Measurements of the magnetic field are one of the most used methods in geophysical exploration. In order to reduce the degree of ambiguity of this technique during inversion and modelling, data acquired by newly available gradiometer systems based on Superconducting Quantum Interference Devices (SQUIDs) are used. These systems provide measurements of the full magnetic gradient tensor of the Earth's magnetic field, which offers a higher directional sensitivity than conventional total field magnetometers. A magnetization vector inversion (MVI) approach has been applied on data sets acquired over a dolerite intrusion in central Germany in order to model the full magnetization vector including remanent and induced components. Two different models have been created: one using only the magnetic total field anomaly (TFA) and the other based on five components of the magnetic gradient tensor. The two models show in principal the same structure, but the model based on the gradient tensor shows better defined structures. Also, magnetization amplitudes are closer to those measured on rock samples in this area. A comparison of the total magnetization vector of the rock samples and the models shows a better agreement in the vector direction of the gradient model compared to the total field model. A separation of induced and remanent contributions to the total magnetization has been performed and again shows better results when the gradient-based model is used. The effectiveness of the separation procedure will be discussed herein. The usage of gradiometer systems in an airborne geomagnetic exploration provides additional directional information, which is very helpful for MVIs. Compared to our model based on conventional TFA data, the gradient-based model features a much better agreement of the shape and magnetization of subsurface structures with those obtained from geologic(-1)al studies. The same, FTMG derived estimates of magnetization are more consistent with the results of measurements on rock samples.

Summary Measuring local perturbations of the Earth’s magnetic field is one of the most successful methods in geophysical exploration, even though it has an inherent ambiguity. In order to avoid misinterpretations, more and more interpreters gravitate towards techniques that allow including not only induced, but also remanent magnetization in the modelling process. One of today’s common approaches is magnetization vector inversion (MVI), which has been successfully applied on many data sets with very promising results. The application of vectorial-type magnetometers or gradiometers such as full tensor magnetic gradiometery (FTMG) systems based on Superconducting Quantum Interference Devices (SQUID), provides additional directional information, which are very useful, in particular in the MVIs. Independent information, e.g. magnetic susceptibility is still necessary in order to perform a reliable separation of the magnetization types. The application of electromagnetic methods, e.g. frequency domain electromagnetics (FDEM) allows to gain independend information on magnetic susceptibility. Here, we combine different approaches, i.e. dc-resistivity (electrical resistivity tomography, ERT), FDEM and FTMG measurements, in order to produce robust subsurface models. This staggered work flow is tested on a small study site in the West Eifel Volcanic field in Germany, where a basaltic lava flow is investigated

ABSTRACTCommonly, geomagnetic prospection is performed via scalar magnetometers that measure values of the total magnetic intensity. Recent developments of superconducting quantum interference devices have led to their integration in full tensor magnetic gradiometry systems consisting of planar‐type first‐order gradiometers and magnetometers fabricated in thin‐film technology. With these systems measuring directly the magnetic gradient tensor and field vector, a significantly higher magnetic and spatial resolution of the magnetic maps is yield than those produced via conventional magnetometers.In order to preserve the high data quality in this work, we develop a workflow containing all the necessary steps for generating the gradient tensor and field vector quantities from the raw measurement data up to their integration into high­resolution, low­noise, and artefactless two‐dimensional maps of the magnetic field vector. The gradient tensor components are processed by superposition of the balanced gradiometer signals and rotation into an Earth‐centred Earth‐fixed coordinate frame. As the magnetometers have sensitivity lower than that of gradiometers and the total magnetic intensity is not directly recorded, we employ Hilbert‐like transforms, e.g., integration of the gradient tensor components or the conversion of the total magnetic intensity derived by calibrated magnetometer readings to obtain these values. This can lead to a better interpretation of the measured magnetic anomalies of the Earth's magnetic field that is possible from scalar total magnetic intensity measurements. Our conclusions are drawn from the application of these algorithms on a survey acquired in South Africa containing full tensor magnetic gradiometry data.

(1) Federal Institute for Geosciences and Natural Resources (BGR), B2: Ground Water and Soil, Hannover, Germany, (2) Federal Institute for Geosciences and Natural Resources (BGR), B4: Geoscientific Information, International Cooperation, (3) Leibniz Institute for Photonic Technologies, Jena, Germany, (4) Supracon AG, Jena, Germany, (5) Federal Institute for Geosciences and Natural Resources (BGR), B1: Energy Resources, Mineral Resources, Hannover, Germany

