GB1594632A

GB1594632A - Device and process for direct and continuous receiving and measuring of electrical magnetic and aciustic signals

Info

Publication number: GB1594632A
Application number: GB4271077A
Authority: GB
Original assignee: Societe National Elf Aquitaine; Societe Nationale Elf Aquitaine Production SA
Current assignee: Societe National Elf Aquitaine; Societe Nationale Elf Aquitaine Production SA
Priority date: 1977-10-13
Filing date: 1977-10-13
Publication date: 1981-08-05

Description

(54) DEVICE AND PROCESS FOR DIRECT AND CONTINUOUS RECEIVING AND MEASURING OF ELECTRICAL MAGNETIC AND ACOUSTIC SIGNALS (71) We, SOCIETE NATIONALE ELF AQUITAINE (PRODUCTION), a French body corporate of France, of Tour Aquitaine, La Defense, 92 Courbevoie, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention is concerned with a direct and continuous receiving and measuring device for electrical, magnetic and acoustic signals and more particularly is adapted to reception or measurement of electro-magnetic natural waves or waves emitted by any type of emitter at a given power and emitted at a given frequency, for example, between 10 Hz and 250 KHz.

It is well known that the artificial electrical currents produced by emitters arranged above ground, such as radio station transmitters or machines moving in air or in water, give rise to telluric currents in the earth's crust which add to the natural currents which, by nature, are random, i.e. they have incessant variations in their direction, sense and their intensity.

The flow of these telluric currents in the ground depends on the structure of the subsoil, i.e. in terms of the resistivity of the rocks or materials of the subsoil.

Moreover it has been shown that the current density decreases exponentially as a function of the depth as the waves are propagated in the subsoil.

Thus exact measurement of the electrical and magnetic components of a given frequency wave being propagated in the subsoil makes it possible to follow the development and passage of the wave in the subsoil and then to know the allocation and distribution of the different materials constituting the subsoil. It is. equally possible to deduce the depth of penetration of this same wave therefrom.

But these continuous varying waves are at relatively low frequencies, generally between 10 Hz and 250 KHz for electrical signal amplitudes as low as 0.04 ,uV/m.

Thus the receivers used until now are not adapted to this dynamic range of frequencies or to signals of such a low amplitude.

In fact, devices comprising direct amplification of the signal picked up, filtering and a system of measurement may be conceived. But these known devices operate on discrete frequencies i.e. preselected frequencies and one is compelled to use a filter according to frequency to be received and this considerably limits the frequencies which may be used because the number of filters cannot be increased in an inconsidered manner without consequently increasing the dimension of the receiver and its cost of production.

Moreover, it is difficult to make a narrow filter which will be stable when the temperature varies between -15" and +50", particularly at low frequency.

Finally, it may be understood that a device constructed as a function of discrete frequencies cannot operate for the frequencies situated outside the predetermined values.

Of course there are receiving devices operating at high frequency or at the very least at frequencies of the order of megahertz and in which there is continuous scanning and measurement of a relatively large band of frequencies. Nevertheless the high-frequency filters used are pass-band filters and the variations in the overvoltage coefficient owing to the variations in temperature cause a variation in the gain on the one hand and a variation in the central frequency of the band on the other hand. It is thus that a change of 1 Hz, which is negligible when working at high frequency becomes unsuitable at very low frequencies because, for a frequency of 10 hz for the signals which are of interest, there would be the risk of adjusting the filter to 9 or 11 Hz which would find expression in non-reception of the signals and large errors in the amplitudes and phases of the signals measured.

The present invention seeks to mitigate the disadvantages mentioned above and to provide a direct and continuous receiving and measuring device for natural or artificial signals of any type, especially electrical signals, magnetic signals or acoustic signals.

According to a first aspect of the invention, there is provided a direct and continuous receiving and measuring device for electrical magnetic or acoustic signals in which the frequency spectrum at the input of the device is divided into a number of low frequency decades, the device comprising means for picking up the signals, a receiver at the input including an amplifier stage, a broadband filtering stage, a local oscillator for supplying a reference signal, a mixer to which the local oscillator and the broadband filtering stage are connected and a lowfrequency filtering stage connected to the mixer and comprising low frequency filters each allocated to one of the decades and a measuring stage for processing the low-frequency filtered signals to provide the desired measurement.

