Technique | Principle | Invasiveness | Strengths | Weaknesses | References |
---|---|---|---|---|---|
Gravimetric | Oven-drying of samples | High (chipping off or drillhole) | Reference method | Low temporal resolution | [7] |
Borehole humidity | Equilibrium between air humidity and rock moisture | Moderate (drillhole needed) | Monitoring possible, good calibration via sorption isotherms | Not suitable for high saturation, values are stuck at 100% for long periods | [60] |
Infrared thermography | Modification of temperature by attenuation and evaporative cooling | None | Quick spatial overview at high resolution | Only qualitative surface information; reflectivity at different angles | [62] |
Electrical resistivity | Electrical current flow between two (or four) electrodes | Moderate (drillholes needed) | Simple, relatively cheap, strong response to water content | Strongly affected by pore water salinity; individual calibration required | [58] |
2D/3D-electrical resistivity (ERT) | Combining automatically switched measurements to a 2D or 3D section | High with drilled electrodes; none when adhesive electrodes are used | Producing clear and instructive sectional views | [55] | |
Microwaves | Attenuation or reflectance of microwaves (1–10 GHz) | None when surface sensors are used | Unaffected by pore water salinity | Calibration required, limited depth information, susceptible to surface roughness | [63] |
Ground-penetrating radar | Attenuation of electromagnetic waves (10–1000 MHz) | None | Measurement along profile lines or 2D arrays | Affected by coupling conditions at uneven surfaces | |
Time-domain reflectometry | Â | Moderate (drillholes needed) | Less error sources than electrical resistivity, only slightly influenced by salts | Physically difficult to install, edge effects along sensor rods | [66] |