Freshwater aquifers on offshore islands are an invaluable resource that directly affects the islands’ economic and social life. Seawater migration into aquifers on islands is a frequent phenomenon that significantly affects the quality and quantity of fresh water in the aquifers. Our research combines electro-geophysical and geochemical methods and has been used effectively to study saltwater intrusion in coastal aquifers. This article presents the results of 16 lines of electrical resistivity tomography (ERT) and 21 physicochemical water samples at the available wells conducted on Coto Island in Quang Ninh province, Vietnam. The obtained interpretation of these data provides new insights into the geological structure, the distribution of aquifers, and the salinity intrusion potential of pore and fissure aquifers in the Coto Island area. The measurement results of 16 ERT lines have determined the thickness and resistivity of the Quaternary pore aquifer and the Ordovic—Siluan fissure aquifer. Resistivity values determined in aquifers closely correlate with EC and TDS measurements of water samples from available wells. The results have determined that the fractured aquifer in the southern area of Dong Tien commune is heavily salinized from two sides. It is confirmed that the northern region of Dong Tien commune has a fissure aquifer with great potential for supplying fresh water for the residents on the island.

The study involved the collection of eighteen surface sediment samples from the coastal shallow water area from Van Don to Tien Yen - Ha Coi in the Northwestern section of the Gulf of Tonkin to analyze their particle size composition. Utilizing the EMMAgeo end-member analysis model, four characteristic particle sizes (4EM) of 0.34, 7.7, 130, and 230 µm, corresponding to clay, fine silt, and fine sand of varying sizes were identified. In conjunction with the sedimentary environment, the spatial distribution analysis of these end members allowed a detailed determination of the formation conditions and distribution of the sediment components. Clay deposits (EM1) are primarily intercalated between the islands. At the same time, fine silt (EM2) is concentrated in the northern part of the study area, transported by flows, and deposited in a low-energy environment. Fine sand sediments (EM3 and EM4) are distributed along the coast of the Van Don peninsula in the Southern part of the study area and likely formed in association with tidal-wave processes under higher energy conditions compared to the North.

Eighteen surface sediment samples were collected from the Van Don-Tra Co coast, northwestern Gulf of Tonkin, for analyzing the concentration of heavy metals (Zn, Cr, As, Cu, Ni, Pb, Co, and Cd) and grain size. The statistical and contamination indices of heavy metals were calculated to evaluate the extent of pollution caused due to heavy metals and their potential harm to the environment. According to the analysis results, the contamination of heavy metals in the study area were higher than the background concentrations, except for Pb. The heavy metal concentrations in the study area followed the descending order of Zn > Cr > As > Cu > Ni > Pb > Co > Cd. The concentrations of heavy metals do not correlate with clay content, and almost heavy metals were non-correlated with silt and sand contents, except for Cr, implying that the grain size does not control heavy metals concentration in surface sediments in the region. The results of pollution indices imply that the contamination in the environment was due to excess accumulation of Cd, in contrast to that of Zn. The ecological pollution risk shows an increased risk of heavy metal pollution. According to the statistical index, the contamination of heavy metals in surface sediments in the study area could come from the same sources under the same environmental conditions and is related to the source of sedimentary material from the mainland near the study area.

