The seismic downhole method provides a designer with information pertinent to the seismic wave velocities of the materials in question (1). The P-wave and S-wave velocities are directly related to the important geotechnical elastic constants of Poisson’s ratio, shear modulus, bulk modulus, and Young’s modulus. Accurate in-situ P-wave and S-wave velocity profiles are essential in geotechnical foundation designs. These parameters are used in both analyses of soil behavior under both static and dynamic loads where the elastic constants are input variables into the models defining the different states of deformations such as elastic, elasto-plastic, and failure. Another important use of estimated shear wave velocities in geotechnical design is in the liquefaction assessment of soils.
A fundamental assumption inherent in the test methods is that a laterally homogeneous medium is being characterized. In a laterally homogeneous medium the source wave train trajectories adhere to Snell’s law of refraction and Fermat’s principle of least time. Another assumption inherent in the test methods is that the stratigraphic medium to be characterized can have transverse isotropy. Transverse isotropy is a particularly simple form of anisotropy because velocities only vary with vertical incidence angle and not with azimuth.
Note 18212;The quality of the results produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities. Agencies that meet the criteria of Practice D 3740
1.1 These test methods are limited to the determination of the interval velocities from arrival times and relative arrival times of compression (P) and vertically (SV) and horizontally (SH) polarized shear (S) seismic waves which are generated near surface and travel down to an array of vertically installed seismic sensors. A preferred method intended to obtain data for use on critical projects where the highest quality data is required is included. Also included is an optional method intended for use on projects which do not require measurements of a high degree of precision.
1.2 Various applications of the data will be addressed and acceptable procedures and equipment, such as seismic sources, receivers, and recording systems will be discussed. Other items addressed include source-to-receiver spacing, drilling, casing, grouting, a procedure for borehole installation, and borehole and seismic cone actual test conduct. Data reduction and interpretation is limited to the identification of various seismic wave types, apparent velocity relation to true velocity, example computations, use of Snell''s law of refraction, and assumptions.
1.3 There are several acceptable devices that can be used to generate a high-quality P or SV source wave or both and SH source waves. Several types of commercially available receivers and recording systems can also be used to conduct an acceptable downhole survey. Special consideration should be given to the types of receivers used and their configuration. Heavily-damped sensors should not be used so that spectral smearing, phase shifting, and latency response between sensors is avoided. These test methods primarily concern the actual test procedure, data interpretation, and specification......
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