Borehole geophysics: Geophysical borehole logging methods used in BGR

The ground geophysics unit in BGR uses geophysical borehole logs primarily for investigations related to groundwater exploration and vadose zone research. Consequently, our available instruments are limited. We measure

• the natural gamma radiation (NGR),
• the electrical resistivity (FEL),
• the spontaneous potential (SP),
• the flow within the borehole,
• the borehole diameter (caliper) and, moreover,
• physical and chemical parameters of the water (temperature, conductivity, pH, oxygen, redox).

The choice of the applied methods is tailored to the research questions. We have no actively radiating instruments at our disposal and can therefore measure neither density (gamma-gamma) nor porosity (neutron-neutron).

Natural Gamma radiation (NGR)

This method permits the discrimination between sandy and clayey layers and is applied in many groundwater related studies for the location of well screens.  Clays and shale often contain some potassium (sometimes even uranium and thorium) and can be detected by higher natural radiation compared with sand. In areas where uranium is present in sandy layers a spectral gamma ray measurement is required to discriminate clay and sand. The tool can be used in cased and in dry boreholes.

Electrical Resistivity (FEL-focused electric log)

This sonde measures the electrical resistivity of the surrounding rock with improved vertical resolution. Resistivity, particularly in connection with natural gamma radiation, gives information of the location of clayey (or shale) and sandy layers since clay lowers the resistivity. In steel cased and/or dry boreholes no measurement is possible. In water filled boreholes with plastic casing the location of the screens can be detected and within the screens the resistivity of the surrounding formation.

Spontaneous potential (SP)

This tool measures the potential difference between the surface electrode and the electrode at the sonde.  As with the FEL sonde a conducting fluid in the borehole is required, no measurement in steel cased boreholes is possible and in boreholes with plastic casing only within the screens information on the formation can be retrieved.  The method is applied to discriminate lower and higher permeable layers but the potential difference depends on the conductivity difference between formation fluid and borehole fluid.

Flow meter log

This tool measures the water flow using an impeller. If the tool is used while pumping the measurement reveals the points of water inflow and the data can be evaluated quantitatively.

Caliper log

This tool (three arm caliper) measures the borehole diameter. In uncased boreholes it may detect borehole outbreaks and in cased boreholes it may reveal uncertain borehole casing details.

Direction sensitive borehole radar sensor

In low conductive media a direction sensitive borehole radar sensor can be used to investigate geological structures in the boreholes surroundings.

The Gamma-Log reveals the depth of the weathering (about 10m), the screen is shown by the FEL log and water inflow is detected along the whole filter screen Source: BGR