Figure 1: GPR measurements depicting changes in soil texture
Soil investigations
In agriculture, site-specific land management is getting more and more important. In order to consider changes of soil conditions when e.g. spreading fertiliser, the site has to be characterised. Recently, ground-penetrating radar (GPR) and resistivity measurements are used to map changes of soil properties. Figure 1 shows several GPR sections which clearly bring out the change from loamy to sandy soil texture within the mapped area.
Figure 2: Robert Meyer carrying out GPR measurements to determine soil moisture on a test area in the ‘Fuhrberger Feld’ close to Hannover.
Soil moisture is an important parameter for modelling mass transports or answer questions concerning agricultural land use. There are numerous techniques to determine soil moisture at selective points, e.g. gravimetric, time-domain reflectometry (TDR), and tensiometry. As soil moisture varies laterally, new methods have to be developed to determine moisture on wide areas in a short time. During the last years, GPR has shown to be a promising tool for this purpose.
The velocity of electromagnetic waves in the ground strongly correlates with soil moisture. Moisture of the topsoil can be determined with the groundwave which propagates in the soil along the top surface. The measuring layout was optimised concerning spatial resolution. Using a specific layout which is depicted in Figure 2, changes in soil moisture can be determined with lateral resolution of approximately 15 cm.
The results of GPR groundwave measurements on grass land on an area of 10 m x 10 m are depicted in Figure 3 (left). The propagation velocities which were determined from the GPR data are transformed in soil moisture contents. The pattern corresponds to natural moisture variations which among others are affected by microtopography or different intensity of the root system.
Figure 3: Results of GPR groundwave measurements on grassland. Natural pattern (left) and pattern affected by former tillage (right).
Figure 3 (right) shows soil moisture pattern on a second grass land area which has been used as an acre as far as two years before the measurement was carried out. One can recognise the regular stripe pattern in the same direction as the former cultivation. Ploughing and tilling seems to have an influence on soil moisture distributions even after a few years.
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