Ground-Penetrating Radar

Ground-Penetrating RadarA Comparison to Seismic ReflectionCovered very briefly in Chapter 14, pgs. 227-231Transmitter/Receiver Vs. GeophonesSeismic ReflectionOne sourceLots of geophonesStacking and Velocity performed in one stepMoveoutPeak Power

GPROne source (Transmitter)One receiverBoth are antennasStacking performed by repeating the EM pulseMoveoutPeak PowerGPR Travel Time Equation

Radar VelocityIn seismic reflection, velocity is found during stacking from the moveout patternsIn GPR, we only have one receiver, so we need a different methodFixed offset survey (i.e. a normal survey)Look for point sources (hyperbolas)Common Midpoint Survey (CMP)Get reflections off of a horizontal reflectorIncrease antenna spacing, repeatProduces a hyperbola

Common Midpoint Surveying

Antennas are moved away from each other by a certain amount each measurement

Survey is performed over a horizontal reflectorGPR Travel Time Equation

Common Midpoint Surveying

Velocity in Fixed Offset SurveysIn a regular, fixed offset survey, radar velocity can be determined only if diffraction hyperbolas are encounteredTypically point sources

Velocity in Fixed Offset Surveys

Velocity in Fixed Offset Surveys

Source Wave PropertiesGround-Penetrating Radar (GPR)Electromagnetic WavesMicrowaves / Radio WavesVelocity (in air) 3x105 km/sI.e. the speed of light (0.3 m/ns)Slower in most geologic materials (0.02 0.2 m/ns)Depends on electromagnetic properties of mediumFrequency 10-2000 MHzDepends on antennaWavelength 30-1.5x10-8 m

Seismic Surveys (Refraction/Reflection)Seismic WavesP, S, R, L wavesVelocity 0.4-14 km/sDepends on mechanical properties of medium (elastic moduli and density)

Frequency = 0.1-100 HzDepends on sourceWavelength = 140-4,000 mGPR Frequency ComparisonHigher FrequencyBetter detailLess penetration (penetration also depends on material)

Attenuation ComparisonGround-Penetrating Radar (GPR)Electromagnetic WavesDecay Exponentially with distance from sourceSpherical spreadingCant penetrate through electrically conductive materialsMetals/Metallic OresSaltwaterClays/Muds

Seismic Surveys (Refraction/Reflection)Seismic WavesDecay Exponentially with distance from sourceSpherical spreadingCant penetrate through inelastic layersFluidsFault/Fracture ZonesScatter wavesAttenuation

What Causes a Reflection?Ground-Penetrating Radar (GPR)Electromagnetic WavesChange in relative permittivitySeismic Surveys (Refraction/Reflection)Seismic WavesChange in acoustic impedance

c = speed of lightr = Relative PermittivityAcoustic Impedance = vRelative Permittivity: a measure of the ability of a material to store a charge when an electric field is applied

Reflection Coefficient:

Visualization of DataIdentical to seismic reflectionWiggle tracesVariable AreaVariable Density