Further to this aspect of the invention, there is provided a direct and continuous receiving and measuring device for electrical. magnetic or acoustic signals in which the frequency spectrum at the input of the device is divided into a number of low frequency decades, the device comprising an input amplifier stage feeding at least one broadband filtering stage, a mixer connected to the broadband filtering stage and to a local oscillator, a low-frequency filtering stage connected to the output of the mixer and comprising low frequency filters each allocated to one of the decades and a measuring stage connected to the low-frequency filtering stage.

According to a second aspect of the invention, there is provided a process for measuring electrical magnetic or acoustic signals, comprising picking up the signals and receiving them in a receiver, subjecting the signals to high frequency filtering, transposing the signals into low-frequency signals, selecting desired frequency decades of the input signals by means of individual filters, each allocated to one of the decade, and processing the low-frequency filtered signals to provide the desired measurement.

The low-frequency filtering stage is necv ary to enable working at lowfrequency and to make it possible to study relatively low-frequency phenomena, such as the natural electromagnetic waves being propagated in the earth's subsoil.

The low-frequency filters allocated to the decades suitably operate in a narrow band.

The local oscillator may be of the type comprising a quartz oscillator connected to a first loop comprising a phase comparator, a voltage controlled oscillator and a divider in series, the said local oscillator being characterised in that a second loop is connected to the output of the voltage controlled oscillator, the second loop comprising at least a second phase comparator and a second voltagecontrolled oscillator the squared wave signal output of which is applied to a divider connected to the second phase comparator.

Thus the local oscillator, may have a sinusoidal signal at its output, this signal retaining it linear characteristics and having a distortion rate less than 3; so as not to be face to face with harmonic pulsation at lower levels. Moreover, relatively reduced response times can be obtained so as to reduce the measurement time.

The frequency spectrum available at the input to be divided into a certain number of low-frequency decades, for example five, each of the low-frequency filters, operating in narrow band and being allocated to one of the decades, which may operate in narrow or normal band, each low-frequency filter defining a narrow band for its own decade which is the normal band for the preceding decade.

Thus all the frequencies of the spectrum may be scanned continuously and it may be determined if the selected and shown frequency is present at the input of the device. Confirmation of the identity between the frequency shown and the frequency supposed to exist at the input may be carried out by means of a verifier switch.

In a particular application of the device in accordance with the invention, it may be used to measure distribution of the impedance of a medium. According to this aspect of the invention, there is provided a measuring process for measuring the distribution of the impedance of a medium, comprising receiving on a receiver natural or artificial waves propagating themselves in the medium and detected by pick-ups, dividing the signals at the input into a number of frequency decades, selecting the desired frequency decades by means of individual filters after transposition to low frequency each filter being allocated to one of the decades, and processing the low-frequency filtered signals to measure the output voltage of each pick-up and the phase difference between any two of the pick-ups by averaging out amplitudes and phases.

In magnetotelluric terms, the natural or artificial waves are electromagnetic by nature and are used to determine the distribution of the resistivity of the subsoil.

The invention will now be described in greater detail, by way of example, with reference to the drawings, in which: Figure 1 is a schematic block diagram giving an overall view of different constituent elements of a device according to the invention, and Figure 2 is a block diagram of a local oscillator in accordance with the invention.

In the drawings, a device according to the invention is shown which comprises a symmetical or an asymmetrical input amplifier depending on the pick-ups used, the inputs of which are connected to the pick-ups which have as their function to detect the particularly electrical, magnetic or accoustic signals which are being propagated at the surface of the soil, the said signals having a frequency varying in any band, for example, between 10 Hz and 250 KHz. In general, adaptable impedance amplifiers known per se and not shown are placed between the pick-ups designated in their entirety by the reference 2, made up of a receiving antenna and the differential amplifier 1.

Rejectors 3 which elimate the continuous components of the signals, such as the 50 Hz and 60 Hz frequencies and their third harmonics, i.e. 150 Hz and 180 Hz, are located at the output of the inDut amplifier 1.

