ELECTROMAGNETICS
GEMSOL UK Ltd is an expert in Electromagnetics method deployment and Electromagnetics Survey.
Applications:
GEMSOL UK Ltd is an expert in Electromagnetics method deployment and Electromagnetics Survey.
Applications:
- Locating sinkholes and sub-surface voids
- Mapping leachate migration
- Mapping and monitoring groundwater pollution
- Mapping saline intrusions
- Mapping buried utilities
- Locating underground storage tanks & buried drums
- Mapping water filled fractures and fissures
- Mapping buried foundations
- Mapping near surface chemical contamination
- Locating the boundaries of landfills
- Mapping buried foundations
- Assessment of ground remediation
- Determination of layer thickness and conductivities
Basic Theory
In electromagnetic (EM) surveying, the electrical conductivity of the ground is measured as a function of depth and/or horizontal distance. Different rocks (and buried structures/objects) exhibit different values of electrical conductivity. Mapping variations in electrical conductivity can identify anomalous areas worthy of further geophysical or intrusive investigation.
The electromagnetic method is based on the induction of electric currents in the ground by the magnetic component of electromagnetic waves generated at the surface.
An alternating current, of variable frequency, is passed through a coil of wire (a transmitter coil). This process generates an alternating primary magnetic field which, in turn, induces very small eddy currents in the earth, the magnitude of which is directly proportional to the ground conductivity in the vicinity of the coil. These eddy currents then generate a secondary magnetic field, a part of which is intercepted by a receiver coil. The interaction between the primary and secondary magnetic flux and the receiver coil generates a voltage that is related to the electrical conductivity of the subsurface, expressed as milliSiemen/metre (mS/m).
In electromagnetic (EM) surveying, the electrical conductivity of the ground is measured as a function of depth and/or horizontal distance. Different rocks (and buried structures/objects) exhibit different values of electrical conductivity. Mapping variations in electrical conductivity can identify anomalous areas worthy of further geophysical or intrusive investigation.
The electromagnetic method is based on the induction of electric currents in the ground by the magnetic component of electromagnetic waves generated at the surface.
An alternating current, of variable frequency, is passed through a coil of wire (a transmitter coil). This process generates an alternating primary magnetic field which, in turn, induces very small eddy currents in the earth, the magnitude of which is directly proportional to the ground conductivity in the vicinity of the coil. These eddy currents then generate a secondary magnetic field, a part of which is intercepted by a receiver coil. The interaction between the primary and secondary magnetic flux and the receiver coil generates a voltage that is related to the electrical conductivity of the subsurface, expressed as milliSiemen/metre (mS/m).
Locating buried structures beneath
This sites are attractive targets for redevelopment
but are riddled with potential pitfalls
and risks to the developer. Geophysics can help
reduce those risks.
Shown to the , the EM clearly marks buried
obstacles such as the foundations of three old
cooling towers and the location of underground
services over the site of a former power station.
The information from the geophysical survey was
used to inform the subsequent intrusive survey which was able to target trial pits more precisely across the foundations and safely avoid the buried services
This sites are attractive targets for redevelopment
but are riddled with potential pitfalls
and risks to the developer. Geophysics can help
reduce those risks.
Shown to the , the EM clearly marks buried
obstacles such as the foundations of three old
cooling towers and the location of underground
services over the site of a former power station.
The information from the geophysical survey was
used to inform the subsequent intrusive survey which was able to target trial pits more precisely across the foundations and safely avoid the buried services
Example (1)
EM-31 ground conductivity data were collected over a closed landfill site to delineate the lateral extent of the waste, and to detect internal variations in waste type. Resistivity imaging and seismic refraction data were also collected as part of an integrated solution for the client. A dGPS (differential Global Positioning System) can be used to accurately locate data. This survey methodology enables rapid coverage of the site area. The plan to the right shows the area covered by the dGPS ‘tracks’ marking the location of recorded EMdata. The lower right figure shows a coloured contoured plot of the data overlain on a plan of the site. The landfill boundary can be interpreted as the boundary between the areas of low and high conductivity values.
EM-31 ground conductivity data were collected over a closed landfill site to delineate the lateral extent of the waste, and to detect internal variations in waste type. Resistivity imaging and seismic refraction data were also collected as part of an integrated solution for the client. A dGPS (differential Global Positioning System) can be used to accurately locate data. This survey methodology enables rapid coverage of the site area. The plan to the right shows the area covered by the dGPS ‘tracks’ marking the location of recorded EMdata. The lower right figure shows a coloured contoured plot of the data overlain on a plan of the site. The landfill boundary can be interpreted as the boundary between the areas of low and high conductivity values.
Detecting Buried Tanks and Ground Contamination
Mapping variations in the electrical subsurface
across a site can highlight anomalously conductive targets such as metallic tanks, pipework, ground contamination, buried waste, solution features and old foundations. In the example shown right,
buried tanks generate a strong response along with services and foundations.
Mapping variations in the electrical subsurface
across a site can highlight anomalously conductive targets such as metallic tanks, pipework, ground contamination, buried waste, solution features and old foundations. In the example shown right,
buried tanks generate a strong response along with services and foundations.