This study investigates the stratigraphic architectures of the South Vietnam continental margin (SVCM) from the Pliocene to the recent period. We have utilized high-resolution seismic data and sediment core data to achieve this. The study reveals that the Quaternary sequences on the continental margin are bounded by composite erosional surfaces, which were formed due to sub-aerial erosion during periods of sea level fall and subsequent reworking processes during sea level rise. The Quaternary sequence can be divided into two subsequences: the Early Pleistocene and the Middle Pleistocene-Holocene. The Middle Pleistocene-Holocene sequences can be subdivided into six fourth-order sequences, each lasting approximately 100 to 120 thousand years. These sequences are well-preserved on the outer shelf at a modern water depth of around 100 to 120 meters. Seismic stacking patterns indicate that regressive deposits predominantly dominate the Quaternary stratigraphy of the SVCM. This means that sediment deposition during the Quaternary period was more pronounced during periods of sea-level fall. The outer shelf margin of the modern continental margin shows a south-to-southeastward migration of prograding clinoforms, indicating sediment progradation in these directions. The thick Quaternary regressive deposits preserved on the outer shelf are attributed to various factors. These factors include the low shelf gradient, tectonic subsidence, regional hinterland uplift, monsoon strengthening, and high sediment supply from the Palaeo-Mekong Delta during sea-level fall. The interaction of these factors facilitated the accumulation of thick sedimentary deposits during falling sea levels. The mid-late Pleistocene stratigraphy of the SVCM shows similarities to many other continental margins worldwide. These margins are mainly shaped under the influence of asymmetric 4th-order Milankovitch cycles. These cycles are driven by variations in Earth's orbital parameters and are characterized by longer phases of relative sea-level fall followed by shorter periods of stabilization and rise.

This paper presents the determination of the Moho and basement depths of Tuchinh-Vungmay basin (TCVMB) offshore southeastern Vietnam by the three dimension direct gravity inversion. The Moho depth was predicted from the mantle residual gravity anomaly with the lithosphere thermal gravity correction. The downward continuation of the basement residual gravity anomaly is also applied to enhance the basement topography’s resolution. The mean depths of the basement and Moho surfaces were constrained by the power density spectrum (PDS) of the residual gravity anomalies and the oceanic bottom seismic (OBS) data. The predicted Moho depth varies from 13.5 km to 23 km and the basement depth is from some hundred meters to 8.5 km. The gravity basement topography has higher resolution and detail than the National Oceanic and Atmospheric Administration (NOAA) seismic basement.

The most important issue in solving the gravity inversion problem is to build a highly reliable initial hypothetical model related to geological boundaries from miscellaneous data sources (seismic or geological data) alongside gravity anomaly data. In many cases, however, the miscellaneous data sources are often very few, less than what is required to build a reliable initial model, yet we nevertheless have to rely heavily on gravity data. Some auxiliary analysis methods have been used to build the initial model such as Bott’s method, Euler convolution, power density spectrum and so on. In this article, we introduce a process to build the initial model for inverting a 2D gravity anomaly profile, in which the normalized full gradient method (NFG) is used to constrain the upper and lower boundaries of the gravity anomaly object. In the calculation of the NFG, we propose to calculate the upward continuation of the gravity anomaly to a certain height before calculating the NFG. Calculating the upward continuation of the gravity anomaly before calculating the NFG’s maximum value curve, allows the optimal harmonic number N to be determined with greater ease and stability. Both the upper and lower boundaries of the gravity anomaly object are estimated by choosing the appropriate harmonic number in the process of calculating the NFG. The obtained result is useful for supporting the construction of an initial model for the 2D gravity inverse problem. The analysis method is tested on a synthetic model as well as actual gravity data from the East Vietnam Sea.

The vertical derivative of the gravity anomaly has a vital role in the methods of geological structure research such as determining fault systems and the location of the field sources. In addition, the vertical derivative is also used to calculate the downward continuation and further clarify the image of the seabed topography. However, determining the vertical derivative according to the traditional Fast Fourier Transform (FFT) method is often unstable and has low accuracy in high-order derivatives for high noise actual data. In this article, we introduce a new calculation method to determine the vertical derivative of gravity anomaly giving higher stable and accurate than traditional methods. The method is verified on synthetic model data and actual data of the Southwest sub-basin of the East Vietnam Sea.

