Source Depth Research Articles

Ca isotopic compositions of zoned granitoid intrusion: Implications for the emplacement and evolution of magma bodies

To investigate the mechanism of magma emplacement of the intruded zoned pluton, we report high-precision Ca isotopic data of 23 granitoids (including granodiorite and quartz diorite), 5 mafic enclaves, and 7 hornblende and plagioclase mineral pairs from the Early Cretaceous Fangshan pluton, Beijing, China. Δ44/40CaHbl-Pl of rocks span from 0.02 ± 0.15‰ to 0.23 ± 0.12‰, which likely reflects equilibrium inter-mineral isotope fractionation. The granodiorites exhibit ∼0.30‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.47 ± 0.08‰ to 0.77 ± 0.09‰. The diorites and the enclaves yield δ44/40Ca of 0.51 ± 0.03‰ to 0.64 ± 0.05‰, and 0.54 ± 0.06‰ to 0.66 ± 0.11‰, respectively, which are lower than that of the upper continental crust (0.72 ± 0.10‰). Ca isotopic variation cannot be ascribed to K decay due to their low εCa values (< ±1.0). Theoretical study showed that Hbl fractional crystallization could counteract Ca isotope fractionation effect of their co-existing Pl crystallization. Source heterogeneity cannot account for Ca isotopic variation because δ44/40Ca does not correlate to (87Sr/86Sr)i. Instead, the Ca isotopic variation is likely controlled by partial melting and residual mineralogy. Specifically, δ44/40Ca of the granodiorites are negatively correlated with (Dy/Yb)N, reflecting various magma pools derived from different crustal depths with clinopyroxene and garnet residua. Modeling calculations suggest that the low Ca isotopic signature can be caused by ∼10% partial melting of sources with various proportions of garnet and clinopyroxene (from 5% garnet + 95% clinopyroxene to 40% garnet + 60% clinopyroxene). The diorites intruded earlier than the granodiorites, which can be slightly contaminated by surrounding basement rock. Ca isotopic compositions of enclaves are similar to those of their host rocks. Considering the spatial structure of the Fangshan pluton, it should be assembled by at least four magma pulses derived from various crustal depths through ballooning. Our results show that Ca isotopes can be used as a potential tool to trace magma source depth and decipher the process of magma emplacement of intrusive pluton, which is important to understand the continental crustal growth.

On the Electrophysiology and Mapping of Intramural Arrhythmic Focus.

Conventional mapping of focal ventricular arrhythmias relies on unipolar electrogram characteristics and early local activation times. Deep intramural foci are common and associated with high recurrence rates following catheter-based radiofrequency ablation. We assessed the accuracy of unipolar morphological patterns and mapping surface indices to predict the site and depth of ventricular arrhythmogenic focal sources. An experimental beating-heart model used Langendorff-perfused, healthy swine hearts. A custom 56-pole electrode array catheter was positioned on the left ventricle. A plunge needle was placed perpendicular in the center of the grid to simulate arrhythmic foci at variable depths. Unipolar electrograms and local activation times were generated. Simulation models from 2 human hearts were also included with grids positioned simultaneously on the endocardium-epicardium from multiple left ventricular, septal, and outflow tract sites. A unipolar Q or QS complex lacks specificity for superficial arrhythmic foci, as this morphology pattern occupies a large surface area and is the predominant pattern as intramural depth increases without developing a R component. There is progressive displacement from the arrhythmic focus to the surface exit as intramural focus depth increases. A shorter total activation time over the overlying electrode array, larger surface area within initial 20 ms activation, and a dual surface breakout pattern all indicate a deep focus. Displacement from the focal intramural origin to the exit site on the mapping surface could lead to erroneous lesion delivery strategies. Traditional unipolar electrogram features lack specificity to predict the intramural arrhythmic source; however, novel endocardial-epicardial mapping surface indices can be used to determine the depth of arrhythmic foci.

