Anomalous Source Research Articles

An integrated geophysical approach for gold from Wayand‐Nilambur granulite terrain, Kerala (India)

The Wayand‐Nilambur granulite terrain in Kerala (India) is well‐known for vein‐type gold mineralization within the quartz veins. Based on the previous geological, petro‐physical and geophysical characteristics of Wayand‐Nilambur gold deposit, the integrated geophysical survey was carried out to delineate the favourable prospecting zones. In the present study, detailed ground geophysical surveys, that is, magnetic, electrical resistivity/chargeability surveys were carried out in an area of 2 km2 in Kattikallu and Kalkulam blocks to delineate the ore deposits in terms of depths and extensions through the structural shear zone, locate the anomalous sources and the geometry of the Au‐rich sulphide zone and its depth continuity in the subsurface. These surveys brought out prominent resistivity and chargeability zones over known magnetite‐quartz veins that are associated with low‐grade sulphide bands at the central part of the Kattikallu block. The prominent potential zone is characterized by strong bipolar magnetic anomalies over the quartz veins. Based on the 2D inversion of resistivity data, the resistivity low zone of order 80–600 Ω m and chargeability of 21–25 mV/V are observed at a depth range of 5–20 m. In Kalkulam block, the magnetic survey has also brought out high intensity anomaly zones over quartz veins, the same quartz veins are mapped by high chargeability of 10–35 mV/V and low resistivity of 185–400 Ω m. The dipole‐dipole configuration produces two parameters, that is, resistivity and chargeability, these methods distinguish the anomalies along the two selected profiles in the study area. An attempt was made for combining the resistivity and chargeability values to identify the anomalous zone boundaries. The results of inversion indicated that the conductive bodies are located at the subsurface, with depths ranging from 5 to 25 m. Based on this integrated geophysical study, we suggested two borehole sites for further geo‐scientific studies in view of mineralization.

3D Monte Carlo geometry inversion using gravity data

Diverse Monte Carlo methods have gained widespread use across a broad range of applications. However, the challenge of 3D Monte Carlo sampling remains due to the curse of dimensionality. To date, only a few works have been published regarding 3D Monte Carlo sampling. This study aims to develop an efficient 3D transdimensional Monte Carlo framework for reconstructing the spatial geometry of an anomalous body using gravity data. Our framework also can quantify the uncertainty of the shape of an anomalous body recovered from geophysical measurements. To improve the computational efficiency of 3D Monte Carlo sampling, we develop a sparse geometry parameterization strategy. This approach adequately approximates the shape of a complex 3D anomalous body using a set of simple geometries, such as ellipsoids. Each ellipsoid can be characterized by a few parameters, such as the centroid, axes, and orientations, significantly reducing the number of parameters to be sampled. During the sampling, we randomly perturb the number, locations, sizes, and orientations of the ellipsoids. To impose prior structural constraints from other geophysical methods, such as seismic imaging, we design a new method by placing a fixed layer oriented along the top boundary of the anomalous body. The fixed layer is then connected to the randomly sampled ellipsoids using an alpha shape, allowing us to estimate the geometry of the anomalous source body. The current work focuses on the reconstruction of salt bodies. We start with a synthetic spherical salt model and then conduct a more realistic study using a simplified 3D SEG/EAGE salt model, which has a much more complex geometry than the synthetic spherical model. Finally, we apply our method to the Galveston Island salt dome, offshore Texas. The numerical results demonstrate that our framework can effectively recover the shape of an anomalous body and quantify the uncertainty of the reconstructed geometry.

Integrated geophysical methods to constrain subsurface structures of Tulu Moye-Bora-Berecha axial volcanic complex, main Ethiopia rift: Implications for geothermal resources

