Ground Magnetic Data Research Articles

Empirical evidence for latitude dependence and asymmetry of geomagnetic spatial variation in mainland China

AbstractSpatiotemporal geomagnetic variation is a significant research topic of geomagnetism and space physics. Generated by convection and flows within the fluid outer core, latitude dependence and asymmetry, as the inherent spatiotemporal properties of geomagnetic field, have been extensively studied. We apply and modify an extension of existing method, Hidden Markov Model (HMM), which is an efficient tool for modeling the statistical properties of time series. Based on ground magnetic measurement data set in mainland China, first, we find the parameters of HMM can be used as the geomagnetic statistical signature to represent the spatiotemporal geomagnetic variations for each site. The results also support the existence of the geomagnetic latitude dependence more apparently. Furthermore, we provide solid empirical evidence for geomagnetic asymmetry relying on such ground magnetic measurement data set.

Reconstructing the Vulcano Island evolution from 3D modeling of magnetic signatures

High-resolution ground and marine magnetic data are exploited for a detailed definition of a 3D model of the Vulcano Island volcanic complex. The resulting 3D magnetic imaging, obtained by 3-D inverse modeling technique, has delivered useful constraints both to reconstruct the Vulcano Island evolution and to be used as input data for volcanic hazard assessment models. Our results constrained the depth and geometry of the main geo-structural features revealing more subsurface volcanic structures than exposed ones and allowing to elucidate the relationships between them. The recognition of two different magnetization sectors, approximatively coincident with the structural depressions of Piano caldera, in the southern half of the island, and La Fossa caldera at the north, suggests a complex structural and volcanic evolution. Magnetic highs identified across the southern half of the island reflect the main crystallized feeding systems, intrusions and buried vents, whose NNW–SSE preferential alignment highlights the role of the NNW–SSE Tindari–Letojanni regional system from the initial activity of the submarine edifice, to the more recent activity of the Vulcano complex. The low magnetization area, in the middle part of the island may result from hydrothermally altered rocks. Their presence not only in the central part of the volcano edifice but also in other peripheral areas, is a sign of a more diffuse historical hydrothermal activity than in present days. Moreover, the high magnetization heterogeneity within the upper flanks of La Fossa cone edifice is an imprint of a composite distribution of unaltered and altered rocks with different mechanical properties, which poses in this area a high risk level for failure processes especially during volcanic or hydrothermal crisis.

Middle‐ and low‐latitude ionosphere response to 2015 St. Patrick's Day geomagnetic storm

AbstractThis paper presents a study of the St Patrick's Day storm of 2015, with its ionospheric response at middle and low latitudes. The effects of the storm in each longitudinal sector (Asian, African, American, and Pacific) are characterized using global and regional electron content. At the beginning of the storm, one or two ionospheric positive storm effects are observed depending on the longitudinal zones. After the main phase of the storm, a strong decrease in ionization is observed at all longitudes, lasting several days. The American region exhibits the most remarkable increase in vertical total electron content (vTEC), while in the Asian sector, the largest decrease in vTEC is observed. At low latitudes, using spectral analysis, we were able to separate the effects of the prompt penetration of the magnetospheric convection electric field (PPEF) and of the disturbance dynamo electric field (DDEF) on the basis of ground magnetic data. Concerning the PPEF, Earth's magnetic field oscillations occur simultaneously in the Asian, African, and American sectors, during southward magnetization of the Bz component of the interplanetary magnetic field. Concerning the DDEF, diurnal magnetic oscillations in the horizontal component H of the Earth's magnetic field exhibit a behavior that is opposed to the regular one. These diurnal oscillations are recognized to last several days in all longitudinal sectors. The observational data obtained by all sensors used in the present paper can be interpreted on the basis of existing theoretical models.

