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Seismic Refraction

Seismic Refraction. Some uses of seismic refraction. Mapping bedrock topography Determining the depth of gravel, sand or clay deposits Delineating perched water tables Determining the depth to the water table Detecting subsurface caverns Estimating rippability

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Seismic Refraction

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  1. Seismic Refraction

  2. Some uses of seismic refraction • Mapping bedrock topography • Determining the depth of gravel, sand or clay deposits • Delineating perched water tables • Determining the depth to the water table • Detecting subsurface caverns • Estimating rippability • Detecting shallow faults and fracture zones • Detecting large boulders

  3. ACTIVE STANDARD: D5777-00(2006) Standard Guide for Using the Seismic Refraction Method for Subsurface Investigation $42.90 for PDF • Developed by Subcommittee:D18.01See Related Work by this SubcommitteeAdoptions:Book of Standards Volume: 04.09 • 1. Scope • This guide covers the equipment, field procedures, and interpretation methods for the assessment of subsurface conditions using the seismic refraction method. • Seismic refraction measurements as described in this guide are applicable in mapping subsurface conditions for various uses including geologic, geotechnical, hydrologic, environmental, mineral exploration, petroleum exploration, and archaeological investigations. • The seismic refraction method is used to map geologic conditions including depth to bedrock, or to water table, stratigraphy, lithology, structure, and fractures or all of these. • The calculated seismic wave velocity is related to mechanical material properties. Therefore, characterization of the material (type of rock, degree of weathering, and rippability) is made on the basis of seismic velocity and other geologic information.

  4. Refraction Lay Out

  5. Seismic Refraction • Advantages • Simple layout • Low manpower requirements • Limited Equipment Requirements • Rapid data reduction and analysis (computer not needed) • Easy interpretation

  6. Seismic Refraction • Disadvantages • Relatively large energy input required • Relatively long layout (10 times depth) • Limited number of model layers • Limited velocity differences • Limited interface geometry (assume smooth)

  7. Rippability versus seismic velocity. (Caterpillar. Handbook of Ripping, 8 th Edition)

  8. Simple Plot

  9. Two Equations for Simple Refraction These two equations should give the same answer. It should be less than half the crossover distance.

  10. Steps in solving for depth • 1 Determine the velocities of both layers in meters per second or feet per second • 2 Determine the crossover distance • 3 Determine the time intercept for V2 • 4 Determine the depth to layer 2 using • Both refraction equations (internal check)

  11. Down Dip Up Dip

  12. Dipping Bed Equations

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