School of Civil EngineeringFaculty of Engineering

Soils and Electromagnetic RadiationDART Workshop

17th September 2013

Dan Boddice

• 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 RadiationWhy EM Radiation?

• 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

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Soils and EM RadiationEM reflections

• 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 RadiationTime Domain Reflectometry

Robinson et al. 2005

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

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

Soils and EM RadiationDiddington Clay Field

Soils and EM RadiationDiddington Clay Field: Permittivity

Soils and EM RadiationDiddington Clay Field: Conductivity

Soils and EM RadiationDiddington Clay Field: Importance of Temperature

Soils and EM RadiationDiddington Clay Field: Temperature

Soils and EM RadiationDiddington Pasture Field

Soils and EM RadiationDiddington Pasture Field: Permittivity

Soils and EM RadiationDiddington Pasture Field: Conductivity

Soils and EM RadiationDiddington Pasture Field: Importance of Temperature

Soils and EM RadiationDiddington Pasture Field: Temperature

Youngs and Poulovassilis 1976

Soils and EM RadiationThe Different Forms of Moisture Profile Development During the Redistribution of Soil Water After Infiltration

Fine Grained and Deep Coarse Grained and Shallow

Soils and EM RadiationDiddington Clay Field: Infiltration

Soils and EM RadiationDiddington Clay Field: Infiltration

Soils and EM RadiationIMKO Probes Vs Campbell Scientific TDR100: VWC/Apparent Permittivity

Thanks to Van Walt Ltd. for the equipment loan

Soils and EM RadiationIMKO Probes VS Campbell Scientific TDR100: BEC

Soils and EM RadiationIMKO Probes Vs Campbell Scientific TDR100: BEC

Thanks to Van Walt Ltd. for the equipment loan

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 RadiationIMKO Probes VS Campbell Scientific TDR100

Thanks to Van Walt Ltd. for the equipment loan

• 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 RadiationWhat have we learnt: Some Thoughts from Ongoing Work?

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)