Patent No. 4198596 Device for direct and continuous receiving and measuring of electrical magnetic and acoustic signals

 

Patent No. 4198596 

Device for direct and continuous receiving and measuring of electrical magnetic and acoustic signals (Waeselynck, et al., Apr 15, 1980)

Abstract

A direct and continuous receiving and measuring device for electrical, magnetic and acoustic signals comprises an amplifier stage and a broadband filtering stage at the input, a local oscillator for supplying a reference signal, a measuring stage, a mixer to which the local oscillator and the broadband filtering stage are connected and a low frequency filtering stage connected to the mixer. The invention also includes the process of carrying out the measurement.

Notes:  

Device for direct and continuous receiving and measuring of electrical, magnetic, and acoustic signals. Filed October 1977, granted April 1980. Assigned to Societe Nationale Elf Aquitaine (production) in France. Device for scanning low frequencies (says between 10 Hz and 250 KHz) of electromagnetic waves either natural or artificial. Says a "direct and continuous receiving and measuring device". Does show that scanning for said low frequencies was done in the 1970s.

BACKGROUND OF THE INVENTION

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 electromagnetic natural waves or waves emitted by any type of of emitter at a given power and emitted at a given frequency, for example, between 10 Hz and 250 K Hz.

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 and 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 K Hz for electrical signal amplitudes as low as 0.04 .mu.V/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.degree. C. and +50.degree. C., 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 over-voltage 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.

SUMMARY OF THE INVENTION

It is an object of the present invention 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 which make scanning all of the frequencies of a given spectrum possible and to choose the most favourable and the most characteristic of the phenomenon studies.

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 comprising an amplifier stage and a broadband filtering stage at the input, a local oscillator for supplying a reference signal, a measuring stage, a mixer to which said local oscillator and said broadband filtering stage are connected and a low frequency filtering stage connected to said mixer.

Further to this aspect of the invention, there is provided a direct and continuous receiving and measuring device for electrical, magnetic or acoustic signals comprising an input amplifier stage, at least one broadband filtering stage fed by said input amplifier stage, a mixer connected to said broadband filtering and to a local oscillator, a low frequency filtering stage connected to the output of said mixer and a measuring stage connected to said low frequency filtering stage.

According to a second aspect of the invention, there is provided a process for measuring electrical magnetic or acoustic signals, said process comprising picking up said signals, receiving said signals in a receiver, transposing said signals into low frequency signals, selecting desired frequency decades by means of low frequency filters and processing said low frequency filtered signals to provide the desired measurement.

Further to this aspect of the invention, there is provided a measuring process for measuring the distribution of the impedance of a medium, said process comprising receiving on a receiver natural or artificial waves propagating themselves in the medium and detected by the pick-ups, selecting the desired frequency decades selectively filtering for said selected decades after transposition to low frequency, and treating said low-frequency filtered signals to measure the output voltage of each said pick-up and the phase difference between any two of the said pick-ups by averaging out amplitudes and phases and bringing the values obtained back to the output of said pick-ups.