a geomatics approach to the interpretation of ground penetrating radar (gpr)

"The challenges associated with Ground Penetrating Radar (GPR) investigations of the Pleistocene fluvial sequences in the Upper Thames and the role that geomatic techniques play in overcoming them.”

Stuart Glenday

http://www.geog.qmul.ac.uk/

Department of GeographyQueen Mary, University of London,Mile End RoadLONDONE1 4NS

s.s.glenday@qmul.ac.uk

Definition 1: Geomatics

“acquisition, modelling, analysis, and management of spatially referenced data”

Definition 2: GPR

• Applied Radar. 25 -1000MHZ.• Dielectric permittivity “ε”

controls behavior of radar waves in subsurface

• When radar wave encounter contrast in “ε” , proportion of energy is reflected.

• In the vadose zone subsurface “ε” dominated by capillary moisture content.

• Capillary context function of the ‘texture’ of the sediment.

• Textures related to primary sedimentary structures and facies.

Examples

Definition 3: Pleistocene of the Upper Thames

Research potential in Upper Thames

• Upper Thames– Additional terrace subdivisions– Dating evidence indicates difference

in stratigraphic development• MIS7 directly overlain by MIS2 at

Latton. (Lewis et al., 2006)– Potential Alluvial Fan development?

• Scope for directed research using “traditional” methods is limited– Relies on serendipitous availability

of sediment exposure in quarries– Limits on scale of investigations

• GPR potential to overcome these issues, especially when used in conjunction with Geomatic technologies

Project conception – Geomatics “Bread and Butter”

• Geomatics has played a role in overcoming challenges during project conception and planning– Spatial compilation of diverse

information types– Similar approach to BRITICE compilation

of glacial landforms and features related to the last British Ice Sheet

• Pinpoint field area– Identified as possible alluvial fan– Sequence diversity > potential to

test stratigraphic relationships– Practical considerations & access

Interpreting Results

– Coarse gravel component• Scattering and attenuation

• Poor response due to:– Presence of clays

• Soil & Subsoil, Component of sediment matrix

• Initial results “Cryptic”

se nature of gravel

Adapted methodology

• More focussed approach– Directed investigations in regions where results

are more positive– Collect data as intersecting grids

• Determine the continuity of reflectors and facies• Understanding of the three-dimensional form of

reflectors and facies to aid interpretation.• Identify signal artefacts

– Integrate with Geo-morphological evidence

• Geomatics facilitates this approach

Spatial data collection

• GPR cart assembly & real-time display– Rapid data collection: Lots of

data in – Instant feedback through

Digital Video Logger– Portable GIS

• GPS integration– X,Y,Z of traces at defined

intervals– RTK-DGPS collects

coordinates at cm precision

Spatial Data

>130 miles of profiles

>1 million traces

Comparison of sampling parameters

200Mhz

Spatially discrete features?

Discrete channel

Domain Conversion

Continuity of reflectors

Mapping sequence boundaries

Claydon Pike: Survey outline

Claydon Pike: Observations

Exposures

References

• Lewis, S.G., Maddy, D., Buckingham, C., Coope, G.R., Field, M.H., Keen, D.H., Pike, A.W.G., Roe, D.A., Scaife, R.G., Scott, K. 2006. Pleistocene fluvial sediments, palaeontology and archaeology of the Upper River Thames at Latton, Wiltshire, England. Journal of Quaternary Science, 21, 181–205.