Passive source depth estimation in shallow water using two horizontally separated hydrophones

A solution to the problem of acoustic source depth estimation in shallow water is presented for the case of two horizontally separated hydrophones or arrays. The solution is based on that the smooth-averaged cross-correlation function (SCF) of signals changes regularly with the source depth and is approximately independent of the source range, allowing that the source depth can be estimated independently without knowing the source range. Using different cost functions, three methods are proposed to estimate source depth by matching the normalized cross-spectral density, temporal envelope and time delay of the SCF. Then, the performance of the proposed methods are evaluated with simulations and two experiments conducted off the coast of China at water depths not exceeding 100m. The mean absolute errors in source depth of the proposed methods are in 2.8–12.8m. Finally, the effect of noise, frequency band, range span, horizontal spacing, sound speed profile, and parameter mismatch are numerically studied. The results show that the methods are more suitable for the case of a low-frequency band and an environment with large negative sound speed gradient.

Deep-learning geoacoustic inversion using multi-range vertical array data in shallow water.

A multi-range vertical array data processing (MRP) method based on a convolutional neural network (CNN) is proposed to estimate geoacoustic parameters in shallow water. The network input is the normalized sample covariance matrices of the broadband multi-range data received by a vertical line array. Since the geoacoustic parameters (e.g., bottom sound speed, density, and attenuation) have different scales, the multi-task learning is used to estimate these parameters simultaneously. To reduce the influence of the uncertainty of the source position, the training and validation data are composed of the simulation data of different source depths. Simulation results demonstrate that compared with the conventional matched-field inversion (MFI), the CNN with MRP alleviates the coupling between the geoacoustic parameters and is more robust to different source depths in the shallow water environment. Based on the inversion results, better localization performance is achieved when the range-dependent environment is assumed to be a range-independent model. Real data from the East China Sea experiment are used to validate the MRP method. The results show that, compared with the MFI and the CNN with single-range vertical array data processing, the use of geoacoustic parameters from MRP achieves better localization performance.

PROPAGATION CHARACTERISTICS OF SURFACE AND IN-DEPTH VIBRATIONS IN SAND GROUNDS: A COMPARATIVE ANALYSIS

This paper aims to investigate the propagation characteristics of blast-induced ground vibrations in loose dry sand under surface and underground vibration conditions by monitoring the particle velocities and dominant frequencies of artificially generated ground vibrations. For this purpose, a ball drop apparatus was used to generate surface and underground vibrations at different depths. The free fall of the ball induced ground vibrations by impact. A total of 60 laboratory-scale ground vibration monitoring tests were performed on 4 physical models placed in a tank designed for this study. The vibrations were monitored on the surface of the sand filling the tank. The obtained results demonstrated that surface vibrations resulted in higher particle velocities than those generated by underground vibrations and that particle velocities measured on the ground surface decreased as the depth of the underground vibration source increased. The frequency analysis emphasized that only low frequencies (&amp;lt;40 Hz) were generated by surface ground vibration monitoring tests whereas 86.67% of those induced by underground vibration monitoring tests were high frequencies (&amp;gt;40 Hz). It was also determined that increasing the depth of the vibration source resulted in decreasing the dominant frequency range within the range of high frequencies (&amp;gt;40 Hz).

Coseismic Deformation Field Extraction and Fault Slip Inversion of the 2021 Yangbi MW 6.1 Earthquake, Yunnan Province, Based on Time-Series InSAR

An earthquake of moderate magnitude (MW 6.1) occurred in Yangbi County, Dali, Yunnan Province, China, on 21 May 2021. Compared to strong earthquakes, the measurement of the deformation fields of moderate earthquakes is more susceptible to errors associated with atmospheric, orbital, and topographic features. We adopted a new time-series InSAR method to process preseismic and postseismic Sentinel-1A SAR time-series images and separated the coseismic deformation signals from various error signals. This method uses preseismic time-series interferograms to estimate the spatially correlated look angle error induced by the digital elevation model and the atmospheric and orbital errors in the master image. The preseismic and postseismic time-series interferograms were then segmented for spatio-temporal filtering to provide a precise estimate of the atmospheric and orbital errors in slave images. Such time-series processing accurately separates various errors from the coseismic deformation signal and prevents the coseismic deformation signal from being included as noise in the error estimation during filtering. Based on this approach, we effectively eliminated the masking of the deformation signal by the errors and extracted coseismic deformation field of the Yangbi MW 6.1 earthquake with high precision. The maximum LOS displacement in the ascending and descending tracks were determined to be −74 and −62 mm, respectively. Subsequently, we used the Geodetic Bayesian Inversion Software to invert the fault geometric parameters of this earthquake, and based on this inverted the rupture slip distribution using the least-squares method. The results showed that the fault orientation is 133.43°, dip angle is 76.98°, source depth is 5.5 km, fault sliding mode is right-lateral strike-slip. The moment magnitude (MW) was calculated to be 6.07.

