Application of Ground Penetrating Radar in Subsurface mapping

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  • 1. Application of Ground Penetrating Radar in Placer Mineral Exploration for Mapping Subsurface Sand Layers: A Case Study Rajesh P. Barnwal Presented at: PLACER 2005, New Delhi Paper by: Loveson, V.J.#, Barnwal, R.P.#, Singh, V.K.# Gujar, A.R.*, Rajamanickam, G.V.$ #Council of Scientific and Industrial Research, CMRI, Dhanbad [INDIA] *NIO, Goa [INDIA] $SASTRA Deemed University, Thanjavur [INDIA]
  • 2. Introduction
    • Effective mapping of subsurface sedimentary layers is needed for exploration of beach placers and its mine planning
    • Conventional techniques and their limitations:
    • Benefits of Ground Penetrating Radar (GPR) Technique:
      • Portable
      • Non-Destructive
      • Near Real-time interpretation
      • Continuous subsurface profiling system
      • High resolution mapping capability
    Methods Limitations Loggings of Trenches very slow, expensive and feasible only if high quality exposures are available Drill Core provides only a narrow column sample, with no information between cores Wirelines Log few boreholes & wells availability, column sample problem etc. Shallow Seismic Methods vertical resolution is limited to 3-4 m
  • 3. What is Ground Penetrating Radar?
    • GPR is a non-destructive geophysical survey tool which is used for mapping subsurface ground layers in short duration of time and greater precision.
    • A typical GPR system essentially comprised of:
      • Control unit: For generating a short electrical pulse
      • Transmitter: For converting electrical pulse into an electro-magnetic pulse of radio frequency and transmitting it into the ground
      • Receiver: For receiving reflected EM pulse
      • Data Logger: For logging reflected signal information into computer
      • Recording Unit: For recording the received signal information in digital form for digital signal processing and post analysis
      • Display unit: For displaying profiles in real time
  • 4. Working Theory of GPR
    • Works on the principal of reflection and refraction theory of electromagnetic waves
    • Uses radio waves to create picture of underground before excavation
    • Profiles is used to locate any variations in the sub-surfaces
    • Electrode magnetic waves emitted into the ground and time measured for wave to be reflected and received
    • Mainly governed by the electrical properties of the ground i.e., the dielectric constant of the media, which is nothing but measure of the ability of the material, which allows the electromagnetic energy to propagate through it.
    • When wave hits areas of change in soil, it is hit back to receiver antenna
    • Changes in soil can include changing of electrical properties of different layers underneath the surface
  • 5.
    • Greater surface difference = Stronger signal
    • Strong signal has large amplitude
    • Weak signal has small amplitude
    • Amplitude wavelength and time are used to create image of what is underground
  • 6. GPR Depth Determination
    • D = (5.9t)/sqrt of (Er)
        • D = depth of target (in)
        • t = wave travel time (nanosec)
        • 5.9 = a constant incorporating speed of light and unit conversions
        • Er = dielectric constant of subsurface material
    • Dielectric constants for common materials
    Air 1 Pure water 81 Fresh water (ice) 4 Granite (dry) 5 Clay (saturated) 8-12 Sand (dry) 4-6 Coastal Sand 10-15
  • 7. Factors affecting Er (Dielectric Constant) & GPR Data
    • Volumetric water contents
    • Mineralogy
    • Grain size
    • Presence of organics
    • Composition of the sediments
    • Orientation of the grains
    • Shape of the grains
    • Packing patterns of the sediments
  • 8. Example of Profiles obtained using GPR at Western Coast of India by CMRI
  • 9. Case Study
    • Nagoor along Tamil Nadu coast has been chosen for the present GPR Study during June 2005
  • 10. Study Area
    • Nagoor (100 48.823’ N and 790 51.057’ E)
    • Located at the confluence point of Vettar River with coast
    • Beach is straight with gentle slope
    • Has narrow tidal area with wider backshore zone, dotted with moderately elevated dunes
    • After the 26th December Tsunami, the backshore zone has been highly disturbed and the dunes were reworked
  • 11. Methodology
    • For the GPR survey, a GSSI Model SIR-20 GPR system was used with 400 MHz and 200 MHz antennae
    • Survey was conducted with an aim to assess the Sedimentological disturbances, if any, caused by the recent tsunami
    • Data was collected just six month after tsunami to study the post tsunami effects on sea beaches of Nagoor area
    • A transact of 60 m parallel to coastline (east-west) has been delineated for GPR profiling just right side of the river Vettar bank
    • For cross check, both 200 MHz antennae and 400 MHz have been used during profiling
  • 12. Methodology contd…
    • In addition, two longitudinal profiles of 10m length each, one using 200 MHz and another with 400 MHz antenna have been taken parallel to the pit in N-S direction
    • A pit has been dug upto 1.25 m along the traverse line at about 32 m from HT line.
