned sheet‐like conductor at the ridge crest is especially prominent in the vertical conductivity. Its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis. The melt zone is at least 100 km wide and is asymmetric, having a greater extent to the west. Off‐axis, and to the east of the ridge, the mantle is more conductive in the direction of plate spreading at depths greater than 60 km. The flat resistive‐conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite, and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations. This suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water (dissolved hydrogen). By contrast, the underlying mantle has retained a significant amount of water.

Magnetotellurics

Peridotite

10.1029/2004jb003598

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Citations (179)

Observations of motional electromagnetic fields during EMSLAB

Journal of Geophysical Research Atmospheres (1989)

Alan D. Chave Jean H. Filloux Douglas S. Luther Lawrie K. Law Antony White

The long‐period (>1 day) behavior of the seafloor electromagnetic fields during EMSLAB is considered in detail with an emphasis on interpretation in terms of oceanic motions. The study begins with a summary of the physics of motional electromagnetic induction, in which the seawater conductivity‐weighted, vertically integrated velocity measured by the horizontal electric field is emphasized. Using frequency‐domain methods, it is shown that seafloor and terrestrial magnetic variations have similar spectral shapes, indicating a common origin, but the seafloor electric field is not consistent with either at periods longer than 4 days, suggesting an oceanic source. The magnetic field variations are highly coherent across the EMSLAB array at periods shorter than 9 days, but the similarity decreases at longer periods, probably due to long‐term instrumental drift. The seafloor horizontal electric field data exhibit the broadband coherence characteristic of ionospheric sources only at periods shorter than 1–2 days and are essentially incoherent between 4 days and a week except in association with certain propagating wave phenomena. The distinction between the depth‐averaged velocity inferred from the horizontal electric field and its point measurement by the vertical electric field is demonstrated with data. Some specific properties of the seafloor electric fields are then considered in detail. The first of these is a 4‐day wave observed to propagate from north‐to‐south along the east flank of the Juan de Fuca Ridge. The wavelength, propagation sense, and association with the ridge are all consistent with topographically trapped Rossby wave behavior at a zero in the group velocity. A second north‐to‐south propagating wave is seen at periods of 8–12 days in the middle of the EMSLAB area with a probable wavelength of order 1000 km. Unresolved low‐frequency structures in the electric field are also mentioned. These observations clearly demonstrate the power of electromagnetic array methods for the study of long‐period oceanic behavior.

Seafloor Spreading

10.1029/jb094ib10p14153

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Citations (34)

Instrumentation to measure electromagnetic fields on continental shelves

R.A. Petitt Jean H. Filloux H.H. Moeller Alan D. Chave

The authors have constructed a set of instruments designed explicitly for shallow water electromagnetic measurements. These units record (with sixteen bit accuracy on EPROM memory cards) the vector horizontal electric field, the vector variations of the magnetic field, pressure, temperature and two components of tilt at a nominal sample rate of 2 Hz for a month. This could be extended to 10 Hz for short experiments as the sensors are easily capable of operation to this point.< >

Instrumentation

EPROM

10.1109/oceans.1993.325954

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Citations (4)

Project NEPTUNE: an interactive, regional cabled ocean observatory in the Northeast Pacific

Oceans 2003. Celebrating the Past ... Teaming Toward the Future (IEEE Cat. No.03CH37492) (2003)

Jennifer Delaney P. Beauchamp Marcia McNutt Chris Barnes Alan D. Chave

Next-generation studies of dynamic, interacting processes in the Earth-ocean-climate system require new interactive, in situ approaches to complement and fundamentally extend the more traditional ship-based expeditionary science that has dominated oceanographic research for more than 100 years. Key to these studies are regional cabled ocean observatories. A US/Canadian consortium is building NEPTUNE, a regional observatory in the northeast Pacific that will be spatially associated with the Juan de Fuca tectonic plate.

Neptune

10.1109/oceans.2003.178480

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Citations (11)

A comparison of techniques for magnetotelluric response function estimation

Journal of Geophysical Research Atmospheres (1989)

Alan G. Jones Alan D. Chave G. D. Egbert D R Auld Karsten Bahr

Spectral analysis of the time‐varying horizontal magnetic and electric field components yields the magnetotelluric (MT) impedance tensor. This frequency dependent 2×2 complex tensor can be examined for details which are diagnostic of the electrical conductivity distribution in the Earth within the relevant (frequency dependent) inductive scale length of the surface observation point. As such, precise and accurate determination of this tensor from the electromagnetic time series is fundamental to successful interpretation of the derived responses. In this paper, several analysis techniques are applied to the same data set from one of the EMSLAB Lincoln Line sites. Two subsets of the complete data set were selected, on the basis of geomagnetic activity, to test the methods in the presence of differing signal‐to‐noise ratios for varying signals and noises. Illustrated by this comparison are the effects of both statistical and bias errors on the estimates from the diverse methods. It is concluded that robust processing methods should become adopted for the analysis of MT data, and that whenever possible remote reference fields should be used to avoid bias due to uncorrelated noise contributions.

Magnetotellurics

SIGNAL (programming language)

Data set

Basis (linear algebra)

Data Processing

10.1029/jb094ib10p14201

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Citations (252)

A multitaper spectral estimator for time-series with missing data

Geophysical Journal International (2019)

Alan D. Chave

SUMMARY A multitaper estimator is proposed that accommodates time-series containing gaps without using any form of interpolation. In contrast with prior missing-data multitaper estimators that force standard Slepian sequences to be zero at gaps, the proposed missing-data Slepian sequences are defined only where data are present. The missing-data Slepian sequences are frequency independent, as are the eigenvalues that define the energy concentration within the resolution bandwidth, when the process bandwidth is $[ { - 1/2,\,\,\,1/2} )$ for unit sampling and the sampling scheme comprises integer multiples of unity. As a consequence, one need only compute the ensuing missing-data Slepian sequences for a given sampling scheme once, and then the spectrum at an arbitrary set of frequencies can be computed using them. It is also shown that the resulting missing-data multitaper estimator can incorporate all of the optimality features (i.e. adaptive-weighting, F-test and reshaping) of the standard multitaper estimator, and can be applied to bivariate or multivariate situations in similar ways. Performance of the missing-data multitaper estimator is illustrated using length of day, seafloor pressure and Nile River low stand time-series.

Multitaper

Interpolation

10.1093/gji/ggz280

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Citations (9)

Bounded influence magnetotelluric response function estimation

Geophysical Journal International (2004)

Alan D. Chave David J. Thomson

Robust magnetotelluric response function estimators are now in standard use in electromagnetic induction research. Properly devised and applied, these have the ability to reduce the influence of unusual data (outliers) in the response (electric field) variables, but are often not sensitive to exceptional predictor (magnetic field) data, which are termed leverage points. A bounded influence estimator is described which simultaneously limits the influence of both outliers and leverage points, and has proven to consistently yield more reliable magnetotelluric response function estimates than conventional robust approaches. The bounded influence estimator combines a standard robust M-estimator with leverage weighting based on the statistics of the hat matrix diagonal, which is a standard statistical measure of unusual predictors. Further extensions to magnetotelluric data analysis are proposed, including a generalization of the remote reference method which utilizes multiple sites instead of a single one and a two-stage bounded influence estimator which effectively removes correlated noise in the local electric and magnetic field variables using one or more uncontaminated remote references. These developments are illustrated using a variety of magnetotelluric data.

Magnetotellurics

Leverage (statistics)

10.1111/j.1365-246x.2004.02203.x

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Citations (287)