Spectroscopic determination of the optical constants and radiative properties of black PMMA for pyrolysis modeling

In-depth radiation absorption significantly affects the ignition time and burning rate of flammable translucent materials. To improve the accuracy of flammability predictions by pyrolysis models, the spectral dependency of radiation sources and the depth of the computational cell in the sample should be considered. This paper provides the absorption spectra of black Poly(methyl methacrylate) (PMMA) in a wide spectral range of 0.25 to 25 μm by measuring the high-resolution spectral transmissivity of thin samples. Compared to clear PMMA, black PMMA has a higher spectral absorption coefficient in the UV–Vis–NIR region (<2.3μm). We extracted the complex index of refraction of black PMMA from the spectral absorption coefficient by applying the Kramers–Kronig transform (KK-transform). Furthermore, to investigate the effect of PMMA temperature on its absorption, the absorbance of the sample was measured with FTIR-ATR for the temperature up to the polymer’s melting temperature. The results showed that the radiation absorption of PMMA may increase for higher material temperature. Implementing the measured optical constants, simulated temperature profiles in a layer of PMMA subjected to the heat flux of 15 and 28 kW/m2 are obtained, showing good agreement with the experimental data. Finally, a set of data for the effective absorption coefficient as a function of source temperature and depth is proposed for the black PMMA to improve the gray modeling of radiation for pyrolysis simulations.

Apatite and zircon geochemistry for discriminating ore-forming intrusions in the Luming giant porphyry Mo deposit, Northeastern China

The Luming giant porphyry Mo deposit in NE China host >0.89Mt Mo, but which intrusion induced the Mo mineralization is still in debated. In this study, apatite and zircon geochemistry from three mentioned ore-forming granites, namely monzogranitic porphyry (MGP), granodiorite (GD) and monzogranite (MG) in Luming ore district are presented in order to constrain the real ore-forming granite. Apatite and zircon U-Pb dating of the MGP, GD and MG yield a crystallization age of ca. 177 Ma, ca. 187 Ma and ca. 186 Ma, respectively. Whole-rock geochemistry, in-situ apatite Sr-Nd and zircon Hf isotopic compositions suggest that these granites formed by partial melting of juvenile lower crust with different depth. Both the MGP and GD have adakitic affinities and the depth of their magma source is > 45 km with garnet as residual phase, whereas the source depths for the MG was < 33 km with plagioclase as residue. Apatite and zircon geochemical features indicated that the magma of MGP and GD have higher oxygen fugacity, water contents and lower Ti-in-zircon temperatures than these of the MG, which make the MGP and GD are more favorable for Mo mineralization, but the less volume of GD limited its metallogenic potential. Decoupled Nd-Hf isotopes of the MGP, GD and MG could be induced by recycled ocean sediment added in their source region, and the addition of recycled sediments could be account for Mo concentrate in the source and regional Mo mineralization.

