- Email: [email protected]
A recording system for the Earth's telluric field with either analog or numerical output
Physics of the Earth and Planetary Interiors, 8 (1974) 19—22 © North-Holland Publishing Company, Amsterdam — Printed in The Netherlands
A RECORDING SYSTEM FOR THE EARTH’S TELLURIC FIELD WITH EITHER ANALOG OR NUMERICAL OUTPUT* G. SIMON and J.C. ROSSIGNOL Institut de Physique du Globe de Paris, Université Paris VI (France) Accepted for publication May 22, 1973. We describe a telluric recording system which gives a record with high sensitivity and fidelity of the earth’s telluric field over short distances. This system, which has been tested many times during the last two years, uses active filtering to suppress natural or artificial noise from the phenomenon considered.
Telluric measurements are more difficult to obtain than magnetic ones, chiefly because of the low level of the signal to be recorded and its high sensitivity to artificial perturbations. The system we describe here (see Fig. 1) enables us to obtain analog telluric records as well as numerical ones directly in field experiments. This flexible systern has been experimented with for two years, either for observatory measurements (Chambon-la-Forêt), or for field experiments at about twenty stations over the Central Massif and Pyrenees.
1. Sensors At the present time there exist different types of nonpolarising electrodes, e.g. Cu—SO4Cu, which are used by prospectors, Cd—SO4 Cd and Pb. There are also the Meunier-type electrodes made of chlorinated silver into potassium or sodium chloride with which the authors are most familiar, We have tested these electrodes in the laboratory several times and noticed that the stabilization time is very short (some hours) and that temperature variation affects them only slightly (some iV/°C).The optimal length of the measurement lines for this type of electrode, taking into account the whole system, is
*This paper was presented at the I.kG.A. Workshop on Electromagnetic Induction, held at the University of Edinburgh, 20—27 September, 1972. Contribution I.P.G.P. No. 66.
between 50 and lOOm. With these lines, we obtained a signal/noise value of about 50 dB for average longperiod telluric activity. This length was defined from field studies of different parameters which are either physical or chemical and which depend on: (1) the electrodes (natural noise, temperature coefficient, PH of the measurement point); (2) the measurement system (natural noise of the electronic components); and (3) the conditions of the experiment: then, while the length of the line increases, the electrofiltration signal/natural telluric signal can increase too because the difference of altitude between the points of measurement usually gets more important as well. Moreover, we must notice that long lines are more often mechanically cut than shorter ones and also they are more liable to wind effects in case of temporary systems. It was found in some cases, after tests with the electrodes, that it is possible to obtain significant measurements with lines of only 15 meters. This has to be considered with Filoux’s results (1973) which are obtained in oceans with the same type of electrodes and with measurement lines of only 2 meters which give significant results.
2. Measurements As the measurement system must not disturb the phenomenon which has to be measured, it is necessary for the impedance of the measurement to be as great as possible compared with that of the elec-
G. Simon and .1. C. Rossignol, A recording system for the earth ‘s tel/uric field
20
DIGITAL OUTPUT A
r-ANALOG
I
OUTPUT
ELECTRODE AgCI—CINa
(Meunier ?yp)
~H
~
Fig. 1. Simplified scheme for the proposed system. It can be seen that the input signal (with lightning protection), has been obtained by compensation of the natural potential between the electrodes; it is then amplified and filtered either statically or numerically (or both) and can be recorded either on digital or analog equipment.
trodes. For this reason, we have put an impedance adaptation floor of 1 M~Zin the circuit while the impedance of the electrodes is only 2 k~2.This systern offers also other advantages such as the reduction of the possibility of polarization of the electrodes and, in the same way, the drift of electrodes whatever recording system has been chosen. Lastly, we notice that in these conditions, the scale value is constant because resistivity variations of the soil-electrodes system do not interfere with the measurement. Another stage of the measurement is to compensate for the natural potential which can exist between the two points of measurement. As a result we have obtained a high-fidelity system which permits us to obtain a long-time compensation of the potential without any noticeable variations (the long-term stability of this system is about 100 jiV/km). A signal is then obtained which is very easy to measure and without any long-term drift; however, does it only represent the natural signal or does it contain any artificial ones? In industrially active regions, there never exist completely natural signals, and thus it is necessary to filter the measurements,
the earth’s telluric field will get scarcer because of industrialization. If we want to measure the earth’s natural telluric field, it is now necessary to filter. There are two possibilities that we have tested, namely, passive and active filtering (see Fig. 2). These two types of filtering offer different possibilities, so it is now necessary to distinguish between them. 3.1. Passive filtering From the technological point of view, these are easier to achieve and cheaper than active filtering systems. On the other hand, they are fully efficient only for artificial perturbations of short periods. This system, the rejection of which is very high, can be used efficiently for 50 Hz and its harmonics. Thus we can prevent the amplification system described in the next section from being saturated, since it is not so efficient near saturation, when distortion of the output signal occurs. Conventionally we have admitted a high cut-off of 3 Hz for this type of filter. Thus we avoid the 50 Hz and its harmonics and we are also certain to have the opportunity, if only this type of filter is applied, to work directly with a 1 Hz phenomenon from the numerical records.