The Main Ethiopian Rift (MER) is a well-known continental rift whose axial sector is characterized by the occurrence of regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of fissural basalts, small mafic scoria cones and numerous young normal faults and fissures. The Tulu Moye-Bora-Berecha volcanic complex is found in the central portion of the MER and includes the Tulu Moye geothermal prospect area. A combination of gravity and magnetic methods was used to better constrain the subsurface volcanic stratigraphy and tectonic structures. Regional and residual anomaly maps were produced from the gravity data and the magnetic data were corrected to produce anomaly and enhanced maps. A complete Bouguer anomaly contour map was produced after the necessary reduction was applied to the gravity data. Due to the geomagnetic field's dipolar nature and since the study area is in the equatorial region (<15°), the reduced-to-equator (RTE) technique was used to minimize external effects and correct the data as if the body had been laid at the magnetic equator. The anomalous source's depth was calculated using a Euler depth solution and spectral analysis approach. Joint 2D forward models on three profiles were developed using GM-SYS of Oasis Montaj software. From the interpretation of the geophysical results, the following conclusions have been reached: (1) the crystalline basement is more raised around Salen ridge and west of it; (2) the main heat source of the geothermal system appears to be a central region of the studied area near Salen ridge and is estimated to be at 4–5 km depth, (3) under Gnaro obsidian dome, the basaltic and silicic volcanic horizon is thin, and the deep-seated regional fault serves as the main conduit for the passage of hot fluid to the surface and (4) the gravity and magnetic anomaly plots, regional-residual maps and enhanced data plots all indicate towards the existence of a major geological feature-a large caldera-comprising of Tulu Moye, Bora and Berecha volcanic centers.

Chemoselective oxidation of aromatic aldehydes to carboxylic acids: potassium tert-butoxide as an anomalous source of oxygen.

Chemoselective oxidation of aromatic and heteroaromatic aldehydes (>45 examples) to their corresponding carboxylic acids has been developed. Potassium tert-butoxide acts as an oxygen source during this transformation that delivers the corresponding acids without chromatographic purifications. The use of bench-top reagents, operational simplicity, and high level of chemo-selectivity with respect to oxidation of the least preferred aldehyde functionality, in the presence of more susceptible functional groups, are some of the highlights of this strategy.

Sequential Anomaly Detection Under Sampling Constraints

The problem of sequential anomaly detection is considered, where multiple data sources are monitored in real time and the goal is to identify the &#x201C;anomalous&#x201D; ones among them, when it is not possible to sample all sources at all times. A detection scheme in this context requires specifying not only when to stop sampling and which sources to identify as anomalous upon stopping, but also which sources to sample at each time instance until stopping. A novel formulation for this problem is proposed, in which the number of anomalous sources is not necessarily known in advance and the number of sampled sources per time instance is not necessarily fixed. Instead, an arbitrary lower bound and an arbitrary upper bound are assumed on the number of anomalous sources, and the fraction of the expected number of samples over the expected time until stopping is required to not exceed an arbitrary, user-specified level. In addition to this sampling constraint, the probabilities of at least one false alarm and at least one missed detection are controlled below user-specified tolerance levels. A general criterion is established for a policy to achieve the minimum expected time until stopping to a first-order asymptotic approximation as the two familywise error rates go to zero. Moreover, the asymptotic optimality is established of a family of policies that sample each source at each time instance with a probability that depends on past observations only through the current estimate of the subset of anomalous sources. This family includes, in particular, a novel policy that requires minimal computation under any setup of the problem.

Determination of thermal structure of the crust beneath the Gongola Basin, Upper Benue Trough, Nigeria

Geothermal energy resources have been established globally to be among the sustainable and environmentally harmless means of energy generation. The data for this study came from the Earth Magnetic Anomaly Grid 2 version 3 (EMAG2V3). It was acquired at a flying elevation of 4 km above the ellipsoid and has a resolution of 2-arc minutes (3.66 km). This research aim to study the thermal structure of the crust beneath the Gongola Basin so as to determine the heat flow, geothermal gradient and the Curie point depth. In this study, the creation of an air satellite magnetic map of the Gongola Basin is accomplished through the use of a fractal magnetization approach. However, Curie point depth (commonly known as bottom most of magnetic sources depth) measure the distance between two points in space, depth to bottom magnetic source (DBMS). Geothermal characteristics of the area was evaluated using Oasis Montaj. So the earth magnetic field is utilized to determine the depth of anomalous sources that ranges from few meters to tens of kilometers. The top depth ranges from 3.4465km to 10.942km, the centroid depth ranges from 14.045km to 31.338km, the geothermal gradient ranges from 10.2566Oc/km to 23.4445 Oc/km and the heat flow ranges from 25.6415mW/m2 to 58.84 mW/m2. The study identify a point (11°63E, 10°12N) with optimal values (26.44OC/km, 66.11mW/m2) at a distance 21.9km which is above the typical optimal values obtained in stable continental craton as obtainable in African plate.

