Session EM1: Laboratory Measurements EM1-1 ELECTRICAL CONDUCTIVITY MEASUREMENT OF GRANULITE SAMPLE UNDER LOWER CRUSTAL PRESSURETEMPERATURE CONDITIONS Kiyoshi Fuji-ta (Kobe University, Department of Earth and Planetary Sciences, 1-1 Rokkodai, Nada, Kobe, Japan, e-mail:[email protected]) Tomoo Katsura (ISEI, Okayama University, Japan) Yoshiaki Tainosho (Faculty of Human Development , Kobe University, Japan) A number of discussions have been done on the cause of Highly Conductive Layer (HCL) in the lower crust. Most of laboratory experiments simulated the rocks in the stable continent saturated by saline water, water or conductive minerals to account for the HCL. On the other hand, the laboratory measurements of relatively dry rocks or homogeneous rocks are not sufficiently done nor properly evaluated. We attempted, therefore, to establish the technique to measure the electrical conductivity of real crustal rocks with relatively low conductivity and complicated mineral components in order to compare with the results given by the Magneto-Telluric (MT) measurements. A granulite sample, representing the lower crustal rock, was obtained from Hidaka Metamorphic Belt (HMB) in Hokkaido, Japan. The granulite sample was sintered under the conditions similar to those of lower crust, and after achieving characterization, the electrical conductivity of synthesized granulite sample was measured. The stable, reversible and reproducible conductivity values were successfully obtained up to about 1000 K under the fixed pressure of 1 GPa. The result accorded with the electrical conductivity structure suggested by the MT data analysis. In conclusion, to consider pore fluid conduction mechanism or the role of accessory minerals in the rock, the mechanisms of electrical conductivity paths in dry or basic rocks should be reconsidered again. EM1-2 LABORATORY MEASUREMENTS OF FREQUENCY DISPERSION OF SEDIMENTARY ROCKS ELECTRIC PROPERTIES AS APPLIED TO GROUND WATER HYDROCARBON CONTAMINATION Hallbauer-Zadorozhnaya V., (Institute for Polimer Science of Stellenbosch University, South Africa, [email protected]), Kamenetsky F. and Shmidbauer E. (Institute of Geophysics of Munich Ludwig-Maximillian University) Preliminary data of laboratory measurements of sedimentary rocks complex admittivity γ (ω ) = σ (ω ) − iωε (ω ) obtained in Institute of Geophysics of Munich University in MarchApril 2002 are reported. The equipment used has been previously developed by Prof. E. Schmidbauer for investigating the complex resistivity of minerals on the basis of the HewlettPackard high-precision LCR-meter HP4284A with frequency range 20 Hz to 1 MHz. The method of variation of samples length has been used to eliminate the influence of electrode polarisation. Several water saturated samples of sandstone and limestone have been measured. The unconsolidated sample of Bavarian sand has been also investigated. It is known from previous field and laboratory experiments that groundwater contamination due to hydrocarbon compounds can essentially change conductivity of rocks and its dependency on frequency. That is way some of the same samples were measured after partial saturation with Diesel fuel. At the initial stage of research all the necessary qualitative co-relations could be hardly observed. Nevertheless, a quite certain quantitative conclusions are obtained. Water saturated consolidated and unconsolidated samples display no or very low conductivity dispersion. Contaminated samples of unconsolidated sand display no dispersion. Contaminated samples of limestone (porosity about 15%) display high conductivity dispersion: about 10.3 times increase of conductivity with frequency. Contaminated samples of sandstone (porosity about 7%) display very high conductivity dispersion: about 320 times increase of conductivity with frequency. Preliminary results obtained promise a new possibility to locate contaminated areas by investigation conductivity dispersion or IP effects in geo-electromagnetics. Frequency dispersion of conductivity of sedimentary rocks samples 14 12 Conductivity mS 10 Contaminated: 1 — limestone σ( ⋅50 is shown), 2 — sandstone (σ100 ⋅ is shown), 3 — sand with 20% water and 10% Diesel. 8 6 4 2 0 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 Frequency, Hz 1 2 3 4 5 6 1,E+06 Water saturated: 4 — limestone, 5 — sandstone, 6 - sand with 30% water. EM1-3 TEMPERATURE, MINERAL COMPOSITION AND ELECTRICAL CONDUCTIVITY OF THE MANTLE Juanjo Ledo and Alan G. Jones. (Geological Survey of Canada, 615 Booth St, Ottawa K1A 0E9, ON, Canada). [email protected] Using laboratory-derived temperature-dependence of electrical conductivity of mantle minerals, we present physical and statistical methods to determine bulk conductivity of mantle mineral assemblages for the ternary Olivine-Orthopyroxene-Clinopyroxene (Ol-Opx-Cpx) system. We determine the physical properties of composite materials by combining variational bounds and percolation theory. We calculate physical properties bounds as a function of the fraction of different phases present; these limits correspond to the extreme situations where the most conducting phase is either interconnected or