3. FWtel-ing 3.2. Active filtering Already nowadays, and more frequently in the future, the points where it will be possible to measure
Passive filters cannot be used when the period
G. Simon and J.C. Rossignol, A recording system for the earth’s tel/uric field
21
41 ~b’
~
STATICALLY
DY NA M ~ALLY
FILTERED
FILTERED
Fig. 2. Examples of natural short-period noise and artificial noise on telluric records. The upper part shows a statically filtered (3 Hz) record. Short periods always exist which make the digitization difficult when we have only analog records. The lower part shows a dynamically filtered (0.01 Hz) record. This record is easier to digitize and it is also very easy to distinguish the artificial perturbations because their continuous component is preserved.
becomes longer. If we want to obtain a high-order response (i.e. 4th order) we then use filters with a high-time response and a high fidelity during a long time. OUTPUT
/ / / /
I
.°°
~
Fig. 3. Response curve of a dynamic filter as a function of the period. This response curve is to be compared with that of a short-period static one.
Such a filtering system presents the following advantages: (1) The trend of the cutting power is very high (see Fig. 3) because it is of the 4th order. The existence of the 4th-order response also permits us to obtain very inexpensive filters with current electronic components. (2) Another advantage which cannot be omitted if we consider this type of filter is that we can obtain a very long time constant. So we can select the periods which are particularly interesting (i.e., diurnal variations and its harmonics with a filtering system which cut periods of 20 or 30 mm). Thus we can obtain recording with a high dynamic range for diurnal varia(3) With this type of filter, it is easier to digitize the analog records when we only have analog recordings. Moreover, if we have numerical recordings, we can obtain pass band frequency records in the same way as we can obtain them with high-frequency passive filters for quick magneto-telluric recording.
22
G. Simon and J. C. Rossignol, A recording system for the earth ~stelluric field
4. Amplification and recording The signal output of the filtering system is linearly amplified 200 times, so we can obtain very high sensitivity recordings. The signal at the output of our system can be recorded in two ways. An analog recorder can be used, the advantage of which is to visualize the phenomena but for which, on the other hand, the disadvantages are to reduce the dynamic range and to increase the difficulty of interpretation in consequence of the transformation from analog to digital. This type of recording is very often used for the study of phenomena whose period is longer than 1 minute and which can be recorded on either potentiometric or galvano. metric recording systems. Afterwards, it is of course necessary to convert the analog signal into a numerical one with an analog to digital converter, such as DEMAC. The second method makes use of magnetic analog tapes, which enable us to pass from 8 Hz. From our point of view, this is of considerable interest because of its price and of the opportunities it presents for improvement of the dynamic range. The price of this system is no higher than the previous one. Last, it is possible to convert the analog magnetic tapes into numerical ones with a numerical system. Lastly, the most expensive system, but one which provides the greatest dynamic range, is the direct numerical recording system. With this system, the magnetic tape recordings are immediately compatible with IBM computers.
5.
Protection against lightning
The last experimental problem concerning telluric measurements is protection against lightning. This
protection appears at two levels, namely at the input signal and at the power supply. The protection against lightning strike can be achieved if we put into circuits some triacs. Their purpose is to follow the lightning front so that they permit to melt a fuse. It is a very easy protection but a very efficient one when one wants to work in mountainous regions. Protection of the recording system is more difficult to ensure. This system can be really efficient only if the power supply system is perfectly independent of the current distribution which are often, after all in particularly mountainous regions, liable to lightning. From our point of view, the only efficient system is to generate alternating current using batteries and an oscillator. The batteries will be charged by a small generator. On the other hand, this system presents a disadvantage in that more manpower is needed.
Conclusion The general scheme of the system as it is presented (Fig. 1) with its filtering system (Fig. 2) enables accurate earth telluric records of a high fidelity to be obtained. We think that the system here described will thus contribute to a better knowledge of the earth’s electromagnetic field. Its very low price would allow us to obtain telluric measurements in greater number all over the earth before industrialization impedes them.
