FR2531233A1 - Device for detecting and measuring the gradient of a magnetic field. - Google Patents

Abstract

Device for detecting and measuring the gradient of a magnetic field. This device consists of at least two proton-resonating systems 2-3 with free precession of protons, which are placed a predetermined distance apart perpendicular to the terrestrial magnetic field. This device makes it possible, with one measurement, to detect low-intensity magnetic field gradients at a given place.

Description

DEVICE FOR DETECTING AND MEASURING THE GRADIENT OF A

MAGNETIC FIELD.

  As we know, the Earth's magnetic field has many inhomogeneities resulting from the presence of ferromagnetic elements, such as compounds

  based on iron, cobalt, nickel or similar elements

  magnetic, such as oil, gas, liquids in large quantities.

  So far, to measure such inhomogeneous-

  births or gradients, - it was necessary to carry out several magnetic field measurements at different locations A field map was thus created

  magnetic from which we calculated the

magnetic field dients.

  One such method is for example to

  measure the proton resonance frequency of the hy-

  drogen and a computer then maps these magnetic field gradients from the

measurements performed.

  All these methods, although very precise and widely used, in particular for deep detection, that is to say of the order of several hundred

  meters> do, however, have the disadvantage when

  that this is to do shallow depth detection

  deur, that is to say less than five meters, to require heavy, complex equipment, to be long and above all not to give immediate results, because it is necessary

  process the information collected.

  The invention overcomes these drawbacks It relates to a device for detecting and measuring the gradient of a magnetic field which, while being precise for shallow depths, that is to say of the order of five meters, nevertheless fast; direct reading of the gradient, economical and easy to carry out and easy to transport, and which allows in a single measurement to detect low magnetic field gradients

intensity and in a given place.

  -2- We know well the so-called "free proton precession" method in a magnetic field HO after prepolarization in a direction perpendicular to the magnetic field HO by a magnetic field H The magnetic field Hp is usually created by a coil whose axis is perpendicular to the HO field and is traversed by an insentity current I for a time equal to two or three times the relaxation time of the resonant proton A sample of the resonant proton is placed inside the coil The assembly is called "proton resonator system" After switching off

  of field H p the coil then collects an in-

  duction corresponding to the variation of the macroscopic oscillating magnetization of the resonant proton at a frequency of:

T = Y HC,

  in which: T denotes the gyromagnetic ratio of the proton (4258 Hz / gauss),

H O the ambient magnetic field.

  The method according to the invention uses this

  known method of free proton precession.

  The device according to the invention for detecting and measuring the gradient of a magnetic field is characterized in that it comprises: a magnetic field sensor means constituted by

  at least two protonating resonator systems

  ques, each formed by a coil of which

  the axis is perpendicular to the field r Oagnéti-

  than to detect and by a resonant proton sample placed inside said coil, a non-ferromagnetic support on which

  the proton resonator systems are fixed

  nics, at a fixed but adjustable distance, intended to allow the displacement of -3 the assembly on the ground a source of direct electric current connected to the terminals of the coils; a means capable of evacuating the energy accumulated in said coils -

  a reader means intended to collect information

  mations given by the coils, constituted by the tuned amplifying organ resonating on the frequency of the magnetic field, a summing organeede of the signals emitted by each coil,

  a member for detecting the beat of

  quence -; a multiplexer intended to connect the coils

  either on the current source or on the means eva-

cookers or readers.

  In other words, this device for detecting and measuring the gradient of a natural magnetic field or

  artificial, consisting of at least two reso-

  proton nators of free precession of the protons placed at a predetermined distance perpendicular to the

Earth's magnetic field.

  This device operates in the following manner.

  For the convenience of the demonstration, we use a

device with two coils.

  Firstly, called "prepolarization", the coils of each resonator system are prepolarized simultaneously by connecting these coils in parallel or preferably in series, to the electric current source.

  (battery), which then directs the magnetization macros-

copy in the H- field

  p, In a second phase, called "evacuation", by means of the switch, the electrical circuit is cut and the electrical energy accumulated in the coils is evacuated by any known means, such as grounding,

  possibly taking care to recover this energy

in the battery.

  -4- Finally, in a third phase called "detection"

  the terminals of the amplification system coils are switched

  tor and we collect the sum of the two signals

  Induction Optionally, depending on the case, you can

  follow this summation, for example on an oscilloscope. After simultaneous prepolarization, each coil collects a frequent radio wave of frequency: 1 l = (H O + i 1 H 0) for the first coil and

  Q 2 = (H O + i 2 H 0) for the second coil.