High-efficient selected area grain boundary diffusion for enhancing the coercivity of thick Nd-Fe-B magnets

Grain boundary diffusion (GBD) is an effective approach for improving the coercivity of Nd-Fe-B magnets with less heavy rare earth consumption. However, it generally only works well for thin magnets since the infiltration depth of diffusion source is limited. Here, a GBD approach that mainly diffuses the source from the edge and corner areas of the magnet, named selected area GBD (SAGBD), is proposed for treating thick magnets. After diffusing Pr-Tb-Al-Cu alloy by SAGBD, the coercivity of a 12-mm thick magnet was increased from 1070 to 1675 kA/m. By this approach, the coercivity enhancement with 1 wt. % Tb was 465 kA·m−1, 1.6 times as much as that by diffusion from two easy magnetization planes. For achieving a coercivity enhancement of ∼600 kA/m, the thickness of treated magnet was increased by 4 mm. Although the diffusion from six planes can further increase the coercivity, SAGBD leads to high efficient use of Tb and less reduction of remanence. The microstructure characterizations and micromagnetic simulation demonstrated that the edges and corners are magnetic “weak” areas of the magnet, and SAGBD can effectively hinder the nucleation of reversed domains in these places and the rapid reversal propagation in whole magnet. The present approach is important for the high-efficient use of the diffusion source and the fabrication of large-size Nd-Fe-B products with high stability.

Redox heterogeneity of picritic lavas with respect to their mantle sources in the Emeishan large igneous province

The oxygen fugacity (fO2) of the Earth’s mantle is a key parameter that regulates fundamental geological processes such as planetary differentiation, mantle melting, and volatile cycling. Large igneous provinces (LIPs) are among the most voluminous mafic volcanic eruptions in the Earth’s history. Constraining the fO2 of LIP rocks is critical for understanding the redox state of the mantle and migration of volatiles from the deep Earth to the atmosphere. Here, we investigated primitive picrites from the Dali and Lijiang areas belonging to the Emeishan LIP, which contain early-crystallized olivine phenocrysts with Fo contents up to 93.5 and 91.9, respectively. The fO2 conditions of primary and derivative melts are determined by the olivine-spinel and olivine-melt V oxybarometers, aided by temperature estimates from olivine-spinel Al and olivine-melt Sc/Y thermometers. The calculated fO2 values are negatively correlated with olivine Fo contents due to fractionation of mafic minerals in a system closed to oxygen. The fO2s of Dali and Lijiang primary melts are −0.8 ± 0.4 and + 0.6 ± 0.3 (1σ) ΔFMQ log units, respectively, more variable than those of primitive MORBs, indicating significant fO2 fluctuations in primary magmas from the Emeishan LIP caused by changes in both redox state of the source rock and depth of melt extraction. Melt barometer constrains the Dali and Lijiang primary melts generated at 4.7 and 3.0 GPa, respectively. After correcting for the depth effect, the Dali and Lijiang sources are more oxidized (with higher Fe3+/∑Fe ratios), in varying degrees, than the ambient upper mantle, likely due to incorporation of recycled surface materials. Under such oxidized conditions, mantle carbon is efficiently extracted during melting in the form of CO2 and released to the atmosphere during massive magma emplacement, which may have played an important role in causing the climatic and environmental changes and triggering the end-Guadalupian mass extinction.

