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Aug 10, 2006 - Richard S. Gross (Geodesy): Jet Propulsion. Laboratory, Pasadena,California USA; richard.gross@ ... Jeffery J. Roberts (Mineral and Rock.
EOS

VOLUME 87 NUMBER 33 15 AUGUST 2006

SECTION news TRANSACTIONS AMERICAN GEOPHYSICAL UNION The Newspaper of the Earth and Space Sciences

Editors John W. Geissman: Dept. of Earth and Planetary Science,University of New Mexico, Albuquerque, USA; [email protected] Wendy S. Gordon: Texas Commission on Environmental Quality; [email protected] Manuel Grande: University of Wales, Aberystwyth; [email protected] Hassan Virji: START; [email protected] Editor in Chief A. F. Spilhaus, Jr.: AGU, Washington, D.C., USA; [email protected]

Corresponding Editors Roland Burgmann (Tectonophysics): University of California at Berkeley, USA; burgmann@seismo. berkeley.edu

William E. Carter (History): Dept. of Civil

Engineering, University of Florida, Gainesville, USA; [email protected]

Millard F. Coffin (Japan): Ocean Research Institute, University of Tokyo, Japan; mcoffin@ori. u-tokyo.ac.jp

Steven C. Constable (Geomagnetism and Paleomagnetism): Scripps Institution of Oceanography, La Jolla, California, USA; [email protected]

Richard S. Gross (Geodesy): Jet Propulsion

Laboratory, Pasadena,California USA; richard.gross@ jpl.nasa.gov

Marguerite Kingston (Public Affairs): USGS (retired); [email protected]

Stephen A. Macko (Education): Dept. of

Environmental Sciences, University of Virginia, Charlottesville, USA; [email protected]

Louise Prockter (Planetary Sciences):

Applied Physics Laboratory, Laurel, Maryland, USA; [email protected]

Paul Renne (VGP): Berkeley Geochronology

Center, Berkeley, California, USA;[email protected]

Justin S. Revenaugh (Seismology): University of Minnesota, Minneapolis, USA; [email protected]. edu

Jeffery J. Roberts (Mineral and Rock Physics): Lawrence Livermore National Laboratory, Livermore, California, USA; [email protected]

Sarah L. Shafer (Paleoceanography and Paleoclimatology): U.S. Geological Survey, Corvallis, Oregon, USA; [email protected]

David Sibeck (Space Physics and Aeronomy): NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA; [email protected]

Ramesh P. Singh (India): Dept. of Civil Engineering, Indian Institute of Technology, Kanpur, India; [email protected]

Maribeth Stolzenburg (Atmospheric and Space Electricity): Dept. of Physics and Astronomy, University of Mississippi, University, USA; [email protected]

Jeffrey M. Welker (Biogeosciences): Environ-

ment and Natural Resources Institute, University of Alaska, Anchorage, USA; [email protected]

Assistant Editors Peter Folger: AGU, Washington, D.C., USA; [email protected]

Catherine O’Riordan: AGU, Washington, D.C., USA; [email protected] Staff Editorial: Randy Showstack, Managing Editor; Ayesha Badhwar, Production Coordinator; Sarah Zielinski, News Writer/Editor; Mohi Kumar, Science Writer/ Editor; Liz Castenson, Editor’s Assistant Advertising Carla Childres, Advertising Assistant; Tel: +1-202-777-7536; E-mail: [email protected] Composition and Graphics: Habib Hastaie, Manager; Valerie Bassett, Carole Saylor, and Nancy Sims, Electronic Graphics Specialists ©2006 American Geophysical Union. Material in this issue may be photocopied by individual scientists for research or classroom use. Permission is also granted to use short quotes, figures, and tables for publication in scientific books and journals. For permission for any other uses,contact the AGU Publications Office. Eos, Transactions, American Geophysical Union (ISSN 0096-3941) is published weekly by the American Geophysical Union, 2000 Florida Ave., NW, Washington, DC 20009 USA. Periodical Class postage paid at Washington, D.C., and at additional mailing offices. POSTMASTER: Send address changes to Member Service Center, 2000 Florida Ave., NW,Washington, DC 20009 USA. Member Service Center 8:00 a.m.–7:00 p.m. Eastern time; Tel: +1-202-462-6900; Fax: +1-202-328-0566; Tel. orders in U.S.:1-800-966-2481; E-mail: [email protected]. Information on institutional subscriptions is available from the Member Service Center. Views expressed in this publication do not necessarily reflect official positions of the American Geophysical Union unless expressly stated.

