Spectral method for processing hydromagnetic survey data at shallow depths

<p>Magnetic surveys are commonly used for solving variety of geotechnical and geological challenges in offshore areas, jointly with a set of other geophysical methods. The most popular technique employed is hydromagnetic surveying with towed magnetometers. One of the most significant challenges encountered during processing of the magnetic data is related to temporal variations of the Earth's magnetic field. Accounting for diurnal magnetic field variations is often done by carrying out differential hydromagnetic surveys, a technique developed in the 1980-s. It is based on simultaneous measurements of the magnetic field using two sensors towed behind the vessel with a given separation. This technique allows to calculate along-course gradient which is free of magnetic field temporal variations. This measurement system resembles a gradiometer, with the distance between two sensors being referred to as the base of the gradiometer. It is possible to calculate anomalous magnetic field by integrating obtained magnetic field gradient. Studies have shown that accuracy of its reconstruction decreases with increasing base of the gradiometer. This becomes most significant when distance between the sensors and sources of magnetic field anomalies is small. This situation occur when the survey area is located in shallow water (i.e. for shallow marine, river or lake surveys).</p><p>An approach for deriving magnetic anomalies and accounting for diurnal variations in differential hydromagnetic surveys based on the frequency (spectral) representation of the measurements was proposed in 1987 [Melikhov, 1987]. This approach utilizes the fact that it is possible to reconstruct the spectrum of magnetic field anomalies along the vessel course from the spectra of measured signals from the first S<sub>1</sub>(ω) and second S<sub>2</sub>(ω) sensors. Assuming that the sensors are located at the same depth, it can be achieved via the following transform:</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.3d3911bac60061487501161/sdaolpUECMynit/12UGE&app=m&a=0&c=ff23bad5ed5181be02f7ef7ab5e8d6e4&ct=x&pn=gepj.elif&d=1" alt="" width="192" height="43"></p><p>where ω - spatial frequency, <em>l</em> - base of the gradiometer, and <em>i</em> - imaginary unit. Assuming that at a single moment in time magnetic field variations equally affect both sensors, resulting Fourier spectrum T(ω) will correspond the spectrum of anomalous magnetic field, free of the magnetic variations. It should be noted that, similar to the along-course gradient integration approach, anomalous magnetic field is restored to a certain accuracy level.</p><p>Estimates made on model examples showed that accuracy of the field reconstruction using this method is comparable to the accuracy levels of modern marine magnetic surveys (±1-3 nT). It could be noted that for gradiometer bases comparable or larger than depths to magnetic anomaly sources, errors of the field reconstruction are significantly lower for the spectral transformation-based approach compared to along-course gradient integration.</p><p>References:</p><p>Melikhov V.R., Bulychev A.A., Shamaro A.M. Spectral method for solving the problem of separating the stationary and variable components of the geomagnetic field in hydromagnetic gradiometric surveys // Electromagnetic research. - Moscow. IZMIRAN, 1987. - P. 97-109. (in Russian)</p><p> </p>

An updated summary digital map of the anomalous magnetic field of Russia as a base for geo-modeling

<p>A digital map of the anomalous magnetic field  (AMF) of Russia has been created over 12 years in the monitoring (update) mode. The map was built from the level of the normal magnetic field <em>Т</em> <sub>n VSEGEI-1965</sub>  at a scale of 1: 2,500,000 using materials that were not previously involved in the process of summary mapping, taking into account modern digital technologies. The base of digital cartographic data contains grids on the network of 2,500 ×2,500 and 5,000×5,000 m and cartographic projects in * .mxd.<br>The anomalous component is of particular interest in the study of geodynamic processes and dynamic environments in the earth's crust and upper mantle. It is believed that the anomalous (short-wave or high-frequency) component, being a quasi-stationary (Lugovenko V.N., 1982) function of the general geomagnetic field, almost does not change over time. However, when calculating it, the primary role is played by the correct registration of the secular variation and the normal field, which change both in time and in space, and these changes are closely related to the dynamic processes inside the Earth. The works of T. Nagata (1969), F. Stacey (1974, 1977), Yu.P. Skovorodkin and L.S. Bezugloy (1980), V.A. Shapiro (1983) and others showed that the anomalous magnetic field of the Earth is also characterized by temporary changes associated with the dynamics of field sources, manifested in anomalies of the secular course. There is a connection between the secular variation anomalies and regional medium-scale anomalies. Within the Manchazh regional anomaly, the anomalous magnetic field increases monotonically at a rate of up to ±5 nT per year. It has been established that the source of the Manchazh anomaly is a block of rocks with increasing remanent magnetization, the mechanism of which is still unclear. The relationship between AMF changes with changes in the seismic regime and with individual earthquakes is evidenced by changes in the amplitudes of temporary changes in the local field from 5-8 nT at the Carpathian geodynamic test site and up to 30-80 nT during the Moneron earthquake on southern Sakhalin. Changes up to the first tens of nT AMFs were recorded several days before the Tashkent earthquake (Ulomov, 1967). During this earthquake, the author of this article observed the glow of the atmosphere, which indicates strong short-term changes in the variable geomagnetic field, which caused ionization processes in the surface layers of the atmosphere.<br>The Earth's magnetic field is 99% generated by its internal sources and reacts sensitively to nonequilibrium phase transitions of a different hierarchical class, which are the basis for the self-organization of the planet Earth system. On the map of magnetic anomalies of Russia, geostructures of different orders of rectilinear, circular, arcuate mosaic forms of anomalies are clearly distinguished, grouped into systems, the shape and size of which allows to reasonably judge the geodynamic conditions of their formation.</p><p> </p>

