GP31A-01
ESA's Magnetic Field Mission Swarm
Swarm is the fifth Earth Explorer mission in ESA's Living Planet Programme. The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution. The Mission shall deliver data that allow access to new insights into the Earth system by improving our understanding of the Earth's interior and climate. The mission is nominally scheduled for launch in 2010. After release from a single launcher, a side- by-side flying lower pair of satellites at an initial altitude of about 450 km and a single higher satellite at 530 km comprise the Swarm constellation. High-precision and high-resolution measurements of the strength, direction and variation of the magnetic field, complemented by precise navigation, accelerometer and electric field measurements, will provide the observations that are required to separate and model various sources of the geomagnetic field. At present the project is at the beginning of the development phase. The current project status, product performance, and on-going scientific studies will be given special attention during the presentation.
GP31A-02 INVITED
Identifying and quantifying ionospheric magnetic field sources on the night side
The understanding and the modeling of the Earth's main and crustal magnetic field has greatly advanced with the availability of magnetic field observations from low altitude satellites. Precise data, not modified by external field sources, are crucial for developing the high-quality magnetic field models. To avoid significant contributions from ionospheric currents night side observations during low magnetic activity are chosen. However, especially at CHAMP altitudes ionospheric plasma processes persist throughout the night. It was shown that currents which are driven by the interaction of the plasma with the gravity and magnetic fields and with the thermospheric winds cannot be ignored. Different night side ionospheric current systems have been identified and quantified using the high quality magnetic field observations on board CHAMP. It is particularly important to correct for these currents when field gradients derived from measurements of two closely space satellites are employed in magnetic field modeling. In order to obtain real progress in characterizing the current distribution it is necessary to consider all drivers at the same time and maintain current continuity. Such a task requires employing coupled, self-consistent models of the ionosphere and thermosphere. In the context of ESA's Swarm mission such an integrated study will be performed handling all relevant currents, and predicting the resulting magnetic signals. The model to be employed in this activity is the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model. We are going to discuss the study's challenges and its first and expected results.
GP31A-03 INVITED
An overview of comprehensive magnetic field modelling and its implications in combining high-resolution data sets
The so called comprehensive magnetic field modelling method (CM) has proven to be a useful approach in isolating the signals from various magnetic sources near the Earth using multiple data sets, especially when those signals have similar spatial and temporal scales. New data sets are becoming available that provide higher resolution in space and time than previously seen and with this comes the need for new models of commensurate accuracy. Although this process of signal separation becomes more difficult at higher frequencies, the CMs are well placed for addressing the problem. However, new algorithms must be developed within the CM framework to achieve this goal. This talk will give an overview of the CM approach and will discuss recent advances in high-resolution modelling in space and time as it applies to satellite, observatory, marine and aeromagnetic data.
GP31A-04
Modelling the secular variation at a regional scale
In the recent years, several modelling techniques were proposed to represent the magnetic field at a regional scale using ground, aeromagnetic and satellite data. The Revised Spherical Cap Harmonic Analysis (R-SCHA) is suitable for representing the data measured at different altitudes. However, it has always been numerically challenging to represent the three components of the magnetic field with data measured on a single surface because a 2D constrain is not sufficient to uniquely solve a 3D problem. In this paper, we address this previous drawback. The new solution of Laplace equation, named R-SCHA-2D, is expressed in terms of associated Legendre functions and a special case of Conical functions that form a complete block-orthogonal basis function on a spherical cap domain. The efficiency of this modelling will be first demonstrated on a set of synthetic data and compared with the standard Spherical Cap Harmonic Analysis (SCHA). In a second step, we perform a spatio-temporal modelling of the french repeat stations data between 1960 to now using temporal splines and R-SCHA-2D functions. We finally compare the regional model with global IGRF secular variation prediction and we discuss the consistency of the obtained small-scale secular variation features over France.