Reference Filloux, J.H., 1973. Phys. Earth Planet. Inter., 7: 323.
A RECORDING SYSTEM FOR THE EARTH’S TELLURIC FIELD WITH EITHER ANALOG OR NUMERICAL OUTPUT* G. SIMON and J.C. ROSSIGNOL Institut de Physique du Globe de Paris, Université Paris VI (France) Accepted for publication May 22, 1973. We describe a telluric recording system which gives a record with high sensitivity and fidelity of the earth’s telluric field over short distances. This system, which has been tested many times during the last two years, uses active filtering to suppress natural or artificial noise from the phenomenon considered.
Telluric measurements are more difficult to obtain than magnetic ones, chiefly because of the low level of the signal to be recorded and its high sensitivity to artificial perturbations. The system we describe here (see Fig. 1) enables us to obtain analog telluric records as well as numerical ones directly in field experiments. This flexible systern has been experimented with for two years, either for observatory measurements (Chambon-la-Forêt), or for field experiments at about twenty stations over the Central Massif and Pyrenees.
1. Sensors At the present time there exist different types of nonpolarising electrodes, e.g. Cu—SO4Cu, which are used by prospectors, Cd—SO4 Cd and Pb. There are also the Meunier-type electrodes made of chlorinated silver into potassium or sodium chloride with which the authors are most familiar, We have tested these electrodes in the laboratory several times and noticed that the stabilization time is very short (some hours) and that temperature variation affects them only slightly (some iV/°C).The optimal length of the measurement lines for this type of electrode, taking into account the whole system, is
*This paper was presented at the I.kG.A. Workshop on Electromagnetic Induction, held at the University of Edinburgh, 20—27 September, 1972. Contribution I.P.G.P. No. 66.
between 50 and lOOm. With these lines, we obtained a signal/noise value of about 50 dB for average longperiod telluric activity. This length was defined from field studies of different parameters which are either physical or chemical and which depend on: (1) the electrodes (natural noise, temperature coefficient, PH of the measurement point); (2) the measurement system (natural noise of the electronic components); and (3) the conditions of the experiment: then, while the length of the line increases, the electrofiltration signal/natural telluric signal can increase too because the difference of altitude between the points of measurement usually gets more important as well. Moreover, we must notice that long lines are more often mechanically cut than shorter ones and also they are more liable to wind effects in case of temporary systems. It was found in some cases, after tests with the electrodes, that it is possible to obtain significant measurements with lines of only 15 meters. This has to be considered with Filoux’s results (1973) which are obtained in oceans with the same type of electrodes and with measurement lines of only 2 meters which give significant results.
2. Measurements As the measurement system must not disturb the phenomenon which has to be measured, it is necessary for the impedance of the measurement to be as great as possible compared with that of the elec-
G. Simon and .1. C. Rossignol, A recording system for the earth ‘s tel/uric field
20
DIGITAL OUTPUT A
r-ANALOG
I
OUTPUT
ELECTRODE AgCI—CINa
(Meunier ?yp)
~H
~
Fig. 1. Simplified scheme for the proposed system. It can be seen that the input signal (with lightning protection), has been obtained by compensation of the natural potential between the electrodes; it is then amplified and filtered either statically or numerically (or both) and can be recorded either on digital or analog equipment.
trodes. For this reason, we have put an impedance adaptation floor of 1 M~Zin the circuit while the impedance of the electrodes is only 2 k~2.This systern offers also other advantages such as the reduction of the possibility of polarization of the electrodes and, in the same way, the drift of electrodes whatever recording system has been chosen. Lastly, we notice that in these conditions, the scale value is constant because resistivity variations of the soil-electrodes system do not interfere with the measurement. Another stage of the measurement is to compensate for the natural potential which can exist between the two points of measurement. As a result we have obtained a high-fidelity system which permits us to obtain a long-time compensation of the potential without any noticeable variations (the long-term stability of this system is about 100 jiV/km). A signal is then obtained which is very easy to measure and without any long-term drift; however, does it only represent the natural signal or does it contain any artificial ones? In industrially active regions, there never exist completely natural signals, and thus it is necessary to filter the measurements,
the earth’s telluric field will get scarcer because of industrialization. If we want to measure the earth’s natural telluric field, it is now necessary to filter. There are two possibilities that we have tested, namely, passive and active filtering (see Fig. 2). These two types of filtering offer different possibilities, so it is now necessary to distinguish between them. 3.1. Passive filtering From the technological point of view, these are easier to achieve and cheaper than active filtering systems. On the other hand, they are fully efficient only for artificial perturbations of short periods. This system, the rejection of which is very high, can be used efficiently for 50 Hz and its harmonics. Thus we can prevent the amplification system described in the next section from being saturated, since it is not so efficient near saturation, when distortion of the output signal occurs. Conventionally we have admitted a high cut-off of 3 Hz for this type of filter. Thus we avoid the 50 Hz and its harmonics and we are also certain to have the opportunity, if only this type of filter is applied, to work directly with a 1 Hz phenomenon from the numerical records.
