Direct Image by Geoprobe

Both the HPT and EC logs below indicate primarily silts and clays from 0 to 9.5 m (with a sandy layer from 4 to 6.5 m). At 9.5 m, the finer soil transitions to primarily sands with some silts evident between 14 and 17 m. However, the HPT pressure response shows high pressure responses around 13.5 and 16 m, indicating clay or other low permeability material is present.

A discrete soil sample was recovered to account for the differences in electrical and hydraulic response. Sample A, taken at 13.5 m, shows a very high clay content, which is consistent with the high HPT pressure response. Sample B was recovered 14 m. Here, in agreement with the HPT pressure response, the sample is sandy but with high clay content. Taken at 15 m, Sample C is sand with very few fines.

In this example, both the EC and HPT responses are accurate, however, the low permeability, fine-grained zones between 13 and 18 m, while causing high pressure response on the HPT, do not display the change in electrical conducvtivity that is typically associated with such zones.

(left to right) Sample A, Sample B, and Sample C