Seismic Module - HT Series
With the optional seismic channel(s) downhole shear wave propagation speed during cone soundings can be measured. The shear wave speed can be used in conjunction with an estimate or measurement of the soil density to determine the dynamic shear modulus of the soil at low strain values. Performing this measurement during CPT soundings is faster and less expensive than standard crosshole or downhole seismic tests.
All of the electronics are in the cone, eliminating the need for drilling and casing bore-holes. The cone itself is in direct contact with the soil and causes less disturbance of the in-situ conditions than an instrument package being lowered into a cased hole. The down-hole orientation of the cone allows the shear waves to propagate down through the soil, rather than across the horizontal layer, thus reducing the effect of refraction on the results.
The depth of each measurement is controlled by the length of the sounding tubes, so positioning is very accurate. Seismic measurements are made when the CPT has stopped. A shear wave is generated at the surface by striking a strike plate with a switched hammer. The resulting wave is recorded for a user defined time interval and transmitted to the computer. The arrival time of the wave can then be determined. The change in depth between the current test and the previous test is divided by the change in the arrival times to determine the average shear wave velocity for that depth interval.
