Photo from: Nadezhda Nadezhdina and Jan CÏermak(2003)Hector Estrada-MedinaSensing technology for the soil environment. CENS2007GPR: What is this?• GPR or Ground Penetrating Radar is an non destructivegeophysical tool used for subsurface earth exploration.Ecological applicationsMonitoring ground water tableMonitoring soil water contentTracking of infiltration of contaminantsAssessing of damage by burrowing animalsRoots (distribution, biomass, size, and structure of tree rootsystems)Earth sciences: stratigraphy and soil descriptions, soil depthassessment.Military: Detection of unexploded land mines.Engineering : structural integrity of roads and buildings, detectionof buried lines, storage tanks, pipes and conduits.Anthropological: Detection of human settlements.Main applicationsGPR: How it looks like?Control unit: Antenna:Contains the electronics thatproduce and regulate the pulseof radar energy that theantenna sends into theground. It also has a built incomputer and hard disk torecord and store data forexamination after fieldwork.The antenna receives theelectrical pulse produced bythe control unit, amplifies itand transmits it into theground. It also receives thesignal reflected to the surface.GPR: How it works?TransmittedwavesReflectedwavesScatteredwavesAttenuationGPR works by sending a tiny pulseof electromagnetic waves (EM) intoa material and recording thestrength and the time required forthe return of any reflected signal tothe surface.Reflections are produced wheneverthe energy pulse enters into amaterial with different dielectricpermittivity (dielectricconstant) from the material it left.While some of the GPR energypulse is reflected back to theantenna, some energy also keepstraveling through the material untilit dissipates or is lost (attenuation).Dielectric permittivityThe capacity of a material to hold and pass andelectromagnetic charge. Varies with a materials’composition, moisture, physical properties, porosity,and temperature.Material DielectricconstantVelocity(mm/ns)Vacuum 1 300Air 1.00054 300Water 81 33Ice 3.2 167Coastalsand10 95Granite 5-8 134-106Limestone 7-9 100-113Material DielectricconstantVelocity(mm/ns)Sand dry 3-6 120-170Sand wet 25-30 55-60Clay dry 3 173Clay wet 8-15 86-110Wood dry 1.5-3 173-245Woodwet10-12 87-95Metals 0 InfiniteAverage velocity is used to calculate depth.Important parameters• Velocity of the electromagnetic signal (v)v= c / (er)1/2Where:v = The velocity of the wave through the subsurface material.c = The speed of light (300 mm/nanosecond).er = The relative dielectric constant.• Signal’s wavelength (!)!= v / fWhere:!= Wavelength.v = The velocity of the wave through the subsurface material.f = Frequency of the antennaThe smaller the wavelength, the better theresolution.Depth of penetrationAntenna frequency is a major factor in depth penetration. The higherthe frequency of the antenna, the shallower into the ground it willpenetrate. Likewise, the higher the frequency the smaller the targetsthat will be identified.Depth Range(approximate)AntennafrequencyAppropriate Application0-1.5 ft0-0.5 m1500 MHz Structural Concrete, Roadways,Bridge Decks, Roots0-3 ft0-1 m900 MHz Concrete, Shallow Soils,Archaeology0-12 ft0-9 M400 MHz Shallow Geology, Archaeology0-25 ft0-9 m200 MHz Geology, Environmental, Utility,Archaeology0-90 ft0-30 m100 MHz Geologic ProfilingFactors that affect GPR useOptimal conditions for GPR use• Sandy drysoilsGood radar media Poor radar mediaDry salt WaterSnow, Ice, and freshwaterMetalsSand (wet or dry) Clay-rich soilsDry soils and rocks Conductiveminerals2D-RadargramsLayers:Appear aslayers.Colors denote:Blue: strong contrastGreen: weak contrastWhite: matrixFeatures:Appear ashyperbolas, orinverted “Us.”Reflection strengthdepends on the change indielectric constantbetween layers.The larger the differencein dielectric constantsbetween two layers thestronger the contrast.Why do single features appearas hyperbolas?When the object isahead of the radar, ittakes more time forthe echo to return tothe antenna. As itpasses over, the timegets shorter, and thenlonger again as itgoes past the object.This effect causes theimage to take theshape of a curvecalled a "hyperbola".3D-ModelingResults from post-processing of 2D radargrams taken from grids.Examples1.5 GHzantennaBox filled with sandMetallic barControlunitSandFloor35 cm10 cmNumber of ScansDepth (m)Foundation??Maximum penetration depth 85 cmStationary antennaSingle homogeneous layerThicknessMaximum penetration depth 90 cmStationary antennaMetallic bar buried at 15 cmNumber of ScansMetallic barDepth (m)Monitoring water table depth after rockextraction in a quarryLevelbeforeextractionLevel afterextraction8mOwners of the quarry are required by the government to leave at least 1 m of rock!!Monitoring water table after rock extraction ina quarryAir-RockinterfaceRock-waterinterfaceSoil depth in restored areas inthe quarry1.30 m on top of the level after rock extractionSoil depth in restored areas in thequarryAir-RockinterfaceSoil-RockInterfaceRock- WaterinterfaceIdentifying roots• Detection of roots is affected by:(1)The electromagnetic gradient existing between aroot and the soil;(2) The size, shape, and orientation of a root;(3) The presence of scattering bodies within the soil;and(4) Antenna frequency.Burtnor et al, 2001. Tree Physiology 21, 1269–1278Identifying roots• Isolated roots > 2.5 cm in diameter can berelatively easy identified under optimumconditions (dry sand conditions).They show strong parabolic curvature and highamplitude areas.• Roots < 2.5 do not show clear hyperbolas.Identification must be through imaging software(OPTIMAS Image Analysis Software, Sigma Scan ProImage Analyses Software, etc.). 3D modeling for sizeand architecture.Looking for rootsMy backyard:•Even surface•Good place for digging•Virtually no rocks•Other features are easilyidentified (i.e. pipes)•Substrate is more or lesshomogeneous•Dry and wet condition can becontrolledLooking for roots (Mybackyard)Air-RockinterfaceMetallic piece at 14 cmRootsRoots> 2 cmUsing GPR to Estimate Tree RootBiomass under optimalconditionsButnor et al, 2005. Soil Sci. Soc. Am. J. 67:1607–1615 (2003).1) GPR grid2) Soil cores3) Air spade excavation4) Analyses of images5) Linear regressionRadar images analyses(A) Raw