ALBERT

Description:    This paper focuses on the study of the correlation between magnetic parameters with the level of contamination by heavy metals in urban soils. We report a magnetic investigation of urban soil samples from Merida, state of Yucatan, Southern Mexico. It appears that most of our samples contain ferrimagnetic minerals as the magnetic carriers, probably coming from the titanomagnetites/titanomaghemites series. This is inferred by the acquisition of isothermal remanent magnetization, which shows that most of samples are almost completely saturated at about 200 mT. The S −200 value (factor characterizing stability of remanent magnetization) is between 0.8 and 1.0, characteristic of ferrimagnetic minerals. The susceptibility vs. temperature measurements also point to some titanomaghemites and titanomagnetites as probable responsible for magnetization. On the other hand, measurements of magnetic susceptibility at high and low frequencies helped us to determine the high content of superparamagnetic grains in the majority of the samples, although not all of these showed high values of magnetic susceptibility. We found that the most contaminated samples by Pb and Zn showed the higher saturation isothermal remanent magnetization values, whereas the higher values in magnetic susceptibility correspond to samples contaminated by Cr. Finally, we noted that a high level in Sr decreases the magnetic susceptibility. Content Type Journal Article Pages 377-388 DOI 10.1007/s11200-011-0021-6 Authors Bertha Aguilar Reyes, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Francisco Bautista, Laboratorio de Suelos y Agua, Centro de Investigaciones en Geografía Ambiental, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Avto Goguitchaichvili, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Ofelia Morton, Laboratorio de Paleomagnetismo, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 2

Description:    The Early Cretaceous may be considered a key period for understanding the evolution of the Earth’s magnetic field. Some still unsolved problems are related to the mode of paleosecular variation (PSV) of the Earth’s magnetic field before and during the Cretaceous Normal Superchron. We report here a detailed rock-magnetic, paleomagnetic and paleointensity investigation from 28 lava flows (331 standard paleomagnetic cores) collected in the Argentinean part of the Parana Flood Basalts (Formation Posadas) in order to contribute to the study of PSV during the early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average paleofield direction is precisely determined from 26 sites, which show small within-site dispersion and high directional stability. Five sites show evidences for the self-reversal of thermoremanent magnetization. 23 sites yielded normal polarity magnetization and only 3 are reversely magnetized. Moving windows averages were used to analyze the sequential variation of virtual geomagnetic pole’s (VGP) axial positions. Interestingly, the axial average VGP path traces an almost complete cycle around the geographical pole and passes near the location of all previously published Paraná Magmatic Province poles. Both paleomagnetic poles and average VGP paths are significantly different from the pole position suggested by fixed hotspot reconstructions, which may be due to true polar wander or the hotspot motion itself. Only 15 samples from 5 individual basaltic lava flows, yielded acceptable paleointensity estimates. The site mean paleointensities range from 25.2 ± 2.2 to 44.0 ± 2.2 µT. The virtual dipole moments (VDMs) range from 4.8 to 9.9 × 10 22 Am 2 . This correspond to a mean value of 7.7 ± 2.1 × 10 22 Am 2 which is 96% of the present day geomagnetic field strength. These intensities agree with the relatively high values already reported for Early Cretaceous, which are consistent with some inferences from computer simulations previously published. Content Type Journal Article Pages 279-309 DOI 10.1007/s11200-011-0016-3 Authors Mabel Mena, Instituto de Geofísica Daniel Valencio, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina Avto Goguitchaichvili, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Miguel Cervantes Solano, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Juan Francisco Vilas, Instituto de Geofísica Daniel Valencio, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 2

