Page 18

3

SCIENTIFIC ISSUES AND RESEARCH OPPORTUNITIES

Issue 1: Dynamics of the Earth's Core and Fluctuations in the Main Field

Core Dynamics and the Dynamo

Self-excited dynamo action—the process by which magnetic fields are continually regenerated within electrically conducting fluids despite ohmic dissipation of magnetic energy—occurs inside most of the planets of the Solar System, within the Sun, and in countless other bodies beyond the Solar System. The details of this process remain obscure.

One of the great challenges in geoscience is to unravel the workings of the Earth's dynamo. The geodynamo can be probed at relatively close range and, in principle, continually in time. Important questions to be addressed include the following:

  • What are the energy sources for core fluid motions? Where are they concentrated in the core, and how do they vary with time?

  • What is the pattern of fluid flow in the core? What is the nature of core turbulence?

  • Is the outer part of the core stratified? Is it stable or unstable from a convective point of view?

  • What are the mechanisms by which the geomagnetic field varies with time? How does the field reverse?

  • Can geomagnetic secular variations be forecast?



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 18
Page 18 3 SCIENTIFIC ISSUES AND RESEARCH OPPORTUNITIES Issue 1: Dynamics of the Earth's Core and Fluctuations in the Main Field Core Dynamics and the Dynamo Self-excited dynamo action—the process by which magnetic fields are continually regenerated within electrically conducting fluids despite ohmic dissipation of magnetic energy—occurs inside most of the planets of the Solar System, within the Sun, and in countless other bodies beyond the Solar System. The details of this process remain obscure. One of the great challenges in geoscience is to unravel the workings of the Earth's dynamo. The geodynamo can be probed at relatively close range and, in principle, continually in time. Important questions to be addressed include the following: What are the energy sources for core fluid motions? Where are they concentrated in the core, and how do they vary with time? What is the pattern of fluid flow in the core? What is the nature of core turbulence? Is the outer part of the core stratified? Is it stable or unstable from a convective point of view? What are the mechanisms by which the geomagnetic field varies with time? How does the field reverse? Can geomagnetic secular variations be forecast?

OCR for page 18
Page 19 Is the toroidal magnetic field in the core comparable to, or an order of magnitude or more greater than, the poloidal field? Core-Mantle Coupling It is well known that the rotation of the Earth is not a perfect time keeper. Changes in the observed length of day are associated with torques exerted on the mantle by a number of sources on a variety of time scales. Torques are applied externally by the atmosphere, the oceans, and the gravitational fields of the moon and sun. They are applied internally to the base of the mantle by the fluid outer core of the Earth. A number of processes might be responsible for the shorter-term variations, but the changes in length of day at a time scale of decades appear to be too large to be due to anything other than the transfer of angular momentum from the core to the mantle. The physical mechanism of core-mantle coupling remains obscure. Dynamo action in the core requires the transport of heat from the core into the mantle, that is, thermal core-mantle coupling. The lower mantle may be a relatively good conductor of electricity, in which case dynamo electric currents can leak from the core into the mantle or can be induced in situ without leakage. The resulting electromagnetic body force exerts a torque on the mantle that may explain decadal fluctuations in the length of day and could conceivably contribute to excitation and damping of the Chandler wobble. Flow of core fluid past seismically detected core-mantle boundary topography may also exert a pressure torque on the mantle. A number of first-order scientific issues are associated with processes of core-mantle coupling, including the following: Which coupling mechanism (electromagnetic, topographic, or other) is dominant? Precisely what is the shape of the core-mantle boundary?

OCR for page 18
Page 20 Is the core flow that produces topographic coupling steady or geostrophic; laminar or turbulent; or magnetostrophic, barotropic, or baroclinic? What fraction of the observed terrestrial heat flux is due to heat flow across the core-mantle boundary? Do motions in the core contribute significantly to the Chandler wobble? At what rate is magnetic energy being dissipated in the mantle? What is the magnitude and distribution of electrical conductivity in the lowermost mantle? Determination of bounds on the depth-integrated conductivity of the lower mantle would serve a twofold purpose. First, as mentioned above, it would constrain the electromechanical coupling between the core and mantle. Second, the attenuation of secular variations with distance from their sources in the core is strongly dependent on electromagnetic induction in a mantle having a large, though finite, conductivity. For a given conductivity, short-period magnetic fluctuations are attenuated more than long-period fluctuations; thus, dissipation in the lower mantle serves to “low-pass” signals emanating from the core. At present, only order-of-magnitude estimates of conductivity are available. Determination of the “cut-off period” of the lower mantle is correspondingly imprecise. In a loose sense, the analysis of secular variations observed at the Earth's surface due to motions in the core allows one to “sound” the conductivity of the lower mantle from the bottom up, or at least to determine an “upper bound” on the depth-integrated conductivity. The analysis of the external/internal coupling relationship can be used to sound the conductivity of the lower mantle from the top down. In actual fact, however, the problem is much more complicated; it is necessary to discriminate between the effects of both external and internal primary sources and their induced counterparts in the solid Earth. This is a

