Additional Measurement Algorithms in the Overhauser Magnetometer POS-1

Transcription

1 Additionl Mesurement Algorithms in the Overhuser Mgnetometer POS-1 O.V. Denisov, A.Y. Denisov, V.A. Spunov (QM Lortory of Url Stte Tehnil University, Mir 19, Ekterinurg, , Russi) J.L. Rsson (Royl Meteorologil Institute, B-5670, Doures, Belgium) Introdution The Quntum Mgnetometry Lortory produes the proessor Overhuser mgnetometer POS-1 sine The mgnetometer is preision instrument for mesurements of the geomgneti field modulus with sensitivity up to 0.01 nt t n opertionl yle of 3 s. Besides the field mesurement the POS-1 hs dditionl possiilities for ontinuous monitoring of the ftors ffeting its funtioning. A sttistil nlysis of digitized periods of the nuler preession signl in single mesurement gives with good ury the signl to noise rtio, the level nd hrter of noise nd the dynmis of period (frequeny) hnges. In prtiulr, the knowledge of these prmeters llows one to estimte the devie sensitivity nd the field ehvior during mesurement, in ft to rry out the dditionl ontrol for oth the mgnetometer nd the mient mgneti onditions. Qulity of Mesurement Conditions In modern wek field nuler-preession mgnetometers, lgorithms re used whih trnsform the signl preession frequeny ω to the field modulus B (frequeny-field onversion) y proessing time-series of signl s zero rossing moments. In generl, mesured period is funtion of zero rossing time moments t i : T = F (t,t,...t ), N 0 where N is the numer of reorded zero rossings over mesurement. In presene of noise, t i 0 differs from the proper vlue t i. The stndrd devition (SD) of the field lulted vlue for n undmped signl nd unorrelted Gussin noise is: σ B 1 N σ N t FN = B, T i= 0 t i where σ t is the SD of the zero rossing time moments t i. The SD σ B is n unised prmeter of mesurement qulity under similr onditions. The distriution lw of the lulted field vlue is ssumed norml. In this se, the ove formul llows one to estimte the onfidene intervl under the given reliility index (onfidene proility), i.e. to ssess the dt reliility. The presented lgorithm for the mesurement rndom error estimtion t onfidene proility 0.68 (in ft SD) is relized in the POS-1. This funtion is lled the Qulity of Mesurement Conditions (QMC). At norml onditions nd yle of 3 s the POS-1 stores t lest thousnd of rossings. This ft llows us to relily estimte σ t nd thus to predit the SD σ B for series of the mesurements rried out t similr onditions nd t onstnt geomgneti field. The QMC is determined y the POS-1 t every field mesurement nd is trnsmitted y the RS232-port with the field vlue. This prmeter is intended for the ontinuous ontrol of sensitivity, whih depends on the internl devie noise nd externl ulturl disturnes. The QMC ws experimentlly tested using POS-1 mgnetometer nd POS-2 grdiometer

2 A numer of experiments were rried out t the nturl mgneti field (Arty oservtory), t RMI mgneti field stndrd (Doures oservtory) nd t the QML y the etlon signl trnsmission from the genertor to the sensor hed y mens of urrent loop. The results of the experiments showed tht for the norml noise the QMC prmeter ws in good qulittive greement with the rel mgnetometer sensitivity, unmiguously refleting the noise sitution hnges. Quntittive disgreement ws up to 100% (the QMC predited the est sensitivity). This n proly e explined y not hving inluded the noise orreltion in the QMC formul. However, suh disgreement is eptle for prmeters of this kind. At the Arty oservtory the QMC nd experimentlly otined SD were ompred while the mgneti field vritions were exluded y mens of the POS-2 grdiometer. Fig. 1 shows n exmple of reorded geomgneti field vritions nd field differene for sensors sped 1.8 m prt. The SD over the presented time mounts to nt nd the QMC verges to nt (the disgreement is 44%). Other dt showed the sme greement. Similr results were otined t the QML testing equipment y provoking vritions of genertor etlon frequeny nd signl mplitude orresponding to rel signl/noise vlues nd Gussin noise (for exmple fig. 3). However, when the signl/noise ws pproximtely 5 nd less, the QMC nd SD showed n rupt disgreement proly euse of normlity of zero rossing times t i. Also essentil disgreements were oserved for non- Gussin externl noise (espeilly impulse noise) t ll signl/noise vlues. This is explined y the nture of the QMC, whih is lulted for norml noise only. The striking exmple is POS-1 testing in the Doures stndrd t 3 vlues of stilized mgneti field (20 µt, 50 µt nd 78 µt). The mesurements were mde y the mnul freezing method: the field ws stilized nd then frozen during the polriztion nd frequeny mesurement to prevent mlfuntion of the field stilizer, whih ws pertured y the POS-1 polriztion mgneti field. Aording to testing results t 20 µt SD = nd verge QMC = 0.022, t 50µT SD = 0,075 nd verge QMC = 0.020, t 78µT SD = nd verge QMC = It is the impulse noise disturne tht uses suh essentil disgreement. Fig.1. Nturl geomgneti field B () nd field differene G () for sensors sped 1.8 m. Arty oservtory. One POS sensor: SD = nt, QMC = t, h As result of the QMC funtion disussion it must e noted: 270