The frequency band 10 Hz-250 KHz in which we are interested is divided into five frequency decades, for example 10 Hz-99.9 Hz, 100 Hz-999 Hz, 1000 Hz9990 Hz; 10 KHz-99.9 KHz, and 100 KHz-250 KHz. In order to eliminate the undesirable frequencies when one is operating in one of the above ranges, highfrequency filters 4 which are five in number, the first filter 4, covering at least 10 Hz--100 Hz, the second covering 100 Hz--1000 Hz and so on, are arranged at the output of the rejectors 3.

A selector shown schematically and designated by the reference 5 is placed between the chain of filters 4 connected in parallel and a mixer 6. A local oscillator 7, whose schematic detail is shown in Figure 2, supplies a signal at a frequency f0 which is applied to the input of the mixer 6 which receives a reference signal on its other input at a frequency fe which is one of the frequencies available at the input of the input amplifier 1. The mixer 6 is in fact an analog multiplier which carries out multiplication of the signal supplied by the local oscillator 7 and representative of the frequency f0 with the signal which has come from the selector 5 at the frequency 1,. At the output of the mixer 6 a signal at the frequency f,=f,-f, is obtained which is of the type 1 Ve sin wetxVo sin wot.

This formular may again be written in the form:

Thus at the output of the mixer a signal is obtained of an amplitude proportional to the input voltage Ve while V0 is a constant because the local oscillator 7 supplies a constant amplitude whatever its oscillation frequency.

The frequencies fe being able to vary in the previously determined decades, five parallel low-pass filters 81 to 85 are connected to the mixer 6.

In each decade of preselected frequencies two selectivities or working bands may be obtained (narrow selectivity or normal selectivity) in accordance with the table below:

Selectivities Dects Frequencies Narrow Normal x ~ 1 10 Hz--99 9 Hz 0.2 Hz 2 Hz 2 100 Hz--999 Hz 2 Hz 20 Hz 3 1000 Hz--9990 Hz 20 Hz 200 Hz 4 10 KHz--99 9 KHz 200 Hz 2 KHz 5 100 KHz-250 KHz 2 KHz I It is evident from this table that the receiver can chose between two emitters the frequencies of which are very close.

If the frequency to be isolated is 300 Hz, for example, the filter 82 allotted to the decade 2 can operate between 299 Hz-301 Hz for the narrow selectivity and between 290 and 310 Hz for the normal selectivity while obviously not taking into account the reflected frequency. If there is another emitter of the frequency 295 Hz, only the narrow selectivity makes it possible to isolate the frequency of 300 Hz because, for normal selectivity there is superposition of the two frequencies 300 Hz and 295 Hz.

The choice of the decade to be used is a function of the frequency to be isolated. This selection is carried out by means of another selector with the reference 9 which is connected to an amplifier 10, the output of which is connected to a measurement apparatus 11 of the type comprising a detector without a threshold so as to be linear, and an integrator constituted by a low-pass filter, this is in order to have a voltage proportional to the mean value of the amplitude of the input signal and constituted by a needle or digital indicator or both.

The local oscillator 7, Figure 2, comprises a quartz oscillator 12 the frequency of which is the reference frequency, a divider 13 by 104 which applies a reference frequency to the input of a phase comparator 14 which is equal for example to 100 Hz if the frequency of the quartz oscillator is 1 MHz.

A loop comprising a low-pass filter 15, a voltage controlled oscillator 16 and a divider by n 17 which is programmable is connected to this comprator 14, the lowpass filter 15 having to supply, at the output, a continuous loop error voltage.

If the divider by n is between 100 and 999, then the frequency supplied by the oscillator 16 is between 10 KHz and 99.9 KHz.

A second loop is connected to the first and also comprises a phase comparator 18 connected to the oscillator 16 via a divider by 103 19, a low-pass filter 20, a voltage controlled oscillator 21, the squared wave output of which is connected to a divider by 10N 22, N being between 0 and 4 in accordance with the selected decade.

In addition, the oscillator 21 supplies a sinusoidal signal at constant amplitude, said sinusoidal signal being applied to the mixer 6.