  If H + a 1 H and H + -a 2 H denote the field o O 0 H O

  magnetic at the location of each coil, the varia-

  the local magnetic field is

H = 4 1 H L 2 H O

  The signals collected by each coil are

  summed up and we observe a frequency battem & nt

  V. = YA H superimposed on the frequency o = H O o.

  If the field H O is the same in the two coils, we then obtain a conventional precession signal, that is to say sinusoidal, of constant frequency and

  single, amortized according to a time constant T 2 o-

  On the other hand, if the field H O is different at the level

  coils, we then observe a beat whose frée-

  quence is equal to the difference of the field between the two coils Knowing the distance separating these two

  coils, it will be possible to deduce the gra-

say.

  Indeed, we can calculate the difference Al H -

O

  2 H and as already said, knowing the distance separa-

  rant the two coils, we can thus measure the value

absolute of the field gradient.

  The orientation of the gradient is given by the orienta-

  tion of the axis joining the two coils In the earth's magnetic field, the resonance frequency (2000 Hz) z

  is in the audible spectrum If we just want to -

  qualitative detection of the gradient, we perceive the beat If the field gradient is weak, the frequency of the beat is difficult to perceive by ear We then superimpose an audible signal of

  frequency greater than 2000 Hz emitted with each beat.

  The repetition frequency of this signal is proportion-

  field gradient. Advantageously: the wires of the resonator systems are placed

  in series at each end of the non-iron element

romagnetic; -

  each coil has a maximum density of turns in amperes-turns with the highest possible Q quality coefficient and the lowest possible overvoltage coefficient, the system has three coils, placed in series or even in parallel on the non-ferromagnetic carrier element, which makes it possible to detect a particular source, even when it is at an equal distance

  of two coils, since in this case the third bo-

  bine is then at a different distance from this particular source and will then provide the signal; the resonant proton sample placed in the coils is made up

  either with water, possibly addition-

  born from an alcohol such as methanol to avoid freezing during in-country detection

cold, -

  either by benzene which has the advantage of having a fairly long relaxation time

  long, therefore to detect a lower gradient

  ble, but however taking great care to increase the prepolarization time either by gels not modifying the relaxation time of the resonant element, such as for example polyacrylamides the coils of the resonator are either solenoidal, or even better toroldales, which is preferable so as not to be hampered by the orientation of the earth's magnetic field -6- the coils are immersed in the resonant proton sample, so as to recover as much as possible of the polarization field; to avoid artificial disturbances of the

  ambient magnetic field, everything around the systems

  resonator temes is made of non-magnetic material

  the multiplexer jointly forms the eva-

  energy booster allowing for a determined time, either grounding or creating a counter voltage; the beat detection member is formed by an oscilloscope or an oscillograph

  the beat detection device is

  killed by headphones worn by the manipulator which are connected to said coils; it is therefore a very simplified detection system;

  the device further comprises an alarm organ

  connected me to the coils which snaps when one of the signals dampens too quickly and disappears, so that the beat would not occur; in fact, in this case, one is generally on one of the coils in the presence of a very intense gradient which, therefore, very quickly damps the signal induced in this coil and disturbs the measurement; this alarm device can, on request, be sent to one or other of the earphones; this alarm system will be triggered for example in the event that one of the signals has a damping which would be at least twice as short as the normal relaxation time of the protons used

  the beat detection device is

  killed by a system linked to a frequency meter making the difference between the different signals emitted; the measuring devices, the detection devices, the amplifier system and the other elements (alarm etc.) as well as the electrical source, are located as far as possible from the resonator systems, for example 7-

  in a bag placed in the back of the manipulator.

  The manner in which the invention can be realized and the advantages which result therefrom will become more apparent from the

  examples of the following embodiments, given for information only

  dicative and not limiting in support of the appended figures.

  Figure 1 shows in summary section a

  detection device according to the invention; Figures 2 and 3 show the connection diagram of this device respectively during the

  prepolarization phase (figure 2), then

  during the phase of storage of stored energy (Figure 3); FIG. 4 represents the connection diagram of this device during the detection phase;

  Figure 5 shows in summary section another

  detection device according to the invention; Figure 6 shows the connection diagram of this device during the detection phase;

  finally Figure 7 is a summary section of a

  magnet usable for the implementation of the invention.