High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound

SUMMARYYasur volcano, Vanuatu is a continuously active open-vent basaltic-andesite stratocone with persistent and long-lived eruptive activity. We present results from a seismo-acoustic field experiment at Yasur, providing locally dense broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. We corroborate our seismo-acoustic observations with coincident video data from cameras deployed at the crater and on an unoccupied aircraft system (UAS). The waveforms contain a profusion of signals reflecting Yasur’s rapidly occurring and persistent explosive activity. The typical infrasonic signature of Yasur explosions is a classic short-duration and often asymmetric explosion waveform characterized by a sharp compressive onset and wideband frequency content. The dominant seismic signals are numerous repetitive very-long-period (VLP) signals with periods of ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). We observe variability in the synchronization of seismic VLP and acoustic sources. Explosion events clearly delineated by infrasonic waveforms are underlain by seismic VLPs. However, strong seismic VLPs also occur with only a weak infrasonic expression. Multiplet analysis of the seismic VLPs reveals a systematic progression in the seismo-acoustic source decoupling. The same dominant seismic VLP multiplet occurs with and without surficial explosions and infrasound, and these transitions occur over a timescale of a few days during our field campaign. We subsequently employ template matching, stacking, and full-waveform inversion to image the source mechanism of the dominant VLP multiplet. Inversion of the dominant VLP multiplet stack points to a composite source consisting of either a dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The derived mechanisms correspond to a point-source directly beneath the summit vents with centroid depths in the range ∼900–1000 m below topography. All mechanisms suggest a northeast trending crack dipping relatively shallowly to the northwest and indicate a VLP source centroid and mechanism controlled by a stable structural geologic feature beneath Yasur. We interpret the results in the framework of gas slug ascent through the conduit responsible for Yasur explosions. The VLP mechanism and timing with infrasound (when present) are explained by a shallow-buffered top-down model in which slug ascent is relatively aseismic until reaching the base of a shallow section. Slug disruption in this shallow zone triggers a pressure disturbance that propagates downward and couples at the conduit base (VLP centroid). If the shallow section is open, an explosion propagates to the surface, producing infrasound. In the case of (the same multiplet) VLPs occurring without surficial explosions and weak or no infrasound, the decoupling of the dominant VLPs at ∼900–1000 m depth from surficial explosions and infrasound strongly indicates buffering of the terminal slug ascent. This buffering could be achieved by a variety of conditions at or directly beneath the vents, such as a high-viscosity layer of crystal-rich magma, a debris cap from backfill, a foam layer, or a combination of these. The dominant VLP at Yasur captured by our experiment has a source depth and mechanism separated from surface processes and is stable over time.

UNDERWATER SOURCE LOCALIZATION USING CYLINDRICAL HYDROPHONE ARRAY AND RIEMANNIAN MATCHED FIELD PROCESSING

The Matched Field Processing (MFP) is for defining the range and the depth of an underwater source effectively. However the algorithm is seldom used for finding the azimuthal direction of the source and is still not considered for multiple field replicas due to the variation of sound velocity. If using cylindrical hydrophone array (CHA) with 32x10 elements (ten ring with each ring of 32 elements) the rough azimuthal angle resolution, , is obtained. Then using TDOA (Time difference of arrival) of the two selected consecutive beams of CHA the system could provide the bearing accuracy up to . Then, the range and the depth of the source are the solution of the proposed Riemannian MFP with the assumption of cylindrical spreading. The proposed RMFP is on the basis of Riemannian distance which is calculated directly by solving the geodesic equation on cylinder surface. The simulations show that only with the data from selected vertical hydrophone array of CHA (there are 32 vertical arrays on cylindrical array) is enough to locate the range and the depth of underwater source precisely.

Source depth discrimination using peak migration line of cross-correlation output in shallow water having negative thermocline

In this paper, the peak migration line of cross-correlation output is utilized to discriminate the depth of an underwater acoustic source in shallow water environment with a negative thermocline through studying the modal arrival structure of the source at different depths. The discrimination can be done according to the position of the migration line when the cross-correlation delay is zero since the type of dominant normal mode is determined by the depth of the source. Firstly, using the beam output of a vertical linear array and the received signal of a single array element to do cross-correlation processing, a two-dimensional intensity image of cross-correlation delay versus elevation angle is obtained. Then, the peaks in the angle domain at different delays can be extracted from the image. The cross-correlation peak migration line can be achieved by piecewise linear fitting which is divided into two parts corresponding to the delay being less than or equal to zero and the delay being more than zero, respectively. Finally, the source depth is determined by the position of the curve where the cross-correlation time delay is zero. Theoretical analysis shows that the migration line is little influenced by the sound velocity profile (SVP) varying with depth, the discrimination threshold is determined by the lower-order normal modes, the identification region is determined by the thickness and depth of the thermocline and the higher strength of the thermocline is instrumental in distinguishing. The numerical results and the experimental results prove the method's effectiveness which can be implemented without SVP details or mode separation, or source movement.

