Theodolite-borne Vector Overhauser Magnetometer: DIMOVER

Transcription

1 heodolite-borne Vector Overhauser Magnetometer: DIMOVER Sapunov V *, Denisov A, Saveliev D, Kiselev S, Denisova O, Podmogov Y Khomutov S 3, Rasson J 4 Quantum Magnetometry Laboratory of USU, Mira str., 9, Ekaterinburg, 6, Russia Amakinsky Expedition, ALROSA Ltd., Saha, Mirnisky region, Ajhal, Judgnaja str.,, 6789, Russia 3 Geophysical Observatory "Klyuchi", Koptyug av., 3, Novosibirsk 639, Russia 4 Institut Royal Meteorologique, Centre de Physique du Globe -567 Dourbes Viroinval, elgique sva@dpt.ustu.ru (Vladimir Sapunov Abstract his report covers results of the uncompleted long-term research directed at developing an absolute vector proton magnetometer based on the switching of bias magnetic fields. he distinctive feature is the attempt of the installation of a miniature Overhauser sensor and optimized Garret solenoid directly on the telescope of the theodolite that, in our opinion, will allow this design (Declination Inclination Modulus Overhauser magnetometer: DIMOVER to be comparable and even better to the universally recognised DIflux device. Preliminary results, which also can be interesting to the experts in vector proton magnetometers, are presented.. rief review of modern proton component magnetometers he vector proton magnetometers were developed practically simultaneously with the availability of the method of nuclear magnetic resonance and are know since about 6 years. he basic drawback of this type of magnetometers was the significant size of the magnetic systems (up to one meter, that is caused by the high requirements for low gradient of the bias fields and the significant sizes of the proton sensors. Figure (a shows an example of such system. In recent years, the development of proton vector magnetometers achieved essential progress due to the based on application of the Overhauser effect that provide stronger proton signal from smaller sensor size. his allows to reduce dimensions of the bias systems and as consequence, allows to improve the sensitivity of measurements. he greatest successes in design and introduction in observatories practice have been achieved by in joint project of GEM Systems Inc., Eotvos Lorand Institute, and Mingeo. he Figure (b shows bias system (9 rings on sphere 3 cm, and one of its co-authors L. Hegymegi. Zhirov G. and Pack V. made the similar development in Kazakhstan and Russia in 993 on the basis of the raunbeck coil (magnetometer MK-, size of magnetic system 8 cm. with the use of the Overhauser sensor OS- developed by Quantum Magnetometry Laboratory of USU. his system was successfully installed and tested during the 994 Observatory Workshop in Dourbes (Figure, c. a b c Figure. Examples of the bias magnetic systems for proton and Overhauser didd magnetometers 59

2 he Overhauser vector magnetometers are used for stable registration of the variations of the components and modulus of the Earth's magnetic field. One of the basic problems is the stability of installation of the magnetic system on the pillar in the observatory or the support in field conditions, which can cause baseline drifts. o solve this task, taking into account of small size of the Overhauser vector systems, GEM System has applied a proven way of vertical suspension. he device is shown in the Figure (a. Size of spherical magnetic system is cm, perpendicular rings (development of the udapest Geophysics Institute. he drift is claimed to be below. n/ C and a long term stability of the geomagnetic component measurement less than n/year at sensitivity up to. n is announced. a b c Figure. Designs of the vector scalar magnetometers (a suspended didd Overhauser GEM magnetometer, b theodolite-borne proton MAGSON magnetometer, c DIMOVER of QMLab he general property of the above-mentioned devices is that they are variometers of the geomagnetic field angles (didd. Presently, the measurement of the absolute values of the field vector or angles I and D is carried out the only by the "DIflux" method that is used in almost all observatories together with a separate proton magnetometer. Some years ago the firm MAGSON has made a development, which tries to unite the advantages of the DIflux method and proton vector magnetometer (Figure (b shows this device. he Garret solenoid and proton sensor is mounted directly on the frame of a nonmagnetic theodolite. We analyzed this design and it has been discussed with the designers of the Russian theodolites. hey have explained that the prisms, which provide measurement of azimuth and vertical angles, are mounted directly in the telescope and that we can benefit from the special systems of self-compensation, in particular of vertical angle index. hus it is desirable to mount our sensor (Overhauser + Garret coil directly on the telescope to meet exact the angular disposition and compensation that is actually used in the DIflux method. Addressing the problems stated in the previous paragraph, the Quantum Magnetometry Laboratory of USU has tried to establish a bias magnetic system and Overhauser sensor directly on the telescope of a non-magnetic theodolite. he Figure (c gives a view on the prototype model (he Overhauser sensor is taken from the solenoid and is shown on the foreground. he development of the given design has required the decision of a number of theoretical and experimental tasks whose results are presented in the report. It is necessary to mention that as scalar magnetic sensors besides proton magnetometers, there is opportunity to use quantum devices based on optical orientation of atoms. In this direction there are some problems, in particular the Cesium sensors have less perspectives taking into account their higher absolute errors. Potassium sensors developed under the direction of the academician E. Alexandrov in State Optical Institute (Russia and GEM Systems is most interesting. he given device has subpicotesla sensitivity at sampling rate up to measurements per second and an absolute accuracy of the field intensity about. n. he drawback of the Potassium sensor, which may limit its use, is the large size of the sensor, the large power consumption about Watts and the limited long-term reliability. While the Overhauser 6

