Use of Geophysics for Transportation Projects (2006) / Chapter Skim
Currently Skimming:
Appendix A - Methods and Techniques
Appendix A - Methods and Techniques
Pages 42-59
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 42... ...
Much of the following text and information presented in this appendix was provided for the synthesis project by FHWA. The documents were prepared as part of the Geophysics Workshop currently in preparation by FHWA.
|
From page 43... ...
□ Pipeline □ Underground storage tanks □ Contaminant plumes Vibration (chapter seven) □ Monitoring vibration □ Specify:
|
From page 44... ...
45 Road Type: □ Country lane □ 2-lane road □ 4-lane road □ Freeway □ Freeway interchange □ Bridge □ No shoulder □ Not applicable □ Specify: Surface Vegetation: □ Trees □ Grass □ Bare □ Shrubs □ Specify: Vegetation Density: □ Sparse □ Light □ Medium □ Heavy Geology: □ Unknown □ Limestone □ Clay □ Sand □ Shale □ Granite □ Specify: Traffic Control: □ Road closed □ Flagger(s) □ provided by: □ Not applicable □ Specify: Target Depth: □ 1–10 cm □ 10–100 cm □ 1 meter □ 10 meters □ 15 meters □ 100 meters □ 1 kilometer □ >1 kilometer □ Specify: Target Size: □ 1–10 cm □ 10–100 cm □ 1 meter □ 10 meters □ 15 meters □ 100 meters □ 1 kilometer □ >1 kilometer □ Specify: Cultural Features Supporting Information: □ Boring logs □ Site history □ Site photographs □ Water table depth □ Specify: Slopes: □ 0°–30° □ 30°–45° □ >45° Above Ground: □ Power lines □ Buildings □ Roadways □ Railroad □ Fences □ Bodies of water □ None □ Specify: Below Ground: □ Utilities □ Abandoned mines □ Landfill □ Pipelines □ UXO □ None □ Specify: Recommended Geophysical Methods OSHA 1910.120: □ None □ Level D □ Level C □ Level B □ Level A □ Radioactive □ Specify Subsurface Characterization: □ Resistivity □ Electromagnetics □ Ground penetrating radar □ Magnetics □ Seismic refraction □ Seismic reflection □ Cross borehole tomography □ MASW □ SASW □ Specify: Engineered Structures Evaluation: □ Crosshole sonic logging □ Crosshole sonic logging tomography □ Gamma–gamma density □ Impact echo □ Ground penetrating radar □ SASW □ Specify: Notes/Action Items/Comments:
|
From page 45... ...
Seismic waves can be divided into two main groups; body waves and surface waves that exist only near a boundary. Body Waves These have the highest velocity of all seismic waves and are called compressional or pressure or primary (P-wave)
|
From page 46... ...
Basic Principles of the Refraction Seismic Method When seismic waves are created on the ground surface they penetrate the subsurface until they encounter layers with different velocities and/or densities. In the case of refraction seismic, the subsurface layers must have successively increasing layer velocities with depth.
|
From page 47... ...
FIGURE A5 Schematic showing a seismic refraction time–distance curve and the refracted waves.FIGURE A4 Main seismic waves and wave partitioning that occurs at a seismic interface.
|
From page 48... ...
Field Data Recording Crosshole seismic testing surveys are conducted using two or more (three are recommended for optimum results) boreholes.
|
From page 49... ...
Table A2 provides values of the relative permittivity, velocity, and electrical conductivity of EM waves for some common materials. These properties often vary with frequency; the table is for frequencies of approximately 100 MHz.
|
From page 50... ...
Because of the dependence of depth penetration on the local ground conditions, such as electrical conductivity and clay content, the success of GPR surveys is site-specific, which sometimes cannot be accurately predicted ahead of the survey. GPR data can be subject to interference from a number of sources.
|
From page 51... ...
Unless the ground is homogeneous, the measured resistivity does not represent that of any particular layer until the data have been interpreted. Therefore, the measured resistivity is called Apparent Resistivity, can be thought of as a composite resistivity that includes contributions from all of the layers under the sounding site to the depth of investigation of the measurement.
|
From page 52... ...
A sounding curve can be plotted showing the Apparent Resistivity versus electrode spacing as illustrated in the upper drawing in Figure A11. Computer software is used to interpret the sounding curve producing a model showing the depths to the top, thickness, and resistivities of the layers under the sounding site.
|
From page 53... ...
These values are shown plotted as a measured resistivity (Apparent Resistivity) versus time plot, also illustrated in Figure A13, called a sounding curve.
|
From page 54... ...
The sounding curve is interpreted using software that iteratively modifies a proposed resistivity model (layer thickness, depths, and resistivities) until the calculated sounding curve matches the field curve.
|
From page 55... ...
FIGURE A14 Schematic showing mechanics of EM induction method for measuring electrical conductivity of the ground. FIGURE A15 Relative responses from vertical and horizontal dipole modes when measuring electrical conductivity of the ground.
|
From page 56... ...
shows conductivity readings taken in the vertical dipole mode over a vertical electrically conductive feature, such as a fracture zone. TABLE A4 DEPTH OF INVESTIGATION Instrument Coil Separation (meters)
|
From page 57... ...
Conductivity measurements taken over this feature in the horizontal dipole mode would not show these diagnostic features. When using this method to locate vertical conductive features, it is important to provide sufficient spatial data density such that the anomaly shape is well defined, otherwise the anomaly may be difficult to recognize.
|
From page 58... ...
By taking measurements with an expanding geophone array a vertical profile can be developed showing the variation in shear wave velocity with depth. Basic Principles of the SASW Method The basis of the SASW method is the phenomenon that Rayleigh waves have phase velocities that depend on their wavelength, called dispersion, when traveling through a layered medium.
|
From page 59... ...
These waves are monitored using two or more receivers, as illustrated in Figure A21. An expanding receiver array is used to avoid near-field effects associated with Rayleigh waves and source–receiver geometry is optimized to minimize body wave signal.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.