From the above, it can be seen that the local oscillator should supply a signal of the frequency f0=fe-f1 where fe is the frequency of the input signal and fj is the intermediate frequency and is equal to half the cut off frequency of the low-pass filter which has been automatically selected. After filtering, only the first term is obtained in f,-f, and one is able to see that two fe are possible for the same fo and the same fj, i.e.

fet=fO+fj or fe2=fO f,.

The receiver does not know how to isolate the reflected frequency f07f, of the actual frequency fO+fl. To this end the device comprises an automatic verifier switch 23 which shifts the frequency of the local oscillator 7 by 2fj. If, when verifying, reception is not modified, then this is because the actual frequency corresponding to a frequency shown f, was obtained.

If reception is modified, it is necessary to lower the frequency shown by 2 fl. In order to do this a decimal coded binary adder-subtractor 24 is used one input of which receives the frequency fa shown and the other fi which is a function of the chosen selectivity and of the type of signal received, according to whether they are artificial or natural.

The device previously described can operate in accordance with a search mode or a synthesized mode which implies a generator synthesizer made up of the local oscillator of the device.

For natural signals which give a continuous audible tone in the two first decades, a frequency foe which has been preselected for example 328 Hz is displayed on the control synthesizer of the local oscillator, said frequency being transmitted to the divider 17 by means of the decimal coded binary adder subtractor 24. In narrow selectivity and for the second decade, the value of fi is 1 Hz from where it follows that the divider n is equal to 327. The phase comparator 14 is thus acted on at one input by a frequency of 100 Hz and at the other input by another frequency of 100 Hz for a quartz frequency of 1 MHz and a value of 104 shown at the divider 13. In order to determine whether the shown frequency is the reflected frequency or the actual frequency the verifier 23 is switched, one position of which corresponds to the value fe-f and the other to the value fe+fi. If in the two positions of the switch 23 the indication remains the same then the frequency shown is the actual frequency, this frequency being present at the input. If on the contrary the indication is modified, the frequency shown is reduced by a value equal to 2 fj so as to recentre on the filter cut off frequency and eventually the verification is restarted for the purpose of verification. The gain values are then controlled so as to be outside saturation. These controls having been carried out, the signal or signals supplied by the low-frequency filtering stage 8 are amplified by the amplifier 10 and then converted into a voltage, possibly rectified and integrated in an interval of time which is a function of the decade in which the shown frequency fa is contained.

Digital voltmeters incorporated in the measuring stage show the voltage at the end of integration which is related back to the output voltage of the pick-ups i.e. a voltage which takes the different fixed or variable gains affecting the integrated signal into consideration.

For the radio signals it is possible to operate directly by the synthesized method as described previously when the frequency to be shown corresponds to that from a known frequency transmitter.

When the frequency from the transmitter is not known, the search mode is used at normal selectivity. In order to do this a frequency selector is activated until a continuous audible tone has been obtained on earphones associated with a voltage-frequency converter, the audible tone corresponding to a frequency which has been derived. Then the synthesized mode is used by setting the frequency thus derived on the synthesizer.

Another advantage of the device in accordance with the invention is to permit calibration. To this end, equal frequencies with respect to 25 Hz for decade 1, 250 Hz for decade 2, 2500 Hz for decade 3, 25 KHz for decade 4 and 250 KHz for decade 5 are set on the synthesizer.

A calibration output connected to the synthesizer is connected in parallel to each of the input amplifiers. For appropriate gain control, the digital voltmeters receiving the integrated signals must supply a voltage equal to 800 uV+ 1 dB if the device is correctly controlled and the relationship between the derived voltages at the input of two of the amplifiers should be close to 0.4 dB in this case.

It will, of course, be appreciated that the meaning device and processes described can be used for measuring the distribution of impedance in a medium. To this end, the process would generally comprise receiving on a receiver natural or artificial waves propagating themselves in the medium and detected by pick-ups, dividing the signals at the input into a number of frequency decades, selecting the desired frequency decades by means of individual filters after transposition to low frequency each filter being allocated to one of the decades and processing the lowfrequency filtered signals to measure the output voltage of each pick-up and the phase difference between any two of the pick-ups be averaging out amplitudes and phases. The natural artificial waves are electromagnetic by nature and the lowfrequency filtered signals are representative of electrical and magnetic components and are processed to determine the distribution of the resistivities of subsoil.

