Full‐tensor magnetic gradiometry: Comparison with scalar total magnetic intensity, processing and visualization guidelines

A former radioactive waste disposal site is surveyed with two non-intrusive geophysical techniques, including magnetic gradiometry and electromagnetic induction. Data were gathered over the site by towing the geophysical equipment mounted to a non-electrically conductive and non-magnetic fibre-glass cart. Magnetic gradiometry, which detects the location of ferromagnetic material, including iron and steel, was used to map the existence of a previously unknown buried pipeline formerly used in the delivery of liquid waste to a number of surface disposal trenches and concrete vaults. The existence of a possible pipeline is reinforced by historical engineering drawing and photographs. The electromagnetic induction (EMI) technique was used to map areas of high and low electrical conductivity, which coincide with the magnetic gradiometry data. The EMI also provided information on areas of high electrical conductivity unrelated to a pipeline network. Both data sets demonstrate the usefulness of surface geophysical surveillance techniques to minimize the risk of exposure in the event of future remediation efforts.

This paper reports on the preliminary integrated geophysical studies including several EM techniques (MT, CSAMT, TEM) and airborne -magnetic gradiometry data in the border of Parnaiba basin for groundwater assessment in Guaribas region, Piaui State in semi-arid north East Brazil. Thirty five MT-CSAMT-TEM broadband soundings (100 KHz – 0.01Hz) were completed in the area. The aerogeophysical data has been acquired and the survey area is 30 Km by 40 Km. The total coverage includes approximately 1200 line-kilometers of flight lines flown in drape survey with a 100 m terrain clearance. Two –dimensional modeling of EM data show a resistive block (resistivities ≥ than 1000 Ω.m.) crossing most of the Guaribas profile at depths varying between about 100 m and 300 m. We can also observe lower resistivity values under the intrusive resistive block. If the conductive material observed at the Guaribas profile is a porous sedimentary layer, it is very likely that it contains a good confined acquifer yet unexplored. A deep exploratory borehole should be drilled in Guaribas in order to confirm the geophysical observations.

We report, for the first time, a prediction of the line shapes that would be observed in the (129)Xe nuclear magnetic resonance (NMR) spectrum of xenon in the cages of clathrate hydrates. We use the dimer tensor model to represent pairwise contributions to the intermolecular magnetic shielding tensor for Xe at a specific location in a clathrate cage. The individual tensor components from quantum mechanical calculations in clathrate hydrate structure I are represented by contributions from parallel and perpendicular tensor components of Xe-O and Xe-H dimers. Subsequently these dimer tensor components are used to reconstruct the full magnetic shielding tensor for Xe at an arbitrary location in a clathrate cage. The reconstructed tensors are employed in canonical Monte Carlo simulations to find the Xe shielding tensor component along a particular magnetic field direction. The shielding tensor component weighted according to the probability of finding a crystal fragment oriented along this direction in a polycrystalline sample leads to a predicted line shape. Using the same set of Xe-O and Xe-H shielding functions and the same Xe-O and Xe-H potential functions we calculate the Xe NMR spectra of Xe atom in 12 distinct cage types in clathrate hydrates structures I, II, H, and bromine hydrate. Agreement with experimental spectra in terms of the number of unique tensor components and their relative magnitudes is excellent. Agreement with absolute magnitudes of chemical shifts relative to free Xe atom is very good. We predict the Xe line shapes in two cages in which Xe has not yet been observed.

Radial basis function (RBF) interpolation algorithm for data transfer on coupling interface is derived from the shape function, and the global D-Lanczos (GLDL) method is introduced to solve the displacement transfer matrix. In order to improve the computational efficiency, a local compactly supported RBF (CSRBF) interpolation algorithm is proposed based on the partition acceleration treatment, and a comparison of the algorithmic complexity is made among the global Multi-Quadrics (MQ), CSRBF and the local CSRBF. Numerical test shows the significant shortening of CPU time by using the local CSRBF interpolation algorithm.

