CA1207024A - Method and apparatus for determining the trace and depth of underground metallic conductors - Google Patents

Abstract

Abstract The subject matter of the present invention is a method and apparatus to determine the trace and depth of metallic conductors (6) located beneath ground level. The substance of the method is that ferrite antennas (16,16') are matched to coupled parallel os-cillating circuits the capacitive element of which provides resonance frequencies equal to the (cp ? 1)-th, preferably (cp - l)-th and cp - th, or (2c ? l)-th, preferably (2c - l)-th and 2c-th upper harmonic of the basic frequency of the alternating current, where c is a properly selected integer higher than 1, the value of p is 1, 2, 3, or 6 as required, while cp is an even number, thereafter the trace of the conductor (6) is localized by detecting the vertical component (5) of the magnetic field at an upper/lower level and the horizontal component (4) at a lower/upper level, on the basis of the ratio of the two components the trace is approximated to a certain extent, then in order to measure the depth (4) of said conductor (6) the hori-zontal component (4) of the magnetic field is detected both at the upper and the lower level and the depth (4) is determined by the known formula. The apparatus com-prises an upper and a lower ferrite antenna (16, 16') at least one of which can be rota-ted in the vertical plane. By means of the method and apparatus provided by the present invention the trace and depth of conduc-tors conducting an alternating current can be determined with a high selectivity and accuracy.

Description

- l -A method and apparatus for determining the trace and depth of underground metallic conductors Technical field The subject ma-tter of the pr~sen-t invention is a method and apparatus for determining the trace and depth of metallic conduc-tors located beneath ground level. According -to the method provided by the in-vention the electromagnetic field generated by the al-ternating current existing in the metallic conductor is detected by ferrite antennas at an upper and a lower level, the trace of the conductor is localized and its depth is determined by means of the charac-teristics of the magnetic field detected, while the apparatus pro-vided by the invention comprises a measuring receiver consisting of a transducer provided with an upper and a lower ferrite antenna to convert the strength of the magnetic field inducted by the alternating current flow-ing in the metallic conductor into a voltage signaly atrace-localizer unit and a depth finder unit to determine the depth of said conductor. The method and apparatus corresponding to the present invention provides for locating the underground metallic conductors faster and with a higher accuracy compared to the methods known heretofore and free of human errors.
Background Art Surveying and registering a densely located sys-tem of public u-tilities raises a lot of problems, as by applying the known me-thods -the determination of the trace.

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and the depth can be accomplished wi-th a relatively low accuracy only, especially in a very noisy' en-vironment, namely in the vicinity of power current lines and equipmen-ts, e.g. tramway and -trolley lines.
The handling of the known equipment is sometimes dif-ficul-t, portable construc-tions are not available, and the result of the me~surements may be deteriorated by significant human errors.
A general method of locating metallic conduc--tors is to indicate the variations of the vertical and/
or horizontal components of the magnetic field gener-ated by the alternating current existing in said con-ductor by means of a suitable probe, usually a trans-ducer comprising ferrite antennas. The alternating current is produced by a generator applied to input an alternating current of the required amplitude and fre-quency to the conduc-tor to be located via its available points. The input can also be accomplished by means of inductivity. A common method is also to connect one pole of the generator to the conductor only, the other pole being grounded.
The locating accomplished by the method des-cribed is based on the fact that if the conductor to be located can be considered as an infinite straight lead, then a concentric magnetic field can be supposed to be existing around it. In the case of such a field pat-tern the vertical and horizontal components of the mag-ne-tic field strength are equal if the lead can be seen at an angle of 45 from the point of the measurements.
Thereby the direction oE the arrangement of the conduc-."

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tor can be determined, while the position o~ the trace of the !conductor can be theoretically established based on the maximum value of -the horizontal component and the zero level of the vertical component. Both measure-ments are, however, uncertain; the measurement of the maximum value may be burdened by serious human errors, and measuring the zero value is illusory, especially in noisy environment. An equipment has been developed by the US company Metrotech utilizing a microcomputer to realize the locating method described wherein the measur-ing process is based on a fix program.
Equipments developed to accomplish the method described above are known from the German Specifications DE-PS 10 64 655, 25 28 511 or 20 37 865 on circuit level.
The solution of particular partial problems are given in these Patent Specifications to increase the accuracy or stability of the detection or to accomplish an energy saving construction of the generator (powering the gener-ator from the conductor to be located is recommended by one of these specifi ations!.
An apparatus is described in the German DE Patent Specification No. 955 348 comprising two coils arranged horizontally one above the other and in a fixed position with respect to each other to find the depth, while a separate antenna is provided to localize the layout which is also rigidly fixed in relation to the coils. ~his apparatus requires a careful calibration when used.
In noisy environments applying the genera-tor raises particular problems as 100 to 500 W high-pow7er units ~L~O~