Alert Classification for the ALeRCE Broker System: The Anomaly Detector

Astronomical broker systems, such as Automatic Learning for the Rapid Classification of Events (ALeRCE), are currently analyzing hundreds of thousands of alerts per night, opening up an opportunity to automatically detect anomalous unknown sources. In this work, we present the ALeRCE anomaly detector, composed of three outlier detection algorithms that aim to find transient, periodic, and stochastic anomalous sources within the Zwicky Transient Facility data stream. Our experimental framework consists of cross-validating six anomaly detection algorithms for each of these three classes using the ALeRCE light-curve features. Following the ALeRCE taxonomy, we consider four transient subclasses, five stochastic subclasses, and six periodic subclasses. We evaluate each algorithm by considering each subclass as the anomaly class. For transient and periodic sources the best performance is obtained by a modified version of the deep support vector data description neural network, while for stochastic sources the best results are obtained by calculating the reconstruction error of an autoencoder neural network. Including a visual inspection step for the 10 most promising candidates for each of the 15 ALeRCE subclasses, we detect 31 bogus candidates (i.e., those with photometry or processing issues) and seven potential astrophysical outliers that require follow-up observations for further analysis. 16 16 The code and the data needed to reproduce our results are publicly available at https://github.com/mperezcarrasco/AnomalyALeRCE.

Ask me anything: Sara Webb

Sara Webb is an astrophysicist and science communicator at the Swinburne University of Technology in Australia. Her research involves using AI and machine learning in astronomy, with a focus on finding and identifying anomalous sources of data in large and complex astrophysical surveys. She is also the mission director of the Swinburne Youth Space Innovation Challenge and in 2022 was named as one of the country’s “Superstars of STEM” – an initiative of Science and Technology Australia.

Estimation of ground depth of radioelements sources in Ogun State, Southwestern Nigeria using gradient techniques

Radioelement exploration has gained economic interest recently due to its usefulness in the detection and delineation of mineral deposits. In this study, the airborne radiometric data were analysed for depth estimation of the radioelement deposit in Ogun state, Nigeria. Three enhancement gradient techniques, namely; Analytical Signal Amplitude (ASA), Horizontal Gradient Magnitude (HGM) and Local Wave Number (LWN) were employed to estimate the possible depth of radioelements for mineralization. Geosoft's (Oasis Montaj) software and Potential Field's (PF) software were used to conduct the estimations. The result obtained revealed shallow sources of 0.584 km (LWN) and 0.387 km (ASA), and deep-seated sources of 5.950 km (ASA) and 5.880 km (ASA) for uranium and thorium, respectively. The shallow source and deep source for potassium are 0.259 km (ASA) and 2.540 km (ASA), respectively. In this study, the position and depth of the source were automatically estimated using linear equations based on derivatives without the use of any a priori knowledge. The three gradient methods are therefore found suitable in estimating the depth to radioelement anomalous source.

Insights into the deep structure of the Bou Azzer-El Graara inlier (Central Anti-Atlas, Morocco): Inference from high-resolution magnetic data, and geodynamic implications

The Bou Azzer-El Graara inlier (BAEI) in the Central Anti-Atlas has been extensively studied due to its famous Pan-African ophiolite and its great mineral potential. In 1998, an airborne survey was flown over this inlier, providing high-resolution magnetic data. This study is concerned with reinterpreting these data to help elucidate the BAEI structure. For this purpose, the aeromagnetic data were processed through a series of filters and transformations. They were then subjected to a detailed analysis, taking into account the study area geology. Qualitative analysis of the reduced to pole data indicates that the observed anomalies are broadly consistent with outcrops of Precambrian mafic and metavolcanic rocks in the BAEI. The extension of this type of rocks below the Paleozoic sedimentary cover could explain similar anomalies found outside the inlier. Additional analysis of the processed data helped to identify various lineaments and structures organized into two main trends: ENE-WSW and ESE-WNW. Furthermore, the calculation of anomalous source depths by Euler deconvolution from aeromagnetic data shows that these vary between 0 and 1000 m within the inlier. However, deeper sources occur beneath the detached and slightly folded Paleozoic sedimentary deposits in the northern part of the inlier. Moreover, a quantitative interpretation using the 3D inverse method was applied to high-amplitude anomalies associated with the Bou Azzer ophiolite (BAO) and the Foum Zguid great dyke (FZGD). The results indicate that (i) the BAO consists of three north-northeast dipping magnetic bodies, (ii) the FZGD corresponds to a deeply rooted vertical structure with lateral thickness variations and (iii) the NE-SW FZGD structure represents the eastern limit of the BAO as indicated by geological and magnetic data. This suggests that the FZGD fracture may have played an important role during the Pan-African collages as a key structure controlling the oblique suturing of the ophiolite slices.