    • The pit was about 5.10 m long along the line (east-west) and 1.0m widths
    • Several GPR readings have been recorded along and across the pit with various gain condition and different antennas
    • The GPR system has been configured for profiling upto 2 m depth of subsurface layers
  • 13. Methodology contd… HTL Sea |----| = 10 m GPR Profile along the pit in N-S direction = Pit (Trench of Length = 5.10 m) 0 10 20 30 40 50 60 32 m Pit Pit is 32 m from HT level Dunes S E W N West End Tree
  • 14. Results and Discussions
    • The data indicates dipping features towards seaside, depicting the coastal influence in deposition
    • Near to 1 m, one can observe a strong disturbance either, magnifying the erosional surface or moisture zone, on which, tsunami deposits are lying
    • This shows that after intensive erosion by tsunami waves, about a meter thick deposit might have deposited with layering of black sands
    East Distance (m) | Pit | West
  • 15. Results and Discussions contd…
    • GPR data clearly spelt out the sequence of many layers in the sub-surface
    • Though the study area was dotted with many sand dunes as the results of strong wind action during post-tsunami scenario, that type of characteristics are missing now
    • The GPR data shows a different situation mainly indicating the accretion due to coastal waves
    • Below 1.00 m depth, disturbed signals shows the influence of seawater intrusion / saturated with water.
    East Distance (m) | Pit | West
  • 16. GPR Profiles at Study Area
    • Full GPR profile using 400 MHz Antennae
  • 17. GPR Profiles at Study Area
    • GPR profile along Trail Pit using 400 MHz Antennae
  • 18. GPR Profiles at Study Area
    • GPR profile along Trail Pit using 200 MHz Antennae
  • 19. Trail Pit View of Trail Pit at Study Site Study Site of Nagoor Beach
  • 20. Correlation with Trail Pit
    • White disseminated sand with heavy minerals has been recorded upto 65 cm, which forms a single massive layer
    • After the white sand of about 10 cm thickness, black sand with dominantly of ilmenite has been observed for another 10 cm thickness
    • It has been followed by the white sand and thin micro varving band of heavy mineral
    • This has been followed by the alternative layers of heavy minerals and white sand finally rested on white sand with shells
    • The basement shell layer is considered to be the erosional surface over which the tsunami sediments deposited.
    White Sand Black Sand Micro Varving White Disseminated Sand White Sand with Shells
  • 21. Correlation with other Parameters
    • The sorting shows that upto 85 cm the sediments are moderately well sorted and from 85 to 110 cm, it is well-sorted
    • These characters are well represented in the GPR data
    • Mostly, all heavy mineral layers shows Leptokurtic character and mostly, white sand layers displays Mesokurtic signatures
    • Appreciable correlation has been noticed upto 95 cm and after that the saturation zone made difficult for further interpretation
    • Skewness types also represent fair correlation upto 95 cm
    • The overall observation indicates that the sorting could be well correlated and kurtosis and skewness could have correlation upto saturation zone.
    Sedimentological Logging GPR Profile White Sand Black Sand Micro Varving White Disseminated Sand White Sand with Shells Moderately Well Sorted Well Sorted Coarse Skewed Symmetrical Fine Skewed Leptokurtic Mesokurtic
  • 22. Conclusion
    • Nagoor beach has been scanned along a selected transect using GPR system with 200 Mhz and 400 Mhz antennas
    • The 60 m long GPR transect provide an appreciable sub-surface data
    • The dipping characters seen towards seaside in the GPR data indicates marine influenced deposition
    • Several GPR profiling along a trail pit reserves good correlation with the visual pit details
    • Heavy mineral layers could be traced all along the GPR profiles and white sand layers made easy tracing in the data
    • After 1.0 m indicates saturated zone or erosional surface over which tsunami deposits rest upon
    • Granulometric details have fairly appreciable correlation especially upto saturated zone
    • This preliminary study needs to be strengthen with more detailed surveys and interpretation.
  • 23. Any Question Please?
  • 24. Thank You!