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| Measured
parameters |
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Water
content and mean pore size of layers at
depth
Aquifer parameter evaluation |
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| Measured
parameters |
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Aquifer
parameter evaluation
- Depth and thickness of water layer
- Layer water content (%)
- Type of aquifer : mean pore size |
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| Technical
specifications |
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DC/DC
CONVERTER UNIT
- Power supply: two 12 V batteries (60 Ah
each)
- 6 to 8 hours reading autonomy
- Capacitance: 0.05F
- Outputs: ± 400 V DC ; 0.5 A
- Two converters may be used in parallel.
TRANSMITTER SPECIFICATIONS
- Supplied by one or two DC/DC converters
- Frequency range: 0.8 to 3 kHz
- Maximum outputs: 4000 V, 450 A
- Pulse amplitude and duration: programmable
- Pulse moment: 100 to 18000 A.ms (loop and
frequency dependant)
RECEIVER SPECIFICATIONS
- Band pass filter width: 100 Hz
- Programmable gain: 10^4 to 10^6
- Noise: less than 10 nV / sqrt(Hz)
- A/D converter: 14 bits
- Sampling frequency: four times the Larmor
frequency
- Calibration procedure for phase reference
TUNING UNIT
- Tuning of the loop to the Larmor
precession frequency by capacitors
- Capacitance of 9 to 30 µF with one tuning
unit and up to 60 µF with two tuning units.
TRANSMITTING / RECEIVING LOOP
- Reels of 100 m wire, 10 mm" section
- Six reels for 150 m investigation:
impedance 1.0 ohm, 1.1 mH
- Four reels for 100 m investigation:
impedance 0.6 ohm, 0.7 mH
- Other loop configuration on request
PC COMPUTER
- Control of the whole system: converter,
transmitter, receiver
- Data processing: DFT and weighted stacking
- Data interpretation: 1D inversion
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 NUMIS Plus, Proton magnetic resonance system
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The
The direct groundwater finder geophysical instrument with modular design
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Modular design with higher output power
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High receiver sensitivity
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PC control
The NUMIS-Plus system allows the direct detection of groundwater through measurements of the relaxation magnetic
field produced by Hydrogen protons from groundwater after they were energized by a current into a loop laid on
the ground. It is a modular and more powerful version of the original NUMIS system.
NUMIS-Plus equipment consists of:
• two converter units powered by two 12 V batteries.
• two tuning units for optimizing the excitation energy.
• a transmitter-receiver unit for pulse generation and signal measurement.
• a wire loop used both as a transmitting and a receiving antenna.
• a PC computer for the control of the whole system, and for data processing and interpretation.
PMR for groundwater:
The field application of the Proton Magnetic Resonance (PMR) method is based on a well established theory.
PMR is the only non-invasive method which directly studies groundwater reservoirs from surface measurements.
Principle of the method:
Hydrogen atom nuclei (protons) of water molecules are energized by pulses of alternative current
at the proper frequency (Larmor frequency), transmitted into a loop laid on the ground. The magnetic
field they produce in return is measured and analyzed for various energizing pulse moments (intensity
x duration):
• the static field (Earth s magnetic field) determines the Larmor frequency of the hydrogen protons.
• the dynamic energizing field (loop magnetic field) produces the nutation of the hydrogen protons
magnetic moment: it tilts away from the static field with some angle, while still precessing at the Larmor frequency.
• once the energizing field has been switched off, the protons come back to equilibrium after a
relaxation decay characterized by its initial amplitude and its decay time constant.
• the water content is proportional to the amplitude of the proton response.
• the pore size of the medium (which is linked to the permeability) determines the time constant of this decay response.
• the depth of investigation is determined by the intensity of the energizing pulse.
PMR Field Measurements
• The transmitting antenna consists of a 150 m wide square loop laid on the ground, allowing a depth of investigation of the order of 150 m.
• The Larmor frequency varies between 0.8 and 3.0 kHz depending on the amplitude of the local Earth's magnetic field. The energizing
current in the loop will reach intensities of 300 - 450 A during pulses of a few tens milliseconds.
• The relaxation field of the protons is measured in the same loop, after the energizing current is turned off.
• The voltage measured in the loop is in the order of a few tens to a few thousands nanovolts. Stacking is used to enhance the signal.
• Taking the readings corresponding to a complete PMR sounding with a full set of pulse moments, usually takes less than one hour per station.
NUMIS Data Acquisition Software:
During the acquisition, the operator monitors on the PC screen the signal curve (the envelop of the
proton response, an exponential decay curve), and a noise curve stacked in the same conditions as
the signal curve. The number of stacks to use depends on the signal/noise ratio and has to be set
by the operator according to the local noise level. In case of high noise conditions, an
eight-shape loop can be used to significantly improve the quality of the measurements, although
it reduces the investigation depth.
Information Obtained:
The interpretation of measurements permits to estimate the water content and the mean pore size of each layer at depth.
These parameters are useful to determine the prospects of a groundwater reservoir before drilling.
Basics of proton magnetic resonance sounding
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INTERPRETATION OF PMR DATA
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The PMR theory states that the investigation depth of a measurement varies with the moment of the
excitation pulse (product of the intensity of current at the resonance frequency by the duration of
the pulse). It is therefore possible to sound the ground with PMR surface measurements. Besides,
it can be shown that the decay time constant of the relaxation field is related to the pore size,
which potentially permits distinguishing between pore free water and clay bound water. For interpreting
a PMR sounding, it is assumed that the underground is stratified at the scale of the loop dimensions.
The inversion gives estimates of the water content, the mean pore size and the depth of each layer,
after processing of the raw data for the whole set of pulse moments.
For inverting a set of field data it is first necessary to compute a matrix giving the theoretical
response of thin water layers located at various depths. This matrix will take into account the
general configuration of the measurements: loop dimension, Earth s field inclination, ground resistivity, &
The computation of this matrix may take several hours on a PC but the results will be valid for all the
soundings of a given survey. Then the inversion itself of one set of data will take only a few seconds:
the results can thus be available in the field before moving the equipment to the next site. The
inversion procedure is fully automatic: no initial model is required. The operator has the ability
to manually change the value of the regularization parameter for smoothing or enhancing the
variations of the water content with depth according to the local context (equivalence properties).
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Basics of proton magnetic resonance sounding
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PMR VERSUS OTHER GEOPHYSICAL METHODS
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PMR is a direct method for groundwater detection, as it directly measures the
response of the water itself (hydrogen protons). The more traditional methods
(DC, TDEM,...), are indirect ones, as they measure a physical parameter which is
only indirectly linked to the presence and to the quantity of water : the electrical
resistivity of the layers is a function not only of the porosity (volume of water) but
also of the resistivity of the water ; besides, the formation resistivity is also
influenced by the conductivity of clay which makes the interpretation sometimes complex.
In terms of depth determination, PMR is influenced, as other geophysical methods, by
equivalence rules, due to the fact that it is an integrating method. However, for PMR,
the eigen parameter is the product of the water content by the thickness of the layer,
which means that the total quantity of water is always fairly well determined.
A particularity of PMR is the non linear relationship between the measured signal
and the energizing pulse intensity. This means that doubling the pulse current does
not mean doubling the signal: instead it increases the depth of investigation. On the other hand,
the PMR signal is
linearly related to the water content of the layers, which makes the interpretation quite quick.
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Copyright
© 1998 IRIS Instruments
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