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Electric field measurements in sea and ground:

Lviv Centre of Institute of Space Research, 5-A Naukova St., 290601 Lviv, Ukraine ABSTRACT
The measurements of slow fluctuations of weak electric fields in the conductive media (soil, water, space plasma etc.) is one of the most difficult problems of measurement science. That is because of extremely lowsignal-to-noise ratio, by this both useful and noise In the last line the first term in brackets is the useful signals have practically the same frequency band.
signal Ux and the second one - the noise Un. The Up to now are two possible methodologies of such separation of these values in order to obtain high measurements. First one mostly applied for sea bottom signal/noise ratio is extremely difficult problem when electrometers includes periodic commutation of extremely low frequency fluctuations of electric field measuring electrodes by mechanic switch gating in and are investigated. That is because Un spectrum is mainly out the useful signal on the noise background. Second centered in the same frequency band and its power is one mostly used in field experiments foresees the much higher as this of Ux. So, all skill of the designer of augmentation of signal-to-noise ratio by separation of the device for the Ux measurement in the conductive the measuring electrodes by great distance, so raising media is concentrated upon the methodology of Un the useful signal on almost the same level of electrodes reduction. For each medium these methods are different, noise. The ways to improve the efficiency of both having one common part - to try to decrease as much as approaches are analyzed in the paper.
possible the ϕ value or at least, according to the second bracket in the equation (2), to make them as 1. INTRODUCTION
equal as possible for two electrodes composing the pair.
The measurements of electric fields in the conductive 2. ELECTRIC FIELD MEASUREMENTS IN THE
media (soil, marine water, space plasma) is one of the most complicated metrological problems. Because ofconsiderable conductivity of all these media the values The most complicated is the situation with the E of electric fields are very low and need high sensitivity investigations in marine water. Because of very high of the measuring instrument. Also it is necessary to note conductivity (typically about 3-4 Sim) the E levels are that there is no physical sensors measuring directly the very low - from unites to fractions of microvolts per electric field intensity E. This value is determined only meter, whereas the Un signal is about hundreds of by calculations on the base of mathematical equations, microvolts in the best case. In such situation the only relating the components of the vector E with other possible way to measure Ux is to separate Ux and Un in physical values, the direct measurement of which is external circuit, because as aforesaid not possible to do possible [1]. In most cases following equation, e.g., for it at the output of the measuring instrument. The unique reliable realization of this procedure is mechanicalchopper, which commutates the electrodes in one way to the external source (water) via so called hydrochannels forming the base x and in the secondway separates the electrodes from the external source are electric potentials in the points 1 and 2 [2]. In the first case we have the output signal according along X axis, separated by a distance x.
to the equation (2), in the second one - according to the As far as electric potentials difference has to be determined, there is the simplest way to make it with thehelp of a couple of electrodes, being in direct contact with the medium and placed in points 1 and 2 Subtracting (3) from (2) we get desired value of E (in all respectively. Unfortunately, together with ensuring ofϕ cases it is supposed that the reference base x of the xi sensing, the direct contact with the medium gives measuring instrument is well known).
rise to the side offset contact potential difference Up to now one of the most reliable constructions of between the electrode and the medium or to the own marine electrometer was built by J. Filloux [2] and the potential of the electrode ϕ . Taking this additional results of its application showed that principally the potential into account, we get on the basis of the long-term measurements of electric field in sea water are possible. Since that time some other more or lesssuccessful attempts were made in different countries and manufactured constructions do not differ in principle from that of Filloux. Main problem what was more or less successfully competed was to provide good and reliable contact of mechanical chopper to the frame in closed position in order to provide full length of the base x and low resistance between electrodes in open
position in order to separate them from external signal.
A new principle of chopper construction, eliminating this problem, was developed in FSU first inNovossibirsk [3], then by the IZMIRAN group [4].
Instead of tight closing of electrode chamber this 3. ELECTRIC FIELD MEASUREMENTS IN THE
construction is principally unhermetized. The modulation of the input signal is executed by changingthe resistance R of the path between electrodes: in The very long period E measurements (about hours and «open» position R is much less than the resistance of days) are most often executed in the soil for the purpose of deep sounding of the Earth’s crust. The best solution c and in the «closed» position R has to n impact decreasing was the x base be achieved as much as 99%. Such system was elongation for hundreds meters for field systems and constructed and showed good results. Its main about few kilometers for stationary ones. By this Ux disadvantages are principally short span in order to term managed to reach big enough values whereas Un term has to be not influenced by the distance between c and the necessity of mechanical driver.