The importance of ground magnetic data in specifying the state of magnetosphere–ionosphere coupling: a personal view

In the history of geomagnetism, geoelectricity and space science including solar terrestrial physics, ground magnetic records have been demonstrated to be a powerful tool for monitoring the levels of overall geomagnetic activity. For example, the Kp and ap indices having perhaps the long-history geomagnetic indices have and are being used as space weather parameters, where “p” stands for “planetary” implying that these indices express average geomagnetic disturbances on the entire Earth in a planetary scale. To quantify the intensity level of geomagnetic storms, however, it is common to rely on the Dst index, which is supposed to show the magnitude of the storm-time ring current. Efforts were also made to inter-calibrate various activity indices. Different indices were proposed to express different aspects of a phenomenon in the near-Earth space. In the early 1980s, several research groups in Japan, Russia, Europe and the US developed the so-called magnetogram-inversion techniques, which were proposed all independently. Subsequent improvements of the magnetogram-inversion algorithms allowed their technology to be applied to a number of different datasets for magnetospheric convection and substorms. In the present review, we demonstrate how important it was to make full use of ground magnetic data covering a large extent in both latitudinal and longitudinal directions. It is now possible to map a number of electrodynamic parameters in the polar ionosphere on an instantaneous basis. By applying these new inverse methods to a number of ground-based geomagnetic observations, it was found that two basic elements in spatial patterns can be viewed as two physical processes for solar wind-magnetosphere energy coupling.

Structural mapping of Chikotra River basin in the Deccan Volcanic Province of Maharashtra, India from ground magnetic data

Ground magnetic data collected over Chikotra River in the peripheral region of Deccan Volcanic Province (DVP) of Maharashtra located in Kolhapur district was analysed to throw light on the structural pattern and distribution of magnetic sources within the basin. In order to isolate the magnetic anomalies showing varying trend and amplitude, several transformation operations including wavelength filtering, and upward continuation has been carried out on the reduced to pole anomaly map. Qualitative interpretation of these products help identify the distribution of magnetic sources, viz., the Deccan basalts, dolerite intrusives and older greenstone and schist belts in the subsurface. Present study suggests that the Chikotra basin is composed of three structural units; a NE–SW unit superposed on deeper NW–SE unit with randomly distributed trap flows on the surface. One of the major outcome of the present study is the delineation of almost 900-m thick Proterozoic Kaladgi sediments below the Deccan trap flows. The NE–SW magnetic sources may probably represent intrusives into the Kaladgi sediments, while the deeper NW–SE trends are interpreted as the northward extension of the Dharwars, underneath the Deccan lava flows, that forms the basement for the deposition of Kaladgi sediments.

Analytic Signal and Euler Depth Interpretation of Magnetic Anomalies: Applicability to the Beatrice Greenstone Belt

We apply the Analytic Signal and Euler depth filtering techniques on magnetic data to identify a magnetic causative body location-depth relationship, two parameters of importance in both geophysical exploration and ore body modelling. We identify a dipping magnetic contact from the interpreted Euler depth anomalies, showing a good agreement with both the Total Field Magnetic (TFM) map and the Analytic Signal (AS) map. The Euler depth anomalies correlate well with the locations and edges of shallow causative bodies. The deeper Euler interpreted sources explain the magnetic high on the regional aeromagnetic map which is coincident with neither geological contacts nor the more recent dolerite intrusions. This suggests that the magnetic highs on the regional aeromagnetic map are due to deep seated sources, otherwise invisible on the regional geological map. The results show the usefulness and relevancy of these two filters not only in interpreting routine TFM data from the study area, but up to a regional scale. While the aeromagnetic data shows that the magnetisation pattern is predominantly divorced from the geological map, the ground magnetic data interpretation points to a more recent magnetisation of the belt, enabling conclusions to be drawn about the geological history and structural geology otherwise not evident on the geological map.

