The direct push downhole seismic test (DPDH) is conducted by pushing a seismic module into the ground using a CPT rig. There are sensors (geophones) in the seismic module. At regular intervals, the push is stopped and a shear beam is struck at the surface sending shear waves through the ground. The shear waves are recorded by the geophones in the seismic module, which provides a measure of the time taken for the shear wave to propagate from the shear beam to the module. Knowing the distance between the shear beam and the geophones in the seismic module, then allows the shear wave velocity (vs) through the soil to be determined.
A cone penetration test (CPT) probe can be attached to the end of the seismic module allowing a CPT test to be done concurrently with a seismic test in a single sounding. In this way the combined tests are referred to as a seismic cone penetration test (SCPT).
Similarly, the seismic module can be attached to flat dilatometer test (DMT) probe, which then allows seismic and DMT tests to be done in the same push. This is then referred to as the seismic dilatometer test (SDMT).
At Ground Investigation we use only the Marchetti dual geophone seismic module, either with a dummy cone (DPDH) or together with a DMT blade (SDMT). This is a true interval system with geophones spaced at 500 mm centres. Tests are done at 500 mm depth intervals. Shear wave velocities are determined by the software using an automated cross-correlation method. Each shear wave velocity measurement is checked by our experienced technicians and corrected if necessary.
For SCPT tests, we do a standard CPT test and then a seismic test as a separate push, either down the same hole or in a new set up adjacent to the CPT test. This is so that the CPT test is not disrupted by stopping for seismic testing.
We do not do SCPT tests using a single geophone (pseudo interval testing). That form of downhole seismic testing is considered potentially erroneous and is no longer considered good practice in New Zealand.
Downhole seismic testing is done in general accordance with ASTM D7400-08.