3 magnetometers are characterized by power about Ws at the smaller speed ( 6 sec and sensitivity of. n to. n.. Method of measurement based on switched bias field and analysis of errors o measure the components of the geomagnetic field by a proton (scalar magnetometer a number of methods are known. We investigate below the set-up based on the switching of bias magnetic fields (cycle +I, I, I = with the measurement of resulting total field. he vector circuit for a switching method (Figure 3 and formula ( for calculation of the field component along bias field are shown: Figure 3. Vector circuit for a switching method Z =. ( ( + It is a well-known formula, but, unfortunately, it gives practically no real information to choose the value and direction of the bias field. Nor does it give the effect of the selected parameters upon the resulting random and systematic measurements errors on Z. We give below such analysis... Sensitivity or random error of the component measurement Using standard methods of error computation in the linear approximation (error is a lot of less of measurement value, the error on the measurement Z can be expressed as: Z Z Z σ ( Z = ( ( σ ( σ + σ +. ( y differentiating in ( and by substituting, we get: Z α = + = ( ( σ ( Z = σ ( + ( + 3 8( σ ( + (3 + 4 σ (. (3 Let's assume identical sensitivity for all measurements of the module field that is σ(т = σ(т = σ(т = σ(т. he value of dispersion of Z component measurement will look as follows 4 ( + ( cos α + ( ( 3cos α σ ( Z = σ ( 4. (4 Figure 4 shows σ(z/σ(: the deterioration of sensitivity of the Z component magnetometer in comparison with sensitivity of the scalar magnetometer. he ratio of the geomagnetic field module to value of the bias field / at various angles α between them are varied. he sensitivity of measurement of components is the best at α =, Т /В =.5 and the loss of sensitivity σ(z/σ( is.58. his optimum corresponds to a direction of the bias fields parallel to the Earth's magnetic field and its value should be approximately double. Unfortunately, this case is not interesting in practice, because the increase of the bias field causes an increase of its gradient that will ask too high requirements from the bias magnetic system. Also, there is a restriction on the bias field amplitude due to the dynamic range of the proton magnetometer (usually n. More interesting is the intermediate case, when the sensitivity of component measurements does not depend on the orientation of the bias magnetic field and total intensity. his bias field should be approximately 7 % of the geomagnetic field, with a factor 3 in the deterioration of sensitivity. 6

4 3 / f π/6 α π/3 π/ Figure 4. An inevitable loss (contoured of sensitivity when vector measurements are made in comparison with a scalar magnetometer.. Some systematic errors of the component measurement o analyze systematic errors caused by the various reasons it is convenient to express that the bias field are and β. he value Z in the switching set-up in this case is: ( β ( β β ( β Z =. (5 β( β he systematic error of the measurement is defined as follows Z = Z Z where Z is the measured component of the field, Z is the true component.... Errors caused by "soft" and "hard" magnetisation of the module sensors Errors caused by the "soft" magnetisation of the magnetometer sensors are defined as = k, = k, = k. Using the formula (5 it is possible to draw a simple but important conclusion: Z k = kz. hus the error on a component measurement caused by the "soft" magnetisation of its absolute magnetometer sensor is fully eliminated by the calibration of the module sensor. he error due to the hard magnetisation of the module sensor means, that the sensor has its own magnetic moment due to some internal magnetic field: = + h where is a result of real measurement, is a true vector without the account of a field h from a magnetic impurity. he final expression of the "hard" error will be Z h = h cosθ, where θ is angle between the bias field vectors and internal field h. hus: he systematic error of component measurement caused by the "hard" errors of the module sensors depends only on the mutual angle of the internal proton sensor s "hard" field and bias field. his does not depend on orientation of Earth's magnetic field. his error can be determined for some orientation of the vector magnetometer with the Earth's field and then be used as a correction at any field angles D and I.his error can be even nulled for a special orientation of the proton sensors to bias field (cosθ =. 6