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Updating the Map of Earth’s Surface Conductance Studying the Earth’s deep conductivity structures, important for developing our understanding of the dynamics of the Earth, is complicated due to effects of the shallow conductive structures on the electromagnetic (EM) responses for periods larger than hours. The results of the deep EM soundings can be heavily distorted by the surface shell conductance, which varies from fractions of siemens (S) inland to up to tens thousand of siemens in the oceans. Thus, separating the effects caused by those variations and by deep conductivity structures is an important step during interpretation of the data. This article reports on efforts to overcome these difficulties by providing high-resolution, global maps of the Earth’s surface shell conductivity structure, from which deep conductivity can be interpolated. Using finescale regional surface schemes of conductance for the shallow structures (S-maps) overlain and compiled into broader spatial maps, scientists will be able to use data products from these efforts to accomplish research goals of the currently running USArray (http://www.emscope.org) and for the planned Euro-Array (http://www.euroarray.org), projects that aim in part to regionally map the conductivity structures at upper and middle mantle depths by using magnetotelluric (MT) and magnetovariation (MV) methods. Changes in the outer part of the Earth’s magnetic field, usually caused by interactions between the solar wind and the ionosphere and magnetosphere, induces an electric ‘telluric’ field in the Earth, and the strength of the telluric field is dependent on the conductivity (resistivity) of the medium. In the MT method, observing the magnetic and electric fields simultaneously, and determining their ratios at varying periods, allows for the derivation of the conductivity distribution with depth. A similar process for determination of conductivity is used in the MV method, but just from magnetic components of the field. Measuring conductivity is a useful tool for distinguishing different rock types, and measurements of the Earth’s subsurface conductivity can shed light on structural geology. The research described here seeks to characterize mantle inhomogeneity against the background of known surface

shell conductance distribution for the European region. This study reveals a number of problems that must be overcome in order to collect reliable mantle soundings, and it highlights the necessity of improving of S-maps.

Titan mission data now available The European Space Agency (ESA) has made public the data from four of the six experiments on the Huygens probe, which landed on the surface of Saturn’s moon Titan in January 2005. Data is available from the following instruments: the Huygens Atmospheric Structure Instrument, which measured the physical and electrical properties of the moon’s atmosphere; the Doppler Wind Experiment, which studied the direction and strength of Titan’s winds; the Aerosol Collector and Pyrolyser (ACP), which collected aerosols; and the Gas Chromatograph and Mass Spectrometer, which analyzed data from the ACP and measured the chemical composition of the atmosphere. Data from the Descent Imager/Spectral Radiometer and the Surface Science Packager will be released in September or October along with the probe’s entry and descent trajectory. The data sets include the data as well as calibration information and documentation needed to process and analyze the data. The data can be accessed from the ESA Planetary Science archive at http://www. rssd.esa.int/PSA Report highlights climate change risk to oceans Human-induced changes in atmospheric carbon dioxide levels will have severe consequences for oceans, concluded a 31 May report from the German Advisory Council on Global Change (WBGU).

The most straightforward way to separate deep conductivity from surface shell conductivity is to numerically simulate EM fields in the frame of the Earth’s layered model, incorporating a surface conducting shell that consists of contributions from the sea water and from sediments (see Figure 1) Obviously, the accuracy of such a modeling depends essentially on the accuracy of the global S-maps used.