Variation of signal-to-noise ratio of vertical array with sound source depth under wind-generated noise background

Wind-generated noise is ubiquitous in ocean environments and highly influences the passive sonar performance. Since it originates from sources near the ocean surface, one of its physical features is that it largely represents only the intermediate- and high-order modes. The array-level signal-to-noise ratio (SNR), which includes the array-sampled sound intensity, background noise power, and array gain, is an essential quantity determining the sonar array performance. What is investigated in this work is how the array-level SNR of the vertical line array (VLA) varies with the source depth in downward-refracting shallow water, contributed by the modal structure of the surface noise. On the assumption that the modes are well sampled, it is theoretically demonstrated that the SNR varying with the source depth can be approximated as a linear combination of the lower-order mode-amplitude intensities varying with the water depth. Particularly, when the surface noise especially dominates and the water channel is highly downward refractive, this variation can be represented nearly only by the 1&lt;sup&gt;st&lt;/sup&gt;-order mode-amplitude intensity varying with depth. The structure is meaningful in practice. It suggests the SNR will be inherently larger when the source is submerged than it is near the ocean surface, and will be maximized at a source depth slightly below the 1&lt;sup&gt;st&lt;/sup&gt;-order mode peak across different source ranges. The above assertions are demonstrated in a typical downward-refracting shallow-water channel; the effects from the dominant degree of the surface noise, sound speed gradient in water column, and array aperture are investigated numerically. The obtained results are shown below. 1) Under certain circumstances, the variation of SNR with source depth is nearly irrelevant to the source range. 2) When the surface noise is more significant, the largest SNR in a certain source range will be more significantly larger than the SNR for the source near the surface, the corresponding source depth will be closer to that presenting the 1&lt;sup&gt;st&lt;/sup&gt;-order mode’s peak, and the variation of SNR with source depth is increasingly irrelevant to the source range. 3) A stronger downward-refracting sound speed also enhances this SNR superiority and irrelevance to the source range, but causes the source depth presenting the largest SNR to be more deviated from the 1&lt;sup&gt;st&lt;/sup&gt;-order mode’s peak. 4) Although the structure is unraveled on the assumption that the VLA spans the full water column, it can be seen when the VLA does not but covers the low-order modes' main part; when the array aperture is insufficiently large it will become approximately periodic in the source range, with the source depth presenting the largest SNR fluctuating lightly and nearly periodically around the 1&lt;sup&gt;st&lt;/sup&gt;-order mode peak.

Source depth estimation using angle-range interference pattern in deep ocean bottom bounce area

Passive sonar limits the ability to sample vertical scale spatiotemporally, and the received signal is indistinct by noise, resulting in the performance degradation or even failure of the source depth estimation method in deep ocean bottom bounce area. When vertical line array is arranged near the sea surface to locate the source by matched-field processing in bottom bounce area, there is great ambiguity in depth dimension. In this work, the problem of source depth estimation in bottom bounce area is addressed. The peak stripe of angle-range interference pattern is modeled and analyzed based on the ray theory, and a source depth estimation method is proposed, which comprises two parts: estimating sound source depth by utilizing peak stripe fluctuation characteristic combined with spatial spectrum analysis, and reconstructing interference pattern using principal component analysis. The flowchart of the method is listed as follows. Firstly, the spatial spectrum corresponding to each range is spliced to obtain the original angle-range interference pattern. Secondly, the original interference pattern is denoised by principal component analysis to obtain the reconstructed interference structure. Finally, the fluctuation period of peak fringes is extracted from the reconstructed interference pattern to calculate the source depth. Under the low signal-to-noise ratio (SNR) condition, the peak stripe destroyed by noise will reappear in the reconstructed interference pattern and the estimation result of sound source depth will be more accurate, making the source depth estimation method suitable for bottom bounce area. The method combines the space-time cumulative gain of the array and the low-rank characteristic of the interference pattern. The simulation results obtained by Bellhop indicate that the fluctuation period of peak stripe depends on the depth and frequency of the source, which is consistent with the modeling result, and the source depth can be estimated precisely by spatial Fourier transform. The simulation results also show that the rank of the interference pattern is very low and the peak stripe can be composed of a few principal components. Monte Carlo experimental results indicate that the estimated results of the source depth by using reconstructed interference pattern are more accurate than those without using principal component analysis at low SNR condition. The proposed method can achieve more than 80% accuracy at –3 dB SNR.

