ALBERT

Description: We have performed an extended palaeomagnetic study of the Oslo Graben volcanics, compared to the study of half a century ago by van Everdingen, using modern techniques and a four times larger amount of sites, plus additional rock magnetic experiments. We conclude that the average direction ( D = 204.0, I = –37.9, k = 46.9, α 95 = 2.0) and associated palaeomagnetic pole ( = 48.3, = 155.5, K = 52.2, A 95 = 1.9) of the Krokskogen and Vestfold volcanics together are statistically identical to those of the earlier study. This gives confidence in the fact that older palaeomagnetic studies can be reliable and robust, even though methods have improved. Our larger number of samples, and better age constraints, enable us to separate the data into two major intervals: the younger, on average, Krokskogen area and the older Vestfold area. The results show firstly that palaeolatitudes are slightly higher than predicted by the latest apparent polar wander path (APWP) for Eurasia by Torsvik et al. These data support an early Permian Pangaea A configuration and do not necessitate a Pangaea B configuration. The larger data set also allows us to assess the distribution of the characteristic remanent magnetization directions of the Oslo Graben in terms of geomagnetic field behaviour, which were acquired during a long period of dominantly single polarity the Permo-Carboniferous Reversed Superchron (PCRS). The distributions show a significantly lower virtual geomagnetic pole (VGP) scatter at the observed (low) latitudes than expected from a compilation from lavas of the last 5 Myr. The data do however show excellent agreement with the scatter observed both during the Cretaceous Normal Superchron and the PCRS. A comparison of the directional distributions in terms of elongation is less discriminating, since the large errors in all cases allow a fit to the predicted elongation/inclination behaviour of the TK03.GAD model.

Description: Lake sediments are excellent sources of palaeoenvironmental and palaeoclimatic information because they provide continuous and high-resolution records. South America is of particular interest because it is the only landmass that stretches southward into the Pacific and Atlantic Oceans towards Antarctica. The aim of this study is to explore the relationship of magnetic parameters with elemental and palaeobiological data of Laguna Potrok Aike to develop a model of lake-level changes and related hydrological and climatic fluctuations. Magnetic measurements were performed on subsamples from 15 500 cal. BP to the present, and associated rock magnetic parameters were calculated to infer magnetic mineralogy, concentration and grain size. According to the model, parameters dependent on magnetic concentration and grain size are directly related to lake-level changes. During dry periods, the remanent coercivity displays high values, whereas the proportion of magnetite is relatively low. Low percentages of greigite are observed, indicating that the water of the lake was stratified at least four times during the studied period, at approximately 10 300, 8900, 8500 and 8300 cal. BP. The preservation of greigite by inhibiting its complete transformation into pyrite is associated with a rapid burial that occurs with high sedimentation rates. Thermal stratification could be caused by a slight cooling in the area triggered by a weakening of the Westerlies and/or low activity of the sun, sum to the effect of a flood of melt water in the North Atlantic.

Description: We illustrate a new imaging method to estimate the depth to the sources of potential fields and the structural index. The method consists of applying the Depth from EXtreme Point (DEXP) transformation to the ratio (R) between two different-order partial derivatives of the field. While the scaling function of the potential field depends on the structural index, we show that the scaling function of R merely depends on the difference between the two used orders of differentiation. This allows three main features to be established for the DEXP transformation of R: (1) it is independent from the structural index; (2) the estimation of the source depths is fully automatic, simply consisting in the search of position of the extreme points of the DEXP image and (3) the structural index of each source is finally determined from the scaling function or the extreme points using the estimated depth. Besides the well-known characteristics of the DEXP transformation, such as high-resolution and stability, the DEXP transformation of R enjoys one more relevant feature: it can be applied to multisource cases, yielding simultaneously correct estimations of structural index and depth for each source in the same image. However, while the DEXP transformation is a linear transformation of the field, the DEXP transformation of R is non-linear, and a procedure is described to circumvent the non-linear effects. The method is tested with synthetic examples and the estimated source parameters show a good agreement with the true values. The method was applied also to real magnetic data from the Pima copper mine, Arizona, USA, Hamrawien area, Egypt and Cataldere, Bala district of Turkey. The results are consistent with the known information about the causative sources.

Description: This paper presents new directional archeomagnetic data from nine Meso-/Neolithic fireplaces, sampled in a cave shelter, at Arconciel, in western Switzerland. Rock magnetic measurements indicate a homogenous magnetic mineralogy in all fireplaces, with magnetite as the main magnetic carrier. The remanent magnetization is stable and generally shows one characteristic directional component. Nine new directions, which were obtained from Arconciel, are combined with 356 other archeomagnetic data from a circular area with a radius of 700 km around this site, to obtain a penalized least square spline fit for the past 9000 yr. We found in general good agreement with other local compilations, such as the Balkan curve, the regional SCHA.DIF.8k model and with lake sediments from UK, Fennoscandia and Switzerland. Nevertheless, a time lag of several centuries is observed for a declination maximum between the archeomagnetic spline fit and the other European data records around 5900 BC. This time lag is also observed in the Swiss lake sediment record; therefore we interpret this shift as a local feature of the Earth's magnetic field.

