MAXIPOL: a balloon-borne experiment for measuring the polarization anisotropy of the cosmic microwave background radiation

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10^′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress.     

Flood-prone area mapping using machine learning techniques: a case study of Quang Binh province,Vietnam

Natural Hazards - Vietnam’s central coastal region is the most vulnerable and always at flood risk, severely affecting people’s livelihoods and socio-economic development. In...     

Geogene Versus Anthropogene Origin of Trace Metals in Sediments in Cua Luc Estuary and Ha Long Bay, Vietnam

In the present study, the geochemistry of 49 surficial-bed and 101 core sediment samples is investigated to clarify the origin of trace metals in the Cua Luc Estuary and Ha Long Bay, which is a famous World Natural Heritage Site in Vietnam. Moreover, the potential mobility of trace metals is also assessed and their relationship with reference elements (Al, Fe, and Ca) and organic matter is established in order to make recommendations for sediment management and monitor future pollution. Generally, trace metals display higher concentrations in Ha Long Bay compared to the Cua Luc Estuary. However, this is controlled by the distribution of the fine (clay?+?silt) fraction, and hence the concentrations of Al, Fe, Ca, and organic matter (OM). The comparison of concentrations of trace metals (normalized towards Al) between the surficial sediments and the subsurface core sediments based on ¹³⁷Cs datings indicates that almost all surficial-sediment data fall inside or deviate slightly from the 95 % prediction interval of a background regression line. In addition, as determined by a Community Bureau of Reference three-step extraction, trace metals mainly dominate in the residual fraction (assumed to relate to crystal lattice of primary and secondary minerals), and this fraction does not change much in recent sediment layers. Therefore, trace metals are supposed to be derived from natural bio/geochemical processes and are characterized by a low potential mobility. Consequently, the established linear regression relationships of trace metal vs. Al or multiple regression relationships of trace metal vs. multi-elements (Al, Fe, Ca, and OM) are useful for the prediction of background levels of trace metals in sediments in future pollution monitoring and assessment programs.     

Cenozoic basement structure of the South China Sea and adjacent areas by modeling and interpreting gravity data

The South China Sea and adjacent areas is a large region with diversely complicated geological conditions. In spite of investigations carried out over the past many years, the marine geological structure in many places has remained poorly understood because of the deficient data, a thick seawater layer as well as of the sensitive areas among the countries in the region. In this paper, the authors study and apply a model-based methodology of the modeling and interpretation of the newest gravity data and others, which are 3D modeling, frequency filtering, horizontal gravity gradient and maximum horizontal gravity gradient, in order to determine clearly the basement structure. The basement features such as the main faults systems, uplift-depression zones and seafloor spreading axis, oceanic boundary in the region have been defined. The achieved results are checked by the seismic data available for the region. From the studied results, the authors have brought out some initial remarks on the structure and form of the basement in the South China Sea and adjacent areas.     

Sustainable groundwater development in the coastal Tra Vinh province in Vietnam under saltwater intrusion and climate change

Three-dimensional transient groundwater flow and saltwater transport models were constructed to assess the impacts of groundwater abstraction and climate change on the coastal aquifer of Tra Vinh province (Vietnam). The groundwater flow model was calibrated with groundwater levels (2007–2016) measured in 13 observation wells. The saltwater transport model was compared with the spatial distribution of total dissolved solids. Model performance was evaluated by comparing observed and simulated groundwater levels. The projected rainfalls from two climate models (MIROC5 and CRISO Mk3.6) were subsequently used to simulate possible effects of climate changes. The simulation revealed that groundwater is currently depleted due to overabstraction. Towards the future, groundwater storage will continue to be depleted with the current abstraction regime, further worsening in the north due to saltwater intrusion from inland trapped saltwater and on the coast due to seawater intrusion. Notwithstanding, the impact from climate change may be limited, with the computed groundwater recharge from the two climate models revealing no significant change from 2017 to 2066. Three feasible mitigation scenarios were analyzed: (1) reduced groundwater abstraction by 25, 35 and 50%, (2) increased groundwater recharge by 1.5 and 2 times in the sand dunes through managed aquifer recharge (reduced abstraction will stop groundwater-level decline, while increased recharge will restore depleted storage), and (3) combining 50% abstraction reduction and 1.5 times recharge increase in sand dune areas. The results show that combined interventions of reducing abstraction and increasing recharge are necessary for sustainable groundwater resources development in Tra Vinh province.

     

Timing of Holocene sand accumulation along the coast of central and SE Vietnam

In Vietnam, the coastal sand barriers and dunes located in front of the steep slopes of the high rising Truong Son Mountains are sensitive to climate and environment change and give evidence for Holocene sea-level rise. The outer barrier sands were deposited shortly before or contemporaneous with the local sea-level high stand along the Van Phong Bay postdating the last glacial maximum (LGM). Optically stimulated luminescence (OSL) dating yielded deposition ages ranging from 8.3 ± 0.6 to 6.2 ± 0.3 ka for the stratigraphically oldest exposed barrier sands. Further periods of sand accumulation took place between 2.7 and 2.5 ka and between 0.7 and 0.5 ka. The youngest period of sand mobilisation was dated to 0.2 ± 0.01 ka and is most likely related to reworked sand from mining activities. At the Suoi Tien section in southern central Vietnam, the deposition of the inner barrier sands very likely correlate with an earlier sea-level high stand prior to the last glaciation. OSL age estimates range from 276 ± 17 to 139 ± 15 ka. OSL dating significantly improves our knowledge about the sedimentary dynamics along the coast of Vietnam during the Holocene.     

