How to record high quality data on moving ice shelf? The geomagnetic observatory at Neumayer Station III, Antarctica

VNA (IAGA code) is the geomagnetic observatory of the German Antarctic research station Neumayer III located on the Ekström ice shelf at 70.7° S and 351.7° E. The station is the currently the only permanent station in Antarctica situated on an ice shelf and as such it raises unique challenges for geomagnetic data acquisition. The station, along with the ice shelf, drifts by 157 m per year to the north and rotates clockwise by 0.25° per year. But high-quality, absolutely calibrated geomagnetic field vector data depend on precise knowledge of the geodetic reference frame and – for the study of secular variation – stable conditions, especially with respect to artificial or crustal fields at the observatory. Here, we present the observatory and our approach to compensate for the observatory's movements for obtaining both near real-time and definitive geomagnetic observatory data. First, the azimuth of the azimuth mark has to be determined and updated regularly, as it also rotates with the ice. The observatory is located in an underground ice cave and therefore celestial objects cannot serve as a fixed reference and instead, the azimuth is determined on a monthly basis by a north-seeking gyro. Second, due to its northward drift, the observatory is moving across a significant gradient of the crustal magnetic field observed first in aeromagnetic surveys over the region as well as in ground profiles of total field strength along the drift path. This results in a temporal change of the crustal bias of approx. 10 nT per year. Based on the observatory data alone, this signal cannot be separated from the secular variation of the core field. In order to correct the observatory time series for this change in crustal bias, vector absolute measurements were performed along the drift path in austral summer 2016/2017. Additionally, the aeromagnetic data was downward continued to obtain the vector field along the drift path of the observatory. In the austral summer 2017/18, the observatory was extended by a high frequency induction-coil magnetometer to study ultra low frequency waves associated with interaction of solar wind and the magneto- and ionosphere

SQUID-based magnetic prospection in interdisciplinary case studies at possible early and high medieval harbour sites in Germany

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Comparison of novel semi-airborne electromagnetic data with multi-scale geophysical, petrophysical and geological data from Schleiz, Germany

In the framework of the Deep Electromagnetic Sounding for Mineral EXploration (DESMEX) project, we carried out multiple geophysical surveys from regional to local scales in a former mining area in the state of Thuringia, Germany. We prove the applicability of newly developed semi-airborne electromagnetic (EM) systems for mineral exploration by cross-validating inversion results with those of established airborne and ground-based investigation techniques. In addition, supporting petrophysical and geological information to our geophysical measurements allowed the synthesis of all datasets over multiple scales. An initial regional-scale reconnaissance survey was performed with BGR's standard helicopter-borne geophysical system deployed with frequency-domain electromagnetic (HEM), magnetic and radiometric sensors. In addition to geological considerations, the HEM results served as base-line information for the selection of an optimal location for the intermediate-scale semi-airborne EM experiments. The semi-airborne surveys utilized long grounded transmitters and two independent airborne receiver instruments: induction coil magnetometers and SQUID sensors. Due to the limited investigation depth of the HEM method, local-scale electrical resistivity tomography (ERT) and long-offset transient electromagnetic (LOTEM) measurements were carried out on a reference profile, enabling the validation of inversion results at greater depths. The comparison of all inversion results provided a consistent overall resistivity distribution. It further confirmed that both semi-airborne receiver instruments achieve the bandwidth and sensitivity required for the investigation of the resistivity structure down to 1 km depth and therewith the detection of deeply seated earth resources. A 3D geological model, lithological and geophysical borehole logs as well as petrophysical investigations were integrated to interpret of the geophysical results. Distinct highly-conductive anomalies with resistivities of less than 10 Om were identified as alum shales over all scales. Apart from that, the petrophysical investigations exhibited that correlating geophysical and geological information using only one single parameter, such as the electrical resistivity, is hardly possible. Therefore, we developed a first approach based on clustering methods and self-organizing maps (SOMs) that allowed us to assign geological units at the surface to a given combination of geophysical and petrophysical parameters, obtained on different scales. © 2020 The Author

DESMEX: A novel system development for semi-airborne electromagnetic exploration

There is a clear demand to increase detection depths in the context of raw material exploration programs. Semi-airborne electromagnetic (semi-AEM) methods can address these demands by combining the advantages of powerful transmitters deployed on the ground with efficient helicopter-borne mapping of the magnetic field response in the air.The penetration depth can exceed those of classical airborne EM systems,since low frequencies and large transmitter-receiver offsets can be realized in practice. A novel system has been developed that combines high-moment horizontal electric bipole transmitters on the ground with low-noise three-axis induction coilmagnetometers, a three-axis fluxgate magnetometer and a laser gyroinertial measurement unit integrated within a helicopter-towed airborne platform. The attitude data are used to correct the time series for motional noise and subsequently to rotate into an Earth-fixed reference frame. In a second processing step, and as opposed to existing semi-airborne systems, we transform the data into the frequency domain and estimate the complex-valued transfer functions between the received magnetic field components and the synchronously recorded injection current by regression analysis. This approach is similar to the procedure employed in controlled-source EM. For typical source bipole moments of 20-40 kAm and for rectangular current waveforms with a fundamental frequency of about 10 Hz, we can estimate reliable three-component transfer functions in the frequency range from 10-5000 Hz over a measurement area of 4 x 5 km2 for a single source installation. The system has the potential to be used for focused exploration of deep targets

Comparison of novel semi-airborne electromagnetic data with multi-scale geophysical, petrophysical and geological data from Schleiz, Germany

In the framework of the Deep Electromagnetic Sounding for Mineral EXploration (DESMEX) project, we carried out multiple geophysical surveys from regional to local scales in a former mining area in the state of Thuringia, Germany. We prove the applicability of newly developed semi-airborne electromagnetic (EM) systems for mineral exploration by cross-validating inversion results with those of established airborne and ground-based investigation techniques. In addition, supporting petrophysical and geological information to our geophysical measurements allowed the synthesis of all datasets over multiple scales. An initial regional-scale reconnaissance survey was performed with BGR's standard helicopter-borne geophysical system deployed with frequency-domain electromagnetic (HEM), magnetic and radiometric sensors. In addition to geological considerations, the HEM results served as base-line information for the selection of an optimal location for the intermediate-scale semi-airborne EM experiments. The semi-airborne surveys utilized long grounded transmitters and two independent airborne receiver instruments: induction coil magnetometers and SQUID sensors. Due to the limited investigation depth of the HEM method, local-scale electrical resistivity tomography (ERT) and long-offset transient electromagnetic (LOTEM) measurements were carried out on a reference profile, enabling the validation of inversion results at greater depths. The comparison of all inversion results provided a consistent overall resistivity distribution. It further confirmed that both semi-airborne receiver instruments achieve the bandwidth and sensitivity required for the investigation of the resistivity structure down to 1 km depth and therewith the detection of deeply seated earth resources. A 3D geological model, lithological and geophysical borehole logs as well as petrophysical investigations were integrated to interpret of the geophysical results. Distinct highly-conductive anomalies with resistivities of less than 10 Omega rn were identified as alum shales over all scales. Apart from that, the petrophysical investigations exhibited that correlating geophysical and geological information using only one single parameter, such as the electrical resistivity, is hardly possible. Therefore, we developed a first approach based on clustering methods and self-organizing maps (SOMs) that allowed us to assign geological units at the surface to a given combination of geophysical and petrophysical parameters, obtained on different scales. (C) 2020 The Author(s). Published by Elsevier B.V