OCR for page 18
Page 21 significant challenge. Meeting this challenge requires long, stable time series from a well-distributed network of observations. Main Field and Secular Variation Knowledge of the main magnetic field and its temporal changes is fundamental to many basic questions relating to the origin and dynamics of the Earth, as discussed above. In addition, global and regional models of the main field have many practical applications in the commercial sector and the military. Such applications include removal of trend from anomaly data for natural resource and crustal exploration; air and sea navigation; surveying; orientation of land and space instrumentation; orientation of drilling tools and instruments in boreholes; and understanding the migration patterns of land and marine animals. For the past few decades, the Earth's main (core) field has been represented at 5-year intervals by spherical-harmonic models based on current observations. Other models have been produced to represent the historical field covering the past 300 years. For the most part, these models have been retrospective and are of limited use for extrapolating more than a few years into the future. This lack of predictability reflects the lack of a physical model describing field change in the geodynamo. One of the major obstacles to accurate modeling of the main field and its secular variation is inadequacy of data. No vector satellite mission has been flown since Magsat in 1980. Secular variation data come from about 180 magnetic observatories worldwide. These data are particularly inadequate because they may be several years old before they become available and also because there are large spatial gaps in the coverage provided by the present network, most notably in the oceans of the Southern Hemisphere. Because of the importance of determining the temporal characteristics of the geomagnetic field, there is great interest in the study of past field

OCR for page 18
Page 22 changes. In view of the success of recent efforts to collect and model historical data from the past three centuries, more data of this type could be utilized by the modeling community. Archaeomagnetic and paleomagnetic studies would provide longer-period variations, in the range of 103 to 105 years, as well as records of more extreme field behavior such as geomagnetic excursions and polarity transitions. The systematic acquisition and ready accessibility of present-day geomagnetic data, historical geomagnetic data, and archaeomagnetic and paleomagnetic data would enable the scientific community to address the following concerns: What is the contribution of external and crustal field components to models of the main field? Impulselike variations (jerks) in the secular change have been detected. Are these real? Are they global? Exactly how fast and how frequently do they occur? What is the relationship between secular change and other geophysical phenomena, for example, Earth rotation and climate? Can paleomagnetic and archaeomagnetic data be used reliably to improve the understanding of the behavior of the main field by producing models for earlier epochs? Recommendations In order to understand the main field of the Earth, the fluid dynamics of the Earth's outer core, and core-mantle coupling, it is essential to study the geomagnetic field as a function of time—on a scale of years to decades, as provided by surface and satellite measurements, and on a scale of hundreds to millions of years, as measured by archaeomagnetic and paleomagnetic techniques. It is also essential to extrapolate surface measurements to the core-mantle boundary (CMB), which requires

OCR for page 18
Page 23 knowledge of the electrical conductivity of the mantle and the effects of crustal magnetization and magnetic fields from external electric current systems. The following recommendations address these issues. Their order does not imply a priority ranking. Long-term, stable time series should be generated from a global distribution of modern, upgraded observatories and repeat stations, tied together with a long-term, preferably continuous, magnetic field satellite monitoring program. In order to optimize the global coverage, new observatories and repeat stations should be installed at selected sites. Some stations would need to be located at relatively inaccessible sites, such as on ocean islands and the seafloor. Costs can be minimized if sites are collocated with existing facilities. Instrument sensitivity and data characteristics should be coordinated with other scientific users. Greater use should be made of archaeomagnetic and paleomagnetic techniques to provide information on the time scales of hundreds to thousands of years. High-quality data should be obtained for paleosecular variation, including paleointensity, magnetic stratigraphy, and reversal transitions. For further progress in understanding core-mantle dynamics, studies should be undertaken to provide better estimates of: (1) core and lower-mantle diffusivities (electromagnetic, viscous, and thermal); (2) topography and other characteristics of the CMB based on seismic tomography; (3) fluid motion at the surface of the core; and (4) toroidal and poloidal magnetic fields in the conducting part of the mantle. Because main field models (such as the IGRF) are important to so many scientific disciplines and societal applications, the updating and upgrading of such models should have a high priority.

