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| Environmental Technology Listing |
Title of Listing: | | High Frequency Electromagnetic Sounding |
Category: | | Characterization Technologies |
Subcategory: | * | Water, Soil, All Listings |
Media: | | Soil, Water, Sludge, Solid, Sediment |
Contaminants: | | Resistivity, Conductivity, Buried Materials, Explosives, Soil Types |
Web Site: | | |
Email: | | |
Technology Description: | |
EM31, EM34-3, EM34-3XL, EM38, EM39, EM16, EM61
The sinusoidally alternating current in the transmitter coil generates an alternating magnetic field in the ground which in turn causes eddy currents to flow, the strength of which is a (relatively complicated) function of the ground conductivity, and is determined by measuring the resultant secondary magnetic field.
The Geonics Ground Conductivity Meters (GCM), such as the EM31, maps geology or underground contaminant plumes by measuring terrain conductivity without electrodes or ground contact, using a patented electromagnetic inductive technique.
Surveys are readily carried out in all regions including those of high resistivity such as sand, gravel and bedrock. The EM31 provides two data sets describing two important properties of the earth; the Quadphase (conductivity) provides information on contaminant presence while the Inphase provides information on the presence of buried ferrous and non-ferrous metallic targets.
Operating under the same principles as the EM31, the EM34 is designed to achieve much greater depth of penetration and also yields much more information about the geo-electric section. The EM34-3 can be used at three intercoil spacings; 10, 20 and 40 meters. Yielding a depth of exploration of 7.5, 15, 30, and 60 meters thus having excellent abilities in plume reconnaissance and fracture detection.
Also based upon the same principles as the EM31, the EM38 is designed to be particularly useful for agricultural surveys for soil salinity. The EM38 has depths of exploration of 1.5 and 0.75 meters depending on the dipole orientation.
The EM39 provides measurement of the electrical conductivity of the soil and rock surrounding a borehole or monitoring well, using an inductive electromagnetic technique. The unit employs coaxial coil geometry with an intercoil spacing of 50 cm to provide a substantial radius of exploration into the host material while maintaining excellent vertical resolution; measurement is unaffected by conductive borehole fluid in the monitoring well or by the presence of plastic casing. The instrument operates to a depth of 500 meters.
The EM61, one of the newest instruments from GEONICS, 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.
The EM61 detects a single 200-litre (55 gallon) drum at a depth of over 3 meters 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.
The EM16 is the most widely used EM instrument of all time. It measures the local tilt ellipticity of very low frequency (VLF) broadcasts, and resolves these values into inphase and quadrature components of VLF response. The EM16 has discovered several base- and precious-metal ore bodies and many water-bearing faults.
The EM16R attaches to the EM16 and, using a pair of electrodes, measures the apparent resistivity of the earth. The TX27 is a portable VLF transmitter supplying a VLF field for surveying with the EM16/16R, if remote broadcasts are weak, intermittent or poorly coupled with the target. For EM16 surveys, the TX27 antenna consists of a long (1 km) grounded wire.
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Performance Status/
Limitations: | |
Technical limitations include a limited dynamic range, setting and maintaining the instrument zero, and limited vertical sounding capability.
Limited dynamic range (1-1000 mmhos per meter). At low values of terrain conductivity it becomes difficult to magnetically induce sufficient current in the ground to produce a detectable magnetic field at the receiver coil. Conversely at high values of conductivity-the quadrature component of the received magnetic field is no longer linearly proportional to terrain conductivity.
Setting and maintaining the instrument zero. Ideally in order to set the zero the instrument would be suspended in free space and the zero set there. The more acceptable alternative is to search out a region of very resistive ground, to accurately measure its conductivity using conventional techniques, and to set the instrumental zero at that location. This is the procedure which is actually followed.
It is necessary that this zero be accurately maintained over long period of time and over the wide variations of temperature encountered during geophysical survey in various parts of the world. This produces tight constraints on the circuitry, with the result that the zero may be in error by up to +/-0.2 mmhos per meter. Such an error would be negligible over the usual range of terrain conductivities; however in the event that measurements are being made on highly resistive ground the zero error can become significant.
Limited Vertical Sounding Capability. In theory it is possible to use a system such as the EM34-3 at a continuum of intercoil spacings to yield more information about electrical layering in the ground. To achieve a wide variety of inter-electrode spacings with conventional resistivity equipment is simple; in the case of the inductive electromagnetic technique the rapid fall-off of the magnetic field from the dipole transmitter introduces a serious dynamic range problem. In due course there will undoubtedly be instrumentation with a wider variety of spacings at the expense of additional complexity.
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Topics
Electromagnetic Fields (EMF), Geophysics, Technologies, Characterization
Additional Topics/Tags/Keywords
Electromagnetic Sounding, Soil |
Organization: | | Geonics Ltd. |
Address: | | 1745 Meyerside Drive
Unit #8 | Phone: | Description | Number |
City: | | Mississauga | | Primary | (905) 670-9580 |
State/Province/Territory: | | . | 2. | | |
Zip/Postal Code: | | L5T 1C6 | 3. | | |
Country: | | United States | 4. | Fax: | (905) 670-9204 |
Branch Locations: | |
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