Application of geophysical investigation methods
Geophysical methods may help to make a preliminary site assessment and site monitoring with a reasonable cost. This is particularly advantageous when investigating large sites. Some of the applied geophysical methods are based on the response of the terrain after application of electromagnetic fields. In this way, information about specific subsoil parameters as for example resistivity or apparent resistivity can be obtained. These methods are based on the alteration of physical characteristics of the subsoil due to contamination. This alteration causes a different response of a contaminated soil compared to a natural soil. Nevertheless, in order to detect this difference a significant alteration of the subsoil characteristics is necessary, which will depend on the contaminating substance and on the contamination quantity present in the study area.
Additionally, the application of these methods will be limited due to geological properties of the area. If the geological structure is very complicated it will be difficult to determine the reasons for changes in the soil response. On the other hand, geological information of the study area is always necessary.
Based on the characteristics and quantity of the contaminants, the geological structure of the study area and on previous available geological information, the selected geophysical method will have different applications:
- Planning of works, especially with regard to location of soil borings.
- Limitation of the extension and thickness of the contamination plume in case of favourable conditions.
Different geophysical methods are available and the use of each of them will depend on the characteristics of the study area. One of these methods is the RMT (Radiomagnetotelluric method). This method is very versatile because:
- it is easy to handle
- it can be applied to every surface type (natural terrain, asphalt, frozen surfaces, etc.) because it is not necessary to drive electrodes into the soil
- it can be applied quickly
This equipment uses frequencies between 10-1000 kHz from commercial radiotransmitters, permitting a varying depth (0,5 m - 100 m) of the analyzed geoelectric section.