Nickel Deposit Depth Estimation Using Source Parameter Imaging and Euler Deconvolution of Aeromagnetic Data of Bakin Kogi and its Environs in Jema’a Local Government Area of Kaduna state

This study aimed to estimate the depth of Nickel deposit in Bakin Kogi – Dangoma region, which lies between latitude 901''and 9027'N and longitude 8000' and 8017' E. The study area is located in Jema’a local government area of Kaduna state. The data was gridded using Oasis Montaj® to produce Total magnetic Intensity Map (TMI) of the area, and further subjected to some filtration processes in order to obtain regional and residual map of the study area. Enhancement methods such as first vertical derivative and analytic signal methods were applied to the residual map to delineate the area with potential Nickel deposit. The quantitativemethod applied were source Parameter Imaging and Euler Deconvolution to determine the various depths of Nickel deposit. From the results of the TMI map and the residual map, the magnetic intensity of the study area was between 32976.5 to 33093.3 nT and 32963.6 to 33072.9 nT respectively with Bakin kogi-Dangoma region, having the highest magnetic intensity of about 33093.3nT and 33072.9 nT respectively. The study area has a high magnetic intensity due to nickel deposit because Nickel ore environment are notable with recognizable “magnetic stratigraphy”, prospective mafic-ultramafic contact, typified by strong magnetic contrast. Abrupt changes in the magnetic intensity infer position of the outer contact of a Nickel bearing zone. The results on the superimposed analytic map on the lineament map showed regional lineaments in Bakin kogi-Dangoma region trending NE -SW, ENE – WSW and NNE – SSW directions. The fractures and faults seen around the study area were strongly associated with minerals deposit. The results of Source Parameter Imaging (SPI) gave a depth of shallower magnetic sources ranges from 92.7 to 116.0 m with an average depth value of 104.35 m while the depth of deeper magnetic sources ranges from 651.2 to 976.2 m with an average depth of 813.7 m. The results obtained from 3D Euler deconvolution which was associated with dykes (SI = 1) anomaly produced at a shallower depth range of 100 - 700 m. Results obtained, shows a correspondence between the depths results obtained from the SPI and that of the Euler Deconvolution methods, indicating correlation in the magnetic depth estimation.

Influence of tropical dipole in the East Indian Ocean on acoustic convergence region

The physical ocean phenomena in the ocean affect the changes of water body, which has an important influence on the sound propagation. A hyperbolic frequency modulation (HFM) signal sound propagation experiment of towed sound source in the East Indian Ocean (EIO) was conducted in summer 2019. The center frequency of the towed sound source is 300 Hz, and the hydrophone receives the data from 4130 m far. This is the first time that we have conducted the underwater acoustic survey in the Indian Ocean. The influence of the physical ocean phenomenon—Indian Ocean Dipole (IOD) on sound propagation is observed. The experimental data of sound propagation from the IOD are processed and analyzed, the effects of sound source depth and water fluctuation on the deep-sea convergence zone (CZ) are analyzed, and the formation mechanism of sound propagation phenomenon in the experiment is explained theoretically. The results show that the second CZ is not formed under the influence of the warm water mass formed by the IOD and the depth of the sound source in the incomplete deep channel environment of the EIO. Owing to the fluctuation of the sound source depth, the deep-sea CZ disappears at the distance where it should have appeared. When the depth of the sound source becomes deeper, it is easier to form the deep-sea CZ. Under the influence of the IOD, the thermocline fluctuation at the location of the second CZ has an important influence on the formation and location of the third CZ. It is found that the location of the third CZ shifts 2–3 km toward the sound source in the experiment. The research results have important significance in guiding the applications in detection and communication sonar in deep-sea complex environment.

