In the working group “Ground-based geophysics” of the department "Geophysical Exploration and Technical Mineralogy" of the division "Groundwater and Soil Science", geophysical borehole probes were used to investigate groundwater quality. The goal is to obtain detailed information about the geological layers, their thickness, composition, porosity, water content, and geotechnical properties.
The available geophysical probes measure
Natural gamma radiation (NGR),
Focused direct current electrical measurement (FEL),
Self-potential measurements (SP).
Borehole diameter (caliber) and
Flow, as well as chemical and physical groundwater parameters (electrical conductivity, pH, redox, temperature, oxygen content).
Natural gamma radiation (NGR)
The measurement of natural gamma radiation is primarily used to distinguish between sandy and clayey sediments, as the latter emit radiation due to the potassium (and sometimes also uranium and thorium) they contain. The measurement can also be performed in a cased borehole. In regions with uranium-bearing sands, even sandy zones exhibit high radiation levels. In such cases a spectral gamma probe, which allows differentiation between potassium and uranium, is necessary.
Focused direct current electrical measurement (FEL)
This probe measures the specific electrical resistance of the surrounding material using a focused electrode array with good depth resolution (a few centimeters). This measurement also aims to differentiate between clayey and sandy sediments, since clayey sediments (or mudstone, for example) have a lower resistivity than sands (or basalt, for example). In water-filled boreholes with plastic casings, the depths of the well screens and through the screens, although with some limitation, the resistivity of the surrounding material can be measured. No information about the surrounding material is obtained in solid casings, and no measurements can be performed in dry or steel-cased boreholes.
Self-potential measurements (SP)
The method measures the potential difference between the surface and the borehole probe and requires a conductive medium (water) in the borehole. The method allows the differentiation between poorly permeable and permeable (clay, dense limestone, shale, and sandstone) horizons, with the potential differences quantitatively depending on the conductivity difference between the formation water and the fluid in the borehole. The measurement cannot be performed in dry or steel-cased boreholes.
Caliber measurements
The caliber log is used to determine the borehole diameter and its changes (e.g., edge breakouts). This measurement is particularly useful in open boreholes or those with unclear development data.
Flow measurement
This probe measures the water flow in the borehole during water production, allowing water inflows to be detected and quantified.
Direction-sensitive borehole radar probe
This probe is used to investigate the geological structures in the vicinity of a borehole in media with low electrical conductivity.
The gamma log shows the depth of the weathering front at about 10m, the position of the filter is shown in the FEL log and the inflow occurs over the entire filter length