5 ... Error caused by instability of the bias field or the power source his is a technically important kind of the error since it formulates the requirements to the stability of the magnetic bias system and source of the switched current. We assume, that the fields +В and В differ, i.e. in the formula (5 factors β = β. We find the error caused by the instability of the bias field or the power source as: ( ( cos α =.5 ( ( α Z β =.5( I I cos. (6 Let's estimate the requirements on stability for the bias field or I/I in conditions close to optimum (Т /В about.5, about 3 and setting error Z n. Using formula 5 the I/I will be about 4. hus requirements to short-term stability of the current and bias field are engineering-executed and we have errors of. n for instability 5 4. It is important to note that the long-term stability does not exert influence in the switching method in contrast to a method of the field components compensation..3. Error caused by the drift of the geomagnetic field It was revealed at experimental research of breadboard models of the Overhauser vector magnetometers in laboratory conditions, which were characterized by fast changes of the geomagnetic field, that the switching method has a dynamic error. Variations of the measured component exceed the real variations. he noticed error grew at increase of speed of variations. A physical explanation is obvious enough. he basic formula for calculating the components makes the assumption of field stability. Estimations of this important kind of a dynamic error are submitted below. o estimate the error caused by a field variation, the jump function model of a variation was simulated. Namely: at the first measurement the geomagnetic field is displaced by h, in the second there is no displacement and in the third the field is displaced by h. A vector proton magnetometer will calculate the Z component of field according to the basic equation (. he error is defined as the difference between the measured and calculated component without the drift h, that is Z = Z Z Z = Z = 3 ( + [ ][ h] h. (7 he analysis of (7 has shown that the drift error is the display of dynamic cross effect: here is an influence (error at the presence of variations of perpendicular components. he error is proportional to speed of a variation to be exact is proportional to change of a field during a cycle of measurement components ( the quantized time derivative of field. Figure 5. Error caused by the drift of the geomagnetic field as result of dynamic cross effect of the proton vector magnetometers (he cycle at calculations is chosen 3 seconds 63

6 Figure 5 shows an example of a computer calculation of the drift error. he top figure is a simulated Z variation. he bias magnetic system is perpendicular and the measurement of H-component is made along to bias field. he variation in H direction is absent. he bottom demonstrates this error. Some discrepancy of variation measurements by the fluxgate and the proton vector magnetometers thus should be observed. Obviously that the drift error can be excluded by some modernization of the basic formula ( or at processing measurements taking into account speed of the components variation. 3. readboard model of the PC-based vector POS- magnetometer he PC-based vector POS- magnetometer was created on the initial stage of our researches to check experimentally of the some theoretical estimation stated above. he raunbeck coils were used as magnetic bias system ( 8 cm. he experimental research has shown the validity of the formulas presented above and of the drift error caused by fast variations in particular. Unfortunately because of volume limitation we address the readers to where the complete text of our poster IAGA 4 report is submitted and where new results in this direction will also be presented. 4. heodolite-borne vector Overhauser magnetometer he device is based on the standard switching circuit of bias magnetic fields (cycle + J, J, J = with the measurement of resulting total field. he distinctive feature is the installation of a magnetic bias system and Overhauser sensor directly on the telescope of the theodolite. Such design provides measurement of the absolute value of a magnetic field component along the telescope optical axis with an accuracy limited potentially only by the accuracy of the theodolite, and also the total geomagnetic field. We name the developed device and principle "DIMOVER" for "Declination Inclination Modulus Overhauser magnetometer" in analogy with the "DIFLUX" which is used presently in almost all magnetic observatories for absolute measurements together with a separate proton magnetometer. he use of the small Overhauser sensor and optimized Garret solenoid gave the physical base for the installation of the vector sensor directly on the theodolite telescope. he usual theodolite 3KP of the Ural Optical & Mechanical Plant (Ekaterinburg, Russia was demagnetized and certificated then as a DIflux magnetometer at Geophysical observatory "Klyuchi" (Novosibirsk, Russia. he Figure (c gives a view on the prototype model of DIMOVER. he electronics of the standard POS- magnetometer is used and an additional block for the power supply of the magnetic system is added. he magnetic system is a solenoid with size 55 5 mm supported by a titanium pipe (the electroconductivity of titanium does not influence on polarization current switching. he internal compensating part of the Garret solenoid was optimized for the maximal time of free precession decay. In spite of the fact that the proposed DIMOVER does not require calibration by external means, there are errors caused by the theodolite magnetic properties and with the solenoid magnetic axis discrepancy ( 3 n depending of orientation. It is supposed that the proposed device will find applications for geological surveys of kimberlite in complex magnetic fields connected with trap formations. here is a modification of the Z-magnetometer without theodolite, which is based on a vertically positioned solenoid so as to speed up the prospecting. We hope that the basic application of the DIMOVER (device or its design can be useful in magnetic observatories and field surveys because: Co-location of the measurement of D, I and that will allow to exclude the d pillar correction Only one device must be transported and powered which is important for field operations 5. Conclusion We hope the brief review of the proton vector magnetometers is interesting and useful from the point of view of development of these devices. We present the analysis of some random and systematic errors in the method of measuring a component by the absolute magnetometer and also the optimization of the bias field value. In particular the drift error caused by dynamic cross effect is shown. Unfortunately, on the one hand, the presented theodolite-borne vector Overhauser magnetometer has a number of shortcomings now; namely its low speed (6 9 seconds and low sensitivity of components measurement (up to.5 n though the total field sensitivity reaches. n. On the other hand we hope that in this case a number of problems on the long-term stability of the measurements and the automation of the base line control can be innovatively resolved. 64