WHAT WE CLAIM IS: 1. A direct and continuous receiving and measuring device for electrical magnetic or acoustic signals in which the frequency spectrum at the input of the device is divided into a number of low frequency decades, the device comprising means for picking up the signals, a receiver at the input including an amplifier stage, a broadband filtering stage, a local oscillator for supplying a reference signal, a mixer to which the local oscillator and the broadband filtering stage are connected and a low-frequency filtering stage connected to the mixer and comprising low frequency filters each allocated to one of the decades and a measuring stage for processing the low-frequency filtered signals to provide the desired measurement.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

on the control synthesizer of the local oscillator, said frequency being transmitted to the divider 17 by means of the decimal coded binary adder subtractor 24. In narrow selectivity and for the second decade, the value of fi is 1 Hz from where it follows that the divider n is equal to 327. The phase comparator 14 is thus acted on at one input by a frequency of 100 Hz and at the other input by another frequency of 100 Hz for a quartz frequency of 1 MHz and a value of 104 shown at the divider 13. In order to determine whether the shown frequency is the reflected frequency or the actual frequency the verifier 23 is switched, one position of which corresponds to the value fe-f and the other to the value fe+fi. If in the two positions of the switch 23 the indication remains the same then the frequency shown is the actual frequency, this frequency being present at the input. If on the contrary the indication is modified, the frequency shown is reduced by a value equal to 2 fj so as to recentre on the filter cut off frequency and eventually the verification is restarted for the purpose of verification. The gain values are then controlled so as to be outside saturation. These controls having been carried out, the signal or signals supplied by the low-frequency filtering stage 8 are amplified by the amplifier 10 and then converted into a voltage, possibly rectified and integrated in an interval of time which is a function of the decade in which the shown frequency fa is contained.

Digital voltmeters incorporated in the measuring stage show the voltage at the end of integration which is related back to the output voltage of the pick-ups i.e. a voltage which takes the different fixed or variable gains affecting the integrated signal into consideration.

For the radio signals it is possible to operate directly by the synthesized method as described previously when the frequency to be shown corresponds to that from a known frequency transmitter.

When the frequency from the transmitter is not known, the search mode is used at normal selectivity. In order to do this a frequency selector is activated until a continuous audible tone has been obtained on earphones associated with a voltage-frequency converter, the audible tone corresponding to a frequency which has been derived. Then the synthesized mode is used by setting the frequency thus derived on the synthesizer.

Another advantage of the device in accordance with the invention is to permit calibration. To this end, equal frequencies with respect to 25 Hz for decade 1, 250 Hz for decade 2, 2500 Hz for decade 3, 25 KHz for decade 4 and 250 KHz for decade 5 are set on the synthesizer.

A calibration output connected to the synthesizer is connected in parallel to each of the input amplifiers. For appropriate gain control, the digital voltmeters receiving the integrated signals must supply a voltage equal to 800 uV+ 1 dB if the device is correctly controlled and the relationship between the derived voltages at the input of two of the amplifiers should be close to 0.4 dB in this case.

It will, of course, be appreciated that the meaning device and processes described can be used for measuring the distribution of impedance in a medium. To this end, the process would generally comprise receiving on a receiver natural or artificial waves propagating themselves in the medium and detected by pick-ups, dividing the signals at the input into a number of frequency decades, selecting the desired frequency decades by means of individual filters after transposition to low frequency each filter being allocated to one of the decades and processing the lowfrequency filtered signals to measure the output voltage of each pick-up and the phase difference between any two of the pick-ups be averaging out amplitudes and phases. The natural artificial waves are electromagnetic by nature and the lowfrequency filtered signals are representative of electrical and magnetic components and are processed to determine the distribution of the resistivities of subsoil.