3. FWtel-ing 3.2. Active filtering Already nowadays, and more frequently in the future, the points where it will be possible to measure
Passive filters cannot be used when the period
G. Simon and J.C. Rossignol, A recording system for the earth’s tel/uric field
21
41 ~b’
~
STATICALLY
DY NA M ~ALLY
FILTERED
FILTERED
Fig. 2. Examples of natural short-period noise and artificial noise on telluric records. The upper part shows a statically filtered (3 Hz) record. Short periods always exist which make the digitization difficult when we have only analog records. The lower part shows a dynamically filtered (0.01 Hz) record. This record is easier to digitize and it is also very easy to distinguish the artificial perturbations because their continuous component is preserved.
becomes longer. If we want to obtain a high-order response (i.e. 4th order) we then use filters with a high-time response and a high fidelity during a long time. OUTPUT
/ / / /
I
.°°
~
Fig. 3. Response curve of a dynamic filter as a function of the period. This response curve is to be compared with that of a short-period static one.
Such a filtering system presents the following advantages: (1) The trend of the cutting power is very high (see Fig. 3) because it is of the 4th order. The existence of the 4th-order response also permits us to obtain very inexpensive filters with current electronic components. (2) Another advantage which cannot be omitted if we consider this type of filter is that we can obtain a very long time constant. So we can select the periods which are particularly interesting (i.e., diurnal variations and its harmonics with a filtering system which cut periods of 20 or 30 mm). Thus we can obtain recording with a high dynamic range for diurnal varia(3) With this type of filter, it is easier to digitize the analog records when we only have analog recordings. Moreover, if we have numerical recordings, we can obtain pass band frequency records in the same way as we can obtain them with high-frequency passive filters for quick magneto-telluric recording.
22
G. Simon and J. C. Rossignol, A recording system for the earth ~stelluric field
4. Amplification and recording The signal output of the filtering system is linearly amplified 200 times, so we can obtain very high sensitivity recordings. The signal at the output of our system can be recorded in two ways. An analog recorder can be used, the advantage of which is to visualize the phenomena but for which, on the other hand, the disadvantages are to reduce the dynamic range and to increase the difficulty of interpretation in consequence of the transformation from analog to digital. This type of recording is very often used for the study of phenomena whose period is longer than 1 minute and which can be recorded on either potentiometric or galvano. metric recording systems. Afterwards, it is of course necessary to convert the analog signal into a numerical one with an analog to digital converter, such as DEMAC. The second method makes use of magnetic analog tapes, which enable us to pass from 8 Hz. From our point of view, this is of considerable interest because of its price and of the opportunities it presents for improvement of the dynamic range. The price of this system is no higher than the previous one. Last, it is possible to convert the analog magnetic tapes into numerical ones with a numerical system. Lastly, the most expensive system, but one which provides the greatest dynamic range, is the direct numerical recording system. With this system, the magnetic tape recordings are immediately compatible with IBM computers.
5.
Protection against lightning
The last experimental problem concerning telluric measurements is protection against lightning. This
protection appears at two levels, namely at the input signal and at the power supply. The protection against lightning strike can be achieved if we put into circuits some triacs. Their purpose is to follow the lightning front so that they permit to melt a fuse. It is a very easy protection but a very efficient one when one wants to work in mountainous regions. Protection of the recording system is more difficult to ensure. This system can be really efficient only if the power supply system is perfectly independent of the current distribution which are often, after all in particularly mountainous regions, liable to lightning. From our point of view, the only efficient system is to generate alternating current using batteries and an oscillator. The batteries will be charged by a small generator. On the other hand, this system presents a disadvantage in that more manpower is needed.
Conclusion The general scheme of the system as it is presented (Fig. 1) with its filtering system (Fig. 2) enables accurate earth telluric records of a high fidelity to be obtained. We think that the system here described will thus contribute to a better knowledge of the earth’s electromagnetic field. Its very low price would allow us to obtain telluric measurements in greater number all over the earth before industrialization impedes them.
Reference Filloux, J.H., 1973. Phys. Earth Planet. Inter., 7: 323.