The recent development of airborne full tensor magnetic gradiometer (FTMG) systems, based on superconducting quantum interference devices (SQUID), allows to obtain the full magnetic gradient tensor of the Earth's magnetic field of large areas (10x10 km). This system allows acquiring all components of the magnetic gradient tensor. This tensor exhibits some advantages over conventional airborne magnetic field data, e.g. a higher spatial resolution and additional directional sensitivity. In this work a FTMG system was applied in the framework of the multidisciplinary INFLUINS project (Integrated fluid dynamics in sedimentary basins) in order investigate different areas in the Thuringian Basin and the neighboring highlands. Main goal was to map magnetic lineaments along major fault zones and to demonstrate the advantages of airborne FTMG. Full tensor data sets have been acquired with very low system noise of only 60 (pT/m). Two different case studies are presented: In the first case study a strong magnetic anomaly in the center of the Thuringian Forest, caused by the magmatic intrusion of the Hohenberger dolerite is analyzed, which exhibits indications of a significant remanent magnetization. Multiple magnetization vector inversions were performed using either the full magnetic gradient tensor or only the total field anomaly data. The inversion results are evaluated using magnetization directions acquired by paleomagnetic sampling and available geological information. In the second case study, a small magnetic anomaly was investigated. It was discovered while mapping magnetic anomalies along the Eichenberg-Gotha-Saalfeld fault zone, which is one of the major fault zones in the Thuringian Basin. The detected lineament is interpreted using the components of the magnetic gradient tensor, additional ground based geo-electrical data and available geological information. The inversion of the magnetic gradients revealed a steeply dipping zone of mostly induced magnetization.

The aim of this work is to deploy a new SQUID (Superconducting Quantum interference device) based instrument for the measurement of the full magnetic gradient tensor of the Earth’s magnetic field in survey scenarios in a sedimentary basin in Thuringia, a local province in Germany. This task requires developing according processing, inversion, and interpretation techniques for this new instrument. The recent state of the instrument and data processing techniques is presented. The new instrument has several advantages compared to commercially available high-resolution aeromagnetic survey instruments. Besides the fact that weaker magnetic anomalies could be detected, it delivers vector data and thus more detailed information even on remanence of the geologic structures. It is required for more enhanced magnetic anomaly delineation and possibly for the determination of the age of intrusive or alteration structures. As a proof of principle a small-scaled magnetic anomaly on the border of the Thuringian basin was selected. The area was mapped in 2013. The results are presented and preliminary results of the inversion discussed which indicate remanent magnetization of the rocks which cause the magnetic anomaly.

Magnetic mapping missions like SWARM and its predecessors, e.g. the CHAMP and MAGSAT programs, offer high resolution Earth’s magnetic field data. These datasets are usually combined with magnetic observatory and survey data, and subject to harmonic analysis. The derived spherical harmonic coefficients enable magnetic field modelling using a potential series expansion. Recently, new instruments like the JeSSY STAR Full Tensor Magnetic Gradiometry system equipped with very high sensitive sensors can directly measure the magnetic field gradient tensor components. The full understanding of the quality of the measured data requires the extension of magnetic field models to gradient tensor components. In this study, we focus on the extension of the derivation of the magnetic field out of the potential series magnetic field gradient tensor components and apply the new theoretical framework to the International Geomagnetic Reference Field (IGRF) and the High Definition Magnetic Model (HDGM). The gradient tensor component maps for entire Earth’s surface produced for the IGRF show low values and smooth variations reflecting the core and mantle contributions whereas those for the HDGM gives a novel tool to unravel crustal structure and deep-situated ore bodies. For example, the Thor Suture and the Sorgenfrei-Thornquist Zone in Europe are delineated by a strong northward gradient. Derived from Eigenvalue decomposition of the magnetic gradient tensor, the scaled magnetic moment, normalized source strength (NSS) and the bearing of the lithospheric sources are presented. The NSS serves as a tool for estimating the lithosphere-asthenosphere boundary as well as the depth of plutons and ore bodies. Furthermore changes in magnetization direction parallel to the mid-ocean ridges can be obtained from the scaled magnetic moment and the normalized source strength discriminates the boundaries between the anomalies of major continental provinces like southern Africa or the Eastern European Craton.

Tilt meters are a widely used tool for monitoring long-term and small-scale deformations. Therefore, they are perfectly suited to test potential subsurface waste repositories. Since November 2005 a high sensitive tilt meter of the ASKANIA-type is recording at a distance of 300m from Mizunami Underground Research Laboratory, where two vertical shafts are under construction. During the construction of those shafts large pumps were used to reduce the groundwater level, leading to two large-and several small-scale pore pressure induced tilt signals. Due to the fault system nearby, the tilt direction does not coincide with the direction towards the pump as would be expected in homogeneous media. In this study we analyze the main surface tilt direction caused by pore pressure induced deformation. Our results show two main directions which are both nearly perpendicular to the fault. Also, the long-term signals show a high correlation with the short-term pore pressure induced tilt signals.

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.