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Environmental Technology Listing
Title of Listing:
PROTEM 37, 47, 57 (Time Domain EM Sounding)
Category:
Characterization Technologies
Subcategory:
 *Water, Soil, All Listings
Media:
Sediment, Groundwater
Contaminants:
Water Table, Resistivity, Buried Objects, Soil Moisture, Bedrock Stratigraphy
Web Site:
Email:
Technology Description:

Transient Electromagnetic (EM) Geophysical Instruments:

          The principles of Time Domain Electromagnetic (TDEM) resistivity sounding are relatively easily understood. The process of abruptly reducing the transmitter current to zero induces, in accord with Faraday's law, a short duration voltage pulse in the ground, which causes a loop of current to flow in the immediate vicinity of the transmitter wire. In fact, immediately after transmitter current is turned off the induced current loop can be thought of as an image in the ground of the transmitter loop. However, because of finite ground resistivity the amplitude of the current starts to decay immediately. This decaying current similarly induces a voltage pulse which causes more current to flow, but now at a larger distance from the transmitter loop, and also at greater depth. This deeper current flow also decays due to finite resistivity of the ground, inducing even deeper current flow and so on. The amplitude of the current flow as a function of time is measured by measuring its decaying magnetic field using a small multi-turn receiver coil usually located at the center of the transmitter loop. From the above it is evident that, by making measurement of the voltage out of the receiver coil at successively later times, measurement is made of the current flow and thus also of the electrical resistivity of the earth at successively greater depths, which process forms the basis of central loop resistivity sounding in the time domain. The technology measures the decay of induced magnetic field in units of Nanovolts per square meter (nV/m2).
Performance Status/
Limitations:

Limitation:

          Noise sources for TDEM ground soundings can be divided into four categories

          (1) circuit noise (usually so low in modern receivers as to rarely cause a problem)

          (2) radiated and induced noise

          (3) the presence of nearby metallic structures

          (4) soil electrochemical effects (induced polarization)

          Radiated noise consists of signals generated by radio and radar transmitters and also from thunderstorm lightning activity.

          The most important source of induced noise consists of the intense magnetic fields from 50/60 Hz power lines.

          The response from metallic structures can be very large compared with the response from the ground. Interestingly, the power lines referred to above can often also be detected as metallic structures, as well as sources of induced noise.

          A rather rare effect, but one which can occur, particularly, in clayey soils is that of induced polarization. Rapid termination of the transmitter current can charge up the minute electrical capacitors in the soil interfaces (induced polarization). The net effect is to reduce the amplitude of the transient response (thus increasing the apparent resistivity) or even, where the effect is very severe, to cause the transient response to become negative over some range of the measurement time.
Topics
Electromagnetic Fields (EMF), Geophysics, Technologies, Characterization
Additional Topics/Tags/Keywords
Electromagnetic Sensor, Subsurface

Organization:
Geonics Ltd.
Address:
1745 Meyerside Drive
Unit #8
Phone:
DescriptionNumber
City:
Mississauga
1.
Primary (905) 670-9580
State/Province/Territory:
2.
Zip/Postal Code:
L5T 1C6
3.
Country:
United States
4.
Fax:(905) 670-9204
Branch Locations: