The Hydraulic Profiling Tool (HPT) is a direct-push screening tool that measures the pressure required to inject a constant flow of water with depth in soil. The HPT can also detect and log electrical conductivity that can be used to infer soil texture (i.e., clay content typically exhibits a higher conductivity).
The HPT was developed by Geoprobe Systems® in the United States (U.S.) and has been used extensively in the U.S., Europe and Canada for mapping formation permeability in the subsurface. In addition to measurement of injection pressure, using a down hole pressure transducer, the HPT can also be used to measure hydrostatic pressure under zero flow conditions. This allows the development of a hydrostatic pressure graph for the estimation of the position of the water table. As a screening tool, the HPT offers many benefits to site investigators:
- Useful for logging both low and high permeable zones in the subsurface.
- Able to detect both coarse and fine-grained soils.
- Works in both saturated and unsaturated soils.
- Real time permeability information is generated, allowing field adjustment of the site investigation program.
- Hydraulic conductivity estimations in the saturated zone based on empirical relationships with data collected on-site.
- Cost-effective in terms of the amount and level of detail of in-situ site characterization data as compared to typical Phase II Environmental Site Assessment techniques.
A constant flow of water is injected into the soil as the HPT is advanced into the subsurface. The down hole pressure transducer measures injection pressure at the HPT screen only and excludes frictional losses through the flow tube of the HPT trunkline. This allows for accurate pressure measurements at the point of injection which is also also necessary for making hydrostatic pressure measurements at the probe. Hydrostatic pressure measurements under zero flow conditions allows for the development of a hydrostatic pressure graph and the estimation of the position of the water table.
The primary use for the HPT system is the view of permeability with depth. The injection pressure correlates well with formation permeability. With this information an experienced site investigator can determine permeable zones, potential contaminant or seepage flow paths, zones that will yield groundwater to samplers, or zones to set monitoring or production wells. Another use for the HPT is the estimation of the saturated zone hydraulic conductivity. Formation hydraulic conductivity can be estimated from HPT data using a empirical relationships developed for the tool.