Description:    In this work we try to better characterise the shallow magnetic signature of hydrocarbon microseepage in oil fields from eastern and western Venezuela. To get a better insight of the processes involved, we attempt to find out the main magnetic phases responsible for the observed oil-related shallow magnetic anomalies. In this way, a new and alternative numerical approach to decompose Isothermal Remagnetization (IRM) curves is introduced. The method is based on a Direct Signal Analysis (DSA) of the IRM curve in order to identify the number and type of magnetic components. Representative wells from western (La Victoria) and eastern (San Joaquín) Venezuelan fields are studied. The DSA approach, together with rock magnetic experimental results, indicates that in the well from western Venezuela the main magnetic mineralogy associated to hydrocarbon microseepage is magnetite. Conversely, in the well from eastern Venezuela, these MS anomalies are mainly caused by the presence of Fe-sulphides (i.e. greigite). These results support the hypothesis of two different authigenic processes. For the well at the western field, we propose that a net electron transfer from the organic matter, degraded by hydrocarbon gas leakage, should occur precipitating Fe(II) magnetic minerals (e.g. magnetite). On the other hand, in the well of the eastern field, high concentrations of H2S at shallow levels, might allow the formation of secondary Fe-sulphides. Content Type Journal Article Pages 343-358 DOI 10.1007/s11200-011-0019-0 Authors Milagrosa Aldana, Dpto. Ciencias de la Tierra, Universidad Simón Bolívar, Caracas, Venezuela Vincenzo Costanzo-Álvarez, Dpto. Ciencias de la Tierra, Universidad Simón Bolívar, Caracas, Venezuela Luis Gómez, Coordinación de Ingeniería Geofísica, Universidad Simón Bolívar, Caracas, Venezuela Carlos González, Coordinación de Ingeniería Geofísica, Universidad Simón Bolívar, Caracas, Venezuela Marisel Díaz, Centro de Física, Instituto Venezolano de Investigaciones Científicas IVIC, Apartado 20632, Caracas, 1020A Venezuela Pedro Silva, Centro de Física, Instituto Venezolano de Investigaciones Científicas IVIC, Apartado 20632, Caracas, 1020A Venezuela Myriam Rada, Coordinación de Ingeniería Geofísica, Universidad Simón Bolívar, Caracas, Venezuela Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 2

Description:    For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacán block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacán block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α 95 = 8.9°. The corresponding mean paleomagnetic pole position is P lat = 75.7°, P long = 166.6°, K = 31, A 95 = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacán Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence. Content Type Journal Article Pages 265-278 DOI 10.1007/s11200-011-0015-4 Authors Jose Rosas-Elguera, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Ingeniería Sísmica - Sede Guadalajara, CUCEI, Universidad de Guadalajara, 44840 Guadalajara, Mexico Bertha Aguilar Reyes, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Avto Goguitchaichvili, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Macario Rocha, Servicio Geológico Mexicano, Pachuca, Mexico Margarita López Martínez, Depto. de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Apdo. Postal 60 Ensenada, B.C. México, Mexico Miriam M. Tostado-Plascencia, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Ingeniería Sísmica - Sede Guadalajara, CUCEI, Universidad de Guadalajara, 44840 Guadalajara, Mexico Luis M. Alva Valdivia, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, Mexico Cecilia Caballero Miranda, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, Mexico Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 2

Description:    The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated by 40 Ar/ 39 Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian statistical parameters are k = 28, α 95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position P lat = 83.9°, P long = 321.6°, K = 34, A 95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study of the scatter of the virtual geomagnetic poles giving S F = 15.9 with S U =21.0 and S L = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35 Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1. Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine sediments at about 590 ka. Content Type Journal Article Pages 311-328 DOI 10.1007/s11200-011-0017-2 Authors Rafael Maciel Peña, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Avto Goguitchaichvili, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Bernard Henry, Paléomagnétisme, Institut de Physique du Globe de Paris and Centre National de la Recherche Scientifique, 94107 Saint Maur, France Leda Sánchez-Bettucci, Departamento de Geología, Area Geofísica-Geotectónica, Facultad de Ciencias, Universidad de la República, 11200 Montevideo, Uruguay Juan Morales, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Bertha Aguilar Reyes, Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica - Sede Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, Mexico Ana María Soler-Arechalde, Laboratorio de Paleomagnétismo, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 México DF., México Manuel Calvo-Rathert, Laboratorio de Paleomagnetismo, Departamento de Física, Escuela Politécnica Superior, Universidad de Burgos, C/Francisco de Vitoria, s/n, 09006 Burgos, Spain Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 2

