Downhole Seismic Testing
The downhole geophysical technique is a test that is performed to obtain mainly the dynamic elastic modulus of the terrain, traversed by a survey, from the measurement of the velocity of the primary and secondary waves (Vp and Vs) along the borehole.
In our geophysical services, the downhole test is defined by the standard ASTM D7400 as standard quality, following the guidelines below:
- The P and S waves are generated at the surface and close to borehole (usually 5m). These are recorded with a triaxial geophone at different depths (usually every meter) throughout the survey.
- The probes must be tubed with PVC pipe and this is solidly attached to the ground by injection of grout, which must be performed from the bottom up to avoid the presence of voids.
Downhole test applications
The geophysical downhole technique is essential in geotechnical foundation designs. Another use of shear velocities is found in the evaluation of soil liquefaction in geotechnical design. As for those cases that require knowledge of dynamic elasticity modules as tunnel portals, etc.
Example of downhole results
Traveltime and associated modules
