The HPT allows the creation of fast, continuous
real-time profiles of soil hydraulic properties in both fine- and
coarse-grained material. The HPT uses a sensitive, downhole transducer
to measure the pressure response of the soil to injection of water.
One primary use of this tool is to locate and define preferential
migration pathways for contaminants in the subsurface. It can also
be used to target zones for injection of remediation material. In
addition, the HPT can be used to select well screen intervals, evaluate
locations to conduct slug tests, and measure static water conditions
across a site.
HPT is designed to evaluate the hydraulic behavior of unconsolidated
materials. As the probe is pushed or hammered, clean water is pumped
through a screen on the side of the HPT probe at a low flow rate.
Injection pressure, which is monitored and plotted with depth in
real time, is an indication of the hydraulic properties of the soil.
That is, a relatively low pressure response would indicate a relatively
large grain size and the ability to easily transmit water. A relatively
high pressure response, however, would indicate a relatively small
grain size and the lack of ability to transmit water. Since the
pressure transducer is located downhole, the HPT system also can
be used to collect profiles of static water pressure data, which
can be used to calculate static water levels. Since the HPT pressure
response is analogous to relative changes in the ability to transmit
water (and therefore the relative change in dominant grain size),
the HPT system can be used to identify potential contaminant migration
pathways. Similarly, it can help identify zones for remedial material
injection or provide qualitative guidance on how difficult injection
may be in different zones of the formation. Static water level calculations
can be made using static pressure and depth data from response testing,
as well as pre- and post-test response data measured at the top
of the reference tube at zero flow. Since the HPT

system
uses an absolute pressure transducer to measure in-situ pressures,
pre- and post-test data must be changed to reference atmospheric
pressure.
A Wenner array is integrated into the HPT probe. This allows a user
to collect soil electrical conductivity (EC) data for lithologic
interpretation. Generally, EC is inversely proportional to particle
size. That is, clays usually have higher conductivity than sands.
Another rule of thumb is the smaller the particle size of the soil,
the lower the permeability. Thus, EC measurements yield important
lithological information and identify the locations permeable and
non-permeable zones. However, other factors can affect EC, such
as mineralogy and pore water chemistry (brines, extreme pH). Conversely,
the HPT pressure response is independent of the chemical and mineralogical
factors.