OCR for page 18
Page 24 Issue 2: Lithospheric Magnetic Anomalies Scientific and Societal Framework Few geophysical methods have had a greater impact on the geosciences than magnetic methods. Magnetic surveys from aircraft, ships, and low-orbit satellites provide key information concerning the geological, tectonic, and thermal state of the Earth's lithosphere. New insights into the character and depth of magnetic source regions have aided investigations of the mechanical and thermal structure of the lithosphere, crustal and oceanic accretion and evolution, true polar wander, the variation of field intensity with time, and the process of field reversals. The large dynamic range of magnetization intensity in rocks (up to five orders of magnitude) permits the detection of otherwise subtle variations in lithology, rock properties, and structure. The persistence of magnetization in the lithosphere makes the magnetic method useful for studying its deep levels. Continental aeromagnetic (and gravity) data are used in the preparation of many geological maps and often provide the only economical means of investigating subsurface geology. Over oceanic areas, magnetic data collected by airborne and shipborne surveys were critical for the discovery of seafloor spreading, which led to the development of the plate tectonics theory. Magnetic surveys continue to be the primary tool for estimating the age and relative movement of tectonic plates. Indeed, the age of most of the oceanic crust is known from magnetic analyses. The power of the magnetic method as a geological mapping tool has increased with time; high-resolution surveying in conjunction with modern processing and graphic routines continues to improve the understanding of the oceanic and continental lithosphere. The magnetic method has many societal applications. Magnetic anomaly studies can delineate features associated with mineral or hydrocarbon accumulations; such features include igneous intrusions, fault

OCR for page 18
Page 25 zones, salt domes, and anticlines. Magnetic anomaly maps have stimulated and focused mineral exploration in many areas of the world, particularly in areas where the basement is concealed by sedimentary cover. Because crustal magnetization is sensitive to metamorphism and hydrothermal alteration, magnetic contrasts in the crust reflect variations in thermal and geochemical history that may be diagnostic for certain energy and mineral deposits. National programs to evaluate earthquake and volcanic hazards, to characterize environmentally contaminated areas, and to permit safe disposal of radioactive waste benefit from magnetic anomaly studies. Tectonic Relevance of Magnetic Anomalies Oceanic Anomalies The magnetic source layer in the ocean basins contains a continuous, high-fidelity record of geomagnetic field history and tectonic motion since the Jurassic. Understanding the processes that control the recording of the Earth's magnetic field by the oceanic crust (the crustal “tape recorder”) and its longevity is an outstanding first-order problem. Such understanding is fundamental to extracting information on paleofield intensity, true polar wander, and the thermal and chemical evolution of oceanic lithosphere. Another important problem is the plate kinematic framework. The response of the lithosphere to major plate reorganizations is recorded in structures delineated mainly by their magnetic signatures. Shipborne, satellite, and deep-tow surveys reveal systematic age-dependent variations in magnetization. Each of these methods is effective at a particular scale of study and provides a unique perspective on the problem. At all scales, these age-related variations in magnetization reflect changes in the source layer (for example, in its thickness or chemical composition), paleofield

