scholarly journals Geophysical study of land subsidence: example from Pokhara basin

2006 ◽  
Vol 34 ◽  
pp. 135-140
Author(s):  
M. Yoshida ◽  
S. R. Pant ◽  
P. C. Adhikary ◽  
V. Dangol ◽  
S. Shrestha

Land subsidence is a serious problem in the Pokhara city and surrounding wide areas consisting of recent debris flow deposits. Owing to their calcareous matrix the debris flow deposits are more-or-less well cemented, even though their age ranges just from 12,000 to 700 years BP. There are numerous caves and caverns in these deposits. To investigate the danger of subsidence, ground penetrating radar and electrical resistivity tomography were employed to detect and map the caves and caverns. The results are quite encouraging and the above methods are found to be quite effective in formulating the control measures to the land subsidence hazard.

2018 ◽  
Vol 25 (4) ◽  
pp. 285-300 ◽  
Author(s):  
Çağlayan Balkaya ◽  
Ümit Yalçın Kalyoncuoğlu ◽  
Mehmet Özhanlı ◽  
Gözde Merter ◽  
Olcay Çakmak ◽  
...  

2020 ◽  
Author(s):  
Jesús Fernández Águila ◽  
Mark McDonnell ◽  
Raymond Flynn ◽  
Alastair Ruffell ◽  
Eric Benner ◽  
...  

<p>Seawater intrusion is a major issue worldwide, as coastal aquifers often act as the primary source of drinking water for more than one billion people. With climate change and projected population increases in coastal areas, this problem is anticipated to become more pressing over the next decades. Effective site characterisation strategies provide a crucial component in understanding subsurface saltwater migration. Density differences cause freshwater to float on seawater creating the classical saltwater intrusion saline wedge. However, tides often control coastal groundwater dynamics causing the emergence of an upper saline recirculation cell beneath the intertidal zone (Intertidal Recirculation Cell, IRC). Here we present the application of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques to characterize the coastal sand aquifer underlying Benone Strand (Magilligan, Northern Ireland) where tides induce an IRC. The aquifer is approximately 20 m thick and rests directly on Lr. Jurassic mudstones.</p><p>2D ERT profiles were generated at Benone beach using the SYSCAL Pro 72 ERI system (Iris Instruments). Two different array configurations (Wenner-Schlumberger and dipole-dipole) were used to provide both improved horizontal and vertical resolution. Because of the homogeneity of the sand, the ERT profiles made it possible to clearly define the configuration of the IRC and the fresh groundwater discharging “tube”. The presence of the tidally-driven recirculation cell causes fresh groundwater to flow below the IRC (“discharge tube”) and discharge in the vicinity of the low water mark. ERT data suggest that the IRC has a resistivity of approximately 1 Ωm and a thickness of 8 m. Resistivity increases below the IRC, but declines moving towards the low water mark. These findings suggest a possible mixing zone between saline water and the freshwater discharge. To verify the accuracy of the resistivity values measured in the ERT profiles, water samples were collected at various distances along a perpendicular transect from the high water mark to the low water mark. The electrical conductivities of the water samples were measured and compared with the resistivities obtained in the ERT profiles using Archie's law. Similar values were obtained in both cases.</p><p>A MALÅ ground penetrating radar system, operating at 50 MHz, 100 MHz and 500 MHz, was used to collect 2D GPR profiles at Benone beach from the low tide mark to beyond the high water mark. Findings suggested that the IRC attenuated the radar signal in all cases. However, GPR profiles were crucially important to demarcate the interfaces between freshwater and saltwater near the ground surface. GPR profiles obtained using higher frequencies (500 MHz) were the most informative.</p><p>The research work carried out at Magilligan allows us to conclude that the application of ERT and GPR techniques is effective in delineating seawater intrusion in aquifers where tides create an IRC. In addition, ERT profiles very clearly identified the IRC through field measurements (which in most cases is studied through numerical models and laboratory tests).</p>


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