Spherical Planting Inversion of GRAIL Data

In large-scale potential field data inversion, constructing the kernel matrix is a time-consuming problem with large memory requirements. Therefore, a spherical planting inversion of Gravity Recovery and Interior Laboratory (GRAIL) data is proposed using the L1-norm in conjunction with tesseroids. Spherical planting inversion, however, is strongly dependent on the correct seeds’ density contrast, location, and number; otherwise, it can cause mutual intrusion of anomalous sources produced by different seeds. Hence, a weighting function was introduced to limit the influence area of the seeds for yielding robust solutions; moreover, it is challenging to set customized parameters for each seed, especially for the large number of seeds used or complex gravity anomalies data. Hence, we employed the “shape-of-anomaly” data-misfit function in conjunction with a new seed weighting function to improve the spherical planting inversion. The proposed seed weighting function is constructed based on the covariance matrix for given gravity data and can avoid manually setting customized parameters for each seed. The results of synthetic tests and field data show that spherical planting inversion requires less computer memory than traditional inversion. Furthermore, the proposed seed weighting function can effectively limit the seed influence area. The result of spherical planting inversion indicates that the crustal thickness of Mare Crisium is about 0 km because the Crisium impact may have removed all crust from parts of the basin.

Paleomagnetism of the Taseeva Group (Yenisei Ridge): on the Issue of the Geomagnetic Field Configuration at the Precambrian–Phanerozoic Boundary

Abstract —We report results of a detailed study of the paleomagnetic record in the sedimentary rocks of the Taseeva Group of the Yenisei Ridge in three typical sections in the lower courses of the Angara, Taseeva and Irkineeva rivers. Our results confirm that the geomagnetic field was in an anomalous state at the Precambrian–Phanerozoic boundary. It is well known that Ediacaran rocks in general have preserved several different paleomagnetic directions that do not conform to the geocentric axial dipole model. For example, Siberian sections display two equally valid groups of paleopoles that cause many debates over the geometry of the geomagnetic field and whether any of the components correspond to its dipole configuration. The paleomagnetic record we studied is unique in that the rocks of the Chistyakovka and Moshakovka formations have captured both these components, which is factual evidence of a synchronous existence of two sources. To explain these findings, we propose an original hypothesis in which the bipolar component that is widely present in the rocks and corresponds to the Madagascar group of paleomagnetic poles is associated to the field of the geocentric axial dipole. The less widespread monopolar component corresponding to the Australian–Antarctic group of poles is reflective of a stationary anomalous source. The recording of this source became possible due to the abrupt decrease in the strength of the virtual dipole moment that probably was at its lowest during the accumulation of the Chistyakovka and Moshakovka formations. The new paleomagnetic pole calculated for the bipolar component – 39.2°N, 61.1°E – plots on the apparent polar wander path for Siberia and can be considered a key determination for the age ~570 Ma.

Early Galaxy Formation and the Hubble Constant Tension

Tension between local determinations of the Hubble constant and the value found in fits to the cosmic microwave background (CMB) acoustic power spectrum have emerged as finer angular scales (higher multipoles ℓ) have been measured in the latter. Independent observations indicate that massive galaxies emerged at higher redshift than anticipated in the standard ΛCDM structure formation paradigm. If genuine, these early galaxies would cause excess gravitational lensing of the CMB, adding an anomalous source of power at high ℓ. I suggest that accommodating this anomalous source of power in multiparameter fits might be the root cause of the Hubble tension rather than systematics in local measurements.

