Nguyen Van Giang

Preliminary Results of Anthropogenic Seismicity Monitoring in the Region of Song Tranh 2 Reservoir, Central Vietnam

Song Tranh 2 hydropower plant and the reservoir containing backed up water are located in the Quang Nam province (Central Vietnam). The region experiences unusual seismic activity related to the reservoir impoundment, with earthquakes of magnitude up to 4.7. In result of cooperation between the Institute of Geophysics, Vietnam Academy of Sciences and Technology and the Institute of Geophysics, Polish Academy of Sciences a seismic network has been built to facilitate seismic monitoring of the Song Tranh 2 area. The network, operating since August 2013, consists of 10 seismic stations. Here we show that the network is sufficient for advanced data processing. The first results of monitoring of the earthquake activity in Song Tranh 2 area in the period between 2012 and 2014, especially the completeness of catalogs, study and comparisons between water level and the seismic activity suggest direct connection between reservoir exploitation and anthropogenic seismicity.

Hydrological and hydrogeological characterization of groundwater and river water in the North Hanoi industrial area, Vietnam

Abstract Degradation of groundwater quality by human activities is a widespread environmental problem in Vietnam. Groundwater there is a major source of water for domestic and industrial purposes. This paper reviews, compiles, and comprehensively analyzes spatiotemporal variations of hydrological and hydrogeological characteristics of shallow and deep groundwater aquifers in northern Hanoi industrial zones and in nearby Red River water. Groundwater level, electrical conductivity, and water temperature were measured in six monitoring wells, complemented by anion, cation, and stable isotope analyses of ground and surface water. The results show that the groundwater in both shallow and deep aquifers was fresh, but mainly calcium-bicarbonate type contaminants and human activities affect groundwater and surface water composition. With the goal of devising sustainable water use regulations, more research must be directed toward long-term monitoring of groundwater and surface water quality, as well as toward detailed investigation of the hydraulic characteristics of local aquifers in the study area.

Geophysical techniques to aquifer locating and monitoring for industrial zones in North Hanoi, Vietnam

Geophysical methods were applied for hydrogeological targets in many countries including Vietnam. This paper presents results of using complex geophysical techniques as well as 2D electrical resistivity imaging (ERI), vertical electrical sounding (VES), very low frequency (VLF), and seismic refraction for geological structure investigation for locating the aquifers and assessing the hydrogeological conditions for groundwater potential in industrial zones of North Hanoi, Vietnam. The locations of two aquifers are determined by their depth and thickness on the basis of resistivity and seismic velocity values which were proved by stratifications of three boreholes to 40–60 m of depth on the study area. There are connections from surface water to shallow aquifer by hydraulic windows, as follows from VLF data. The deeper aquifer can be considered as a potential groundwater supply, but the water level is descending in time, as shown by hydrological monitoring. However, with careful use and by reducing sources of pollution, groundwater can continue to be an important natural resource for future.

Clustering and Stress Inversion in the Song Tranh 2 Reservoir, VietnamClustering and Stress Inversion in the Song Tranh 2 Reservoir, Vietnam

The seismicity in the vicinity of the Song Tranh 2 reservoir that commenced in late 2010 is an example of reservoir‐induced seismicity. Moment tensor (MT) solutions are mostly normal‐faulting mechanisms with shearing as the main component of full MT, which suggest reservoir exploitation stress field changes as a triggering origin of this seismicity. However, the local tectonic stress field plays a role in the seismogenic process, which is accelerated by the reservoir exploitation. Two main orientations of discontinuities reactivated by reservoir exploitation are identified, with clustering of events located in two main areas: northern and southern. In addition to the northern cluster (NC) and southern cluster (SC), further smaller clusters are distinguishable, assuming both clustering of epicenters and similarity of focal mechanisms as the clustering criteria. Maximum seismic activities in clusters appear in different periods, with high seismicity in the SC preceding that in the NC. The results indicate significant differences between the NC and SC. It is shown that the SC is an example of rapid triggering, whereas the NC is a delayed triggering type. Also, magnitude distribution differs between the NC ( b≈1.1b≈1.1 ) and SC ( b≈1.3b≈1.3 ).

Groundwater investigation on sand dunes area in southern part of Vietnam by magnetic resonance sounding

In the last five years, magnetic resonance sounding (MRS), as a non-invasive geophysical method, has emerged as a new technique for ground water investigation in Vietnam. In this paper, we present the general theoretical basis of this method together with acquisition, processing, and interpretation of the MRS data. We show a case study of MRS surveys in sand dunes area in order to characterize aquifers situated in the southern part of Vietnam. From the interpretation of MRS soundings we delimited an aquifer layer in the subsurface with strong lateral variations for which we determined the depth at 44 m and water content between 3% and 9.5%. The longitudinal relaxation constant T*1 is about 250 m s, while the transverse relaxation T*2 is between 150–200 m s. That indicates fine to medium grain size and thus low to medium hydraulic permeability. These results are confirmed by the observations from the well LK1 between 45 to 70 m. The results of other MRS measurements showed the presence of a low water bearing aquifer and were confirmed by the observations in two other wells.

Kirchhoff migration for specifying velocity model in ground penetrating radar method

The Kirchhoff migration plays an important role in converting seismic data into images of geological layers according to viewpoint in the applied geophysics. In this paper, the application of the Kirchhoff migration was used in the case of the high frequency electromagnetic method for specifying GPR velocity model. The measurements of raw GPR data collected by Ramac X3M with 500 MHz frequency in HoChiMinh city were processed on the basis of the Kirchhoff migration to velocity model by establishing a set of different velocities. The migrated results of a diffraction case were analyzed to choose the optimal velocity. The optimal velocities of being chosen from Kirchhoff migration in two cases were considered medium environment velocities.

Investigation of sinkholes on the roads by GPR: Application to HCM city, Vietnam

HCM city is the largest city in Vietnam with about 3,000 km of transport roads for 8 min people. There were many sinkholes appeared on the roads during the period of 2009-2010 years of different diameter. The location of those sinkholes is required to be done to forecast safety transportation in future for. We proposed to use GPR technique for location of these sinkholes in the city by Ramac X3M with 500 MHz frequency equipment and other for deeper target for instance Pulse Ekko100A with 200 and 100 MHz frequency antennas. The majority of anomalies was found by GPR measurements related with subsurface holes or soft-material by 0.5-2 m of diameter and 0.5-4 m of depth. In this paper, we present result of processing and interpreting GPR data for 3-D image for location of 3 sinkholes on the road in the city by using network of 16 × 17 GPR lines measurement. To obtain accurate parameters of GPR anomaly by data processing, we used CMP configuration for estimation of the velocity in-site. The GPR images reveal the distribution of the sinkholes and other weakness zone by comparing with dug holes.