Identification of magnetic anomalies based on ground magnetic data analysis using multifractal modelling: a case study in Qoja-Kandi, East Azerbaijan Province, Iran

Abstract. Ground magnetic anomaly separation using the reduction-to-the-pole (RTP) technique and the fractal concentration–area (C–A) method has been applied to the Qoja-Kandi prospecting area in northwestern Iran. The geophysical survey resulting in the ground magnetic data was conducted for magnetic element exploration. Firstly, the RTP technique was applied to recognize underground magnetic anomalies. RTP anomalies were classified into different populations based on the current method. For this reason, drilling point area determination by the RTP technique was complicated for magnetic anomalies, which are in the center and north of the studied area. Next, the C–A method was applied to the RTP magnetic anomalies (RTP-MA) to demonstrate magnetic susceptibility concentrations. This identification was appropriate for increasing the resolution of the drilling point area determination and decreasing the drilling risk issue, due to the economic costs of underground prospecting. In this study, the results of C–A modelling on the RTP-MA are compared with 8 borehole data. The results show that there is a good correlation between anomalies derived via the C–A method and the log report of boreholes. Two boreholes were drilled in magnetic susceptibility concentrations, based on multifractal modelling data analyses, between 63 533.1 and 66 296 nT. Drilling results showed appropriate magnetite thickness with grades greater than 20 % Fe. The total associated with anomalies containing andesite units hosts iron mineralization.

Giant pulsations on the afternoonside: Geostationary satellite and ground observations

AbstractGiant pulsations (Pgs) are a special class of oscillations recognized in ground magnetometer records as exhibiting highly regular sinusoidal waveforms in the east‐west component with periods around 100s. Previous statistical studies showed that Pgs occur almost exclusively on the morningside with peak occurrence in the postmidnight sector. In this paper, we present observations of Pgs extending to the afternoonside, using data from the GOES13 and 15 geostationary satellites and multiple ground magnetometers located in North America. For a long‐lasting event on 29 February 2012, which spanned ∼08–18h magnetic local time, we show that basic Pg properties did not change with the local time, although the period of the pulsations was longer at later local time due to increasing mass loading. There is evidence that the Pgs resulted from fundamental poloidal mode standing Alfvén waves, both on the morning and afternoonsides. Oscillations of energetic particles associated with the field oscillations exhibited an energy‐dependent phase, which has previously been reported and explained by drift resonance. A statistical analysis of the ground magnetic field data (L = 3.8–7.4) covering 2008–2013 confirms that afternoon Pgs are not unusual. We identified a total of 105 Pg events (about 70% (30%) of the events occurred during non‐storm (late storm recovery) periods), 31 of which occurred on the afternoonside. The afternoon Pgs occur under solar wind and geomagnetic conditions that are similar to the morning Pgs, but the afternoon Pgs tend to have short durations and occur frequently in winter instead of around spring and fall equinoxes that are favored by the morning Pgs.

Application of Magnetic Survey in the Investigation of Iron Ore Deposits and Shear Zone Delineation: Case Study of Mutomo-Ikutha Area, SE Kenya

The main objective of this research was to use ground magnetic survey to delineate shear zone and iron ore deposit within the Neoproterozoic rocks of Mutomo-Ikutha area of south eastern Kenya. Total field magnetic data was recorded by using high resolution proton precision geometric magnetometer which recorded total components of the ground magnetic anomaly data running through sixteen traverses. The field data was qualitatively and quantitatively interpreted and the results gave values for the total component measurements of ground magnetic anomaly that varied between a minimum negative peak value of about 250 nanoTesla and a maximum of about 1000 nanoTesla. 550 nanoTesla was considered to be threshold of the iron mineralization within the area. The results indicated that the western part of Mutomo-Ikutha was sheared, faulted and contained iron ore mineralization trending in the north-south direction. Areas with high anomalous values were geochemically proven to contain magnetite.

Electric currents of a substorm current wedge on 24 February 2010

AbstractThe three‐dimensional “substorm current wedge” (SCW) was postulated by McPherron et al. (1973) to explain substorm magnetic perturbations. The origin and coherence as a physical system of this important paradigm of modern space physics remained unclear, however, with progress hindered by gross undersampling, and uniqueness problems in data inversion. Complementing AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) space‐derived radial electric currents with ground magnetic data allowing us to determine currents from the ionosphere up, we overcome problems of uniqueness identified by Fukushima (1969, 1994). For a substorm on 24 February 2010, we quantify SCW development consistently from ground and space data. Its westward electrojet carries 0.5 MA in the more poleward part of the auroral oval, in Region 1 (R1) sense spanning midnight. The evening sector electrojet also feeds into its upward current. We thus validate the SCW concept and obtain parameters needed for quantitative study of substorms.

