Ground-based Magnetic Surveys Research Articles

Drone-based magnetometer prospection for archaeology

Magnetometry is one of the most efficient and successful methods of archaeological prospection. Drone-based prospecting is increasingly being used in many fields of remote sensing but with respect to magnetometry, with rather poor results. For magnetic surveys, drone prospecting comes with the problem that magnetic and mechanical disturbances originating from the aircraft decrease the quality of the measurements.Here we present the adaptation of a commercial three-axis fluxgate magnetometer setup, which can be suitable for archaeological prospection, after applying appropriate filter methods and minimising the flying altitude and speed. For approval of the system, we performed drone-based surveys in high spatial resolution (50 cm line spacing) at constant, ultra-low sensor altitudes of 45 ± 10 cm and 75 ± 10 cm. We chose an archaeological site from the roman period as survey site (3.8 ha), where high-quality ground-based caesium magnetometer data was available. This allows us to demonstrate the first detailed comparison of drone-based and ground-based magnetic survey data for archaeology. For further evaluation, the influence of the drone on the measurements is assessed, drift and sensor resolution checks are carried out, and suitable data filtering methods are evaluated.Our results show that we can detect main archaeological features such as ditches, pits, fireplaces and remnants of stone fundaments with the drone-based setup, which are the relevant structures for the majority of archaeological prospections worldwide. Further, we can prove that the data quality can significantly be increased by lowering the flight altitude and speed. We conclude that drone-based magnetometry serves above all to cover large and inaccessible areas in short time. Our findings are a first step towards the development of standards for drone-based archaeological prospection approaches.

Technology of low-altitude aeromagnetic survey for prospecting for iron ores as a direction of sustainable development of modern geology

Exploration is essential to ensure the achievement of the UN Sustainable Development Goals through the provision of ferrous and non-ferrous metal resources. At the same time, geology as an industry should be based on the principles of sustainable development. The purpose of the study is to develop and test the technology of aeromagnetic survey for prospecting for iron ores that meets the requirements of the sustainable development of modern geology. Unmanned aerial vehicles with GNSS receivers, a camera, a magnetometer, as well as Trimble Business Center, Agisoft Metashape Professional, and QM Center software were used. A search for magnetic radiation anomalies over an area of 6.20 square kilometres was carried out. Areas were established where the radiation level is 59000–65860 nanotesl (background – 57803 nanotesl), that is, there are reserves of magnetite ores. The binding of these sites to the exact geographical coordinates has been completed. The results of the low-altitude aeromagnetic survey are compared with the control data of the ground-based magnetic survey performed earlier, and the correctness of the low-altitude aeromagnetic survey data is shown. Since ground surveys provide more detailed information, it is advisable to use low-altitude aeromagnetic surveys as a method of fast, low-cost screening of large areas. This will make it possible to make decisions on the expediency of carrying out expensive ground works. The study found that labour productivity in low-altitude aeromagnetic surveys exceeds that in ground-based surveys by about 75 times. In addition, low-altitude aeromagnetic surveys have virtually no impact on ecosystems and do not lead to greenhouse gas emissions. This corresponds to the main directions of sustainable development of geology.

Integrative Geophysical Studies at the Novaya Kurya-1 Cemetery in the Kulunda Steppe

We outline the results of prospection studies at the Novaya Kurya-1 cemetery in the south of Western Siberia, using remote sensing methods such as aerial photography, ground-based magnetometry, high-precision aeromagnetic survey, electromagnetic profi ling, and electrotomography. Original techniques were used to construct relative relief maps, and an inversion of data from ground-based magnetic survey at various altitudes was carried out. The fi rst technique reduces the effect of natural relief, and highlights anthropogenic altitudinal anomalies, making the analysis of digital elevation models more effi cient. The second technique is helpful for assessing the thickness and depth of anomalous magnetic bodies or horizons, not only providing planigraphic information but enabling us to evaluate two- and threedimensional geometric properties of the detected objects. As a result of the analyses, at least 14 kurgans were identifi ed at the cemetery, six of which lack salient outward features. Structural details suggest that most of them date to the Early Scythian time (800–400 BC). On the basis of the interpretation of the results of highly effi cient prospection analyses using the UAV platform, offering the possibility of surveying a large area (about 25 ha), the boundaries of the site were determined. Several features were detected. To identify these, further studies are needed.