3 1. In spite of some quntittive disgreement the QMC funtion reflets dequtely the rel mesurement onditions t norml noise. This prmeter llows one to ontrol the qulity nd dt uthentiity, to estimte the mient noise sitution nd to pre-tune the devie. All stndrd mgnetometers of POS fmily re supplied with this useful QMC funtion. 2. To inrese the QMC funtion ury the development of nother lgorithm is neessry, in prtiulr one, whih tkes into ount the noise orreltion. Time Derivtive Mode The seond feture to e tested is the dditionl funtion Time Derivtive Mode (TDM), whih is in development stge. This funtion is intended for mesurements of mgneti field vritions. Besides the field mesurement, the mgnetometer hs proessing lgorithm for extrting the time derivtive of field modulus (db/dt) in single mesurement. The ide of TDM onsists in using the zero rossing times rry umulted during mesurement time for the lultion of field vritions. This is relized y the lultion of two funtions for verge period determintion y N 1 rossings with dely n (n + N 1 = N): -1-1 ( F (t,t,...t ) F (t,t,...t ) B B D N 0 1 N N n n 1 n N ) t n The N 1 nd n prmeters re hosen suh tht the lgorithm error is minimized. The theoretil estimtions show tht for stndrd POS-1 sensor t mesurement time of 1.5 s (totl yle time is 3 s), the sensitivity (SD) for TDM is up to nt/s. For the lgorithm evlution the stndrd POS ws equipped with the dditionl TDM funtion. The tests were rried out in the nturl geomgneti field of Doures oservtory with n dded swtooth rtifiil field vrition supplied y oil system nd y mens of urrent genertor (see for exmple fig. 2-5). The results showed good greement of the TDM experimentl sensitivity nd theoretil predition. Fig.2. Nturl geomgneti field with dded swtooth rtifiil vrition y the oil system. Doures oservtory. Mesured field modulus B (). Derivtive db/dt determined y two sequentil mesurements (). Derivtive db/dt lulted y TDM (). 271

4 It is to e noted tht for the long-time vrition oservtion the TDM is t disdvntge n reltion to the method using two sequentil mesurements (fig. 2-5, nd ). Theoretilly, for equl yle times, the TDM sensitivity is pproximtely smller y ftor 11 thn the σ[b] = 0.01 QMC = σ[db/dt] = σ TDM [db/dt] = two-point method. If the yle time for TDM is pproximtely 2.6 times s lrge s the yle for the two-point method, the lgorithm sensitivities re lose. In ft, to hieve the sensitivity of the two-point method, speil sensor hed with relxtion time of the working sustne 2.6 times s lrge s for the stndrd POS-1 hed is neessry. However, when ssessing the TDM versus the two-point method, we must tke into ount tht the ltter my e strongly ffeted y lising errors, when one tries to ompute the derivtive for two mesurements widely seprted in time, wheres TDM will orretly estimte the two derivtives. The dvntges of the TDM re the simultneous determintion of the derivtive with the mgneti field mesurement nd orret response of the TDM funtion to rpid vritions with periods of the order of the mesurement time. The estimtion showed tht for the equl time intervl t (on whih derivtive B/ t is lulted) for the oth methods, the TDM funtion hs n dvntge ftor of 1.4. In onlusion it is possile to sum up the TDM nlysis: 1. The TDM funtion showed results in greement with the theoretil preditions. However the sensitivity of the TDM hieved y the stndrd POS-1 sensor is not enough for useful oservtion of stndrd vritions of the geomgneti field. The plnned sensitivity n e hieved y employing of speil sensor hed with long relxtion time of proton signl. 272 Fig.3. Exmple of reords B nd db/dt t simultion of undmped proton signl y the genertor signl. Signl mplitude pproprite to working signl of the POS-1. Mesured field modulus B (). Derivtive db/dt determined y two sequentil mesurements (). Derivtive db/dt lulted y TDM ().

5 2. The TDM funtion ury improvement is possile not only y wy of sensor speiliztion, ut lso t the expense of lgorithm improvement. 3. The TDM is t n dvntge in omprison with the stndrd methods when mesuring rpid vritions (up to 40 %). 4. The TDM funtion n e used s n dditionl ontrol prmeter of externl mgneti onditions for utonomous oservtories with low yle rtes nd for geologil explortions. 5. The development of the TDM nd the use of other estimtions of the rel mgneti sitution re mjor ontriution to the retion of smrt high-preision mgneti equipment 273

6 Fig.4. Simultion of field vrition 0.5 nt/s y the genertor signl, t whih field ws modulted y swtooth lw. Mesured field modulus B (). Derivtive db/dt determined y two sequentil mesurements (). Derivtive db/dt lulted y TDM (). Fig.5. Simultion of field vrition 0.25 nt/s y the genertor signl, t whih field ws modulted y swtooth lw. Mesured field modulus B (). Derivtive db/dt determined y two sequentil mesurements (). Derivtive db/dt lulted y TDM (). 274