Study’s Excerpt:• Aeromagnetic survey was used to identify mineral zones in Zamfara, NW Nigeria's basement terrain.• Total magnetic field intensity data were reduced to the equator to enhance anomaly detection.• Residual field intensity was extracted using regional-residual separation techniques in processing.• Enhancement tools, first vertical derivative and analytic signal, highlighted key structures.• Anomalies suggesting mineral zones were mainly found in SE Zamfara at depths under 200 meters.Full Abstract:The importance of mineral exploration to Nigeria’s present economy cannot be overemphasised, especially in the northern states where crude oil explorations have not come to light. To aid in this quest, an investigation of subsurface structures was conducted in this study to identify and delineate potential mineralization zones in the Zamfara area of North-western Nigeria. To achieve this, aeromagnetic data were analysed and interpreted. The total magnetic field intensity (TMI) data were first reduced to the magnetic equator. Regional-residual separation was then carried out to obtain the residual field intensity map of the area, after which several qualitative enhancement techniques – first vertical derivative and analytical signal – were used to further analyse the data in the Oasis Montaj environment. The results revealed that the area's total magnetic field intensity falls within the range of -78 to +108 nT after a background field of 33,000 nT was removed. The regional magnetic field intensity trends in the NE-SW direction. Both the first vertical derivative and analytical signal indicated a cluster of anomalies in the south-eastern part of the study area, interpreted as faults, folds, contacts, and shear zones, which are indicative of mineralized zones. Source parameter imaging analysis of the data revealed that the anomalies of interest were primarily located at depths of less than 200 m.

Solid-state CP/MAS {sup 31}P NMR spectra were measured for a series of phosphido-bridged diiron complexes Fe{sub 2}(CO){sub 6}({mu}-X)({mu}-PPh{sub 2}), where X is a three-electron-donor hydrocarbyl ligand, and for the isostructural series M{sub 2}(CO){sub 6}({mu}{sub 2}-{eta}{sup 2}-C{triple bond}CR)({mu}-PPh{sub 2}) (M = Fe, Ru, Os; R = Bu{sup t}, Pr{sup i}, Ph). The isotropic solid-state shifts correlate rather well with the corresponding shifts in solution, the largest difference being 8.9 ppM, indicating that there are no phase-dependent structural features. Analysis of the principal elements of the shielding tensor by the spinning-sideband method showed a good correlation between the individual tensor components {sigma}{sub 11}, {sigma}{sub 22}, and {sigma}{sub 33} or composite measures of these and the M-P-M bond angle. The increase of isotropic shift with M-P-M appears to reflect a large change in the {sigma}{sub 33} tensor component, and the correlations observed suggest that all of the compounds studied have common shift tensor orientations with respect to the molecular frame. Experimental techniques for optimizing the acquisition of solid-state {sup 31}P spectra are discussed. 15 refs., 7 figs., 4 tabs.

The induced magnetisation of a magnetic source is proportional to the ambient magnetic field and varies in response to natural geomagnetic variations, such as diurnal changes, storm fields and pulsations. In contrast, the remanent magnetisation is independent of changes in the ambient field. The local perturbation of the geomagnetic variations arising from a subsurface magnetic body can be determined by simultaneous monitoring of geomagnetic variations at two sites: one within the static magnetic anomaly associated with the body and another at a remote base station. Total field measurements can only provide a qualitative indication of the relative contributions of remanent and induced magnetisation to the anomaly. Monitoring of all three field components at the on-anomaly and base stations, however, allows the components of the second order gradient tensor of the anomalous pseudogravitational potential to be determined. This tensor depends only on the source geometry and the measurement location and is independent of the nature (remanent or induced), magnitude or direction of the source magnetisation.Without making any assumptions about source geometry or location, the Koenigsberger ratio (Q), the direction of remanence and the direction of total magnetisation can be obtained from the components of this tensor. This information can constrain magnetic modelling prior to drilling and remove a major source of ambiguity in magnetic interpretation. The direction to the centre of a compact source can be determined directly from diagonalisation of the tensor. Values of Q constrain the magnetic mineralogy of the source and the remanence direction can discriminate sources of different ages or geological histories. Thus the method can also alleviate the geological ambiguity that afflicts magnetic interpretation.Field trials of differential vector magnetometry (DVM) at several sites, including the Tallawang magnetite deposit, New South Wales, have demonstrated the validity of the proposed in situ method. However, a number of technical difficulties must be resolved before this method can be used routinely. Accurate characterisation of departures from mutual orthogonality of the components measured by each vector sensor, and the relative orientation of the anomaly and base station sensors, are crucial to the successful implementation of the method.

