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Week 2 geophysical survey


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Week 2 geophysical survey

  1. 1. Week 2
  2. 2. Learning OutcomesStudent should be able to: Describe the various methods of Soil Investigation Methods.(CO1-PO10) Derive information from these investigations. (CO1-CO10) Discuss the limitations and advantages of each methods. (CO1-PO10)
  3. 3. Learning OutcomesStudent should be able to: Explain how SPT are done (CO1-PO10) Derive information from the bore-log. (CO1-PO10) Discuss sampling method and the laboratory tests to determine the soil properties.(CO1-PO10)
  4. 4. Geophysical Investigations Initial Site Exploration/Preliminary Surveys • Assist with Placement of Borings/In-Situ Tests Difficult Locations • Gravels, Cobbles, Boulders, Debris • Difficult Terrain • Contaminated Sites Supplementary Exploration • Observe Variations Between Borings/Soundings/Outcrop, etc. • Locate Irregularity
  5. 5. Example of Geophysical Investigation:1) Seismic refraction survey2) Resistivity survey
  6. 6. Geophysical InvestigationsAdvantages geophysical method:1 - Continuous subsur face profile2 - Quick to conduct enable the coverage of a large area relatively quickly3 - Non-intrusive methods: geophysical methods do not involve destruction and excavation. Most of the methods are silent and environmentally friendly, and can therefore be implemented at all hours of the day, including in densely populated areas. 
  7. 7. Applications of Seismic Reflection & Refraction Seismic reflection and refraction have numerous potential applications to a variety of environmental and geotechnical problems, including: Depth and characterisation of bedrock surface Buried channel definition Depth of water table Mapping of faults and other structural features Location of karst features 7
  8. 8. Seismic Refraction Survey (summaries)- Conduct by impact the ground and observing the arrival of the waves at various point. Recorded by geophones. Velocity of the waves is related to the properties of various layers.
  9. 9. Seismic Reflection Reflections of sound waves (compression waves) from the subsurface arrive at the geophones some measurable time after the source pulse. If we know the speed of sound in the earth and the geometry of the wave path, we can convert that seismic travel time to depth. By measuring the arrival time at successive surface locations we can produce a profile, or cross-section, of seismic travel times. A simple concept. In practice, the speed of sound in the earth varies enormously. Dry, sand might carry sound waves at 250 m/s or less. At the other extreme, unfractured granite might have a velocity in excess of 6,000 m/s. The more layers between the surface and the layer of interest, the more complicated the velocity picture. Various methods are used to estimate subsurface velocities including refraction analysis, borehole geophysical measurements, estimates from known lithologic properties, and analysis of reflection times at increasing offsets. Generally, a combination of velocity estimation methods will give the best results. 9
  10. 10. Seismic Reflection Source: 10
  11. 11. Seismic RefractionWhen a sound wave crosses an interface between layers of two differentvelocities, the wave is refracted. That is, the angle of the wave leavingthe interface will be altered from the incident angle, depending on therelative velocities. Going from a low-velocity layer to a high-velocity layer,a wave at a particular incident angle (the "critical angle") will be refractedalong the upper surface of the lower layer. As it travels, the refractedwave spawns upgoing waves in the upper layer, which impinge on thesurface geophones.Sound moves faster in the lower layer than the upper, so at some point,the wave refracted along that surface will overtake the direct wave. Thisrefracted wave is then the first arrival at all subsequent geophones, atleast until it is in turn overtaken by a deeper, faster refraction. Thedifference in travel time of this wave arrival between geophones dependson the velocity of the lower layer. If that layer is plane and level, therefraction arrivals form a straight line whose slope corresponds directly tothat velocity. The point at which the refraction overtakes the direct arrivalis known as the "critical distance", and can be used to estimate the depthto the refracting surface. 11
  12. 12. Seismic Refraction 12