PreviousNext No AccessProceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021Determine the initial model for solving 2D gravity anomaly inversion problem by normalized full-gradient methodAuthors: Nguyen Nhu TrungTran Van KhaBui Van NamNguyen Nhu TrungInstitute of Marine Geology and Geophysics and the Graduate School of Science and Technology, VAST, Hanoi, VietnamSearch for more papers by this author, Tran Van KhaGraduate School of Science and Technology, VAST, Hanoi, VietnamSearch for more papers by this author, and Bui Van NamInstitute of Marine Geology and Geophysics and the Graduate School of Science and Technology, VAST, Hanoi, VietnamSearch for more papers by this authorhttps://doi.org/10.1190/segj2021-038.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract The most important issue of solving the geophysical inversion problem in general and gravity in particular is to build a highly reliable initial hypothetical model related to geological boundaries from the other data sources (seismic or geological data) and gravity anomaly data. In may cases, however, the other data sources are often very few, not enough to build a reliable initial model, but we have to rely heavily on gravity data. Some auxiliary analysis methods have been used to build the initial model such as Bott's method, Euler's convolutional method, energy spectrum method, derivative filtering methods to determine the depth of boundary faces or the position of the horizontal boundaries. In this article, we introduce a new interpretation process of the normalized full gradient (NFG) method in estimating the uper and lower boundaries of the field sources. The result supports usefully to construct an initial model for solving the 2D inverse gravity problem. The analysis method is tested on synthetic model and actual gravity data in the East Vietnam Sea. The results showed that the initial model given by NFG method has high reliability and quite accuracy. Keywords: gravity inversion, normalized full gradient, East Vietnam Sea, initial modelPermalink: https://doi.org/10.1190/segj2021-038.1FiguresReferencesRelatedDetails Proceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021ISSN (online):2159-6832Copyright: 2021 Pages: 349 publication data© 2021 Published in electronic format with permission by the Society of Exploration Geophysicists of JapanPublisher:Society of Exploration GeophysicistsSociety of Exploration Geophysicists of Japan HistoryPublished Online: 29 Nov 2021 CITATION INFORMATION Nguyen Nhu Trung, Tran Van Kha, and Bui Van Nam, (2021), "Determine the initial model for solving 2D gravity anomaly inversion problem by normalized full-gradient method," SEG Global Meeting Abstracts : 138-141. https://doi.org/10.1190/segj2021-038.1 Plain-Language Summary Keywordsgravity inversionnormalized full gradientEast Vietnam Seainitial modelPDF DownloadLoading ...

In this paper, we present a procedure based on Parker's three-dimensional direct gravity inversion to determine the sedimentary basement depth rapidly and objectively. In the foregoing procedure, the Bouguer gravity anomaly is continued downward near the seabed before inversion, and the mean depth of the basement is constrained by the power density spectrum of the gravity anomaly. The bulk density of sediment and basalt rocks are also constrained by the logging data obtained from Expedition 349 Scientists (Expedition 349 Scientists, in: International Ocean Discovery Program Preliminary Report, 349, (2014). doi:10.14379/iodp.pr.349.2014). The sedimentary basement depth of the Southwest Sub-basin (SWSB) of the East Vietnam Sea (South China Sea) derived from the inversion of the downward continued Bouguer anomalies has more detail and accuracy than the sediment basement depth inverted from Bouguer anomaly at the sea level. The calculated basement depth and the sedimentary thickness in the SWSB vary from 4.0 to 6.5 km and from 0.5 to 2.0 km, respectively. The crustal thickness beneath the SWSB ranges from 4.0 to 7.5 km. Our model is consistent with sedimentary basin structure where spreading ridge valley is presented as the deepest points in the sedimentary basement and the terrain along the two banks of the valley is elevated. In addition, the SWSB is divided into four structure domains that are useful for the geodynamic and tectonic implications of the SWSB and surrounding area.

The calculations which determine the chemical composition of the primitive magma are simple but they show changes in the temperature and pressure states of the magma source. The method is based on the addition of the chemical composition of the Olivine to the major element composition of the eruptive rocks which follows the formula: Ci = Ci-1+ 0.1 * Ci-1Ol. In accordance with the characteristics of the study area, we have made new additions to the calculation method. The calculation results are highly accurate when tested and compared with the chemical composition of the eruptive rocks. The chemical composition of the primitive magma solution is used to calculate the temperature and pressure states in the magma source. The results show that there is a difference in temperature and pressure in the source at different tectonic positions in the study area. Accordingly, the South Central coast region and the adjacent continental shelf are divided into two main types of eruptions. The first type of volcanic eruptions occurs at locations where major faults intersect and they are located north of the study area. The second type of volcanic eruptions in the form of a single volcano is located to the south of the study area and the southeastern continental shelf, and occurs in intracontinental extension structure.