Geochemistry and petrogenesis of Permian ultramafic-mafic complexes of the Jinping–Song Da rift (southeastern Asia)

The Jinping–Song Da rift structure in the Emeishan Large Igneous Province is composed of Permian high- and low-Ti volcanic and volcanoplutonic ultramafic-mafic associations of different compositions and genesis. High-Ti picrites, picrobasalts, basalts, and dolerites are enriched in LREE and depleted in HREE and show low Al₂O₃/TiO₂ ratios (~4), commensurate ε_Nd(T) values (+0.5 to +1.1), and low (Th/Nb)_PM ratios similar to those of OIB-enriched mantle source. The established geochemical characteristics evidence that the parental melts of these rocks were generated from garnet lherzolite at the depths of garnet stability (~200 to 400 km). Later, high-Mg low-Ti volcanics (komatiites, komatiitic basalts, and basalts) and associating small peridotite-gabbro massifs and komatiite-basalt dikes were produced as a result of ~20% partial melting of depleted water-poor (≤0.03 wt.% H₂O) peridotite substratum from the hottest upper part of mantle plume at relatively shallow depths (100–120 km). The LREE-depleted komatiites and komatiitic basalts are characterized by low (Ce/Yb)_CH values, ¹⁸⁷Re/¹⁸⁸Os = 0.05–1.2, ⁸⁷Sr/⁸⁶Sr = 0.704–0.706, positive ε_Nd(T) values (+3 to +8), γ_Os = –0.5 to +0.9, and strong negative anomalies of Ba, K, and Sr on the spidergrams. The scarcer LREE-enriched komatiites, komatiitic basalts, and basalts vary greatly in chemical composition and values of ε_Nd(t) (+6.4 to –10.2), ⁸⁷Sr/⁸⁶Sr (0.706–0.712), and γ_Os (+14.8 to +56), which is due to the different degrees of crustal contamination of parental magmas. The Rb-Sr isotopic age of basaltic komatiite is 257 ± 24 Ma. The Re-Os age determined by analysis of 12 komatiite samples is 270 ± 21 Ma. These data agree with the age of flood basalts of the Emeishan Large Igneous Province. The komatiite-basalt complex of the Song Da rift is still the only Phanerozoic PGE-Cu-Ni-complex of this composition. The geochemistry of accompanying Cu-Ni-PGE-ores confirms their relationship with komatiite-basaltic magmatism.     

Meshless BEM and overlapping Schwarz preconditioners for exterior problems on spheroids

We consider the exterior Neumann problem of the Laplacian with boundary condition on spheroids. We propose to use spherical radial basis functions in the solution of the boundary integral equation arising from the Dirichlet-to-Neumann map. Our meshless approach with radial basis functions is particularly suitable for handling scattered satellite data. We also propose a preconditioning technique based on an overlapping domain decomposition method to deal with ill-conditioned matrices arising from the approximation problem.     

An Application of HFE-D for Evaluating Sea Water Intrusion in Coastal Aquifers of Southern Vietnam

The coastal aquifers and inland waters of the Long Xuyen Quadrangle and Ca Mau Peninsula of southern Vietnam have been significantly impacted by sea water intrusion (SI) as a result of recent anthropogenic activities. This study identified the evolution and spatial distribution of hydrochemical conditions in coastal aquifers at this region using Hydrochemical Facies Evolution Diagram (HFE-D) and Geographical Information System mapping. Hydraulic heads and water chemistry were measured at 31 observation wells in four layered aquifers during dry and rainy seasons in early (2005), and more recent (2016), stages of agricultural development. Hydrochemical facies associated with intrusion or freshening stages were mapped in each aquifer after assigning mixing index values to each facies. The position of groundwater freshening and SI phases differed in Holocene, Upper Pleistocene, Middle Pleistocene, and Lower Pleistocene aquifers. The geographic position of freshening and intrusion fronts differ in dry and rainy seasons, and shifted after 11 years of groundwater abstraction in all four aquifers. The spatial and temporal differences in hydrochemical facies distributions according to HFE-D reflect the relative impact of SI in the four aquifers. The study results provide a better understanding of the evolution of groundwater quality associated with SI in a peninsular coastal aquifer system, and highlight the need for improving groundwater quality and management in similar coastal regions.     

Effects of riverbed incision on the hydrology of the Vietnamese Mekong Delta

The hydrogeomorphology of the Vietnamese Mekong Delta (VMD) has been significantly altered by natural and anthropogenic drivers. In this study, the spatiotemporal changes of the flow regime were examined by analysing the long-term daily, monthly, annual and extreme discharges and water levels from 1980 to 2018, supported by further investigation of the long-term annual sediment load (from the 1960s to 2015), river bathymetric data (in 1998, 2014 and 2017) and daily salinity concentration (from the 1990s to 2015) using various statistical methods and a coupled numerical model. Then, the effects of riverbed incision on the hydrology were investigated. The results show that the dry season discharge (i.e., in March–June) of the Tien River increased by up to 23% from the predam period (1980–1992) to the postdam period (1993–2018) but that the dry season water level at My Thuan decreased by up to −46%. The annual mean and monthly water levels in June at Tan Chau and in January and June–October at My Thuan in the Tien River decreased statistically, even though the respective discharges increased significantly. These decreased water levels instead of the increased discharges were attributed to the accelerated riverbed incision upstream from My Thuan, which increased by more than three times, from a mean rate of −0.16 m/year (−16.7 Mm³/year) in 1998–2014 to −0.5 m/year (−52.5 Mm³/year) in 2014–2017. This accelerated riverbed incision was likely caused by the reduction in the sediment load of the VMD (from 166.7 Mt/year in the predam period to 57.6 Mt/year in the postdam period) and increase in sand mining (from 3.9 Mm³ in 2012 to 13.43 Mm³ in 2018). Collectively, the decreased dry season water level in the Tien River is likely one of the main causes of the enhanced salinity intrusion.