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The seismic crosshole test provides dynamical soil parameters down to depths of 100 or 150 m based on the determination of wave velocities. Seismic waves are generated in a source borehole and recorded in a receiver borehole.
The focus of the crosshole test is on the determination of the shear wave velocity (s-wave velocity). Therefore a s-wave source has to be applied (e.g. a s-wave sparker or a borehole hammer). A borehole geophone is installed in the receiver hole and tightly coupled to the borehole walls by means of a pneumatic packer. A good coupling is needed since s-waves cannot be transmitted by the groundwater only. Example signals: shear_wave_signals.pdf.
Source and receiver are always installed at same depths and moved parallel along the boreholes to achieve a velocity profile with depth. The usual vertical spacing between two measuring points is 1 to 2 m. The shear modulus (i.e. the dynamic stiffness) can be calculated if a value of the material density is assumed.
The measurements can be performed below and above the groundwater table. A grouted casing (e.g. PVC) with a diameter between 3 and 6 inch needs to be prepared. Typical surface distances between source and receiver borehole are between 5 and 20 m.
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