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Applications
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Metal Detection (EM-61)
The EM61 is a time-domain metal detector which detects both ferrous and non-ferrous metals. A powerful transmitter generates a pulsed primary magnetic field in the earth, which induces eddy currents in nearby metallic objects. The eddy current decay produces a secondary magnetic field measured by the receiver coil.
By taking the measurement at a relatively long time after the start of the decay, the current induced in the ground has fully dissipated and only the current in the metal is still producing a secondary field. The responses are recorded and displayed by an integrated data logger.
The EM61 detects a single 200-litre (55 gal) drum at a depth of over 3 metres beneath the instrument, yet is relatively insensitive to nearby cultural interference, such as fences, buildings and power lines. The response is a single, sharply defined peak, greatly facilitating quick and accurate location of the target. Depth of the target can usually be estimated from the width of the response.
The system can be pulled around as a trailer with odometer mounted on the axle to trigger the data logger or it can be carried by a single operator with a shoulder harness.
Terrain conductivity (EM-31)
Terrain conductivity (TC) meters measure apparent conductivities (the inverse of resistivity). TC investigations do not require direct coupling to the earth, are faster, require less data-processing time than resistivity methods, and are a cost-efficient means to investigate a relatively large area.
TC methods utilize a magnetic field, generated by a transmitting coil located in the end of the instrument's boom. Time-varying primary fields induce eddy current loops in the earth. These induced currents in turn generate a secondary magnetic field which is measured, together with the primary field, by the receiver coil. The ratio between the primary and secondary fields, within certain constraints, is linearly proportional to the earth's conductivity.
The depth range from which data can be collected is a function of the distance between the transmitting and receiving coils, or the intercoil spacing. The closer the coils are to one another, the shallower the depth of penetration. Two basic terrain conductivity meters are commonly used; fixed-distance and variable-spaced intercoil.A fixed-distance intercoil instrument, such as the Geonics' EM-31 terrain conductivity meter shown above, is used for shallow investigations and has a depth of penetration of approximately 6 meters. This instrument provides a very rapid reconnaissance of an area. It is commonly used to detect changes in apparent conductivity and to locate buried metallic objects. A variable-spaced intercoil instrument, such as the Geonics' EM-34 terrain conductivity meter, is used for apparent conductivity investigations from 7.5 to 60 meters below ground.
Time-domain Electromagnetics
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Applications
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Time-domain electromagnetics (TDEM) methods measure the decay of the secondary electromagnetic field with time. TDEM methods may be appropriate in investigations where the target depths are as great as 200 meters. The resulting TDEM data are measured in millivolts versus time which can be modelled to conductivities and depths with ground-truth information. In general, TDEM methods have better vertical resolution for mapping interfaces characterized by conductivity contrasts than other electromagnetic techniques.
This site was last updated on March 26, 2004.
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