disconnected. Percolation theory is used to determine the probability of having a connected or interconnected conducting phase depending on the dominant lattice geometry. The bulk physical property is then calculated through a weighted arithmetic mean of the physical bounds. The relationships presented here between temperature, mineral composition and electrical conductivity allows constraining one of them given the other two. As an example of the application of our approach, we discuss an area in the Yukon, northwestern Canada, where there is xenolith evidence for a bimodal upper mantle mineral assemblage (harzburgite and lherzolite). This locality coincides spatially with an upper mantle region of low Vp, determined by teleseismic data, and low electrical conductivity, determined by long period magnetotelluric data. The teleseismic data were taken to suggest an up to 200 °C localized increase in mantle temperature, compared to neighbouring regions that are more lherzolitic (Cpx-rich) in nature. Our mixing results can exclude this interpretation, suggesting that compositional variation would be a more appropriated explanation. EM1-4 SCALING OF APPARENT RESISTIVITY OF FERROUS QUARTZITES S.S.Krylov, (St.Petersburg State University. St.Petersburg, Russia, [email protected]) N.Yu.Bobrov, V.A.Lubchich (St.Petersburg State University. St.Petersburg, Russia) Ln < ρa > As it was demonstrated by the set of investigations, the banded iron formations (BIF) have a specific fractal structure. To a first approximation these rocks can be considered as a bicomponent media composed of conductor (magnetite) and insulator (quartz). Such structures can be studied using the percolation theory approaches. It is well known that the effective conductivity depends on the system size according to the power low at the distances less than correlation length of the system. We have performed the laboratory investigations of DC resistivity of the sample of ferrous quartzite from the Pechguba BIF deposit (Kola Peninsula). The sample had quasi-plane shape and its sizes were approximately equal to 0.9х0.7х0.2 м. The apparent resistivity was measured at the upper polished plane with the Wenner’s array. This plane was approximately perpendicular to the quartz bands. The electrodes were placed at the nodes of rectangular grid with the cell size of 1 cm. The distance between the electrodes was varied from 1 to 10 cm. For each fixed distance about 100 measurements in the different points were carried out. Apparent resistivity was derived from the measured values of voltage and current. The values obtained at each distance were then averaged and log-log plot of resistivity mean value versus distance was drawn. This dependence occured to be best fitted by the straight line. The correlation coefficient of the least-square linear fit exceeded 0.99. The 18 power exponent µa was 1.49±0.04 for the four-electrode array and 1.61±0.06 for the three-electrode array. 17 Then the electronic two-color image of the sample’s surface was obtained with the use of a scanner. This image 16 was used for the calculation of fractal dimension by the boxµa=1.49 counting method and effective conductivity by the random15 R2=0.994 walks method. It is obvious that the results of these calculations are to strongly depend on the brightness of the image. In our work the brightness was selected in such a way, 14 that the calculated power exponents to coincide with the 0 0.4 0.8 1.2 1.6 2 Ln L other ones determined in the experiment (taking into account Logarithm of average apparent resisrivity that not the conductivity but the apparent resistivity was versus logarithm of distanse. measured). For the selected image the fractal dimension (four-electrode array) evaluated by the box method is equal to 1.816, that is in a good agreement with the value 1.83 obtained earlier by G.Yu.Ivanyk. During the field work at the Pechguba BIF deposit TEM soundings with coinciding horizontal loops have been carried out and anomalous polarization of ferrous quartzites have been revealed. Analysis of the field sounding curves showed that the dependence of dielectric constant on the frequency could be described by the “universal” Jonscher’s function. The assessment of the crossover frequency for this model is compartible with the frequency, for which the value of electromagnetic wave length is corresponding to the correlation length evaluated by the measurements of apparent resistivity scaling. Acknowledgements: The presented investigations were partly financed by the grant of Russian Ministry of Education and Federal Program “Integratsiya”. EM1-5 PROMOTION OF GRAPHITE FORMATION BY TECTONIC STRESS A LABORATORY EXPERIMENT Georg Nover (Mineralogisches Institut, Universiitat Bonn , [email protected]) Johannes Stoll (Institut fur Geophysik, Universittat Goettingen) Graphitisation of less ordered hexagonal carbon was studied under in-situ pressure and temperature conditions on anthracite, black shale and a synthetic calcite/anthracite mixture at upper greenschist facies conditions. Anthracite exhibited a continuous loss of volatiles in the temperature range from 100C up to 850C (9.9 weight % at 450C) as detected