Description:    This paper presents an approach to determine the gradient of curvature of the normal plumblines at a point P above the ellipsoid and introduces a new geometrical object which is the isocurvature line. The assumed facts are the coordinates of the point P and the formula for the normal gravity potential U. For the determination of the gradient of the normal plumbline curvature k at the point P we define a small circle on the meridian plane of P whose center is at the point P. The circle has the radius of one meter and interior D. In this circle we construct a curvature replacement function to approximate the curvature function k. This replacement function is a quotient of polynomials hence it is easy to find its partial derivatives at the point P. For the construction of replacement function we make the assumption that in the interior of the circle D the first order partial derivatives of U behave linearly and the second order partial derivatives have constant values which equal their value at the point P. Then we set the gradient of the curvature function to be equal with the gradient of the aforementioned replacement function at P. An isocurvature line of the normal gravity field passing through a point P is a curve such that the value of the function of the plumblines’ curvature k is constant and equals k(P). We give a formula to find the direction of the isocurvature line on the meridian plane and we prove that there are infinitely many isocurvature lines passing through the point P and they all lie on a special surface, the isocurvature surface. Content Type Journal Article Pages 501-514 DOI 10.1007/s11200-011-0030-5 Authors Gerassimos Manoussakis, Department of Surveying Engineering, National Technical University of Athens, Iroon Polytechniou 9, Zografos, 15780 Athens, Greece Demitris Delikaraoglou, Department of Surveying Engineering, National Technical University of Athens, Iroon Polytechniou 9, Zografos, 15780 Athens, Greece Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 3

Description:    A spherical approximation makes the basis for a majority of formulas in physical geodesy. However, the present-day accuracy in determining the disturbing potential requires an ellipsoidal approximation. The paper deals with constructing Green’s function for an ellipsoidal Earth by an ellipsoidal harmonic expansion and using it for determining the disturbing potential. From the result obtained the part that corresponds to the spherical approximation has been extracted. Green’s function is known to depend just on the geometry of the surface where boundary values are given. Thus, it can be calculated irrespective of the gravity data completeness. No changes of gravity data have an effect on Green’s function and they can be easily taken into account if the function has already been constructed. Such a method, therefore, can be useful in determining the disturbing potential of an ellipsoidal Earth. Content Type Journal Article Pages 455-464 DOI 10.1007/s11200-011-0026-1 Authors Elena M. Mazurova, Moscow State University of Geodesy and Cartography (MIIGAiK), Moscow, Russia Mary I. Yurkina, Central Research Institute of Geodesy, Aerosurveying and Cartography (TsNIIGAiK), Moscow, Russia Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 3

