Moisture in Geophysics and Geotechnology

In all geophysical and geotechnological Applications the water content of the investigated soils and physical properties derived from it are playing an important role. Examples for applications are to be found in agriculture, in environmental technology, in monitoring of waste dumps, in geophysical investigations etc.

Opposite to other areas of application, where mass based moisture definitions are used, soil moisture is measured in volume based values. This leads to some specifics in the design and use of measuring technology, in it´s calibration and for the used reference methods. But finally the volume based moisture definitions are closer to the physical meaning of water and it´s effects than the mass based. Most of the todays used indirect measuring methods have their interaction in the volume. So the connection to the volume based moisture definitions allows a better characterization of the resulting effects.

Currently known methods for soil moisture measurement are either directly working but destructive or indirectly and in some cases non-destructive or even non-invasive. The simplest indirect methods are based on conductivity or other geoelectrical measurements. They are strongly influenced by the salinity of soils.

Dielectric moisture measuring methods have found wider spreading. They are used in different kinds of moisture meters, the best known of them TDR probes are. TDR probes are small and can be handled easily, but are designed for stationary use and show high consumption of electrical power. In most cases they are connected to complete measuring stations or integrated in measurement networks for monitoring.

In many applications fast information about soil moisture is needed, or moisture profile in depth direction are required. In those applications the mounting of more probes is not the desired way. Users like to have a quick portable measurement.

Microwave based moisture measurement methods are dielectric methods too. Because the frequencies used are very high, they are not influenced by salinity. Their capability of real time measurements and manyfold possibilties for sensor design make them best suited for the solution of the tasks described above.

In this context the MOIST series of microwave moisture meters was an ideal basis for the design of applicators for moisture measurements in soils and stones.

The chosen applicators show a good coupling of microwaves in the material under test. They allow interaction in different measuring volumina (20 cm³ to 100 l). Those large measuring volumina make sure that the meter itselfs averages over smaller inhomogenities.

Using the penetration applicator MOIST BIO already singular measurements give representative information about moisture content. The penetration depth of the electromagnetic is 30 to 50 cm radially, the resolution in depth direction between 5 cm and 10 cm.

Measurements are carried out by inserting the sensing head in the soil or in a guiding tube adapted to the sensor. The guiding tube can be used for monitoring measurements more times. For this case it´s end is sealed with a cap.

In combination with the handheld MOIST 200 depth profiles of different places can be stored in the meter as profile.

Soil moisture measurements in upper layers can be done with the volume sensor MOIST P too. MOIST P allows a penetration depth of 20 to 30 cm depending on soil type and moisture.

There are many applications for the use of moisture meters in soil moisture measurements. Irrigation control today is done by balancing the soil water content by means of climatical water balances and occasional measurements of the current soil moisture. This way it´s hard to get information about water transportation in the soil. For cyclical or continous moisture measurements a high potential is existing in such applications.

Moisture monitoring in waste dumps is an important application too. More and more waste have to be closed. In many cases this is accomplished with thick mineral layers from clay or other non-porous materials. Such clay covers have the problem that they must have certain moisture for not becoming leaking. Moisture measurements and moisture monitoring helps to keep the moisture level constant and high enough.

In biological soil redevelopment water content plays an essential role. The development of microorganisms necessary for that is depending on the soil moisture content.

Another application is the moisture monitoring in dikes and dams.

For the future a development towards more monitoring and monitoring of larger areas is to expect. Such measurements will give more qualified information than todays singular measurements or profiles.