by Differential-Thermo-Analysis (DTA) and Thermo-Gravimetry (TG). Energy dispersive Xray diffraction (EDX) revealed a broad amorphous 002 graphite reflection while after p,Ttreatment nearly perfect crystallised graphitic carbon was detected. The electrical conductivity was measured at the same time in the frequency range from 0.7 up to 100 kHz. As a function of time the bulk resistivity was decreased by about three orders in magnitude at constant pressure and temperature conditions (0.7 GPa, 450C), while the complex response exhibited a continuous decrease of the imaginary part of the impedance. "Quasi-metallic" conduction now dominates the charge transport. Application of pressure, stress, temperature and time caused an increase in ordering and the degree of interconnection of the formerly random oriented carbon sheets. Thus graphitization requires temperatures in the order of 300500C and pressures of several 100 MPa. This result corresponds with the occurence of graphite in overthrusts and nappe structures. EM1-6 INFERENCES OF MULTIPLE MELT CONNECTIVITY FROM MANTLE CONDUCTIVITY Stephen K. Park (IGPP, University of California, Riverside, USA) Mihai N. Ducea (Dept. Geological Sciences, University of Arizona, Tucson, Arizona, USA) Models of the effective conductivity of mixed phases (e.g., melt + solid) based on laboratory measurements are often used to relate electrical conductivity inferred from magnetotelluric studies to the physical and chemical state of the upper mantle. Rarely do these models permit the inclusion of multiple phases, however. Hashin-Shtrikman (1962) bounds are commonly used to provide upper and lower bounds for mixed phases. While rarely used for more than two phases, these bounds do permit the inclusion of multiple phases. Unfortunately, these bounds do not adequately represent the connectivity that might exist between multiple conducting phases in the mantle. A new model is presented here which allows for different connectivities between three phases; this model can be generalized to many phases, however. We use this model to show that the predicted connectivity between two conducting phases (sulfide melt and basaltic melt) beneath the Sierra Nevada matches that observed in laboratory experiments. In particular, sulfide melts form isolated blebs on the surface of the solid phase and the basalt melt is the interconnected phase. This result suggests that electrical conductivity may be used to infer in situ melt properties in the mantle. Hashin, Z., and S. Shtrikman, A variational approach to the theory of effective magnetic permeability of multiphase materials, J. Appl. Phys., 33, 3125-3131, 1962. EM1-7 ELECTRICAL CONDUCTIVITY OF WATER-BEARING WADSLEYITE: IMPLICATIONS FOR THE WATER CONTENT OF THE TRANSITION ZONE Brent T. Poe (Bayerisches Geoinstitut, University of Bayreuth; now at: Istituto Nazionale di Geofisica e Vulcanologia,Via di Vigna Murata 605, 00143 Roma) Claudia Romano (Dipartimento delle Scienze Geologiche, University degli Studi di Roma Tre, Rome, Italy) James A. Tyburczy (Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 [email protected]) Recent laboratory work has demonstrated that the electrical conductivity profile of the mantle can be reasonably approximated from the measured conductivities of the major mineral constituents such as olivine, wadsleyite, ringwoodite and silicate perovskite. Where the approximation shows lesser agreement with recent geophysical electrical conductivity models, particularly at transition zone depths, consideration of the effects of minor constituents that might affect the electrical properties of minerals is required. For the transition zone, only the conductivities of nominally anhydrous minerals have previously been measured, yet both wadsleyite and ringwoodite, which compose a substantial proportion of the transition zone, can incorporate up to several weight percent water into their structures. Knowing the effect of dissolved water on the electrical conductivities of these minerals may thus provide some constraints on the amount of water in the transition zone. We have obtained preliminary results for the electrical conductivity of hydrous wadsleyite at P,T conditions of the Earth's upper mantle. The sample material was synthesized at 12 GPa and 1100 C in a welded Pt capsule using a multianvil apparatus. The recovered material was cut into 0.5 mm thick disks for in-situ measurement by complex electrical impedance spectroscopy also at 12 GPa in a multianvil apparatus. Pre-and post-run analyses by secondary ion mass spectrometry (SIMS) indicate that the samples lost some water during the high P,T conductivity runs. Post run analyses were 0.40 wt % and 0.50 wt % H2O, respectively for two conductivity runs performed. We find that the water-bearing wadsleyite is approximately 1/2 to 1 orders of magnitude more conductive than the nominally dry wadsleyite (Xu et al., 2000) over the temperature range 600 to 1050°C and that conductivity increases with water content. The activation energies are all similar with values of about 1.0 eV. Furthermore, analysis of nominally dry wadsleyite prepared in the same manner as that of the previous