EM 31 & 34
Background
Electromagnetic surveys operate by projecting a simple magnetic field into the subsurface. This initial field forms eddy currents within metallic or conductive objects present below ground, which then produce a secondary magnetic field that can be detected by a receiver. The actual physical property measured is conductivity (the ability of a material to carry electrical currents). Highly conductive materials, such as metals, produce a strong secondary field thus giving a high reading whilst poorly conductive materials will struggle to produce eddy currents and thus fail to create a detectable field.
The equipment used depends on the type of survey being carried out, but both techniques used require two wire coils, a transmitter coil and a receiver coil. Unsurprisingly, the transmitter coil is responsible for producing the primary magnetic field while the receiver coil detects the secondary field produced by the subterranean body. An EM 31 survey has the two coils located at opposite ends of a long pole held horizontally above the line of survey, while the EM 34 uses two separately cased coils held at a predetermined distance from each other, linked to a control panel by a series of cables. When conducting an EM 34 survey it is imperative that the two separate coils are aligned with each other in the …show more content…
A single line will typically take less than half an hour and the data is simple and easy to interpret. The only significant downfall with EM methods is the disturbance caused by foreign objects buried in the near surface. Common objects, such as metal pipes or wires will have a high conductivity, so will create erroneous secondary fields. In the Sourton area, these EM disturbances were very rare so did not pose a significant issue, but in a more urbanised environment, removing background noise from data is an important first step in data