This paper reports the results of the third Russian–Vietnamese expedition (POI FEB RAS and the Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology) in the Gulf of Tonkin, South China Sea (April 2016) and field work outcomes from 2016-2017. The studies revealed new specific features of the distribution and origin of gas-geochemical fields in sediments within the rift zone of the Red River along a 150-km profile. Four zones with high amplitude anomalies of hydrocarbon gases, helium, hydrogen, carbon dioxide, and carbon monoxide were revealed. The distribution of the anomalies reflects the tectonic structure of the area and points to the presence of several lithospheric sources of gases including gases of deep origin. The studies were carried out within the scope of the Joint Vietnamese–Russian Laboratory for Marine Geosciences (POI FEB RAS and the Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology). The article is dedicated to the year of friendship between Russia and Vietnam.

The article mentions the results of the new analysis of gas-geochemical and high resolution shallow seismic data acquired by the sea surveys in 2016, which revealed a remarkable coincidence between the seabed gas anomalies at river mouth area of Gulf of Tonkin and the gas distortion/accumulation phenomena in the near-surface sediment layers, especially right above the large tectonic fault systems, such as Red river, Lo river and Chay river. The gas-geochemical analysis reveals the existences of thermogenic and metamorphic methane and carbon dioxide gases within the faults of Red river and on Cat Ba island, anomalies of helium and hydrogen as well as the “heavy” isotopic carbon signals of methane and carbon dioxide in the areas of near-surface active faults. These new results suggest the initial conclusions about the relationship between gas-geochemical field and tectonic fault activities in study area.

Представлены результаты третьей Российско-Вьетнамской (ТОИ ДВО РАН — ИМГГ ВАНТ) комплексной геолого-геофизической экспедиции в Тонкинском заливе Южно-Китайского моря (апрель 2016 г.) и полевых исследований на севере Вьетнама в 2016–2017 гг. В результате исследования выявлены новые особенности распределения и генезиса газогеохимических полей в осадочных отложениях в пределах рифтовой зоны Красной реки по профилю длиной 150 км. Выявлены четыре зоны с высокоамплитудными аномалиями углеводородных газов, гелия, водорода, двуокиси и окиси углерода, распределение которых отражает тектоническое строение района и характеризует наличие нескольких литосферных источников газов, в том числе глубинного происхождения.

In this paper, the authors present the result of three-dimensional (3D) direct inversion of gravity data to determine the Moho depth along the Red River Fault (RRF) zone and adjacent areas in the land territory of Vietnam and the Gulf of Tonkin. The Moho depth was calculated by 3D Parker inversion supplemented with the gravity anomaly power density spectrum. The given Moho depth and the available teleseismic and magnetotelluric Moho depth are well matched. A structural Moho map of the study region was constructed from the interpretation results. The Moho surface was uplifted along the RRF zone. The shallowest Moho depth was determined in the Red River Basin (RRB), where it ranges from 22 +/- 1.0 km to 24 +/- 1.0 km in the depocenter of the RRB in the Gulf of Tonkin to 27 +/- 1.0 km to 29 +/- 1.0 km in the Red River Delta. It is 29 +/- 1.0 km to 32 +/- 1.0 km from Vietri to Laocai provinces. The deepest Moho depth was found in the northwesternmost and northernmost land territory of Vietnam, where it ranges from 33 +/- 1.0 km to 35.8 +/- 1.0 km. A percentage of the Earth's crustal compensation is also investigated. The obtained crustal thickness and compensation state are useful for their implications regarding the geodynamics and tectonics of the RRF and surrounding region.