The crustal structure of Onega-Kandalaksha paleorift identified by complex analysis of the anomalous magnetic field of the White Sea

Geological and geophysical studies recently conducted in the White Sea and the adjacent territory have provided new data on the deep structure of this region. Our study aims to conduct complex analysis of the anomalous magnetic field and the geological and geophysical data on the Onega-Kandalaksha paleorift located in the White Sea basin and the adjacent southeastern land area, and to develop a model showing its deep structure. The basis for analysing the magnetic field is the anomalous magnetic field (AMF) map constructed by the authors using the magnetic survey data consolidated by the Marine Arctic Geological Expedition (MAGE) in 2003–2008 and supplemented by the survey data of the Institute of Oceanology RAS in 2001–2004. The parameters of the magnetically active layer are estimated by the independent complementary methods of quantitative interpretation developed by the Laboratory of Geophysical Fields, P.P. Shirshov Institute of Oceanology RAS. This article describes a model showing the structure and formation of the magnetically active layer of the White Sea paleorift. Our study shows that the magnetically active layer of the paleorift system has a complex structure reflecting all the main stages in the evolution of tectonic activity in the White Sea region, from the Middle and Late Riphean to the last glaciation of the Quaternary period. The model includes three structural layers, each corresponding to a certain stage. The bottom structural layer is the base of the magnetically active layer, which reflects the continental rifting stage in the evolution of the White Sea mobile belt in the Middle and Late Riphean. The middle structural layer reflects the Middle Paleozoic (Late Devonian) stage of rifting reactivation, which is characterized by alkaline-ultrabasic magmatism and represented by swarms of alkaline dykes and diatremes, including kimberlite pipes. The top structural layer reflecting a high-frequency component of the AMF is related to the highly magnetic sources of anomalies located in the upper part of this structural layer. The characteristics of the top structural layer suggest that it formed in the Late Pleistocene – Holocene and developed during the final stage the tectonic activation of this region. The deep crustal structure of the White Sea basin is specified in our model showing the magnetically active layer for the low-frequency component of the AMF. In the southeastern part of the basin, magmatism products of the basic (Riphean – Vendian) and alkaline-ultrabasic (Middle Paleozoic) composition are abundant in the crust and provide for a strong magnetic source of anomalies, the lower edges of which are traced at the depths to 30 km. This probably reflects the most active plume-lithospheric interaction. Wedging and uplifting of the magnetically active layer northwestward along the Onega-Kandalaksha rift is related to the White Sea (Belomorsky) deep fault. This fault is a long-lived conduit that channels magma from the central portion of the plume, as evidenced by the igneous bodies of the basic composition in the basement and central parts of the sedimentary wedge in the Kandalaksha graben. The complex analysis of the AMF in the White Sea region suggests the presence of morphologically different igneous bodies in the upper crust in the study region.

Anisotropic atomic-structure related anomalous Hall resistance in few-layer black phosphorus

Specific anisotropic-atomic-structure of atom-thin black phosphorus causes the anomalous magnetic-field dependence of the Hall resistance, which opens doors to novel quantum phenomena and innovative two-dimensional atom-thin devices.