Description:    Spherical harmonic transformation is of practical interest in geodesy for transformation of globally distributed quantities such as gravity between space and frequency domains. The increasing spatial resolution of the latest and forthcoming gravitational models pose true computational challenges for classical algorithms since serious numerical instabilities arise during the computation of the respective base functions of the spherical harmonic expansion. A possible solution is the evaluation of the associated Legendre functions in the Fourier domain where numerical instabilities can be circumvented by an independent frequency-wise scaling of numerical coefficients into a numerically suitable double precision range. It is then rather straightforward to commit global fast data transformation into the Fourier domain and to evaluate subsequently spherical harmonic coefficients. For the inverse, the computation of respective Fourier coefficients from a given spherical harmonic model is performed as an inverse Fast Fourier Transform into globally distributed data points. The two-step formulation turns out to be stable even for very high resolutions as well as efficient when using state-of-the-art shared memory/multi-core architectures. In principle, any functional of the geopotential can be computed in this way. To give an example for the overall performance of the algorithm, we transformed an equiangular 1 arcmin grid of terrain elevation data corresponding to spherical harmonic degree and order 10800. Content Type Journal Article Pages 489-500 DOI 10.1007/s11200-011-0029-y Authors Christian Gruber, GFZ German Research Centre for Geosciences, Section 1.2: Global Geomonitoring and Gravity Field, c/o DLR Oberpfaffenhofen, D-82230 Wessling, Germany Pavel Novák, Research Institute of Geodesy, Topography and Cartography, 25066 Zdiby, Czech Republic Josef Sebera, Technical University in Prague, Thákurova 7/2077, 166 29 Praha 6, Czech Republic Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 3

Description:    The classical least-squares (LS) algorithm is widely applied in practice of processing observations from Global Satellite Navigation Systems (GNSS). However, this approach provides reliable estimates of unknown parameters and realistic accuracy measures only if both the functional and stochastic models are appropriately specified. One essential deficiency of the stochastic model implemented in many available GNSS software products consists in neglecting temporal correlations of GNSS observations. Analysing time series of observation residuals resulting from the LS evaluation, the temporal correlation behaviour of GNSS measurements can be efficiently described by means of socalled autoregressive moving average (ARMA) processes. For a given noise realisation, a well-fitting ARMA model can be automatically estimated and identified using the ARMASA toolbox available free of charge in MATLAB® Central. In the preliminary stage of applying the ARMASA toolbox to residual-based modelling of temporal correlations of GNSS observations, this paper presents an empirical performance analysis of the automatic ARMA estimation tool using a large amount of simulated noise time series with representative temporal correlation properties comparable to the GNSS residuals. The results show that the rate of unbiased model estimates increases with data length and decreases with model complexity. For large samples, more than 80% of the identified ARMA models are unbiased. Additionally, the model error representing the deviation between the true data-generating process and the model estimate converges rapidly to the associated asymptotical value for a sufficiently large sample size with respect to the correlation length. Content Type Journal Article Pages 537-556 DOI 10.1007/s11200-011-0033-2 Authors Xiaoguang Luo, Geodetic Institute, Karlsruhe Institute of Technology (KIT), Englerstr. 7, 76131 Karlsruhe, Germany Michael Mayer, Geodetic Institute, Karlsruhe Institute of Technology (KIT), Englerstr. 7, 76131 Karlsruhe, Germany Bernhard Heck, Geodetic Institute, Karlsruhe Institute of Technology (KIT), Englerstr. 7, 76131 Karlsruhe, Germany Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169 Journal Volume Volume 55 Journal Issue Volume 55, Number 3

Description:    The frequency-dependent anisotropy of magnetic susceptibility (AMS) can be most easily determined through subtracting directional susceptibilities measured at two operating frequencies along each of the measuring directions and the differences subsequently processed using standard methods for AMS computation. The effect of the measurement precision on the accuracy of the determination of the frequency-dependent AMS is investigated through mathematical simulation of the measurement process using the statistical theory of measuring the AMS of rocks. The accuracy of the AMS determination is presented in terms of the errors in determining the principal susceptibilities and principal directions. By modelling the measuring errors, the limits are discovered for the investigation of the frequency-dependent AMS. The modelling shows extremely high requirements for measurement accuracy met by the most sensitive instruments, only. Content Type Journal Article Pages 1-14 DOI 10.1007/s11200-011-9009-5 Authors František Hrouda, AGICO Ltd., Ječná 29a, 621 00 Brno, Czech Republic Jiří Pokorný, AGICO Ltd., Ječná 29a, 621 00 Brno, Czech Republic Journal Studia Geophysica et Geodaetica Online ISSN 1573-1626 Print ISSN 0039-3169