Analysis of post-sunset F-region vertical plasma drifts during Counter Electrojet days using multi frequency HF Doppler Radar

In this paper, through a case study, an attempt has been made to bring out the relationship between post noon E-region electric field and post sunset F-region vertical plasma drift on quiet time Counter Electrojet (CEJ) days. Study carried out using the data from a multi frequency HF Doppler Radar and Digital Ionosonde located over Trivandrum (8.5° N; 77° E; 0.5° N dip lat.) a geomagnetic dip equatorial station in India during quite time CEJ days of the years 2004 and 2006, revealed some interesting aspects of the E region electrodynamics and post sunset F region electrodynamics. It has been observed that, in contrast to the normal electrojet (EEJ) days, the Pre-Reversal Enhancement (PRE) is either weakened or inhibited on CEJ days and the field reversal takes place much earlier than that on a normal day. It is suggested that even after the effects of the field reversal ceases to show up in the ground magnetic data, the reversed field may persist and shows up as a decrease in the PRE experienced by the F-region. In other words, the study indicates that the EEJ associated electrodynamics have a significant role in controlling the PRE.

Interpreting subsurface volcanic structures using geologically constrained 3‐D gravity inversions: Examples of maar‐diatremes, Newer Volcanics Province, southeastern Australia

AbstractWe present results and a method to geophysically image the subsurface structures of maar volcanoes to better understand eruption mechanisms and risks associated with maar‐forming eruptions. High‐resolution ground gravity and magnetic data were acquired across several maar volcanoes within the Newer Volcanics Province of southeastern Australia, including the Ecklin maar, Red Rock Volcanic Complex, and Mount Leura Volcanic Complex. The depth and geometry of subsurface volcanic structures were determined by interpretation of gridded geophysical data and constrained 2.5‐D forward and 3‐D inverse modeling techniques. Bouguer gravity lows identified across the volcanic craters reflect lower density lake sediments and pyroclastic debris infilling the underlying maar‐diatremes. These anomalies were reproduced during modeling by shallow coalesced diatremes. Short‐wavelength positive gravity and magnetic anomalies identified within the center of the craters suggest complex internal structures. Modeling identified feeder vents, consisting of higher proportions of volcanic debris, intrusive dikes, and ponded magma. Because potential field models are nonunique, sensitivity analyses were undertaken to understand where uncertainty lies in the interpretations, and how the models may vary between the bounds of the constraints. Rather than producing a single “ideal” model, multiple models consistent with available geologic information are created using different inversion techniques. The modeling technique we present focuses on maar volcanoes, but there are wider implications for imaging the subsurface of other volcanic systems such as kimberlite pipes, scoria cones, tuff rings, and calderas.

Development and validation of inversion technique for substorm current wedge using ground magnetic field data

AbstractThe classic substorm current wedge model represents ground and space magnetic perturbations measured during substorms. We have developed an inversion technique to calculate parameters determining the intensity and geometry of the current system using magnetic field data at midlatitudes. The current wedge consists of four segments: a sheet‐like field‐aligned current downward to the ionosphere postmidnight, a westward current across the auroral bulge, an upward sheet‐like current from the westward surge premidnight, and an eastward current in the equatorial plane. The model has five parameters including the current strength, the locations, and breadths of the two field‐aligned current sheets. Simultaneous changes in the ring current are represented by the superposition of a symmetric ring current and a partial ring current characterized by three additional parameters. Parameters of the model are determined as a function of time based on midlatitude ground magnetometers, using realistic field lines and accounting for Earth's induction. The model is validated by a variety of techniques. First, the model predicts more than 80% of the variance in the observations. Second, the intensity of the current wedge and the ring current follows the same trends of the westward electrojet and the ring current indices. Third, the intensity of the westward electrojet agrees extremely well with the intensity of the current wedge. Finally, spacecraft observations of the aurora correspond with the evolution deduced from the model. This model of the substorm current wedge provides a valuable tool for the study of substorm development and its relation to phenomena in space.

