Electromagnetic Ground Conductivity Surveying

 

Why use Ground Conductivity (EM)?

EM has the advantages of not requiring probes to be placed into the ground. This means soil conductivity (and hence resistivity) can be measured over hard or high resistance surfaces.

Electromagnetic surveying can be used for a number of applications. Included in the range that Stratascan offer are;

  • Detection of buried metal objects
  • Detection of cavities
  • Contaminated land mapping including plumes
  • Buried foundations and mine shafts
  • Geological feature mapping
  
 
 An EM31 being used on a construction site

How it works

The principle of operation relies on a transmitter coil inducing eddy currents in conductive sub-surface materials. These eddy currents in turn produce their own electromagnetic field which is detected by a receiver coil in the instrument.

Different orientations


Data can be collected with the instruments in different orientations or modes. The vertical dipole mode has very low sensitivity to near surface materials. The sensitivity then increases with depth, becomes a maximum and then falls away beyond that depth. In the horizontal dipole mode the relative sensitivity is greatest to material at the near surface and then decreases with depth. This means that deeper anomalies are detected in the vertical mode and shallow anomalies in the horizontal orientation.

  
 The depth of detection depends on the coil separation, frequency used, the conductivity of the soils and whether in the vertical or horizontal dipole mode. The following table is based on the Geonics range of instruments.
A diagram showing variation of sensitivity
with depth for each dipole orientation

Instrument Coil Separation Frequency

Typical Depths

 Resolution
(Approx. 1/5th of the coil separation)

 

 

 

 Vertical dipole Horizontal dipole

 
EM38
EM31
EM34
EM34
EM34		 1m
3.66m
10m
20m
40m		 14.6KHz
9.8KHz
6.4KHz
1.6KHz
0.4KHz		 1.5m
6m
7.5m
15m
30m		 0.75m
3m
15m
30m
60m		 0.2m
0.7m
2m
4m

   
The example above shows the response produced in vertical mode over an old paper factory. The site was surveyed with an EM31 in both vertical and horizontal dipole modes. The data sets highlight the fact that where anomalies appear strongest in the horizontal mode they are relatively shallow. Conversely strong features in the vertical mode indicate deeper seated features. The survey outlined two areas where foundations where thought to still exist beneath the ground. These target areas were later excavated to reveal concrete foundations.
In the second example a playing field site was surveyed to locate swallow or sink hole features. The plot highlights one such feature with a weak channel leading to it from the north west. This is thought to be a passageway for water movement into the swallow hole.