Imaging Probe Techniques
Before a remediation project can be planned, the subsurface and its structure must be thoroughly understood. Imaging probing methods provide continuous, high-resolution data on soil structure, hydraulic properties, and contaminant distribution—all directly on-site, without having to wait for time-consuming laboratory analyses.
Sensatec uses Geoprobe®’s DirectSense or DirectImage technology for this purpose: Various probe types are inserted into the subsurface using dynamic and static methods, providing continuous depth profiles that enable virtually complete vertical and horizontal mapping of the subsurface and any existing contamination. Depending on the specific problem at hand, we combine these methods to reliably assess both the geological stratification and the contamination situation.
EC (Electrical Conductivity) Measurement
The purpose of this procedure is to create a geological cross-section of the subsurface.
The Geoprobe® Electrical Conductivity (EC) System enables the determination of soil classes by applying an electric current and measuring conductivity. Higher electrical conductivities are typically characteristic of fine-grained sediments, such as silts and clays, while sands and gravels exhibit significantly lower conductivities.
The EC probe is available in two different configurations: dipole array and Wenner array. Both configurations operate in the same way. A current is sent through the soil surrounding the probe between two probe contacts. This current is measured along with the resulting voltage, and the resulting conductivity is calculated.
MIP (Membrane Interface Probe) sounding
Using MIP (Membrane Interface Probe) surveys, site investigations for volatile contaminants in the saturated and unsaturated zones can be conducted using imaging techniques without the need for sampling, down to any depth.
The MIP probe is driven into the ground using either the driving or pushing method. The probe heats its surroundings to approximately 121 °C via an integrated heating block. This ensures that the heating temperatures remain as constant as possible and do not fall below 100 °C. The organic contaminants dissolved in the groundwater or sorbed onto soil particles are mobilized (thermoresorption), adsorbed by a polymer membrane, and directed into the probe.
The contaminants then pass into the carrier gas (nitrogen) contained in the capillary tube within the assembly and are transported by the gas toward the surface.
The gas is ionized by detectors. In a laboratory unit, a semi-quantitative measurement of the gas or pollutants is performed as a total concentration parameter. The measurement is carried out using a combination of PID (photoionization detector), FID (flame ionization detector), and DELCD (dry electrolytic conductivity detector).
Simultaneously with the pollutant measurements in the soil and groundwater, a geological subsurface profile is recorded (MIP/EC coupling).
HPT (Hydraulic Profiling Tool) Survey
The HPT system was developed by Geoprobe Systems for the hydrogeological characterization of soils. Using the HPT method, exploratory boreholes are drilled to map the hydraulic conductivity and lithology of the subsurface in a non-intrusive, sample-free manner as the borehole progresses to greater depths. In particular, the method allows for the investigation of preferred migration paths of groundwater and contaminants in the subsurface.
The HPT probe is driven into the ground using the ramming or pushing method at a constant feed rate of 2 cm/s.
During the drilling process, water is continuously pumped through a membrane in the HPT probe at a flow rate of 300 ml/min. The resulting injection pressure is recorded over depth, which allows conclusions to be drawn about the hydraulic properties of the soil. A low pressure response would indicate higher hydraulic conductivity. Conversely, a high pressure response would indicate comparatively low hydraulic conductivity.
The conductivity of the surrounding soil material is recorded in parallel with the pore water pressure. Higher electrical conductivities are typically characteristic of fine-grained sediments, such as silts and clays, while sands and gravels exhibit significantly lower conductivities. The measurement of relative electrical conductivity is performed using the Wenner configuration (4-pole).