OCR for page 18
Page 26 intensity, or processes operative during the evolution of the crust. Detailed studies of along-strike variations in the magnetization of ocean ridge crests are being carried out to address the cause of morphologically and geochemically defined segmentation of the ridge axis in relation to heterogeneity in magma composition and supply. Paleomagnetic studies of contemporaneous sequences of sedimentary rocks exposed on land or cored on the seafloor will help to distinguish geomagnetic field behavior from the effects of thermal and geochemical processes. Continental Anomalies High-resolution aeromagnetic surveys, such as the statewide survey recently completed by the Minnesota Geological Survey (Figure 3-1), provide extremely powerful tools for geological studies of continental lithosphere and set the standard for future aeromagnetic surveys. Inversions of airborne and satellite data have been performed to improve the understanding of the deep structure of the continents and to guide the systematic exploration for geothermal resources. However, meaningful interpretations require an understanding of magnetic mineralogy at depth. Analysis of lithospheric magnetic anomalies provides insight into paleogeography, thereby constraining paleoclimatic conditions. The ocean-continent boundary frequently displays a distinct, short-wavelength magnetic anomaly in airborne and shipborne surveys; however, substantial controversy exists concerning its expression in satellite data. This fundamental lithospheric boundary deserves additional study. Global and Regional Anomalies The Polar Orbiting Geomagnetic Observatory (POGO) and Magsat missions have mapped the Earth's magnetic field at a resolution sufficient to reveal previously unknown intermediate-to-long-wavelength (400 to 4,000 km) lithospheric magnetic anomalies without complications from

OCR for page 18
Page 27 secular variation. Many of these anomalies were previously unknown because of biases inherent in patching together individual data sets from aeromagnetic surveys. Imperfect knowledge of the reference field, lack of anomaly resolution in existing satellite data, and inadequate information on the magnetic properties of the lower crust and upper mantle limit the interpretation of satellite-derived regional magnetic anomalies. External field contamination reduces data quality, especially at the equator and poles. Nevertheless, these anomalies have been exploited for information on the spatial/temporal variations in thickness, thermal gradients, and composition of the lithosphere. Higher-resolution satellite data and systematic airborne surveys will provide data that can be used to interpret the geological and tectonic evolution of the lithosphere, especially in concert with global gravity and topography data sets. FIGURE 3-1 Shaded relief image of high-resolution aeromagnetic data from Minnesota (courtesy of the Minnesota Geological Survey). Most of the data used to produce this image were acquired under the supervision of the Minnesota Geological Survey; additional data were contributed by the U.S. Geological Survey, USX Corporation, and the Geological Survey of Canada.

OCR for page 18
Page 28 U.S. Magnetic Anomaly Map The Magnetic Anomaly Map of North America, published in 1987 by the Geological Society of America, has proven invaluable for interpretations of regional magnetic structures and for addressing regional geological problems. In spite of its general application, the U.S. portion of this map has many shortcomings, mainly due to inconsistencies in the numerous sets of magnetic data used to construct it. These inconsistencies are a product of disparate survey specifications, nonuniform treatment of regional fields, and inadequate coverage in some regions. This map could be improved with a few new surveys designed to stitch together existing data, replace substandard data, and fill in gaps in coverage. It would be useful to extend the aeromagnetic data offshore, at least to the limits of the Exclusive Economic Zone (EEZ) (320 km offshore).

OCR for page 18
Page 40 current requires research-oriented satellites. Ongoing studies of the ring current make considerable use of data from the limited number of properly equipped satellites in geosynchronous orbit and in low-altitude polar orbits. These studies would benefit from three satellites, widely spaced in local time, in each of these orbits. Filling gaps in the current global network of ground magnetometers, facilitating the dissemination and archiving of the relevant data, and mounting focused campaigns in selected regions would greatly increase the scientific value of all of these studies. Recommendations Magnetospheric physics is primarily concerned with study of the interaction of the solar wind with the Earth's magnetic field to create the various current systems described above. The study of magnetic fields and currents in the magnetosphere and ionosphere requires a suite of simultaneously recording instruments on spacecraft and on the ground. In addition, data from ground-based geomagnetic observatories are essential in order to characterize the disturbance magnetic field induced in the solid Earth due to currents in both the ionosphere and magnetosphere. These data are used both for modeling the complete current system and for the creation of magnetic indices that provide a measure of the level of large-scale magnetic disturbances. The following recommendations address needs for spacecraft- and ground-based instrumentation. The order of the recommendations does not imply a priority ranking.

