Geoelectrical imaging combining resistivity and induced polarization (IP) surveying has shown great potential for environmental monitoring at landfill sites and other locations that have been polluted, for example old industrial sites. Detection of landfill gas migration has been shown to be possible using resistivity and IP.
Resistivity and IP surveying is based on measurement of the potential distribution arising when electric current is transmitted to the underground via electrodes (see principle sketch below). The electrical resistivity of soil materials generally depends on the moisture content, temperature, porosity, and on the pore water salinity. Multi-electrode data acquisition systems (see below) allow time and cost efficient mapping of the subsurface space in 2D and 3D, and can also be configured for 4D (monitoring).
The principle of slingram methods is to generate a primary electromagnetic field in the ground through a transmitter coil. The secondary electromagnetic field arising from induced currents in the ground are then detected in a reciever coil, and the measurements results in 2D-mapping of electromagnetic properties (electric conductivity and magnetic susceptibility) of the ground. The methods does not require instrument contact with the ground, and large areas can therefore quickly be explored. There are several versions of slingram instruments available, with different arrangement of the transmitter and receiver coil. Metal detectors are one kind of slingram instruments. When the coils are placed on a rod, the distance between them determines the depth penetration and lateral resolution of the measurements (for example EM38 and CMD Explorer). EM61 consist of two vertically arranged coils shaped as frames, and is example of an effective instrument for metal detection.
Surface wave seismics makes use of the most powerful seismic wave types, and gives the best resolution in the uppermost 10 m of the soil volume. The energy rich surface waves allows a sledge hammer to be used as a source for shallow (0-10 m) applications. It also allows for profiling measurements using a landstreamer with the geophones mounted on metal plates. The landstreamer could be towed by a vehicle producing long profiles with high resolution in a day. The method is based on the measurement of the velocity of the seismic surface waves generated by a source, ie a sledge hammer. The velocity is correlated to the stiffness of the material, the higher velocity the stiffer material. By producing a shot every 10 m and then moving the array in a roll along manner a detailed velocity profile is produced, and 2D or 3D pictures could be produced. In this project the MASW interpretation technique has been applied.
Håkan Rosqvist at hakan.rosqvist(at)tyrens.se