Efficiency of geophysical survey in evaluating the bedrock configuration and aquifer extent in St. Martin’s island, Bangladesh
About the Event:
St. Martin's island is the only coral-bearing offshore island in Bangladesh; it supports a population of thousands. Groundwater in the shallow subsurface is the only source of potable water on the island. However, the extent (both lateral and vertical), distribution and geometry of the aquifer are unknown.
Seismic refraction (12 channels) and Vertical Electrical Sounding (VES) were carried out with the Schlumberger array in order to assess the bedrock configuration and evaluate the extent of the aquifer, respectively. We compare both types of geophysical data with very shallow auger data, dug well data and outcrop information.
A seismic velocity contrast can differentiate two velocity layers separated by a single interface. The shale-dominated Miocene Bokabil Formation from the Upper Surma Group acts here as the bedrock or the lower layer. The depth of bedrock varies from 0.85 to 3.8m below the land surface. Three south-dipping thrust faults with vertical displacements from 0.8 to 1.8m can be seen in the bedrock elevation contour map, with very steep to vertical dip angles. Another two normal faults are also identified.
Our interpretation of the resistivity show that the top seismic layer can be divided into two distinctive resistivity layers. Below an upper unsaturated zone, the aquifer is present from a depth of 0.25 to 2.3m. The resistivity value of this layer ranges from 60 to 200 Ωm. The bedrock can be identified below 1.43 to 4.18m, which is consistent with the seismic data. Variations in the resistivity value (>150 Ωm) show that water in the eastern part of Uttar Para, Pashim Para and Madhya Para should be fresher than all other areas of the island. However, a detailed lithological study is necessary to confirm the interpretation from the VES data. Further, we find that the aquifer is not continuous across St. Martin’s island because in some areas a silty clay layer is directly in contact with the bedrock.
About the Speaker:
Passionate about earth and environmental science, Md. Golam Muktadir completed his Bachelor’s degree in Geology and recently completed his Master’s degree in Geophysics at the Department of Geology, University of Dhaka, Bangladesh. He joined DUEO (the Dhaka University Earth Observatory) and has been working with EOS as a Field Coordinator on the TREMBLE project (Temporary Receivers for Monitoring Bangladesh Earthquakes) since 2016. He was awarded a fully funded travel grant to attend the Chevron-SLS (Student Leadership Symposium) program and the 2017 SEG International Exposition and 87th Annual meeting in TX, USA. He has also been trained in RGNIYD TN, India on SDGs in 2018 and visited Bhutan as a Bangladeshi Student Delegate. Now he is also working with Vanderbilt University TN, USA and LDEO (Lamont Doherty Earth Observatory) of Columbia University as a Research Assistant on a project titled “Long Term Monitoring Research and Analysis of Bangladesh Coastal Zone (Sustainable Polders Adapted to Coastal Dynamics)’’ and seeking a PhD position to further his expertise and experience in geology and geophysics.