Stable downward continuation of the gravity potential field implemented using deep learning

Downward continuation (DC) of the gravity potential field is an important approach used to understand and interpret the density structure and boundary of anomalous bodies. It is widely used to delineate and highlight local and shallow anomalous sources. However, it is well known that direct DC transformation in the frequency domain is unstable and easily affected by high-frequency noise. Recent deep learning applications have led to the development of image recognition and resolution enhancement using the convolutional neural network technique. A similar deep learning architecture is also suitable for training a model for the DC problem. In this study, to solve the problems in existing DC methods, we constructed a dedicated model called DC-Net for the DC problem. We fully trained the DC-Net model on 38,400 pairs of gravity anomaly data at different altitudes using a convolutional neural network. We conducted several experiments and implemented a real-world example. The results demonstrate the following. First, several validation data subset and test data prediction results indicate that the DC-Net model was sufficiently trained. Moreover, it performed better than the traditional strategy in refining the upscaling of low-resolution images. Second, we performed tests on test datasets with changing levels of noise and demonstrated that the DC-Net model is noise-resistant and robust. Finally, we used the proposed model in a real-world example, which demonstrates that the DC-Net model is suitable for solving the DC problem and delineating the detailed gravity anomaly feature near the field source. For real data processing, noise in the gravity anomaly should be reduced in advance. Additionally, we recommend noise quantification of the gravity anomaly before network training.

Electromagnetic Signatures of Possible Charge Anomalies in Tunneling

We reconsider some well-known tunneling processes from the point of view of Aharonov-Bohm electrodynamics, a unique extension of Maxwell’s theory which admits charge-current sources that are not locally conserved. In particular we are interested into tunneling phenomena having relatively long range (otherwise the non-Maxwellian effects become irrelevant, especially at high frequency) and involving macroscopic wavefunctions and coherent matter, for which it makes sense to evaluate the classical e.m. field generated by the tunneling particles. For some condensed-matter systems, admitting discontinuities in the probability current is a possible way of formulating phenomenological models. In such cases, the Aharonov-Bohm theory offers a logically consistent approach and allows to derive observable consequences. Typical e.m. signatures of the failure of local conservation are at high frequency the generation of a longitudinal electric radiation field, and at low frequency a small effect of “missing” magnetic field. Possible causes of this failure are instant tunneling and phase slips in superconductors. For macroscopic quantum systems in which the phase-number uncertainty relation ΔNΔφ∼1 applies, the expectation value of the anomalous source I=∂tρ+∇·j has quantum fluctuations, thus becoming a random source of weak non-Maxwellian fields.

An enhancing precision method for downward continuation of gravity anomalies

Downward continuation of the gravity field observed on the surface is a method applied to calculate the anomalous field below the surface close to deep sources. However, the downward continuation is mathematically incomplete and uncertain, and its accuracy depends on the sampling rate and coverage of the gravity data. To improve the accuracy of the downward continuation of gravity anomalies, we constructed a quasi-neural network that is made up of multiple layers of neurons. Each of the neurons analyzes the attributes of the input data to determine the best control parameters for the downstream output of the current layer, and to judge whether the results of the downstream extension are close to the main anomalous sources. We define the singularity statistic parameter and singular probability of the signal to characterize the singularity attribute of each-layer's continuation results. If the continuation data contain large singularity, they are corrected by the minimum curvature method to weaken the influence of the interference of shallow random sources. Numerical simulation tests showed that the proposed method can effectively weaken the interference of shallow random sources, and obtain more accurate information on deep-sources. A real-world example demonstrated that the method successfully delineate deep geological structures.

Crustal structure of the Qiangtang and Songpan-Ganzi terranes (eastern Tibet) from the 2-D normalized full gradient of gravity anomaly

Numerous geophysical studies have revealed the lithospheric structure of the Qiangtang and the Songpan-Ganzi terranes in the eastern Tibetan Plateau. However, crust–mantle evolution and crustal response to the Indian lithospheric subduction are still controversial. Answering these questions requires additional information regarding crustal structure. In this study, the 2-D normalized full gradient (NFG) of the Bouguer gravity anomaly was used to investigate anomalous sources and interpret the crustal structure underneath the Qiangtang and Songpan-Ganzi terranes. The NFG-derived structures with low-order harmonic numbers (N = 33 and N = 43) showed that an anomalous source beneath the southern Qiangtang terrane had a characteristic northeastward-dipping shape, suggesting the northeastward motion of the crustal material induced by underthrusting Indian lithospheric mantle. The NFG images with harmonic number N = 53 showed a large-scale anomalous source in the lower crust of the transformational zone from the Qiangtang terrane to the Songpan-Ganzi terrane, consistent with thickening crust and resistance of lower crustal flow. The anomalous source demonstrated by the NFG results with harmonic number N = 71, located in the upper crust underneath the Ganzi-Yushu fault, suggested a seismogenic body of the 2010 MW6.9 Yushu event.