ТРАНСФОРМАЦИЯ АНОМАЛИЙ СИЛЫ ТЯЖЕСТИ В ПРЕДЕЛАХ БОЛЬШИХ ТЕРРИТОРИЙ (НА ПРИМЕРЕ КУРИЛЬСКОЙ ОСТРОВНОЙ ДУГИ)

An algorithm and a program for transformation of gravity anomalies for large territories are presented. The spherical shape of Earth is taken into account and the geodetic coordinates of measuring points are used. The calculations use the «quasi-ellipsoidal» model of V.V. Kavraisky, which allows the transition from geodesic to spherical coordinate system with increased accuracy. The algorithm is based on a source-like approximation of the field values, located in the nodes of a regular network, with the depths of the equivalent sources, depending on the latitude. To determine the source masses, the system of linear algebraic equations is solved by the fastest gradient descent method with an accelerated step calculation. The results of transformation of the gravity field in the Bouguer reduction for the Kuril island arc, the adjacent water areas and parts of land, located within 40°–54° N, 142°–162° E are presented with a total area of about 2.4 million km2. A slight difference between the recalculation results of gravity anomalies into the upper half-space using the «spherical» and «quasi-ellipsoidal» models of the Earth was experimentally established.

ВУЛКАН БОЛЬШОЙ ПАЯЛПАН (СРЕДИННЫЙ ХРЕБЕТ, КАМЧАТКА). К ПРОБЛЕМЕ КОНВЕРГЕНТНОСТИ «ОСТРОВОДУЖНЫХ» И «ВНУТРИПЛИТНЫХ» ПЕТРОЛОГО-ГЕОХИМИЧЕСКИХ ПРИЗНАКОВ В МАГМАТИЧЕСКОЙ СИСТЕМЕ

The paper presents the data on age, mineralogy, geochemistry, and isotope composition of rocks from the Bolshoi Payalpan Volcano (Sredinny Range, Kamchatka). We compared these data with those on the Nosichan and Belogolovsky volcanoes, located within the Belogolovsky volcanic center. The basalts of the neck and the upper lava complex of Bolshoi Payalpan are compositionally similar to the intraplate-type trachybasalts of the Belogolovsky Volcano, and the basaltic andesites of the lower lava and the cone complex are similar to the island arc rocks of the Nosichan Volcano. Analysis of the data obtained evidences that spatial and temporal manifestations of intraplate and island-arc volcanism at the Bolshoi Payalpan Volcano are not accidental, but may be a consequence of a change in the degree and depth of melting of the same deep source with the involvement of a mantle diapir. The Belogolovsky volcanic center formed in a setting of the Late Miocene-Early Pliocene rifting. Its evolution, right up to its extinction, proceeded in the same geodynamic setting with an increase in depth of the mantle source and a decrease in the degree of its melting. Rock compositions of the Lower-Middle Pliocene Nosichan Volcano remain of the island-arc type under conditions of rifting, since they are associated with the mantle reservoir located at a shallower depth, which has experienced a higher degree of melting. There is good reason for considering large volcanic centers as spontaneously-developing geological entities. As the endogenous activity dies down, the degree of melting decreases and the depth of melting increases with the replacement of island-arc volcanism by intraplate volcanism. The volcanic center becomes extinct.

Depth and Thickness of Tectonic Tremor in the Northeastern Olympic Peninsula

AbstractTectonic tremor has been explained as a swarm of low‐frequency earthquakes (LFEs), which are located on a narrow fault at the plate boundary. However, due to the lack of clear impulsive phases in the tremor signal, it is difficult to determine the depth of the tremor source with great precision. The thickness of the tremor region is also not well constrained. The tremor may be located on a narrow fault as the LFEs appear to be, or distributed over a few kilometers wide low shear‐wave velocity layer in the upper oceanic crust, which is thought to be a region with high pore‐fluid pressure. Lag times of peaks in the cross‐correlation of the horizontal and vertical components of tremor seismograms, recorded by small‐aperture arrays in the Olympic Peninsula, Washington, are interpreted to be S minus P times. Tremor depths are estimated from these S minus P times using epicenters from a previous study computed with a multibeam backprojection method. The tremor is located close to the plate boundary in a region no more than 2–3 km thick and is very close to the depths of LFEs. The tremor is distributed over a wider depth range than the LFEs. However, due to the uncertainty on the depth, it is difficult to conclude whether the source of the tremor is located at the top of the subducting oceanic crust, in the lower continental crust just above the plate boundary, or in a narrow zone at the plate boundary.