workers yields a water content of 0.16 wt % H2O, suggesting that the previous 'nominally dry' results contained a small amount of water. Based on these laboratory results, Earth conductivity profiles in the transition zone (420-670 km depth) can be matched by those of dry wadsleyite, that is, no water is required to exist in the region. EM1-8 LABORATORY MEASUREMENTS OF THE ELECTRICAL PROPERTIES OF GEOTHERMAL RESERVOIR ROCKS FOR RESERVOIR ANALYSIS, FLUID TRACKING, AND FRACTURE DETECTION Jeff Roberts (Geophysics and Global Security, Lawrence Livermore National Laboratory, Livermore, CA, 94551; [email protected]) Abelardo L. Ramirez, Russ Detwiler, Steve Carlson, Bill Ralph and Brian Bonner (Geophysics and Global Security, Lawrence Livermore National Laboratory, Livermore, CA, 94551) EM geophysical methods are often used in the geothermal industry for exploration, reservoir evaluation, to track changing reservoir conditions and to optimize fluid reinjection strategies. The effective use of such geophysical tools requires an understanding of the physical properties that affect electrical conductivity. Laboratory measurements of the electrical resistivity of intact and fractured representative geothermal reservoir rocks were performed to investigate the resistivity contrast caused by active boiling and to infer saturation and fracture location in a field test. Laboratory measurements were performed to simulate field test conditions with confining pressures up to 100 bars and temperatures to 145°C. These measurements are a first step toward making the search for fractures using electrical methods quantitative. Intact samples showed a gradual resistivity increase when pore pressure was decreased below the phaseboundary pressure of free water while fractured samples show a larger more abrupt resistivity increase at the onset of boiling. The change in resistivity for intact samples at the onset of boiling is controlled by the pore size distribution. The ratio of boiling to liquid-saturated sample resistivity ranged between 2 and 5. Analysis of a field test provided the opportunity to evaluate fracture detection using electrical methods at a larger scale. Interpretation of electrical resistance tomography (ERT) images using resistivity contrasts determined by laboratory experiments indicates that actively boiling fractures can be identified. Rapid changes in resistivity as determined by ERT indicated active fractures and changing fluid pathways. Borehole extensometers showed simultaneous fracture movement in the area of most rapidly changing electrical resistivity, corroborating the interpretation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng 48. EM1-9 MANTLE CONDUCTIVITY MEASURED IN THE LABORATORY: A HISTORY T. J. Shankland (Geophysics Group, Los Alamos National Laboratory, Los Alamos NM 87545; [email protected]) Since the electrical conductivity profiles of Lahiri and Price (1939) indoor geophysicists have attempted to answer the question, what does it all mean? How do we interpret such profiles of Earth's mantle? More than half a century has gone into trying to get it right. As we now know, the basic materials are olivines, pyroxenes, spinels, garnets, and their high-pressure, high-temperature polymorphs. However, beginning in the late 1940s, researchers, in the best geophysical tradition, plunged in by simply measuring conductivities in ultramafic rocks. As inconsistencies appeared over the next couple of decades, it was necessary to define minerals in terms of condensed matter physics—an approach made necessary by the need to extrapolate to extremes of mantle conditions not available in the laboratory. By these standards mantle minerals are insulators, and for insulators electrical transport properties are difficult to measure reliably. Achieving chemical buffering (principally of oxygen fugacity by Duba and colleagues) in the early 1970s had two big effects: (1) it threw into doubt most of the previous quarter-century of work, and (2) it introduced nearly unprecedented reproducibility. Improved laboratory measurements permitted the role of iron in charge transfer to be defined and interpreted in terms of oxygen-sensitive defect populations. For mantle olivine (~10% fayalite content) there was actually general agreement among several groups for measurements at mantle temperatures. [In both field and laboratory conductivity measurements half an order of magnitude appears to be the level at which disagreements become academic.] Attention to chemical buffering has led to other advances. For instance, measurements of mineral conductivity in multi-anvil devices and diamond anvil cells have become possible at mantle pressures and/or temperatures. The role of crystallographic phase transitions was elucidated. An unusual and surprising agreement with geophysical observations has been achieved. In another case, "water" in its various chemical species appears to enhance conductivity, at least in the uppermost mantle. Elemental carbon could also play a role in this region. In summary, mistakes as well as successes in this field illustrate how geophysical science moves toward improved understanding of the Earth.