Magma propagation and emplacement within the central intrusive complex of Summer Coon stratovolcano, Colorado

A field-based study used geologic mapping and a ground-based magnetic survey to investigate the exhumed central intrusive complex (CIC) of Summer Coon stratovolcano in Colorado. The CIC comprises a group of diorite to granodiorite stocks and sub-vertical domains of conduit-filling breccia hosted within basaltic-andesite breccia associated with cone building. The stocks serve as the focal points for 53 basaltic-andesite (mafic) and 36 andesite to rhyolite (evolved) radial dikes mapped within the CIC. The evolved dikes with outcrop lengths (strike length) of <650 m are confined to the central areas of the CIC, cut through the stocks, and have steeply plunging terminal segments. In contrast, most evolved dikes with outcrop lengths >1600 m are excluded from the central areas of the CIC, do not cut through the stocks, and have terminal segments that plunge shallowly toward their focus. Assuming the dikes are blade-shaped and that they originated directly below the CIC, the propagation direction of the evolved dikes was estimated using the plunge angles, spatial distribution, and dike outcrop lengths. The relatively long dikes may have ascended toward the level of exposure along inclined paths. These dikes remained exclusively within the basaltic-andesite breccia and, probably due to the higher relative stiffness of the stocks, were unable to propagate through the stocks as they ascended. Upon approaching the level of exposure, the same dikes encountered a stress barrier likely generated by the gravitational load of the edifice. This barrier altered the dike propagation paths from inclined to sub-horizontal, significantly increasing their outcrop lengths. In contrast, the dikes with short outcrop lengths ascended along primarily sub-vertical paths, intersecting the level of exposure within the central portions of the CIC. To propagate sub-vertically through both the relatively stiff stocks and a potential stress barrier, the magma overpressures within the shorter dikes may have been higher relative to the longer sub-horizontally propagating dikes. It is probable that at active volcanoes, only dikes with sufficiently high overpressures can ascend through the central intrusive complex of mature stratovolcanoes to feed eruptions near the summit. Perhaps more frequently, stress barriers and existing intrusions stall or deflect dikes with lower relative overpressures toward the slopes of the volcano where they may feed flank eruptions.

Sub-surface structure of La Soufrière of Guadeloupe lava dome deduced from a ground-based magnetic survey

In this study, we present the analysis and interpretation of a new ground magnetic survey acquired at the Soufrière volcano on Guadeloupe Island. Observed short-wavelength magnetic anomalies are compared to those predicted assuming a constant magnetization within the sub-surface. The good correlation between modeled and observed data over the summit of the dome indicates that the shallow sub-surface displays relatively constant and high magnetization intensity. In contrast, the poor correlation at the base of the dome suggests that the underlying material is non- to weakly-magnetic, consistent with what is expected for a talus comprised of randomly oriented and highly altered and weathered boulders. The new survey also reveals a dipole anomaly that is not accounted for by a constant magnetization in the sub-surface and suggests the existence of material with decreased magnetization beneath the Soufrière lava dome. We construct simple models to constrain its dimensions and propose that this body corresponds to hydrothermally altered material within and below the dome. The very large inferred volume for such material may have implications on the stability of the dome.

High-Resolution Ground-Based Magnetic Survey of a Buried Volcano: Anomaly B, Amargosa Desert, NV

High-Resolution Ground-Based Magnetic Survey of a Buried Volcano: Anomaly B, Amargosa Desert, NV

The first aeromagnetic survey in the Arctic: results of the &amp;lt;i&amp;gt;Graf Zeppelin&amp;lt;/i&amp;gt; airship flight of 1931

Abstract. In July of 1931, on the eve of International Polar Year II, an Arctic flight of the Graf Zeppelin rigid airship was organized. This flight was a realization of the idea of F. Nansen, who advocated the use of airships for the scientific exploration of the Arctic territories, which were poorly studied and hardly accessible at that time. The route of the airship flight was Berlin – Leningrad – Arkhangelsk – Franz Josef Land – Severnaya Zemlya – the Taimyr Peninsula – Novaya Zemlya – Arkhangelsk – Berlin. One of scientific goals of the expedition was to measure the H and D geomagnetic field components. Actually, the first aeromagnetic survey was carried out in the Arctic during the flight. After the expedition, only preliminary results of the geomagnetic measurements, in which an anomalous behavior of magnetic declination in the high-latitude part of the route was noted, were published. Our paper is concerned with the first aeromagnetic measurements in the Arctic and their analysis based on archival and modern data on the magnetic field in the Barents and Kara sea regions. It is shown that the magnetic field along the flight route had a complicated structure, which was not reflected in the magnetic charts of those times. The flight was very important for future development of aero- and ground-based magnetic surveys in the Arctic, showing new methods in such surveys.