OCR for page 18
Page 41 Spacecraft-based instrumentation: A permanent platform (such as an L-1 satellite) should be established for monitoring the interplanetary magnetic field (IMF) and solar wind for a variety of scientific and societal applications. It is essential that solar wind data with at least 5-minute temporal resolution be continuously available to users in near real time in order to support research and operational activities. Inside the magnetosphere, at geosynchronous orbit, the normal complement of two Geostationary Operational Environmental Satellites (GOES) does not allow for a complete coverage in local time. Another magnetometer at geosynchronous orbit should be added at 6 to 8 hours in local time from both GOES East and GOES West. Between geosynchronous and low-Earth orbit, available magnetic field data are inadequate for developing accurate models of the magnetospheric field. When new scientific and operational missions in this region of space are developed, they should include a research-grade magnetometer to support modeling of the magnetospheric field as a function of IMF direction and substorm activity. Ground-based instrumentation: Existing permanent observatories should be upgraded to record the magnetic field digitally at sampling rates of at least one vector per minute for normal operation and up to two vectors per second (0.5 hertz) for special epochs. The latitudinal spacing of these permanent observatories should be no greater than 10° at equatorial and middle latitudes in order to obtain adequate spatial resolution of the main field and field variations due to magnetospheric and ionospheric sources. In the

OCR for page 18
Page 42 subauroral and auroral regions and the polar caps, latitudinal spacing of 3° or less is desirable. For specific research purposes, temporary stations are needed with spacing of approximately 100 km in both latitude and longitude. Where possible, arrays should be deployed conjugately in the Northern and Southern Hemispheres and should take advantage of complementary measurements from other ionospheric instruments such as radars, riometers, and optical imagers and spectrometers. Issue 4: Electromagnetic Induction in the Solid Earth and Oceans Imaging Earth Conductivity The dynamic processes of metamorphism, magmatism, convection, and deformation in the Earth give rise to anomalously high temperatures, volatiles, and magmas. Electrical conductivity is the physical property most sensitive to these manifestations, particularly to the configuration and chemistry of fluids and other conductive grain-boundary phases. Electromagnetic (EM) measurements made at or near the Earth's surface are used to delineate conductivity structure and are therefore appropriate tools for understanding the evolution of the planet. EM methods also have important applications to several problems of societal concern, including energy, mineral, and water resource exploration; the reliability of electric power grids; and water quality and waste management. The Earth's oceans have a significant influence on climate through long-term storage and transport of heat. Ocean water is a good electrical conductor; it generates easily measurable electric fields as it moves through the geomagnetic field. Therefore, EM measurements in the ocean are an extremely useful tool for probing large-scale ocean dynamics and monitoring long-term variability of the sea.

OCR for page 18
Page 43 Background on Natural EM-Field Methods Transient magnetic field disturbances from sources in the ionosphere and magnetosphere produce electromotive forces that drive electric currents in the Earth. These currents, in turn, cause secondary magnetic fields over a wide range of scales and amplitudes. Observations of these magnetic and electric fields at the surface can be used to characterize the conductivity structure of the solid Earth. The magnetic variation (MV) method (also called geomagnetic deep sounding, or GDS) measures magnetic fields; the magnetotelluric (MT) method measures the orthogonal components of the horizontal electric field (the so-called telluric field). For a medium of given conductivity, long-period signals decay less rapidly with depth than short-period signals. Therefore, by estimating the response of the Earth over a range of increasing periods, one can progressively sound the electrical conductivity to increasingly greater depths. Interpretation of EM data is usually broken into three steps. The first extracts a set of one or more frequency-dependent response functions from long time series of magnetic and electric field data observed simultaneously at one or more sites through estimation of frequency-domain transfer functions between measured field components. Data reduction is complicated because the Earth is multidimensional, external sources may not be ideal, and noise processes are commonly non-Gaussian. The second step inverts these transfer functions to obtain Earth structure. Besides the nonuniqueness problems shared by all inversions of incomplete and inaccurate data, this inverse problem is both difficult and numerically intensive because it is unstable, nonlinear, and commonly multidimensional. However, progress in recent years has significantly improved the ability to image electrical structure. The third step relates conductivity to physicochemical processes in the Earth. This step requires laboratory measurements of relevant materials at carefully

OCR for page 18
Page 44 controlled conditions and can often be improved by incorporation of independent geophysical and geological information. Monitoring Large-Scale Fluid Motions in the Oceans The motion of seawater through the geomagnetic field induces electric currents in the ocean through the usual dynamo process. The resulting electromagnetic fields contain information about a variety of oceanographic processes: surface waves, internal waves, and steady flows. In the case of large-scale ocean currents, for instance, the electric field at the deep seafloor is closely related to mass transport of the water column above the point of measurement. Although the net vertically integrated electric current is small, direct measurements of the electric field have been found to be an excellent means to monitor large-scale barotropic flows. For example, an 8-year time series of transport in the Florida Current has been derived from measurements of electric voltage using a cable that spans the Florida Strait. Other oceanic flows, such as surface and internal waves, produce appreciable magnetic as well as electric fields. These flows can be measured using EM methods, especially on the continental shelves and the floors of shallow seas. Research Needs for Electromagnetic Induction Studies Crust Electromagnetic methods are useful for understanding the distribution and character of fluids in the crust. Four fundamental classes of questions can be addressed by these methods:

