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NUMISPlus schematic diagram |
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 atoms 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 magneto field they produce in return is measured and analyzed for various energizing pulse moments (intensity x duration).
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.
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The static field B, (Earth’s magnetic field) determines the Larmor frequency of the H protons: F(Hz) = 0.04258' Bo (nT)
The dynamic energizing field B, (loop magnetic field) produces the nutation of the H protons magnetic moment M, @ it @its away from the static field with an angle 0, while still processing at ft Larmor frequency.
Once the energizing field has been switched off, the protons come back to equilibrium (Mo aligned with Bo) after a relaxation dewy characterized by an initial amplitude Eo, and a time constant T2*.
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 dewy 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 thou- sands 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.
INTERPRETATION OF PMR DATA
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 to distinguish 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 possibility 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).
PMR VERSUS OTHER GEOPHYSICAL METHODS
PMR is a direct method for groundwater detection, as it directly measures the response of the water itself (H 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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NUMISPLUS MAIN FEATURES
NUMISPLUS is a modular equipment designed to allow measurements at remote locations, as each component of the system weighs 25 kg or less, making it one man portable. The Tx/Rx unit is the core of the system. It ensures the production of the energizing pulses at the Larmor frequency, also the measurement of the PMR response with filtering, amplification and analog to digital conversion. The PC computer receives raw data, then process, display and store them for further interpretation. The two DC/DC converter units are required for a maximum investigation depth of 150 m), to energize the 150 m side square loop (600 m total length). However, if an investigation of 100 m is sufficient one converter unit only is required with a I 00 m side square loop (400 m total length) The two tuning units must be used at lower magnetic latitudes (for an Earth’s field lower than 31 000 nT with the 150 m side square loop, or 37 000 nT with the 100 m side square loop), while one tuning unit only is needed at medium and higher latitudes.
NUMISPLUS
DC/DC CONVERTER UNIT
Power supply: two 12 V batteries (60 Ah each)
6 to 8 hours reading autonomy
Capacitance: 0.05F
Outputs: d: 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 dependent)
RECEIVER SPECIFICATIONS
Band pass filter width: 100 Hz
Programmable gain: 104 to 106
Noise: less than 10 nV / sqrt(Hz)
AID 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 uF with one tuning unit and up to 60 uF with two tuning units.
TRANSMITTING / RECEIVING LOOP
Reels of 100 m wire, 10 mm2 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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Description |
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Order Number |
| NUMIS plus | 175-850-0211b |
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