WHAT WE CLAIM IS: 1. A direct and continuous receiving and measuring device for electrical magnetic or acoustic signals in which the frequency spectrum at the input of the device is divided into a number of low frequency decades, the device comprising means for picking up the signals, a receiver at the input including an amplifier stage, a broadband filtering stage, a local oscillator for supplying a reference signal, a mixer to which the local oscillator and the broadband filtering stage are connected and a low-frequency filtering stage connected to the mixer and comprising low frequency filters each allocated to one of the decades and a measuring stage for processing the low-frequency filtered signals to provide the desired measurement.
2. A direct and continuous receiving and measuring device for electrical,

magnetic or acoustic signals in which the frequency spectrum at the input of the device is divided into a number of low frequency decades, the device comprising an input amplifier stage feeding at least one broadband filtering stage, a mixer connected to the broadband filtering stage and to a local oscillator, a lowfrequency filtering stage connected to the output of the mixer and comprising low frequency filters each allocated to one of the decades and a measuring stage connected to the low-frequency filtering stage.
3. A device as claimed in Claim I or 2, wherein the low-frequency filters operate with a narrow band.
4. A device as claimed in Claim 1,2 or 3, wherein the local oscillator comprises a quartz oscillator connected to a first loop comprising a phase comparator, a voltage controlled oscillator and a divider in series, a second loop connected to the output of the voltage-controlled oscillator, the second loop comprising at least a second phase comparator and a second voltage-controlled oscillator, the output of the squared signal of which oscillator is applied to a divider connected to the second phase comparator.
5. A device as claimed in Claim 4 wherein a decimal coded binary addersubtractor is associated with the divider of the first loop, the adder-subtractor receiving the value of the frequency of interest on one of its inputs and receiving on its other input the value of the frequency from the mixer which is applied to the low-frequency filtering stage.
6. A device as claimed in Claim 5 wherein a verifier switch is associated with the decimal coded binary adder-subtractor for verifying whether the desired frequency is present at the input of the device.
7. A device as claimed in Claim 4 or 5 wherein a low-pass filter is placed between the phase comparator and the voltage-controlled oscillator in the first and second loops of the local oscillator.
8. A device as claimed in any one of Claims 1 to 7, wherein the broadband filtering stage comprises as many filters as the low-frequency filtering stage.
9. A device as claimed in Claim 8, wherein a first selector is arranged between the low-frequency filtering stage and the measuring stage and a second selector is arranged between the broadband filtering stage and the mixer.
10. A device as claimed in Claim 3 or any claim appendent directly or indirectly thereto wherein each low-frequency filter allocated to a decade can operate in narrow band and normal band.
11. A device as claimed in Claim 10, wherein each low-frequency filter defines a narrow band for its own decade which is the normal band for the preceding decade.
12. A device as claimed in Claim 3, or any claim appendent directly or indirectly thereto, wherein the local oscillator constitutes a generator which, for each decade, supplies a predetermined frequency or its multiples by 10 at a constant voltage for use in calibration of the modes of measuring for each decade.
13. A process for measuring electrical magnetic or acoustic signals, comprising picking up the signals and receiving them in a receiver, subjecting the signals to high frequency filtering, transposing the signals into low-frequency signals, selecting desired frequency decades of the input signals by means of individual filters, each allocated to one of the decades and processing the low-frequency filtered signals to provide the desired measurement.
14. A measuring process for measuring the distribution of the impedance of a medium, comprising receiving on a receiver natural or artificial waves propagating themselves in the medium and detected by pick-ups, dividing the signals at the input into a number of frequency decades, selecting the desired frequency decades by means of individual filters after transposition to low frequency each filter being allocated to one of the decades and processing the low-frequency filtered signals to measure the output voltage of each pick-up and the phase difference between any two of the pick-ups by averaging out amplitudes and phases.
15. A process as claimed in Claim 13, wherein the natural artificial waves are electromagnetic by nature and the low-frequency filtered signals are representative of electrical and magnetic components and are processed to determine the distribution of the resistivities of subsoil.
16. A direct and continuous receiving and measuring device substantially as described herein with reference to the drawings.
17. A process for measuring electrical magnetic or acoustic signals substantially as described herein with reference to the drawings.
18. A measuring process for measuring the distribution of impedance of a medium substantially as described herein with reference to the drawings.