Geophysical Mapping: Method Details Printer Friendly Version   43 of 48 Method Name: Time-domain electromagnetics - Sounding Method Type:   Electromagnetic Methods Assigned Problems: + Depth of Overburden-bedrock interface Civil Engineering + Groundwater table Groundwater + Host sediments, hydogeological settings Hazardous Waste + Permafrost and ice detection Natural Hazards + Quality and thickness (Natural resources) Natural Resources + UXO detection Hazardous Waste + Weapon Forsenic Investigations 0 Aquifer pollution Groundwater 0 Characteristics of hazardous waste Hazardous Waste 0 Contaminant plumes Hazardous Waste 0 Dead body Forsenic Investigations 0 Fractures Groundwater 0 Gravel, clay, limestone, salt exploration Natural Resources 0 Ice thickness Natural Hazards 0 Landslides Natural Hazards 0 Location of buried materials Hazardous Waste 0 Porosity / Permeability Groundwater 0 Quality / Thickness of aquifer/aquitard Groundwater 0 Quantity/ Thickness Hazardous Waste    '+' = Technique applicable; '0' = Application possible/limited use Principle:   The time-domain electromagnetic method measures the electrical resistivity of the subsurface by inducing pulsed currents into the ground. The decay of the induced currents results in a secondary magnetic field, which is measured. Keywords:   Time domain EM; TDEM Souding; transient electromagnetic; magnetic fields; sounding curves; 1-D resistivity-depth functions 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:   Time domain soundings provide average values over subsurface volumes. The lateral extension of these volumes increases with increasing depth from a few tens to several hundreds meters2. Normally, 3 - 4 layers can be resolved Typical depth of investigation: from a minimum depth of a few meter to a few hundreds of meters (maximum a few km) 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:geological information for reliable interpretation 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: depending on instrument and configurations few tens to few hundreds of measurements per day Processing: requires 2 - 3 days per acquisition day Equipment rental costs: intermediate Parameters to specify:   Side length of transmitter loop (accuracy of less than 1%) Loop configuration Measurement separations Receiver gate location / Measuring period / Integration time (usuall less than 1 mnx for engineering - scale investigations) QC Documents:   At least 2 measurements per measuring point Field notes (e.g., all activities, effective time schedule, present personnel) Optional: Map of buried cables, roads Products:   Raw data (reciever output voltage vs. Time) Apparent resistivity vs. time functions Inversion results: 1-D or 2-D resistivity vs. Depth functions Interpretation Printer Friendly Version