Context. The Parker Solar Probe (PSP) is operational since 2018 and has provided invaluable new data that measure the solar vicinity in situ at smaller heliocentric distances than ever before. These data can be used to shed new light on the turbulent dynamics in the solar atmosphere and solar wind, which in turn are thought to be important to explain long-standing problems of the heating and acceleration in these regions. In recent years, it was realized that background inhomogeneities in magnetohydrodynamics could influence the development of turbulence and might enable other cascade channels, such as the self-cascade of waves, in addition to the well-known Alfvén collisional cascade. This phenomenon has been called uniturbulence. However, the precise influence of the background inhomogeneity on turbulent spectra has not been not studied so far. Aims. In this work, we study the influence of background roughness on the turbulent magnetic field spectrum in PSP data, including data from encounter 1 up to and including encounter 14. Methods. The magnetic spectral index αB receives our highest attention. Motivated by the presumably different turbulent dynamics in the presence of large-scale inhomogeneities, we searched for correlations between the magnetic power spectra and a measure for the degree of inhomogeneity. The latter was probed by taking the standard deviation (STD) of the total magnetic field magnitude after applying an appropriate averaging. The data of each PSP encounter were split into many short time windows, of which we subsequently calculated both αB and background STD. Results. We find a significant impact of the background STD on αB. As the variations in the background become stronger, αB becomes more negative, indicating a steepening of the magnetic power spectrum. We show that this effect is consistent in all investigated PSP encounters, and it is unaffected by heliocentric distance up to 50 R⊙. By making use of artificial magnetic field data in the form of synthetic colored noise, we show that this effect is not simply due to the fluctuations imposed on the total magnetic field, but must have another as yet unidentified cause. Conclusions. There is a strong indication that the background inhomogeneity affects the turbulent dynamics, possibly through uniturbulence. This leads to a different power spectrum in the presence of large-scale total magnetic field variations. The fact that it is present in all investigated encounters and at all radial distances up to 50 R⊙ suggests that it represents a general and ubiquitous feature of solar wind dynamics. The analysis with the synthetic colored noise indicates that the observed steepening effect is not to be attributed simply to the small-scale fluctuations superposed on the total magnetic field. This conclusion is confirmed by the fact that no similar consistent steepening trend is observed for the magnetic compressibility Cb instead of background STD. The steepening trend is instead a real physical effect induced by the large-scale variations in the background magnetic field.

We derive an unambiguous apparent conductivity from total magnetic field TEM data for fixed-loop geometry. For single-component fixed-loop TEM measurements, apparent conductivity is either dual-valued or undefined. The ambiguity or non-existence is particularly evident for readings taken outside the transmitter loop, both for step and impulse response data. Therefore, computing apparent conductivity from single-component fixed-loop TEM data can be problematic, especially at intermediate delay times. However, if multi-component fixed-loop magnetic field data is available, an unambiguous apparent conductivity can be derived from |B (t )| at all times, except in the inductive limit. Impulse response measurements can be time-weighted and summed to yield “quasi-|B |” data. Apparent conductivity derived from quasi-|B | amplitudes is dual-valued, but usually only one of the alternatives is geologically plausible. Computing apparent conductivity from |B | or quasi-|B | amplitudes expedites generation of conductivity-depth sections from fixed-loop TEM. A field data example with multi-component SQUID data shows significant improvement in the conductivity-depth section when |B (t )| is transformed rather than the vertical component, Bz , alone; the lateral extent of a conductive target is under-estimated in the Bz CDI

The results of analyses of the airborne gamma radiation and total magnetic field survey flown for the region identified as the Elko national Topographic Map NK11-12 are presented. The airborne data gathered are reduced by ground computer facilities to yield profile plots of the basic uranium, thorium and potassium equivalent gamma radiation intensities, ratios of these intensities, aircraft altitude above the earth's surface, total gamma ray and earth's magnetic field intensity, correlated as a function of geologic units. The distribution of data within each geologic unit, for all surveyed map lines and tie lines, has been calculated and is included. Two sets of profiled data for each line are included, with one set displaying the above-cited data. The second set includes only flight line magnetic field, temperature, pressure, altitude data plus magnetic field data as measured at a base station. A general description of the area, including descriptions of the various geologic units and the corresponding airborne data, is included.

The results of analyses of the airborne gamma radiation and total magnetic field survey flown for the region identified as the Challis National Topographic Map NL11-12 is presented in Volume I and II of this report. The airborne data gathered is reduced by ground computer facilities to yield profile plots of the basic uranium, thorium and potassium equivalent gamma radiation intensities, ratios of these intensities, aircraft altitude above the earth's surface, total gamma ray and earth's magnetic field intensity, correlated as a function of geologic units. The distribution of data within each geologic unit, for all surveyed map lines and tie lines, has been calculated and is included. Two sets of profiled data for each line are included with one set displaying the above-cited data. The second set includes only flight line magnetic field, temperature, pressure, altitude data plus magnetic field data as measured at a base station. A general description of the area, including descriptions of the various geologic units and the corresponding airborne data, is included also.