GP31A-05 INVITED
Satellite Induction Studies Based on Spatiotemporal Analysis of Low-Earth Orbit Magnetometer Data
An overall perspective is given of geomagnetic induction based on spatiotemporal analysis of low-Earth orbit satellite magnetometer data. The goal of geomagnetic induction is to map 3-D electrical conductivity of Earth's mantle. This is a primary objective, for example, of the swarm multi-satellite mission. The eventual aim of satellite geomagnetic induction is to perform 3-D inversions of low-Earth orbiting satellite magnetometer data to map 3-D electrical conductivity distribution of the mantle and to compare with global seismic tomographic images. There are certain advantages to performing this work directly in the spatiotemporal domain, i.e. by performing track-by-track fits instead of estimating frequency-domain response functions. The main challenges arise from the fact that the geological signal from induced currents exhibits length-scale dependent heterogeneity and is small in magnitude. Furthermore, the external Sq and Dst source geometries are poorly represented by the F10.7 and Dst indices. A 1-D time-domain inversion of storm-time CHAMP data reveals a jump in mantle electrical conductivity deeper than transition zone depths, consistent with an earlier magsat analysis. Some future possibilities for satellite induction studies are outlined at the conclusion of the talk.
GP31A-06 INVITED
New insights into the structure of Norwegian continental margins from modern aeromagnetic compilations
We present the aeromagnetic compilation of the Norwegian mainland and its shelf area and its importance for geological models and tectonic studies. The combined data-set reveal that the bedrock structures are continuous from the Baltic Shield under the Caledonian orogen into the continental shelf and that the late-Caledonian collapse of the Caledonian orogene has influenced the style of extension on the Norwegian shelf. On the margin, modern high-resolution aeromagnetic surveys with small line-spacing, more accurate navigation and sensitive magnetometers have revealed the existence of significant magnetic anomalies arising from sedimentary layers. Sub-cropping Late Paleozoic to Tertiary sedimentary units along the Trøndelag-Nordland coast produce a very distinct anomaly pattern. The asymmetry of the anomalies, with a steep gradient and a negative anomaly to the east and a more gentle gradient to the west, relate the anomalies to a strata gently dipping westward. Susceptibility measurements on core samples, hand specimens and in situ on bedrock exposures are essential for the interpretation of these anomalies. Remapping of the oceanic crust has also improved our under-standing of the Tertiary opening of the North Atlantic as previously interpreted oceanic fracture zones zones do not exist; these were artefacts of poor navigation and wide line spacing of the vintage datasets. Tectonic reconstruction has shown that the opening of the Norwegian-Greenland Sea between the Jan Mayen and Senja fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies and without jumps in spreading axis. Transfer zones have previously been associated with oceanic fracture zones along the Mid-Norwegian and East-Greenland margins. Transfer zones are important entry points for sedimentary drainage systems, a relationship that has also been suggested for the transport of Cretaceous sands to the mid-Norwegian margin. Our new interpretation has consequently implications for evaluating the petroleum potential in the Vøring Basin, Mid-Norway. A high-resolution survey of the Oslo Graben changed our understanding of this Permian Rift. The magnetic field data of the Oslo Graben area are unique since two surveys recorded at different flight altitudes (50 m above ground and 3400 m above sea level) exist. The magnetic anomaly reaches values up to 1500 nT over an elongated area of 75 by 30 km. Local structures, such as calderas, ring fracture zones and their associated circular-shaped magnetic anomalies are a first order feature in the aeromagnetic data. Analysis of the depth-to- bottom of the magnetic sources and forward modeling indicates the presence of a more than 15 km thick intrusion beneath the Oslo Graben. The intrusion is interpreted to indicate a differentiation series from base (gabbro) to top (granite). This petrological variation with depth is not detectable in the gravity anomaly because of the small density contrast to the surrounding crust. http://www.ngu.no
GP31A-07
A new method for mapping depth to the Curie-temperature isotherm in the Great Basin from aeromagnetic anomalies