Section News

cont. on page 331

Fig. 1.The current global surface S-map with a global conductivity distribution of the Earth’s interior and extracted S-map for the European region.The initial S-map is obtained from sediment thicknesses (left) and the new corrected S-map is obtained by MT soundings in situ (right). Sediment thicknesses are from Laske and Masters [1997].

news In Brief

Improving S-maps

The WGBU wrote the report, which was released in English on 26 July, in order to draw attention to the need to develop climate change mitigation and adaptation strategies and to contribute to the development of a European Union ocean policy. The report identifies several threats to world oceans from climate change, including increasing ocean acidification, melting of sea ice, increasing destructive force of tropical cyclones, rising sea level, and negative impacts on marine ecosystems. For each of these items, WBGU established ‘guard rails,’ quantitative limits on damage that if breached would lead to intolerable consequences. For example, an increase in mean global surface temperature above 2°C would lead to climatic changes that have intolerable consequences for marine conservation. In addition, the report makes several recommendations to avoid exceeding these guard rails. They include decreasing anthropogenic greenhouse gas emissions by 2050 to half of 1990 levels, establishing 20–30% of any marine ecosystem as a protected area, sequestering carbon dioxide under the sea floor only as a transitional strategy and not a permanent solution, and constructing international agreements to accommodate refugees from coastal areas affected by climate change. The report is available in English at http:// www.wbgu.de/wbgu_sn2006_en.html and in German at http://www.wbgu.de/wbgu_ sn2006.html Witnesses urge measures to protect oceans To better manage the nations oceans, the United States should establish a national

ocean policy and ratify the United Nations Convention on the Law of the Sea, several witnesses testified at a 3 August hearing before the Senate Commerce, Science, and Transportation Subcomittee on National Ocean Policy Study. “Our oceans are in crisis,” said Leon Panetta, co-chair of the Joint Ocean Commission Initiative. Panetta cited several issues threatening oceans, including ocean acidification, increasing amounts of pollution, lost wetlands, depleted fisheries, invasive species, and a fragmented U.S. governance system. Michael Orbach, director of the Duke University Marine Laboratory, said,“If we are to be perceived as a leader in ocean science and policy, we have to demonstrate that we are, in fact, part of the international community.” He explained,“We cannot solve the ocean policy or scientific or human problems by ourselves, so accession to [the Law of the Sea] treaty is very important.” The United States is the only industrialized nation that has not yet ratified the treaty. Although the convention has wide support in the Senate, a small group led by Sen. James Inhofe (R-Okla.) has prevented a vote on ratification. Other countries are beginning to take advantage of the treaty. Several—including Russia, Canada, and Norway—are preparing or already have submitted applications to extend the limits of their continental shelf, said Paul Kelly, a member of the U.S. Commission on Ocean Policy.“With tight oil and gas supplies and growing demand, these countries are beginning to look at the Arctic for future exploration,” he said. The United States has the technology to map its continental shelf and expand the limits, but “we have not even started the mapping,” Kelly noted.

—Sarah Zielinski, Staff Writer

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VOLUME 87 NUMBER 33 15 AUGUST 2006

Section News cont. from page 326

The modern global S-maps are mainly based on the bathymetry, and global sediment thicknesses (on land as well as under water) given by Laske and Masters [1997]. Additionally, improvement in the oceanic conductance data recently was achieved, taking into account the salinity, temperature and pressure of the sea water [cf. Manoj et al., 2006]. However, the methods used to create these global S-maps seem to be insufficient do to the ambiguity introduced by applying heuristic procedures to convert sediment thicknesses into conductance values [cf. Everett et al., 2003]. One way to improve the global S-map on the continents is to merge the existing regional S-maps that have been compiled all over the world for many years. These regional S-maps often have better resolution and, most probably, higher reliability since they use a priori information, for example, the shallow seismic and gravity investigations, or electrical and EM prospecting [cf. Harinarayana and Naganjaneyulu, 2003]. The authors of this article started updating the global S-map on the continents by incorporating the existing regional S-maps, and welcome the contribution of regional conductance data in any form for subsequent correction of the global S-map. The data can be sent to article lead author Ján Vozár (geofSmap@ savba.sk), Geophysical Institute, Bratislava, Slovak Republic. The modeling results for the regions of interest with the updated global S-map will be available on request.