  Example 1: Reference is made to FIG. Lo The device for detecting and measuring the gradient of the magnetic field comprises: a support bar (1), for example one and a half meters in length in a non-magnetic material such as a

  aluminum alloy duraluminium of 10 mm dia-

  metre; two proton-resonator systems (2) and t (3)

  attached to each end of the bar (1) consisting

  killed respectively by a cylindrical container (4) or (5) in

  transparent or translucent plastic

  cide, for example 10 cm long and 5 cm in diameter, provided with a plug (6) or 4-7)

  for filling with the sample liquid

  lon of resonant proton (8), for example water added with lake or benzene 8 - a coil (9) or (10) solenoid of 8 x 3 cm in diameter with contiguous turns on a

  centimeter thick, surrounded by the li-

  that resonant (8); these coils (9) or (10) placed in series and connected by the connecting wire (11) which runs along the bar (1), are made of a wire of one mm square or rectangular section, copper or aluminum, to reduce the weight of the assembly; a handling handle (12), welded to the center of the bar (1), also made of a non-magnetic material,

  terminated by a handle (13) for handling

  put move of the whole horizontally also

as close to the ground as possible.

  First, the terminals (14-15) of the system

  resonant teme are connected to a light waterproof battery (16) (see figure 2), for example of the cadmium / nickel type of 24 volts and 10 Amps / hour to prepolarize

the two coils (9) and (10).

  At the end of a time at least equal to twice the relaxation time of the resonant liquid, namely for water at 25 ° C. for two seconds and for benzene: 10 seconds, in a second so-called "evacuation" time, by means of the three-way multiplexer (17) which comprises: a first channel for connecting to the battery

(16),

  a second way to connect to the detection organs, a third way to connect to the sensor organs,

  We cut the electrical circuit and we evacuate the energy

  gie accumulated in the coils (9) and (10) by placing their terminals (14) and (15) to ground, -for example by connecting them to the negative of the battery (16) (see

figure 3).

  When all the accumulated electrical energy has 9 - been discharged, that is to say in a period of time less than about a second, the circuit is cut in series of the two coils (9) and (10) to put these here in

  parallel (see figure 4) We place the inputs of cha-

  that coil (9) and (10) to ground and the coils or outputs of these coils (9) and (10) are connected by the

  multiplexer (17) to a differential amplifier ac-

  strung (20) after passing through an alarm device (25-

  26) for example of the voltage comparator type The tuned amplifying member (20) is itself connected to the reading member (21) formed either by an oscillograph

  either by headphones or by frequency-

metre.

  This easily transportable device is suitable for

  specifically for the detection of archaeological sites (archaeomagnetism). In a variant, the assembly (bar 1, handle 12,

  ) is removable to be easily transportable.

  Example 2: (see Figures 5 and 6) Here, the bar (1) carries a third resonator (30) with its coil (31) in series between the other two end coils (9) and (10) and the bar rectilinear handling (12) has been replaced by a U-shaped bar (32) welded to the horizontal bar (1), this bar (32) carrying two handling handles (33)

and (34).

  The first two phases of prepolarization and

  evacuation takes place as before, that is

ie coils in series.

  As before, during the de-

  tection, the coils (9-30 and 10) initially placed in series are cut to be placed parallel (see Figure 6) Thus, during this phase, the inputs of the coils are placed to ground while their outputs are connected respectively : first to the multiplexer (7), then to two tuned amplifiers (35-36) -

  after passage through an alarm device (37, 38, 39) also

  also of the voltage comparator type; two headphones (40-41), which allows you to

stereophonic detection.

  These headphones (40-41) can be replaced as before either by a two-channel oscillograph, or by a two-track oscilloscope or by two frequency meters. This device makes it possible, thanks to the three coils, to detect a signal even when the magnetic source is equidistant from two coils, and thanks to the alarm system, to detect very intense gradients, therefore rapidly absorbed, and also an immediate indication of the

  direction o is the source to be detected.

  In this way, this system can be successfully used to detect an artificial field gradient

  created by ferromagnetic or paramagnetic materials

  ticks at average depths For example, we can detect a magnet placed at four meters while the best comparable devices available today

  hardly allow to go beyond a meter.

Example 3

  For the detection of corsp buried under snow, using a device according to one of the two

previous examples.

  The skier just needs to carry it

  a magnet that will automatically orient in the direction

  north-south with respect to the vertical in order to obtain

the optimum effect.

