Direct Wave Arrivals - LSU Geology & Geophysics · Direct Wave Arrivals ... Horizontal Cylinder -...
Transcript of Direct Wave Arrivals - LSU Geology & Geophysics · Direct Wave Arrivals ... Horizontal Cylinder -...
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Direct Waves – Velocity from x-t
Direct Wave Arrivals
Head Wave Distance vs. Time
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Thickness from Intercept Time
Thickness from Crossover Distance
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Critical Distance (no head waves)
Picking Arrivals on a Seismogram
Seismogram Data
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Mohorovicic Discontinuity
Multiple Horizontal Layers
2 Interface Example
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2 Interface x-t Plot – computed from RefractModel
Field Seismogram
2 Horizontal Interface x-t Plot –from seismogram (note: just 1
direct wave first arrival)
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Horiz. Interface -> Sym. x-t plots
Dipping Interface: Velocity not equal to 1/slope
Horizontal Interface: Symmetric x-t
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Dipping Interface: asymmetric x-t
Intercept Time gives up-dip direction
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Velocity = Frequency x WavelengthFrequency = 10 to 1000 MHzWavelength = cm to meters
Velocities of Air, Water, and Earth Materials
Reflection and Transmission
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Radargrams - record time that energy reflected back to surface
at many locations
Diffractions off sudden change in interface of same dimensions as
wavelength of radar waves
Diffractions appear on Radargram as a Hyperbola
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Wave Attenuation and Amplitude -Spherical Spreading and Absorption
Energy Partitioning
Amplitude of Reflection
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Depth of Penetration depends on Frequency and Material
Reflections at Interfaces -Common Offset Technique
Other Techniques - WARR and CMP/CDP
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Dielectric Constant & Velocity
Resolution depends on frequency and the material
Imaging Sediments through Ice and Water
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Diffractions/ Water Table
Monitor Contamination Plume (in time)
Buried Objects
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Rebar in Concrete Slab - High Frequency Radar Diffractions
Buried Roman Road with ditches and cart ruts
Grave Site Survey
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Grave Site Survey
Memorial Tower Survey
Parade Grounds After Rain
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Lacoste-Romberg GravimeterHooke’s Law (Strain proportional to Stress)
Gravity From a Buried Sphere
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Gravity over a Sphere
Gravity of a Horizontal Cylinder
Horizontal Cylinder - Radius
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Reverse Fault
Normal Fault