The Magnetization Vector became important in Surface Geology and Chromite Prospecting within Ultramaphic Massifs

Ground-based magnetic survey was performed on two ultramaphic massifs, Albania, mapping magnetic anomalies possibly connected with tectonic and cumulate sections, grade of horst rocks serpentinization and/or shallow chromite deposits of variable size. The results were contradictory. We have implemented a scheme and computer code for contouring rock surface with abnormal magnetization utilizing the simplicity of magnetic bell shaped field anomaly with so-called vertical and the analytic signal for the 2-d thin sheet model. Rapid estimations for the edge of the sources, depth, intensity and angle of total magnetization for the observed shallow anomalies surveyed-line data are obtained. As an alternative of filtering technique we have used the magnetic computed field intensity map of the area with constrained source parameters. The computed magnetic field anomaly map with magnetization vector collinear within the range revealed from chromite laboratory measurements show characteristic, which hardly notices in the survey magnetic anomaly map. The predicted magnetic data on an area 4 km2 are well correlated with the shallow deposits or surface occurrences or controlled associated structures in the harzburgite host rocks. Image of total magnetization inclination data derived from ground magnetic survey acquired over a 30 km2 resembles the geology than the total field magnetic anomaly map. These data show that the total magnetization of thick residual dunites, rich in basaltic impregnation, reveals an opposite direction data against the underlain harzburgite rocks

Geophysical Modelling of Structure and Tectonostratigraphic History of the Longford Basin, Northern Tasmania

A new gravity survey combined with ground-based magnetic surveys have enabled detailed modelling of basement structures in the Longford Basin of northern Tasmania. This basin began to form in the mid-Jurassic, but contains only Cainozoic sediments. The models suggest that the basin has been formed by a series of predominantly northwest-trending Jurassic normal faults which have produced a series of half-grabens in which blocks were downthrown to both east and west with dips of up to 10° W.The basin blocks incorporate Palaeozoic to Mesozoic Parmeener Supergroup sedimentary rocks intruded by pre-to syn-fault Jurassic dolerites. These units overlie an older, geophysically distinct, Devonian fold–thrust terrane. Much of the tectonic activity from the Palaeozoic to the Cainozoic appears to have been controlled by the Tiers Fault system, a major gravity and magnetic lineament within the Tasmanian crust.In contrast to many other Tasmanian Mesozoic–Cainozoic basins, the Longford Basin is exclusively continental, with up to 800 m of Paleocene to Quaternary fluvial sediments having been drilled in two wildcat oil wells.The basin also contains intercalated Late Eocene to Pliocene basaltic volcanic rocks. Magnetic data indicate three eruptive centres localised in reactivated Jurassic faults. Massive flows, now lateritised, have infilled an Early Eocene to Pliocene drainage system.

The Blairgowrie magnetic anomaly and its interpretation using simplex optimisation

AbstractThe large magnetic anomaly which runs along the Highland Boundary Fault of Scotland immediately to the NW of Blairgowrie has been studied by both a ground-based magnetic survey and a palaeomagnetic investigation. A newly developed optimisation program, making use of a simplex algorithm, was used to model the long wavelength component of this effectively two-dimensional anomaly. The consequent model consists of a vertical, rectangular body, 3 km wide and 13 km deep, whose top surface is 2 km below ground level, with a magnetisation directed vertically downwards. The composition of this body is most likely to be that of an ultra-basic, metamorphic complex which has been brought up to its present position between two of the many near-vertical faults in the area. The medium wavelength structure of the anomaly was modelled using the common technique of trial and error, and can be interpreted in terms of a pair of Devonian andesitic lavas and a small extension of the main ultrabasic block. Subsequent palaeomagnetic remanence measurements confirmed that the magnetisation of the two lavas recorded a Devonian polarity reversal. The magnetisation of the ultrabasics is directed vertically downwards, exactly as deduced from the earlier modelling work.