We have revisited the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm and tested our new methodology in an attempt to provide an independent estimate of heat flow in the Great Basin. Such methods typically assume that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose their spontaneous magnetization (e.g., 580°C for magnetite). They usually operate in the Fourier domain by analyzing the shape of the power-density spectrum calculated from aeromagnetic anomalies and critically depend on assumptions about the distribution of crustal magnetization. Early methods assumed that crustal magnetization is a completely random function of position characterized by a flat power- density spectrum. In this study, we attempted to incorporate more realistic geologic models for crustal magnetization and applied the method to newly released aeromagnetic compilations for Nevada and North America. We assume that crustal magnetization has fractal properties, as suggested previously by others, so that the power-density spectrum of the magnetization is proportional to the wavenumber raised to a power -β, where β is related to the geologic terrane. In this case, the theoretical power spectrum, as derived by Maus et al. (Geophys. J. Int., 129, 163-168, 1997), depends on three independent parameters: the depths to the top and bottom of the magnetic source layer and the fractal exponent β. We estimate these parameters by first calculating a three-dimensional matrix representing the misfit between the power spectrum computed from observed data and a variety of theoretical spectra calculated from a range of realistic parameter values. We then search the matrix for the set of parameters that leads to the minimum misfit. This operation was performed on overlapping sliding windows that were swept across the entire magnetic map. A matrix was developed for each window, thereby providing lateral variations in the depth to the bottom of magnetic sources. We tested this methodology on synthetic aeromagnetic data and applied it to aeromagnetic compilations from the Great Basin. Preliminary results obtained by assuming β is constant throughout the Great Basin show spatial variations in the depth to the bottom of magnetic sources that, in general, do not depend on the assumed value of β or on the size of the window. However, our observed variations also do not correlate to large extent with observed surface heat-flow anomalies. They may reflect real variations in crustal magnetic thickness, due either to undulations of the depth to the Curie-temperature isotherm not reflected in surface heat-flow measurements, or to lateral variations of shallower magnetic interfaces. Alternatively, they may be artifacts caused by variations in geologic terrane (i.e., variations in β). Future studies will attempt to include β explicitly, using mapped geology as a guide to help distinguish which of the observed patterns reflect real variations in depth to Curie-temperature isotherm.
GP31A-08
Circum-Arctic Magnetic Anomalies – Challenges of Compilation and the Value of Regional Interpretation in a Frontier Area
Important societal issues are driving increased attention to polar regions. The arctic, in particular, is the focus of scientific studies relating to climate change as well as resource exploration and territorial claims. The news and entertainment media are picking up on polar themes and driving interest within popular culture. Part of the attraction and mystique of the ends of the Earth lies in their relative inaccessibility and harsh environment. These same attributes make it difficult to conduct even basic scientific investigation, and therefore, the arctic remains a scientific frontier in many respects. Delineation of a robust tectonic framework for the top of the world is an essential prerequisite to resource assessment. The difficulty of making direct geologic observations beneath ice and sea requires remote measurement. Regional magnetic anomaly mapping provides important constraining information for the development of tectonic models for this structurally complex region. In addition to the obvious logistical challenges to detailed magnetic field measurement in the high arctic, noise and instability in the magnetic field itself at high latitudes presents difficulties. Nevertheless, regional magnetic anomaly data have been collected over the past 50 years for much of the arctic. The available surveys are diverse in vintage and survey design; the amplitude and frequency content of measured anomalies are widely variable. Availability of metadata and other documentation are also inconsistent for these surveys. This leads to significant challenges in constructing accurate regional magnetic anomaly maps. Preliminary maps from a new international cooperation effort (CAMP-GM, under the direction of Carmen Gaina, Geological Survey of Norway) provide the most consistent view yet of magnetic anomalies for the tectonically complex arctic basins and surrounding continents. Careful attention to digital compilation details allows the new grids to be mathematically filtered to assist in the regional characterization of magnetic domains and boundaries. The frequency content, amplitudes, and patterns of regional magnetic anomalies provide a window into the tectonic character and structure of the crust. Continental, oceanic, and various types of transitional crust each have a distinctive magnetic anomaly signature that can be used to define a fundamental tectonic framework of the circum-arctic. Interpretation can be extended by including additional data such as regional bathymetry (an indicator of crustal buoyancy and isostatic equilibrium) and free air gravity (an independent indicator of crustal density balance and composition). Used together with magnetic domains these data reveal a composite geodynamic subdivision of the arctic. This subdivision provides a framework for investigations of mineral and energy resource potential, tectonic reconstruction, and long-term climate dynamics.