Modeling EM Responses With a New Regional S-map In a demonstration of the distorting effect of the nonuniform conductance distribution on EM responses in the European region, the layered model of the Earth shown in Figure 1, which incorporates a thin spherical shell, was used with updated regional conductance for northern, central, and eastern Europe. This S-map was compiled with spatial resolution of 1° × 1° as part of two projects: Baltic Electromagnetic Array Research (BEAR) [Korja et al., 2001] and Central

Europe Mantle Geoelectrical Structure (CEMES) [Semenov et al., 2003]. The new regional S-maps are shown in Figure 1. Induction equations have been solved numerically according to the three-dimensional scheme by Kuvshinov et al. [2005]. Figure 2 presents examples of the theoretically calculated apparent resistivity and impedance phase values for the MT and magnetovariational geomagnetic deep soundings (GDS) methods at periods of 0.25 days (more results are available at ftp://gpi. savba.sk/Smap). The two methods respond in different ways to the near-surface inhomogeneities, and moreover, the MT responses depend on the field polarization. The distortions of the apparent resistivity obtained by the MT method are much higher than the impedance phases, as expected. The apparent resistivity values obtained for the GDS method (apart from the MT responses) are less sensitive to the surface conductance variations than the MT method. Note that the inconsistency between the two methods due to the lateral variability of the conductance decreases as the period increases. These modeling results show that effects caused by surface shell conductance inhomogeneities are significant and can be predicted with required detail and accuracy if the reliable conductance maps are available. Once these high-resolution S-maps have been made, it then would soon be possible to model deep conductivity structures by subtracting shallow components from the overall signal. Hopefully, this would lead to a better understanding of mantle evolution and plate tectonics.

Acknowledgments CEMES Experimental Team [Semenov et al., 2003] members from the following institutions participated in the preparation of the S-map of central and eastern Europe: Institute of Geophysics, Warsaw, Poland; Geophysical Institute, Prague, Czech Republic; Institute of Geological Sciences, Minsk, Belarus; Geodetic and Geophysical Research Institute, Sopron, Hungary; Geological Survey of Romania, Bucharest;

Fig. 2. (top) The apparent resistivities and (bottom) phases of impedances modeled for the Europe region by (left) the GDS and (center and right) MT methods (for two polarizations).The period is 0.25 days. Geophysical Institute, Bratislava, Slovakia; Institute of Geophysics, Kiev, Ukraine; and State University of Moscow, Russia. Thanks are due to the Polish Committee of Scientific Research, which has supported the investigations through grants 6P04D-01220 and 2P04D-02329, and to the Centre on Geophysical Methods and Observations for Sustainable Development, Warsaw, Poland, which has supported the joint work of the authors. Additionally, C. Manoj thanks V. P. Dimri, National Geophysical Research Institute, Hyderabad, India, for permission to publish this article. J. Vozár is grateful to Vedecká Grantová Agentúra (VEGA) for support of this work through grant number 02/6045/26.

Korja, T., et al. (2002), Crustal conductivity in Fennoscandia: A compilation of a database on crustal conductance in the Fennoscandian Shield, Earth Planets Space, 54, 535–558. Kuvshinov, A., H. Utada, D. Avdeev, and T. Koyama (2005), 3-D modelling and analysis of the Dst EM responses in the North Pacific Ocean region, Geophys. J. Int., 160, 505–526. Laske, G., and G. Masters (1997), A global digital map of sediment thickness, Eos Trans. AGU, 78(46), Fall Meet. Suppl., Abstract S41E-1. Manoj, C., A. Kuvshinov, S. Maus, and H. Luhr (2006), Ocean circulation generated magnetic signals, Earth Planets Space, in press. Semenov,V.Y., W. Jozwiak, and J. Pek (2003), Deep electromagnetic soundings conducted in TransEuropean Suture Zone, Eos Trans. AGU, 84(52), 581, 584.

References

—Ján Vozár, Geophysical Institute of SAS, Bratislava, Slovak Republic; E-mail: geofjavo@ savba.sk; Vladimir Y. Semenov, Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland; Alexey V. Kuvshinov, Danish National Space Center, Copenhagen, Denmark; and Chandrasekharan Manoj, Magnetotelluric Division, National Geophysical Research Institute, Hyderabad, India.

Everett, M. E., S. Constable, and C. G. Constable (2003), Effects of near-surface conductance on global satellite induction responses, Geophys. J. Int., 153, 277–286. Harinarayana, T., and K. Naganjaneyulu (2003), Regional surface electrical conductance map of India. J., Geol. Soc. Ind.ia, 61(6), 724–728.

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