  In this case, it is desirable that the orientation of the magnet is such that its field is in the same direction as the earth's magnetic field. This is obtained by the effect of gravity. To do this, the magnet is mounted. movable in all directions and its center of gravity is positioned in such a way that - the greatest possible field gradient is obtained in the search zone generally located

above the magnet.

  This magnet (see Figure 6) (50) which has the shape of a truncated sphere, is placed in a spherical bubble (51) of plastic material whose residual volume is filled with oil (52) A dragone (53 ) allows the skier to pass this magnet around his arm under the anorak so as not to be embarrassed during the practice

ski.

  If unfortunately the skier is buried under the snow, the location of the magnet (50) will be precise thanks to the devices according to example 1 or 2 and the

skier can be saved.

  The device according to the invention has many

  significant advantages compared to current comparable devices. One can cite the ease of construction and implementation even by non-specialists;

  the lightness that makes it transportable and self-

  nome the possibility of having an immediate reading and on the spot a very markedly improved sensitivity since, as already said, one can detect objects at depths of the order of five meters against one meter currently with the most efficient systems does that any signal necessarily matches

  at a field gradient, which avoids false detections

  tions: In this way, this device can be used with

  success in detecting ferro or paramagnetic elements

  ticks at shallow depths, such as elements

  natural ': archaeological sites, minerals, detection

ground faults,

  artificial: accidental body or objects buried

  - so much in mud, in sand, in 12 swampy grounds - in ponds or under snow or even bodies or objects buried voluntarily such as boundary stones of ground, buried conduits and their limits, explosive mines, etc. 13 -

Claims (10)

  1 / Device for detecting and measuring the gradient of a magnetic field, characterized in that it comprises a magnetic field sensor means constituted by   * at least two proton-resonator systems   ques (2-3) each formed by a coil (9-10) whose axis is perpendicular to the magnetic field to be detected and by a sample (   resonant proton tillon (8) placed inside   of said coils (9-10), a non-ferromagnetic support on which   the proton resonator systems are fixed   ques (2-3), at a fixed but adjustable distance   ble, intended to allow the movement of the assembly on the ground, a source of direct electric current (16) connected to the terminals (14-15) of the coils (8 9); means (17) capable of evacuating the energy accumulated in said coils (9-10)   a reader means intended to collect information.   tions given by the coils (9-10) constituted by a tuned amplifying member (20,35,36)   resonating on the frequency of the magnetic field   tick, a device for summing the signals emitted by each coil (9-10),   a bat detection member (21,40,41)   frequency a multiplexer (17) intended to connect the coils (9-10) either on the current source (16) or to the spillway or reader means.   2 / Device according to claim 1, characterized in that the coils (8-9) are placed in series at   each end of the support (1).   3 / Device according to claim 2, characterized 14 - in that it comprises three coils (8,9,31) placed in series on the support (1).   4 / Device according to one of claims 1 to   3, characterized in that the proton sample resonates   nant (8) placed in the coils (9,10, 31) is constituted by an element chosen from the group consisting of water, water containing alcohol, benzene and gels which do not modify the relaxation time of the reson element nant (8).   5 / Device according to one of claims 1 to 4,   characterized in that the coils (8,9,30,31) are placed   created perpendicular to the Earth's magnetic field to detect and are toroidal.   6 / Device according to one of claims 1 to 5,   characterized in that the bat detection member   elements (21) and formed by an oscillograph, an oscillos-   cope or headphones or a frequency meter   connected to amplifiers (20,35,36).   7 / Device according to one of claims 1 to 6,   characterized in that it further comprises an alarm member (25-26-3738-39) connected to the coils (8-9,31) which is triggered in the event that one of the signals emitted by the one of the coils (9-10-31) has a damping at least twice as short as the normal relaxation time protons.   8 / Device according to one of claims 1 to 7,   characterized in that the multiplexer (17) forms a joint   energy evacuating system by allowing for a determined time either to ground or by creating a counter tension.   9 / Device according to one of claims 1 to 8,   characterized in that during the detection phase, the coils (9-1031) initially placed in series are   cut to be placed in parallel.   10 / Device according to one of claims 1 to 9,   characterized in that the source of electric current (16), the means (17) able to evacuate the energy, the assembly -   reader: amplifier (20-35-36), the summing organ   tion and the detection unit (21-40-41) and the multi-   plexers (17) are located in a bag placed in the back of the manipulator.