School of Civil EngineeringCollege of Engineering and Physical Sciences

Using Time Domain Reflectometry (TDR) to Monitor the Geophysical Properties of

Archaeological Residues

Dan Boddice, Laura Pring, Dr. Nicole Metje, Dr. David Chapman

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesThe Problem

• DART is focused on improving the detection of archaeological sites through both aerial remote sensing and geophysical techniques

• Many of these use Electromagnetic (EM) radiation and require a contrast in EM properties

• Ground penetrating radar (GPR)

• Airborne multi and hyper-spectral sensors

• Low frequency EM slingrams (e.g. EM38)

• Other techniques use electrical properties

• Electrical resistance surveying

1. EVIDENCE FOR SOIL CONTRASTS IS ANECDOTAL

2. POOR DETECTION RATES ON CERTAIN SOILS

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesThe Problem

KEY QUESTION: Can prior knowledge of geotechnical and climatic factors improve our understanding of the geophysical contrast factors between archaeological residues and the surrounding soil matrix

• To map the soil and archaeological residues geophysical properties and identify the contrasts between them through a long term monitoring strategy.

• Find ways to use the geotechnical properties of the soil, and weather data to predict sensor response and inform sensor choice.

• Develop an appropriate methodology for similar or follow-on projects studying other geologies and archaeological feature types

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesEM radiation and Permittivity Basics

• EM transmission is governed by complex permittivity which varies according to changing ground conditions

• Real permittivity is the ability to store and release energy through charge separation

• Can occur due to separation of charges in atom, movement of ions or rotation of molecules in line with applied field

• Governs the speed of EM radiation propagation as produces displacement electrical field

• Difference in layers will determine size of reflection

• Imaginary Permittivity causes loss of signal energy• Relaxation mechanisms – lag in polarisation with applied field

• Conductivity - causes the energy to attenuate as a function of distance

SOIL TYPE DIELECTRIC PERMITTIVITY

Air 1

Water 80-81

Dry Sand 3-5

Dry Silt 3-30

Asphalt 3-5

Limestone 4-8

Granite 4-6

Shale 5-15

Clay 5-40

Organic rich surface soil 12

Saturated sand 20-30

Adapted from Conyers 2004

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesPermittivity

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesTDR Operating Principals

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesDesign and Installation

Requirements

• Needs to take hourly readings of permittivity, conductivity and temperature

• Needs probes both in and out of archaeological feature and at different depths

• Needs constant power supply

• Needs some kind of communication

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesDesign and Installation

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesDesign and Installation: clay sites

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesDesign and Installation: simpler sites

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesDesign and Installation

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesWaveform Interpretation

• Identifying start and end reflection points takes time

• 16 permittivity per hour

• c.11520 per station per month

• Currently 6 stations –too many to do this manually!

• Conductivity data is simpler

• Quantity is the same as for permittivity

SOLUTION: Script needed

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesOther Challenges Since Installation

• Loggers behaving erratically• Skipped readings and scans

• Unexplained stopping

• Solved by slowing the scan rate and switching CF card brand

• Animal Damage• Weather Station cables eaten

• Probes re-excavated

• Telemetry system will reduce future loss of data

http://free-extras.com/images/elmer_fudd-5189.htm

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesExample data: Permittivity

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesExample data: Conductivity

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesExample data: Temperature

Using TDR to Monitor the Geophysical Properties of Archaeological ResiduesThe future?

• Complete geotechnical analysis on soils from the lab to categorise them

• Create a link between permittivity, moisture content and bulk density using the TDR on prepared samples in the lab

• Analyse frequency dependence of soils in different states in the lab

• Continue to analyse data flow using statistical tests and time delay correlation

Progress UpdateAcknowledgments

The Rest of the DART Project• Dave Stott , Dr. Anthony Beck and Dr. Anthony Cohn (University of

Leeds)

• Rob Fry and Dr. Chris Gaffney (University of Bradford)

• Dr. Keith Wilkinson (University of Winchester)

• The other stakeholders and academic consultants

Also• Giulio Curioni and Andrew Foo (Mapping the Underworld)