Soils and Electromagnetic Radiation
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This document summarizes a workshop on using electromagnetic radiation to detect archaeological sites. It discusses how different soil properties like water content, organic matter, and temperature can affect the permittivity and conductivity measured by ground penetrating radar and other electromagnetic techniques. Case studies from two fields in Diddington show how these measurements vary over time with rainfall, infiltration, and temperature. The document also compares measurements from IMKO probes to a Campbell Scientific TDR100, finding the probes less accurate but easier to install long-term. The overall aim is to better understand how soil characteristics influence electromagnetic readings and how these techniques can be used for long-term monitoring of archaeological sites.
Soils and Electromagnetic Radiation
- 1. School of Civil Engineering Faculty of Engineering Soils and Electromagnetic Radiation DART Workshop 17th September 2013 Dan Boddice
- 2. • DART is focused on improving the detection of archaeological sites through both aerial remote sensing and geophysical techniques • Many of these use EM radiation • Ground penetrating radar (GPR) • Airborne multi and hyper-spectral sensors • Low frequency EM slingrams (e.g. EM38) • Have different operating frequencies Soils and EM Radiation Why EM Radiation?
- 3. • Reflection of applied signal (Ar) is proportional to incident signal (Ai) and a reflection coefficient defined using the EM impedance • Impedance is dependent on magnetic permeability (μ), dielectric permittivity (ε) and electrical conductivity (σ) and can defined • We can take μ as reasonably constant but the other two vary seasonally and with soil conditions 21 21 ZZ ZZ ' ' j j Z ir AA Soils and EM Radiation EM reflections
- 4. • EM technique used in soil science research • Broadband EM pulse is sent through coaxial cable to the probe • Relative voltage is measured and plotted as function of time allowing reflections due to changes in impedance to be identified • Measures hourly readings of • Apparent Relative Dielectric Permittivity from travel time (linked to water content via different models) • Bulk Electrical Conductivity from signal loss after multiple reflections Soils and EM Radiation Time Domain Reflectometry Robinson et al. 2005
- 5. Soils and EM Radiation What affects the Permittivity and Conductivity? • Water Content • Variation is based on rainfall but water behaviour is affected by soil properties and interactions with soil particles • Particle surface area • Density • Porosity • Organic Matter • Chemistry • Contrasts in soil water content are affected by differences in • Storage – show as long term differences in values • Infiltration – show as time variance between rain and TDR readings
- 6. Soils and EM Radiation What affects the Permittivity and Conductivity? • Frequency of Signal • Depends on instrument used • Causes variations in measured values • Soil Temperature • Variation is Seasonal and Diurnal • Temperature affects water behaviour-bound water, viscosity, ion mobility etc. • The effect on geophysical properties and the extent of its importance is debated
- 7. Soils and EM Radiation Diddington Clay Field
- 8. Soils and EM Radiation Diddington Clay Field: Permittivity
- 9. Soils and EM Radiation Diddington Clay Field: Conductivity
- 10. Soils and EM Radiation Diddington Clay Field: Importance of Temperature
- 11. Soils and EM Radiation Diddington Clay Field: Temperature
- 12. Soils and EM Radiation Diddington Pasture Field
- 13. Soils and EM Radiation Diddington Pasture Field: Permittivity
- 14. Soils and EM Radiation Diddington Pasture Field: Conductivity
- 15. Soils and EM Radiation Diddington Pasture Field: Importance of Temperature
- 16. Soils and EM Radiation Diddington Pasture Field: Temperature
- 17. Youngs and Poulovassilis 1976 Soils and EM Radiation The Different Forms of Moisture Profile Development During the Redistribution of Soil Water After Infiltration Fine Grained and Deep Coarse Grained and Shallow
- 18. Soils and EM Radiation Diddington Clay Field: Infiltration
- 19. Soils and EM Radiation Diddington Clay Field: Infiltration
- 20. Soils and EM Radiation IMKO Probes Vs Campbell Scientific TDR100: VWC/Apparent Permittivity Thanks to Van Walt Ltd. for the equipment loan
- 21. Soils and EM Radiation IMKO Probes VS Campbell Scientific TDR100: BEC Soils and EM Radiation IMKO Probes Vs Campbell Scientific TDR100: BEC Thanks to Van Walt Ltd. for the equipment loan
- 22. ADVANTAGES • Faster and easier to install • Minimal soil disturbance • Capable of identifying trends in VWC and BEC • Telemetry gives a data stream minimising site visits • Simpler interface and no need to process data DISADVANTAGES • Undefined measurement volume • Plastic tube and electrode coating makes BEC determination problematic • Model may not fit all soils and hard even with conversion to permittivity to fit other models because of tube effects make changes smaller-Needs empirical calibration to overcome this rather than existing models What is the overall aim of the experiment? Soils and EM Radiation IMKO Probes VS Campbell Scientific TDR100 Thanks to Van Walt Ltd. for the equipment loan
- 23. • Greatest difference seems to be in water held in the bottom ditch fill for both sites for both apparent permittivity and BEC-field capacity is higher • Magnitude of difference is greater in coarser grained soils • Infiltration tends not to affect below the top 30 - 40 cm except in cases of extreme drying beforehand • Temperature has very minor role on permittivity but quite a large role on BEC especially at saturation • How to monitor soil properties long term • Flooding • Animal damage • Settling-should we wet the probes in? • Work is Still Ongoing • Two more sites at RAC, Cirencester • Need to link behaviour to soil properties Soils and EM Radiation What have we learnt: Some Thoughts from Ongoing Work?
- 24. Soils and EM Radiation Acknowledgments • EQUIPMENT LOANS • Van Walt Ltd. • Utsi Electronics Ltd. • OTHER SUPPORT • Giulio Curioni and Andrew Foo (Mapping the Underworld) • Nicole Metje and David Chapman (Birmingham University)