Three-dimensional potential field modelling of the subsurface morphology of complex maar volcanoes - Examples from the Newer Volcanics Province, Western Victoria

SUMMARY Potential field geophysical modelling techniques can be applied to better understand the subsurface morphology of volcanoes, and when linked with observations of surface geology can be used to develop a more complete understanding of the volcanic centres eruptive history. High resolution ground gravity and magnetic data were acquired across several maar volcanoes located within the Newer Volcanics Province (NVP) of Western Victoria. The maar volcanoes surveyed represent a range of the different sizes and styles of eruptions observed within maar volcanoes of the NVP. Gravity and magnetic data were subject to 2D forward and 3D inverse modelling in order to reveal details on the depth, geometry and petrophysical property distributions within subsurface volcanic structures. Gravity lows with corresponding magnetic highs are observed across the maar craters and were reproduced during modelling with the presence of a diatreme. Smaller wavelength gravity and magnetic anomalies detected in the centre of the more complex volcanic craters can be explained by the presence of intrusive dykes or vents filled with a higher proportion of denser volcanic debris. Modelling suggests that multiple coalescing diatreme structures exist below the volcanic edifices, some containing intrusive dykes or a denser central vent filled in with volcanic debris. Multiple diatreme structures suggest a complex eruption history involving vent migration, while preserved dykes within the diatreme suggest short-lived fluctuations between phreatomagmatic and magmatic eruption styles.

Tilt angle interpretation of dipping fault model

Abstract The interpretation of tilt angle transformed total magnetic intensity data for a dipping contact model is studied, where it is shown that if the trace of a dipping contact on a horizontal surface is determined then other source parameters, such as the dip and the depth of burial of the contact can be estimated using simple formulas. The trace of the dipping contact can be obtained from the location of the peak of a profile of the first order horizontal derivative of a tilt angle. The tilt angle response and its horizontal derivative are found to be sensitive to noise; however, it is shown that upward continuation operation which does not alter the location of the trace of the contact can improve the tilt angle interpretation for the measured noisy data. Use on synthetic model studies and subsequently on field examples on an aeromagnetic profile across the San Ysidro Fault in the Rio Grande Ridge, U.S.A., and also on an extracted profile from 2D ground magnetic data across Son-Narmada Fault in India demonstrates the applicability of the tilt angle interpretation to delineate the subsurface architecture.

Interpretation of airborne and ground magnetic and gamma-ray spectrometry data in prospecting for base metals in the central-north part of the Itabuna-Salvador-Curaçá Block, Bahia, Brazil

Interpretation of airborne and ground magnetic and radiometric geophysical data is fundamental to provide support for geologic mapping and exploration of mineral resources. This is particularly important of the Itabuna-Salvador-Curaçá Block component of the São Francisco Craton in the state of Bahia, Brazil, which bears important base metal mineralization in ultramafic intrusions. The interpreted airborne data covered part of the northern segment, and the ground data comprised an area of [Formula: see text] in the northern segment of that Archean block. The radiometric interpretation was only qualitative, and the magnetic interpretation was qualitative and quantitative. The qualitative interpretation defined distinct configurations of the geophysical signatures. So, it furnished possible regional and local geologic models and also helped to infer the distribution of magnetization and radiometric content in the rocks. The quantitative interpretation started with Werner deconvolution for a preliminary identification of the magnetic sources, and 2D magnetic modeling improved the definition of the anomalous bodies in terms of geometry, depth to the top, and magnetic susceptibility. The combined qualitative and quantitative interpretation allowed mapping of the rocks that contain magnetic and metallic minerals, as well as felsic lithology, such as granites and orthogneisses, and mafic lithology, such as gabbros and amphibolites. It also suggested that most of the northwest–southeast lineaments related to felsic and mafic magnetic intrusions. The analysis may extend to other areas of the block and be integrated with field and laboratory geologic, geophysical, and geochemical studies for an adequate exploration of nickel, copper, and chromium that are potentially present in the region.