All the disadvantages of the mechanical device with electrodes. Practically it is not so because of different moving parts under the high pressure and in such soil, humidity, temperature etc. at so large distances, aggressive medium as marine water are clear.
what leads to the considerable raise of Un.
Numerous attempts were made to use the electronic So, to have the electrode line length reasonably small commutator instead of mechanic one. One of the (about 100 m or less) can be recommended. By this the proposed solution was the use of so called ring magnetic requirements to have as low as possible electrode drift modulator as contactless electrometer [5] which reacts or the differential drift of electrode pair become very on the current density flowing through the inner hole.
Unfortunately, theoretical limit of sensitivity of the In spite of some progress in the electrodes construction device doesn’t allow to get the resolution better than their drift still remains much greater than useful signal.
unites of mkV/m with reasonable size of this modulator.
The application of special PC-based methodology of Another attempt was to replace the mechanical matching pairs selection, which requires a set of commutator by the electronic one [6]. Its principle was laboratory experiments, allows to reduce considerably to use additional pair of electrodes for periodic shielding the differential drift. The efficiency of this methodology of measuring electrodes from the signal. The is illustrated by Table 2. Here the results of the choice investigation showed that unfortunately there is a of 4 matching pairs from 14 electrodes are presented, by deadlock solution because by no way the possible this in column a) the drift data for selected matched number of equations could be made equal to the number pairs are given and in column b) the data for the same electrodes but randomly combined. The selection was For the moment all activity in this branch is made basing on 20-days laboratory tests of the whole concentrated on the chopper construction improvement and the input amplifier development [7]. In the last direction new version was created, based upon theAD204JN insulation amplifier. The comparative set of amplifier’s parameters for marine electrometer are Table 1. Low noise amplifier with galvanic insulation So, it is evident that this methodology allows to selectmatched pairs with enough low differential drift.
But still more important question is the methodology of electrodes conservation and installation which has to be directed to the maintaining of achieved low drift level.
Best way to stock the electrodes between operationperiods is to keep selected pairs in the pure solution of The problem discussed in this paper still is a open the salt with the same radical as manufactured question in metrological practice. Also the calibration of electrodes. In our case for Cu-CuSO4 electrodes CuSO4 electric field meters, about what was nothing solution is used. Then following proceeding by mentioned, is may be the most difficult task, solved very poorly. Nevertheless the importance to study • Look for the place for electrode installation with experimentally the electric fields forwards still new preferably clayey ground and dig the hole up to the attempts in this branch and more or less reliable wet ground, but not shallower than 70 cm.
experimental results are from time to time reported. The • Make a deepening in the bottom with about the same task of the authors was to show that the methods and diameter as electrode and put there the electrode - devices described in the paper do can provide the electric field measurement both in the sea and in the • Fill the hole bottom by preliminary prepared ground with enough high for practice quality.
suspension till the sensitive part will be covered by3-5 cm layer, wait about 10-20 minutes and bury the REFERENCES
To make intimately the suspension is also very [1] P. Sopruniuk, S. Klimov, V. Korepanov, Electric important. It has to be made from the ground taken from fields in space plasma (in Russian), Naukova dumka, nearest proximity to the electrode and CuSO conserving solution and carefully stirred in plastic [2] J. Filloux, «Instrumentation and experimental methods for oceanic studies», Geomagnetism, Vol.1, A good indicator of properly placed electrode pair is its Acad. Press, USA, pp. 143-248, 1987.
low output resistance which has to be around1000±500 Ohm. With time this value can increase up to [3] V. Selatitsky et al., USSR Patent N 996957, 2-3 kOhm and if the output resistance becomes greater than 5 kOhm then the electrodes have to be reinstalled.
[4] M. Bogorodsky et al., USSR Patent N 1594453, Certainly the output resistance measurement method in no way has not to allow the loading of electrodes by [5] R. Berkman and V. Korepanov, IIS-81 Conference external currents or their short-circuiting. Many on information systems. Part II, Lviv, pp. 77-78, possibilities to realize this method in given restrictions Fig. 1. Experimental data of electric field measurements.
LCISR has already long practice to manufacture and to use Cu-CuSO4 electrodes in field practice, mainly for [7] V. Korepanov and A. Marussenkov, Electric field magnetotelluric study. An example of field measurements at sea bottom, in: XXI General experimental data obtained during summer 1998 Assembly of JUGG, Boulder, Colorado, p B114, campaign in Estonia is given on Fig. 1. The data was not subjected to any processing and both low level ofmeasured signals and practical absence of long-termdrift are clearly seen.
[6] V. Korepanov, USSR Patent n 1442942, Cl.G01R


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