Report on Time Domain Electromagnetic (TEM) system for remote sensing of

blockmindlessΠολεοδομικά Έργα

16 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

54 εμφανίσεις

Report on Time Domain Electromagnetic (TEM) system for remote sensing of

Uranium deposits

Electronics & Instrumentation Division, IGCAR is developing an eddy current based
time domain Electromagnetic survey system. This system will be used by AMDE&R for
e
xploration of Uranium. It was proposed to develop the system in three stages along with EM
simulation.

Theory of Operation

The TEM system consists of a transmitter and a receiver coil assembly. The system
employs an on
-
off transmitting current that prod
uces a non
-
continuous primary field. The
transmitter excitation current pulse is periodic with a well defined on and off time. During the
on time a steady current (approx. 300A) is caused to flow for a sufficient time after which the
transmitter current is

terminated in a controlled fashion. In accordance with the Faradays law,
rapid reduction of the transmitter current and also of the primary magnetic field induces an
emf in the nearby subsurface conductor (survey target). The magnitude of emf induced is
Eddy currents

Conductor

Receiver

Transmitter

Primary field

Secondary field

0A

300A

0A

300A


s


s


ms

Transmitter excitation Current


ms

OFF_time

ON_time

p
roportional to the time rate of change of primary magnetic field at the conductor. This emf
causes eddy currents to flow in the conductor with a characteristic decay, whose time
constant is a function of the conductivity and geometry of the survey target.
The decaying
currents generate a proportional secondary magnetic field, the time rate of change of which is
measured by the receiver coil as induced voltage during off time. Analysis of the nature of
the transient decay is carried out by sampling the rece
iver amplitude at numerous intervals of
time. From circuit theory consideration the subsurface target is represented as a wire loop
with lumped resistance( R ) and inductance( L ). The receiver coil output voltage (
e
0
) is
proportional to the time rate of c
hange of secondary magnetic field and is of the form,


e
0



⠱(⼠

⤠e



(t /


,
where



= L/R is the characteristic time constant of the target

It is evident from this expression that conductive targets, having small value of resistance and
hence large
value of


yield signals with small initial amplitude that decays relatively slowly
with progress of time. Signals from poorly conducting targets that have large resistance value
and small


have high initial amplitude but decay rapidly with time. The time

sampled voltage
when fed to interpretation algorithms gives information about the survey target conductivity
and geometry.

EM simulation

Simulation studies are being carried out using FEM based EM modeling software package
Magnet
-
3D as per the Earth mode
l specified by the AMDE&R. Composition of the earth,
conductivity & depth of different layers, excitation current, size of the air
-
box are some of the
parameters analysed during the simulation.

Stage
-
0 Experiments & Simulations

To verify the principles of
TEM (eddy currents and the associated secondary fields) an in
-
house setup was developed. The system configuration is given below,

Transmitter geometry

Radius: 12.5cm

Current: 3 ampere

Number of turns: 5

Transmitter waveform

Unipolar trapezoidal with 21.5us

fall time with 25Hz. repetition or base frequency (25%
duty cycle, ontime: 10ms,offtime: 30ms)

Receiver geometry

Number of turns: 1000, 2000

Radius: 1.25cm

Receiver electronics

Preamplifier gain: 1000

Oscilloscope

Experimental setup and results

The setup
consists of the TX coil hung from a support with concentric receiver coil. The
output of the receiver coil was given to the preamplifier, which in turn was connected to a
digital storage oscilloscope. The response plots are shown below along with the simul
ation
results carried out using electromagnetic modelling software. The target used were thin
aluminum sheets


Project with IITM, Chennai

The design and fabrication for
Stage
-
I

(1/10
th

of the Stage
-
III final version) involving
the following

sub
-
systems were carried out in collaboration with IITM, Chennai (a)
Simulation& Modeling Studies (b) Transmitter and Receiver coils (c) An electronically
controlled high current source. In addition, IITM will provide the design for Stage II and Stage
-
II
I.

Hardware Development and Experiments of Stage
-
I

Transmitter coil of size 1.25 m radius was fabricated and Power electronics meant of Stage
-
I
was developed. Several runs of experiments with varying specimens have been conducted


Without Target

Cylindrical

Rectangular

Without Target

Cylindrical

Rectangular

Stage
-
1 Experiments & Si
mulations

A transmitter current source of 30 ampere and a transmitter
-
receiver coil assembly was
fabricated at IIT chennai. The system configuration is given below,

Transmitter geometry

Radius: 1.25m

Current: 30 ampere

Number of turns: 3

Transmitter wavefo
rm

Unipolar trapezoidal with 250us fall time with 25Hz. repetition frequency (50%duty
cycle, ontime: 20ms,offtime: 20ms)

Receiver geometry

Number of turns: 100,300,500

Radius: 12.5cm

Receiver electronics

Preamplifier gain: 2000

Oscilloscope cum data record
er

Experimental setup and results

The setup consists of a transmitter coil hung from a cantilever support with a concentric
receiver coil. The output of the receiver coil was given to the preamplifier, which in turn was
connected to a data recorder cum dig
ital storage oscilloscope. In this setup the cantilever
support had a small pulley through which the distance between the coil and target was varied
and receiver voltage was recorded at different height. The response of earth surface was also
measured at t
wo different heights. The experimental setup and results are shown below

Transmitter and receiver coil assembly along with the target



Voltage induced in the receiver coil with and without Aluminum target at two different
heights












Voltage in
duced in the receiver coil indicating earth response

Conclusion

Practical results have confirmed that

1) The conductive target exhibits its characteristic longer decay compared to free space
response.

2) Decay amplitude decreases with increase in separati
on between target and transmitter.

Results were also verified with simulation.