A method to predict electric field spectra from empirically modeled geomagnetic ULF activity

[1] A model for prediction of the Earth background electromagnetic field spectra in the ULF range (1 mHz to 10 Hz) is developed. The possibility to model the hourly integrated magnetic wave power spectra with two different mathematical models, a power law or a fourth‒order polynomial, is investigated. Spectral properties of 3 months of ground magnetic data show that the temporal evolution of the power law parameters can be modeled based on the hourly planetary magnetic activity, the Kp index. Furthermore, the parameters of the polynomial model are related to the magnetic wave power in two spectral bands within the Pc3 and Pc1 pulsation bands. Empirical models of the magnetic wave power in these bands are developed based on the diurnal variation and the correlation with solar wind velocity of geomagnetic pulsations. Comparison with observations shows that the power law model represents the spectra well for low frequencies. However, the polynomial model based on solar wind velocity provides a better representation for the bulk of the ULF domain with mean errors between 2 and 7 dB, increasing with decreasing frequency. The modeled magnetic wave spectrum and knowledge of the underlying electrical conductivity profile can be used to predict the induced ground electric field spectra. An apparent electric conductivity profile was found via model‒based inversion, and the predicted electric wave power is compared to observations from an electrode systems offshore of the test site. The mean error between the observed and predicted electric field amplitudes is 2–7 dB and is consistently lower than the 95% central range of the data set.

Comments on quiet daily variation derivation in "Identification of the IMF sector structure in near-real time by ground magnetic data" by Janzhura and Troshichev (2011)

Abstract. The description presented in the paper of the relations of the solar wind sector structure to the derivation of the quiet daily variation (QDC) in polar magnetic recordings used for calculation of polar cap (PC) indices is found to be unclear and not properly justified. The presented example on inclusion of a solar sector term in an actual QDC series is found to be questionable even on the authors' premises.

Delineation of the subsurface structures and basement surface of the Abu-Rodaym area, Southwestern Sinai, using ground magnetic data

The present study deals with the analysis of data from a ground magnetic survey that was conducted in the Abu-Rodaym area of the Southwestern Sinai Peninsula, Egypt. This survey was carried out to delineate the subsurface structural framework, and to identify the thickness of the sedimentary basin of the study area. Locating these structures, and determining the localities of maximum sedimentary thicknesses that consist mainly of sandstone, serves as a preliminary process in exploring the confined aquifer beneath the surface of the Abu-Rodaym area. This will greatly benefit Bedouins who suffer greatly from a lack of water in the driest region in the country. The processing, analysis, and interpretation, of the total intensity magnetic data shows that there are three sets of faults striking mainly in the N-S, NW-SE, and NE-SW, directions. The depth to the basement surface was found to fluctuate from about 45 m, to more than 100 m, beneath the ground surface. It was also found that the variations in magnetic observations were produced by the striking structures that are mainly responsible for the variations in thicknesses of the sedimentary rocks in the area.

Statistical study of geomagnetic field line resonances observed by CHAMP and on the ground

Studies of single events showing the signatures of Pc3 pulsation field line resonances (FLRs) observed in magnetic field data by CHAMP above the E‐region ionosphere and at conjugate ground stations at L < 2 have previously been reported. Notable features of FLRs observed in these investigations were a Doppler shift in the satellite azimuthal component and a 90° rotation of the polarization ellipse between CHAMP and the ground. In this report, we extend the previous work by means of the first statistical investigation of the same features for a large number of events using CHAMP and ground magnetic field data. Our statistical study confirms the Doppler shifts to higher or lower frequencies relative to the ground FLR frequencies for equatorward or poleward passes of CHAMP over the ground station. Our statistical investigation shows the amount of rotation of the FLR polarization ellipses between the ionosphere and the ground to be ~100° on average, rather than the theoretically predicted 90°. This is explained by the polarization ellipses on the ground being oriented close to N‐S irrespective of the orientation above the ionosphere, which is −13° on average. We find no evidence of ionospheric conductivity gradients associated with sunrise and sunset affecting the amount of rotation of the polarization ellipses as predicted by more recent theoretical advances. We are not able to provide conclusive evidence on the relationship between the polarization ellipse rotation and azimuthal wave number as predicted in recent numerical modeling.