Description: Reconstructions of the Holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long-term solar-terrestrial relationships and as relative dating tools for geological and archaeological archives. Here, we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts, igneous rocks and sediment records. A new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data. To reduce the influence of a few individual records all sedimentary data are resampled in 50-yr bins, which also means that more weight is given to archaeomagnetic data during the inversion. The sedimentary declination data are treated as relative values and adjusted iteratively based on prior information. Finally, an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties, often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model. As a result of the data adjustments, power has been shifted from quadrupole and octupole to higher degrees compared with previous Holocene geomagnetic field models. We find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr. The new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder, possibly originating from the equator.

Description: Measurement of magnetic vector or tensor quantities, namely of field or field gradient, delivers more details of the underlying geological setting in geomagnetic prospection than a scalar measurement of a single component or of the scalar total magnetic intensity. Currently, highest measurement resolutions are achievable with superconducting quantum interference device (SQUID)-based systems. Due to technological limitations, it is necessary to suppress the parasitic magnetic field response from the SQUID gradiometer signals, which are a superposition of one tensor component and all three orthogonal magnetic field components. This in turn requires an accurate estimation of the local magnetic field. Such a measurement can itself be achieved via three additional orthogonal SQUID reference magnetometers. It is the calibration of such a SQUID reference vector magnetometer system that is the subject of this paper. A number of vector magnetometer calibration methods are described in the literature. We present two methods that we have implemented and compared, for their suitability of rapid data processing and integration into a full tensor magnetic gradiometry, SQUID-based, system. We conclude that the calibration routines must necessarily model fabrication misalignments, field offset and scale factors, and include comparison with a reference magnetic field. In order to enable fast processing on site, the software must be able to function as a stand-alone toolbox.

Description: This paper presents new directional archeomagnetic data from nine Meso-/Neolithic fireplaces, sampled in a cave shelter, at Arconciel, in western Switzerland. Rock magnetic measurements indicate a homogenous magnetic mineralogy in all fireplaces, with magnetite as the main magnetic carrier. The remanent magnetization is stable and generally shows one characteristic directional component. Nine new directions, which were obtained from Arconciel, are combined with 356 other archeomagnetic data from a circular area with a radius of 700 km around this site, to obtain a penalized least square spline fit for the past 9000 yr. We found in general good agreement with other local compilations, such as the Balkan curve, the regional SCHA.DIF.8k model and with lake sediments from UK, Fennoscandia and Switzerland. Nevertheless, a time lag of several centuries is observed for a declination maximum between the archeomagnetic spline fit and the other European data records around 5900 BC. This time lag is also observed in the Swiss lake sediment record; therefore we interpret this shift as a local feature of the Earth's magnetic field.

Description: Surface nuclear magnetic resonance (surface-NMR) is a promising technique for exploring shallow subsurface aquifer structures. Surface-NMR can be applied in environments that are characterized as a 1-D layered Earth. The technique utilizes a single loop and is referred to as magnetic resonance sounding. The technique referred to as magnetic resonance tomography (MRT) allows complex 2-D aquifer structures to be explored. Currently, MRT requires multiple loops and a roll along measurement scheme, which causes long survey time. We propose a loop layout using an elongated transmitter and an in-loop receiver arrays (ETRA) to conduct a 2-D survey with just one measurement. We present a comprehensive comparison between the new layout and the common approaches based on sensitivity and resolution analyses and show synthetic and field data. The results show that ETRA generates subsurface images at sufficient resolution with significantly lower survey times than other loop layouts.

Description: We studied ionospheric responses to the 2012 April 11 M w 8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CIDs) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (~1 km s –1 ). Resonant atmospheric oscillation with a frequency ~4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes world-wide with moment magnitudes ( M w ) 6.6–9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the M w increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, that is small vertical crustal movements for their magnitudes.

Description: Very low frequency (VLF) electromagnetic waves that penetrate conductive magma-filled dykes generate secondary fields on the surface that can be used to invert for dyke properties. The model used for the interpretation calculates currents induced in a conductive strip by an inducing field that decays exponentially with depth due to the conductivity of the surrounding medium. The differential equations are integrated to give an inhomogeneous Fredholm equation of the second kind with a kernel consisting of a modified Bessel function of the second kind. Numerical methods are typically used to solve for the induced currents in the strip. In this paper, we apply a modified Galerkin–Chebyshev method, which involves separating the kernel into source and field spectra and integrating the source terms to obtain a matrix equation for the unknown coefficients. The incident wave is expressed as a Chebyshev series. The modified Bessel function is separated into a logarithmic singularity and a non-singular remainder, both of which are expanded in complex Chebyshev polynomials. The Chebyshev coefficients for the remainder are evaluated using a fast Fourier transform, while the logarithmic term and incident field have analytic series. The deconvolution then involves a matrix inversion. The results depend on the ratio of strip-size to skin-depth. For infinite skin-depth and a singular conductivity distribution given by $\tau _0 a/\sqrt{a^2 - z^2 }$ (where 0 is the conductance, a is the half-length and z the distance from the centre), Parker gives an analytic solution. We present a similar analytic series solution for the finite skin-depth case, where the size to skin depth ratio is small. Results are presented for different ratios of size to skin depth that can be compared with numerical solutions. We compare full-space and half-space solutions. A fit of the model to VLF data taken above a magma filled dykes in Hawaii and Mt Etna demonstrates that while properties such as depth to top, conductivity ratio, tilt and dip can be determined, the depth to bottom is indeterminate due to the exponential decay of the inducing field.