61_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

61_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 - •...

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X2 Geophysical Methods (non-invasive, quick, characterize soil mass, seldom definitive) o Seismic Refraction Measure velocity of sound waves through soil - a function of soil stiffness (E, G, and Jl). Use to determine layering also. Analysis sometimes difficult. Point source (hammer, explosion) with geophones (accelerometers) to detect and record wave arrival. (figures from USACOE manual) r-- a SHOT POINT GEOPHONES - -~;;TH OF FIRST ARRIVAL. \ HEAR BY GEOPHOHES \ .-IJf \ )a \ \ r-- I I I I I I LAYER 1 OISTANce 1,127 ftIsec 4,850 ftIsec 1,500-3,000 ftIsec 3,000-6,000 ftIsec 5,000 ftIsec 5,000-10,000 ftIsec 12,000 ftIsec 22,000 ftIsec 16,500 ftIsec 'c -- -- -,- -.:::::.§ -- "2 -- /". . /1 / 1 /j'; 1 / "I 1 / III ! .. Air, sea level, 20 ac Fresh Water, 20ac Dry Sand Clay Saturated Loose Sand Weathered Rock Shale Hard Granite I Limestone Steel
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Unformatted text preview: • Generally determine velocity from first arrival time of wave after impact. • Compression waves: particle motion parallel to propagation of wave (fastest) • Shear waves: particle motion perpendicular to propagation of wave (slower) • Rayleigh wave: surface wave, particle motion elliptical (slowest) • Waves refract through layers at depth. Refracted waves obey Snell's law-refraction angle inversely proportional to wave speed. If deep layer is faster, then refracted wave may arrive before direct wave- defines depth to layer. May help define water table depth and depth to rock. • Typical wave speeds:...
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