The results of analyses of the airborne gamma radiation and total magnetic field survey flown for the region identified as the Lander National Topographic Map NK12-6 are presented. The airborne data gathered are reduced by ground computer facilities to yield profile plots of the basic uranium, thorium and potassium equivalent gamma radiation intensities, ratios of these intensities, aircraft altitude above the earth's surface, total gamma ray and earth's magnetic field intensity, correlated as a function of geologic units. The distribution of data within each geologic unit, for all surveyed map lines and tie lines, has been calculated and is included. Two sets of profiled data for each line are included, with one set displaying the above-cited data. The second set includes only flight line magnetic field, temperature, pressure, altitude data plus magnetic field data as measured at a base station. A general description of the area, including descriptions of the various geologic units and the corresponding airborne data, is included also.

The results of analyses of the airborne gamma radiation and total magnetic field survey flown for the region identified as the Llano National Topographic Map NH14-5 are presented. The airborne data gathered are reduced by ground computer facilities to yield profile plots of the basic uranium, thorium and potassium equivalent gamma radiation intensities, ratios of these intensities, aircraft altitude above the earth's surface, total gamma ray and earth's magnetic field intensity, correlated as a function of geologic units. The distribution of data within each geologic unit, for all surveyed map lines and tie lines, has been calculated and is included. Two sets of profiled data for each line are included, with one set displaying the above-cited data. The second set includes only flight line magnetic field, temperature, pressure, altitude data plus magnetic field data as measured at a base station. A general description of the area, including descriptions of the various geologic units and the corresponding airborne data, is included also.

Sub-Audio Magnetics (SAM) is a method by which a total-field magnetic sensor may be used to simultaneously acquire both total magnetic intensity (TMI) and total-field electromagnetic induction (TFEMI) responses from buried metal. In situations where public safety is involved, regulators, such as the U.S. Environmental Protection Agency, can mandate the use of both magnetic and electromagnetic sensors for the detection of potentially hazardous items such as unexploded ordnance (UXO) and metal drums. SAM has now completed a program of evaluation and certification as a state-of-the-art technology, acceptable for application in the litigation-sensitive environmental hazard remediation market.During its certification program, SAM was applied to controlled U.S. test sites at the Aberdeen Proving Ground, Maryland, McKinley Range, Alabama, Waikoloa, Hawaii, and at the Newholme facility in Australia. It was also applied to situations in Montana where live UXO was present. In these demonstrations, a direct comparison was made between SAM results and common, state-of-the-art alternative geophysical detecting technologies. Based on demonstrations at these sites, it has been possible to quantify the key performance parameters: level of effort, detection performance as a function of depth and target size, false-alarm rejection achieved through dual-mode sensing, and the cost-benefit ratio of SAM compared with contemporary sensor technologies such as the Geonics EM61 and EM63 electromagnetic detectors, and G-tek’s TM-5EMU electromagnetic detector and TM-4 total field magnetometer.SAM has proven to be very successful in detecting a wide range of metallic UXO items to depths exceeding achievable detection by all contemporary sensors. In the Montana live sites, where the smallest item of UXO was a 76 mm projectile, false-alarm rates were reduced from 7 per UXO, using TMI only, to 0.7 per UXO using combined SAM TMI and TFEMI data, without diminishing the probability of UXO detection. Survey cost per hectare using SAM was determined to be less than one quarter that of conducting TMI plus EMI surveys separately.The application of SAM to extremely magnetic lava flows at Waikoloa in Hawaii showed that SAM cannot yet compete with the ground-balancing capability of the TM-5EMU in this environment. Advanced signal-processing development may reduce this present limitation.

This research includes the derivation of an updated total magnetic intensity map of Iraq given from the satellite data. The World Magnetic Model, (WMM) 2023, which was obtained from satellite orbitography adopted to determine the total magnetic intensity field (F) values for each grid point that covered the whole study region of Iraq. The WMM software was used to calculate the F value for 272 grid points that covers the whole study region. This region was gridded with suitable interstitial distances of 50 Km to cover such region representatively. The calculations of F were achieved in two steps. The first step represents the F calculation at sea level (altitude=0), whereas the second one calculated the F by considering the elevation meter value of altitude for each grid point. It was proposed to deduce the residual F by subtracting these values at altitude = 0 field from that altitude field. Then, the deduced residual surface was reflected the magnetic intensity variations across the study region for the rocks overlain the reference ellipsoid. Residual Fsurface was treated mathematically within the 2D domain by using the software. The results were provided a vision of the highest and lowest gradients of F and for interpreting their variations were shown to be coincided with the major tectonic boundaries of Iraq that been detected by previous studies. The given values of F were found to be mostly dependent on the magnetic susceptibility subsurface basement and / or crustal rocks depth. It was concluded that the increase of F values towards the N and NE direction is related to the increasing thickness of crustal rocks or the depth to the Moho discontinuity, and also such thickness or depth is found to be increased towards the E-W direction but with lesser values.