The anomalous electoral advantage: Evidence from over 17,000 mayoral candidacies in Poland

Relying on a dataset covering mayoral elections in Poland held over the period 2006–2018, we contribute to the ongoing debate on the anomalous sources of advantage enjoyed by certain candidates contesting close electoral races. While our analysis lends support to earlier conjectures that such anomalies tend to be confined to small jurisdictions, we also point to context-specific clientelistic strategies behind the anomalous advantage. The last is driven largely by an experience of holding a mayoral office in the 1990s, an era when Polish mayors were elected by local councils rather than directly by citizens. When interpreting these findings, we emphasize fundamental characteristics of post-transition Polish local democracy, including feudal legacy, pauperization of the electorate and decentralization of public spending. • We study electoral anomalies at mayoral elections in Poland. • Electoral anomalies characterize small local communities. • We identify the subset of mayoral candidacies largely responsible for the anomalies. • We interpret the anomalies as an expression of clientelistic relations.

Crustal and upper mantle density structure below the Indian Ocean Geoid Low based on 3-D constrained potential field modelling: Inferences on causative sources

The Indian Ocean Geoid Low (IOGL) in the Central Indian Ocean is the largest geoid depression on the globe. Spectral decomposition of geoid anomalies reveals that a significant fraction (up to ~90%) of the IOGL is due to mass anomalies lying within the lower mantle (~ > 700 km). The crustal density structure and Moho depth variations obtained from 3-D gravity inversion along with the upper mantle density structure derived from SL2013sv tomographic model are used to compute the total geoid anomaly up to 700 km depth and compare it with degree-10 residual geoid anomaly map. Comparison of these two maps necessitated the presence of relatively shallow low- density sources within the upper mantle that were not resolved in the 3-D modelling. The presence of two anomalous sources: i) low S-wave velocities at the base of the LAB (4.4 km/s) and ii) an anomalous source of Vs = −1.0% at a depth of 320–400 km within the sub-lithospheric mantle observed in the tomographic model accounted for this difference. We model the geometry of these low-density anomalous bodies using 2-D joint gravity-geoid modelling, which reveals the total contribution of density structure up to the upper mantle depths (700 km) accounts for only 10% of the IOGL whereas the remaining part is primarily caused by mass anomalies in the lower mantle probably related to high-density subducted slabs or plume sources at the core-mantle boundary as proposed by earlier studies.

Interpretation of Magnetic Anomaly Data in the Andesitic Rock Prospect Area of Kutasari Subregency, Purbalingga Regency, Central Java, Indonesia

DOI:10.17014/ijog.8.3.345-357Interpretation of magnetic anomaly data has been carried out in the andesitic rock prospect area, Kutasari Subregency, Purbalingga Regency, Central Java, Indonesia. Geographically, this area is located within 109.2788° - 109.3072°E and 7.3032° - 7.3319°S. The study has been done in April – September 2019 with the purpose to map the distribution of andesitic rocks based on the local magnetic anomaly data. The data that are acquired in this study have the values ranging between -1,238.13 - 1,892.40 nT. The results of qualitative interpretation on the local magnetic anomaly data having been reduced to the pole show the distribution of strong anomalous sources in the northwest area interpreted as massive andesitic rocks. Whereas the results of quantitative interpretation through 2D-forward modeling on the local magnetic anomaly data show six anomalous bodies, with magnetic susceptibility values ranging from 0.0025 to 0.0350 cgs and depths range between 7.16 - 505.97 m. The highest magnetic susceptibility is 0.0350 cgs interpreted as a massive andesite intrusion forming a very dense dike; whereas the lowest magnetic susceptibility is 0.0025 cgs interpreted as undifferentiated igneous rocks, volcanic breccias, lava, and tuff. Based on the study results, the correlation between the results of qualitative and quantitative interpretations occurs.