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SJ Geophysics Ltd. - Contracting Services: Electromagnetic Surveys Applications:
Examples:
UTEM UTEM The UTEM-3 transmitter passes a low-frequency current of precise triangular wave form through the transmitter loop. The magnetic field sensed at the receiver coil is the time derivative of the transmitted magnetic field, so that in "free-space" a precise square wave voltage would be induced in the receiver. In the presence of subsurface conductors the received waveform is substantially distorted from a square wave. This distortion is principally a measure of the conductance of the materials in the region beneath the receiver coil. The UTEM-3 measures this distortion by determining the amplitudes at 10 decay times (averaging over windows) which are spaced along the decay curve in a binary geometric progression. UTEM Profiles Because the UTEM decay curve will exhibit relatively large amplitudes
at early times and small amplitudes at late times, it is appropriate to
plot the channels of data at different amplitude scales. The profile plots
of the data collected outside the loop and the sounding data are therefore
plotted on three horizontal axes. The inside loop data on two axes and
the electric field on one axis. On the three axes plots the latest time, channel 1, is plotted on the lower axis, the mid time, channels 2 to 4, on the middle axis, and the early times, channels 4 to 10, (channel 4 is repeated on the upper axis) on the top axis. On the two-axes plot channel 1 is plotted on the lower axis and the remaining 9 time channels on the second axis. The symbols used to identify the channels on all plots as well as the mean delay time for each channel is shown here. The amplitude of the data and the type of normalization and reduction used are displayed on each profile plot. MaxMin - Horizontal Loop EM Method
The MaxMin is a frequency domain EM system where the primary field is established by sending an alternating current through a coil of wire. The receiver measures both the inphase and quadrature (out-of-phase) components of the resultant field. A cable connecting the transmitter and receiver provides specifications of the primary field, which is subtracted from the measured field to yield amplitudes of the secondary field, expressed as a percentage of the primary. In the horizontal loop mode, the transmitter and receiver coils are horizontal and kept at a fixed distance apart. 
Characteristics of the maxmin profiles are determined by two main factors: the geometry and attitude of the conductive source and the geometry of the receiver and transmitter coils. In the horizontal coplanar configuration, a conductive response to a vertically oriented plate-like body typically appears as a negative peak, flanked by two lower amplitude positive shoulders ~1.3 x the coil separation apart. A suite of Slingram (horizontal loop) anomalies has been compiled based on small scale model measurements. They can be seen here. By comparing field results to these type curves a quantitative analysis of shape, orientation, depth and conductivity thickness characteristics of the conductor can be determined. A qualitative interpretation of the conductor can be determined from an inspection of these profile characteristics. The relative amplitudes of the inphase and quadrature components are indicative of the conductivity thickness of the source. The relative amplitude of the response at different frequencies is a measure of the conductivity. The asymmetry of the shoulders is a measure dip of the source. Depth to the source can be estimated from the amplitude of a response but is more accurately determined by comparing the results between profiles using different coil separations. As a general guideline, this system will detect a vertically oriented plate to a depth of ~ 0.7 x the coil separation and a flat-lying plate to a depth of ~ 0.6 x the coil separation. Absolute measurements of a conductive half-space can be made to a depth of ~ 1.5 x the coil separation. References Click here to learn more about data processing for Electromagnetic Surveys. |
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The UTEM-3 uses a large
fixed horizontal transmitter loop (a varnished single wire) as its signal
source. For typical surveys, the total field (primary and secondary) is
then measured in the near field zone (exterior and interior to the TX
loop) with the receiver system. The vertical component (Hz) of the magnetic
field is always measured, and provisionally the horizontal components
(Hx and Hy) of the magnetic field. Since the measurements are made in
the on-time the electric field components (Ex and Ey) can also be measured.
All UTEM-3 survey results
are presented as profiles. Profiles of UTEM data are not right away easy
to interpret because various styles of reduction, normalization and plotting
of the data are necessary to maximize their use for interpretation.
A wide variety of electromagnetic techniques are used to map conductivity
variations within the earth. Electromagnetic techniques operate in either
the frequency or time domains. In either instance, a time varying magnetic
field is established by passing an electrical current through a coil or
very long wire. This primary field will generate eddy currents in a conductive
medium. These eddy currents will in turn generate a secondary EM field
which is diagnostic of the electrical characteristics of the conductive
medium excited by the primary field. A wide range of frequencies and coil
configurations are available, each with advantages and disadvantages with
respect to the geometry and attitude of the conductors.