OCR for page 18
Page 45 1. What are the present microscale and macroscale configurations of crustal fluids? 2. How are fluids emplaced and distributed in the crust? 3. How are fluids modified by the ambient geological environment? 4. How do fluids affect and modify the geological/tectonic environment? Fluid-rich sedimentary rocks and oceanic crust are transported in subduction zones to depths that require dewatering and conversions to higher metamorphic grades. Do fluids persist in the deep crust over long geological times? What paths do they take in returning to the surface? Fluids trapped at depth have a significant effect on the rheology of the lower crust and upper mantle. In the shallow crust, high pore pressures have been implicated in large-offset horizontal thrusting, in the low strengths of strike-slip faults such as the San Andreas fault, and in controlling rupture during earthquakes. Because fluids can have a large effect on electrical conductivity, EM methods are appropriate tools for investigating these problems. Many of these problems can be addressed with new, more accurate EM data collected in a more systematic fashion. MV studies involving large-scale arrays of simultaneously recording magnetometers on the Earth's surface have historically served two functions: (1) to support solar-terrestrial physics campaigns to study the temporal and spatial morphology of current systems in the magnetosphere and ionosphere; and (2) for reconnaissance of electrically conducting features in the Earth's interior, such as sediment-filled basins, anomalies associated with fluids in the deep crust, and thermal anomalies in the upper mantle. The collection and interpretation of MV data from arrays are valuable prior to detailed MT profiling to help ensure optimum profile location with respect to structures of interest and to constrain the three-dimensional context in which MT interpretations are made. MV surveys are also useful in areas with too much topographic relief or cultural noise for conventional MT measurements. These MV arrays can also be used

OCR for page 18
Page 46 during MT profiling to understand and correct for the effects of external source complications. As in the case of modern MT systems, modern MV systems require broadband capabilities, remote referencing, and infield processing. Upper Mantle The conductivity observed in the outer 200 km of the mantle through long-period MT and MV studies is controlled by such intrinsic factors as the composition, temperature, and mineralogy of the crystalline matrix, as well as by such extrinsic factors as the composition, quantity, and connectivity of interstitial pore fluids and the presence of intergranular partial melts, graphite, and sulfides. Given this plurality of effects on conductivity, what independent constraints are required to make unique interpretations? Dynamical considerations suggest that temperature and melt gradients should produce order-of-magnitude lateral variations in conductivity. Can such lateral variations in conductivity in the upper mantle be mapped? Theoretical studies suggest that MT methods, when deployed with adequate spatial coverage, may be a more effective tool than presently available seismic methods for studying melt segregation zones. Observed conductivities in the upper mantle are as much as three orders of magnitude lower than conductivities of candidate rocks and rock-forming minerals measured in the laboratory at the same temperatures. Does this difference reflect the influence of one or more of the intrinsic factors discussed above, such as bulk composition, or is it an effect of extrinsic factors, such as interstitial phases along grain boundaries?

OCR for page 18
Page 47 Middle and Lower Mantle Evidence is strong for a steep rise in conductivity in the seismic transition zone (approximately 400 to 700 km) in the mantle. Existing data are consistent with steplike increases in conductivity at the seismic discontinuities at 400 and 670 km, but such “steps” are not required by the data, and features with scales shorter than 200 km are poorly resolved. The conductivity may approach about 1 siemens per meter below 800 km, but resolution deteriorates rapidly below 1,000 km. There are significant differences in ultra-low-frequency response functions at different magnetic observatories. How much of this signal is due to deep lateral heterogeneity? How much is due to biases associated with shallow conducting features such as the oceans with inadequately represented external source field morphologies? Improving the knowledge of deep-mantle conductivity and understanding the constraints it provides on composition, physical state, and dynamics of the Earth's interior require a multifaceted approach. Both the maximum depth of inference and the resolution of features within that depth range can be improved with the following: more accurate data in the presently available bandwidth; improved observatory coverage; and new data at longer periods. Better areal data coverage is required to map lateral variations and to characterize the external source properly. In ocean areas without suitable islands, this requires ocean bottom facilities. Abandoned submarine telephone cables offer an attractive possibility for electric field measurements and are also likely to be an important tool for ocean flow studies. Extending the low-frequency limit requires improved separation of temporal fluctuations from the Earth's core and magnetosphere. In turn, knowledge of the conductivity structure of the deep mantle can contribute to studies of the magnetic fields associated with hydromagnetic processes

OCR for page 18
Page 48 in the core dynamo. Finally, additional studies of candidate lower-mantle materials under carefully controlled thermodynamic conditions can be used to interpret lower-mantle conductivities. For instance, interpretation of the conductivity increase between 400 and 700 km requires additional laboratory studies of the electrical properties of β- and γ-(Mg,Fe)2SiO4 (spinel structure). Material Properties Laboratory studies of electrical conductivity provide the link between models of conductivity and the physical and chemical processes occurring within the Earth. Because conductivity is sensitive to environmental parameters, interpretation of mantle conductivity requires extrapolation of data obtained under the limited conditions accessible in the laboratory. This requires a thorough understanding of the basic mechanisms that control conductivity. Oxygen and sulfur fugacities, pressure, and the chemical environment of surrounding minerals can affect conduction and are particularly important for adequately constraining thermodynamic conditions. Other fundamental questions remaining to be addressed relate to stability and interconnectedness (pore geometry) of conducting fluids (aqueous and partial melts), and grain-boundary phases (such as carbon and sulfides) over geological time. Extrapolation of laboratory studies to in situ conditions requires improved modeling of the bulk electrical response of composites and networks and is critical to understanding crustal and upper-mantle conductivities. Recommendations The study of crustal and mantle conductivities and the constraints they provide on composition, physical state, and dynamics of the Earth's interior requires a multifaceted approach. It should include new data,

OCR for page 18
Page 49 improved observatory coverage, and improved laboratory measurements of mantle minerals under controlled thermodynamic conditions, as addressed by the following recommendations. The order of these recommendations does not imply a priority ranking. Existing magnetic observatories should be upgraded with higher-quality, faster-acquisition-rate instruments. New observatories should be established at additional sites, including relatively inaccessible locations such as the seafloor. A small task force should be established to estimate the cost and feasibility of installing long-term observatories on the ocean bottom. This group should include scientists and engineers with ocean floor instrumentation experience. The instrument base should be expanded and upgraded to satisfy the growing interest in utilizing electromagnetic methods for geophysical and geological investigations. Newly acquired MT instruments should be mobile and easily deployable, fully remote, and referenced for cultural noise cancellation, with complete in-field processing to ensure that quality data are obtained. MV array facilities, consisting of up to several dozen digitally recording, three-component fluxgate magnetometers, should also be acquired for use with MT profiling. Some of these instruments should have the capability for electric field recording to enable collection of low-frequency MT data. The limited number of ocean bottom electrometers should be augmented to support water motion, ocean dynamics, and mantle studies. The monitoring of long-term variability of the geoelectric field for both MT and deep ocean studies requires long, grounded dipoles. The use of abandoned submarine cables for this purpose is promising and should be explored by scientists in cooperation with telephone companies.

OCR for page 18
Page 50 Improved laboratory measurements of mantle minerals under controlled thermodynamic conditions should be undertaken to extrapolate laboratory results to Earth conditions. In particular, the influence of minor elements such as hydrogen, nickel, and aluminum on the point defect populations that control solid-state conduction in olivines and pyroxenes should be determined. New experimental techniques are needed to study nonequilibrium electrical properties of water-saturated crustal rocks at elevated temperatures (50 to 500°C). Systematic experimental and theoretical studies of the electrical response of multiphase aggregates and networks should be undertaken to improve understanding of upper-crustal conductivities. Effects of the presence and distribution of other conductivity-enhancing phases such as carbon, magnetite, sulfides, and partial melts should be investigated using advanced experimental techniques that can carefully control thermodynamic variables. Conductivity and complex impedance measurements linked to physical properties such as porosity, permeability, and acoustic velocity in porous, water-saturated crustal rocks should also be collected.