|
| | |
 |
Method Name: |
|
Time-domain electromagnetics - Profiling |
|
| | |
|
| | |
|
Method Type: |
|
Electromagnetic Methods
|
|
| | |
|
|
|
Assigned Problems: |
|
|
|
|
| | |
|
Principle: |
|
Time domain electromagnetic measures the lateral variations of electrical conductivity of the subsurface using the temporal decay of a magnetic field resulting from induced electromagnetic currents. In contrast to geoelectric profiling no galvanic ground coupling is required. |
|
| | |
|
| | |
|
Keywords: |
|
Time domain EM Profiling; metal detector; magnetic fields; apparent resistivity; resistivity profiles; resistivity contour maps |
|
| | |
|
| | |
|
Prerequisites: |
|
- Target must be characterized by a resistivity contrast
- Buried wires, metal pipes, metal fences may influence measurements
- Urban areas may cause high noise levels (e.g. stray currents)
- High-voltage power lines, railways and antennas may influence measurements
- Coins, metallic belt buckle may influence measurements
- Interference from atmospheric storms
- May not work well in very resistive materials
- Induced polarization occurs clay-rich environments
- Prohibited use: pronounced 2 - or 3 D dimensional subsurface geometry
|
|
| | |
|
| | |
|
Resolution: |
|
Lateral resolution is determined by the spacing between the transmitter and receiver coil and the station spacing between measurements.
The variation of conductivity is detectable up to 1000 m. |
|
| | |
|
| | |
|
Expected Results: |
|
- Measured parameter: magnetic fields resulting from induced currents are recorded with induction coils [mV]
- Data analysis: voltages are plotted in form of apparent resistivities as profiles or contour maps.
- Interpretation: often qualitative. Areas displaying anomalously high or low values, or anomalous patterns can be identified. Depth of objects can be roughly estimated.
|
|
| | |
|
| | |
|
Combination with other Methods: |
|
- Required additional information: rough estimates of target depths
- Related add-on information: surface-based geoelectrical data; electrical data
- Independent additional information: georadar data; magnetic data; seismics data
|
|
| | |
|
| | |
|
Operating Expense: |
|
- Crew size: 1 key person, 1-2 assistants
- Acquisition speed: maximum of around 3 km profile length per day depending on field conditions
- Processing: requires 1 - 2 days per acquisition day
- Equipment rental costs: low
|
|
| | |
|
| | |
|
Parameters to specify: |
|
- Spacing between measurements (few m to few tens of m)
- Line spacing
|
|
| | |
|
| | |
|
QC Documents: |
|
- Around 2 - 5 % of repeated measurements
- Field notes (e.g., all activities, effective time schedule, present personnel)
- Optional: Map of buried cables, roads
|
|
| | |
|
| | |
|
Products: |
|
- Profile data
- Contour maps
- Interpretation
|
|
|