  13. 13. oscilloscopeSeismic Refraction ASTM D 5777 Note: Vp1 < Vp2Determine depth t1to rock layer, zR t2 t3 Vertical Geophones Source t4 (Plate) x1 x2 x3 Soil: Vp1zR x4 Rock: Vp2 13
  14. 14. oscilloscopeSeismic Refraction“Shortest path not necessarilythe fastest one” Vertical Geophones Source (Plate) Fastest path is the shortest one Soil: Vp1 Fastest path is NOT the shortest one Rock: Vp2 14
  15. 15. Geophysical Properties P - Wave Velocities Steel Intact Rocks Weathered Rocks Ice Till Resistivity Values (ConeTec & GeoProbe, 1997) Sand ClayWeathered Rocks Sea Water Glacial Till Fresh Water 0 1000 2000 3000 4000 5000 6000 7000 8000 Sands & Gravels Compression Wave Velocity, Vp (m/s) Loose Sands S - W ave V elocities Loam Steel Clay Intact Rocks 1 10 100 1000 10000 Weathered Rocks Ice Bulk Resistivity, ρ (ohm-meters) Till Sand Clay Sea Water Fresh Water }V s =0 0 1000 2000 3000 4000 Shear Wave Velocity, VS (m/s) 04/17/12 15
  16. 16. Results from Seismic Refraction04/17/12 16
  17. 17. Resistivity Survey (summaries)- Electrical current is input into the ground and the resistivity of the various layers is measured. Different soil materials has different resisitivity.
  18. 18. Surface Wave TestsNon invasive method based on the geometric dispersion of Rayleighwaves, which are waves that travel along the ground surface resultingfrom a vertical impact or continuous vibration source (like waves inthe sea).The relationship between velocity of propagation of Rayleigh wavesand frequency can be determined experimentally analysing theparticle motion induced on the ground surface by the propagation.Seismic waves are generated using either impact sources or vibratorsand are detected using vertical velocity transducers (Geophones).The recorded ground motion is then analysed in the frequencydomain to estimate the experimental dispersion curve (therelationship between frequency and velocity).The experimental dispersion curve is finally used in an inversionprocess to estimate the variation with depth of the velocity ofpropagation of shear wave, which is linked to the small strainstiffness of the soil. G = ρ⋅ V 2 o sThe inversion process is based on the numerical propagation ofRayleigh wave propagation in layered linear elastic media. 18
  19. 19. Electrical MethodsElectrical properties are among the most useful geophysical parametersin characterizing earth materials. Variations in electrical conductivity (orits inverse, resistivity) typically correlate with variations in watersaturation, fluid conductivity, porosity, permeability, and the presence ofmetal. Depending on the particular site, these variations may be used tolocate contaminant plumes, salt water intrusion, stratigraphic units,sinkholes, fractures, buried drums and tanks, and any other featurewhose electrical properties contrast with the surrounding earth.Ground conductivity can be measured either directly, using the galvanicresistivity method, or inductively, using electromagnetic induction (EM).Because EM requires no direct contact with the ground surface, data canbe acquired more quickly than with resistivity. Resistivity, however, canprovide better vertical resolution and is generally less sensitive to sourcesof "noise" such as fences, buildings and overhead powerlines. 19
  20. 20.  Concept of Electrical Resistivity Method used to determine the sub surface profile: The electrical resistivity use the concept that electrical resistance varies significantly enough among dif ferent types of soil and each materials, to allow identification of specific types once their DV resistivities in = 2π fieldπare measured. In P the I = 2 DR the field electrodes are used and electrical current are supplied. The voltage drop in the soil material within the zone created by the electrodes electric field is measured by voltmeter. The soil electrical resisitivity is computed:
  21. 21. Electrical Resisitivity Measurements
  22. 22. ElectroMagnetic Induction Combined 3-D Plot/Contour Map of EM Induction Data 22
  23. 23. Electromagnetic Conductivity(EM)
  24. 24. Applications of Electrical Methods EM and Resistivity can be applied to a wide variety of problemsencountered in environmental, groundwater, geotechnical, and archaeologicalwork, including: Location of buried drums, tanks, trenches, and utilities Location of landfills and bulk buried materials Delineation of contaminant plumes Depth of water table and aquifer identification and mapping Continuity of stratigraphic interfaces such as clay layers Mapping of faults and fractures Location of karst features 25