are used in attempting to generate a high level alternating current in the conductor sufficient to suppress the effect of the own current of the conductor considered to be dis-turbing. The power requirement of this accomplishment is very high, developing a mobile equipmen-t is difficult and -the generator must be carefully watched for the measurements.
A source of additional difficulties is the measuring frequency, the selection of which can be commonly considered as incidental~ it is an odd-order multiple of the mains frequency or approximates this value (e.g. 1450 Hz in the case of the instrument of the sEsA company, or 1030 Hz in the RFT equipment), thereby a suffici-ently selective measuremen-t cannot be always effec-ted, the relative value of the noise may be signi-ficant.

Disclosure of the Invention The object of -the present invention i5 to eliminate the disadvantages described above.
It has been realised, that localizing the layout and measuring the depth can be effectively accomplished by applying ferrite an-tennas placed at -two different levels and rotatable with respect to each other for the measurements. Another impor-tant realization is tha-t a generator is not always ~0 necessary to apply just in a noisy environment if a sufficiently strong alternating current is flowing in the conductor, therefore, it is advis-able to make a dis-tinction between the operation with and without a . ~ , :LZ1370~

generator and to select the measuring frequency accordingly. The main point of this latter realiz-a-tion is that such multiples of the basic frequency are selected as measuring frequency when a generator is applied by which a high effective signal level can be attained with respect -to -the in-terfering noise (this is a properly selected even upper harmonic), while in an operation mode without genera-tor there is suffici-ent signal level (-this is one of the odd upper harmonics as a consequence of the Fourier-analysis of the alter-nating current).
Based on the realizations described above the object of the present invention is to provide a method and apparatus applicable to determine the layout and the depth of underground metallic conductors with a high accuracy and practically free of human errors.
A method has been developed for the realization of the object described above, whereby in order to de-termine the layout and depth of metallic conductors the magnetic field generated by the alternating cuxrent flowing said metallic conductor is detected by ferrite antennas at an upper and a lower level, the trace of the conductor is localized and its depth is determined on the basis of the measured characteristics of the magnetic field, and according -to the present invention the ferrite antennas are matched to two coupled paral-lel oscillating circui-ts the capaci-ty of which provides resonance frequencies equa~ to the (cp + l)--th, pre~
ferably (cp - l)-th and cp-th, or ~2c -~ l)-th, prefer-ably (2c l)-th and 2c-th upper harmonics of the basic frequency of the al-terna-ting current, where c is a properly selected integer higher than 1, the value of p is 1, 2, 3 or 6 as required, while cp is an even number, thereafter the trace of the conductor is local-ized by sensing the ver-tical component of the magnetic field at the upper/lower level and the horizontal com-ponent at the lower/upper level, the trace is approxi-mated to a cer-tain ex-tent on the basis of the ra-tio of the two components, -then in order -to de-termine the depth of the conduc-tor, -the horizontal components of the magnetic field is detected at bo-th the upper and the lower level and -the depth is calculated by means of the known formula. With the proper selection of the resonance frequencies, preferably selecting the 48-th and 47-th upper harmonics a high selectivity and thereby the sufficient accuracy of the measurements can be attained, while the human errors can be prevented by means of forming ratios, by processing -the ratio as required, preferably by forming the logarithms of the absolute values.
The depth is preferably established by means of analog circuits as thereby a relatively simple construc-tion can be attained.
The object of the present invention is also served by the apparatus provided by the invention, which is applicable to implement the method described above, having a measuring receiver comprising a transducer pro-vided with an upper and a lower ferrite antenna to con-vert the magne-tic field strength generated by the alter-nating current flowing in a metallic conductor into a vol-tage signal, a trace-localizing unit and a depth measuring unit, wherein according to the present inven-tion said trace-localizing uni-t connected to the trans-ducer sensing the magnetic field generated by -the alter nating current induced by a generator ~ .

when required is realized as a unit pointing out a corridor of predetermined half width depending on the depth of the conductor, the depth-measuring unit also connected to the transducer is built of analog circuit elements, the transducer is provided with two band-pass filters consisting of passive and active filter elements, one of which comprises the upper ferrite antenna as its inductive element and the other one comprises the lower ferrite antenna, wherein at least one of the upper and lower ferrite antennas can be rotated in the vertical plane, furthermore their capacitive element contain capacitors whereby the resonant frequency of each of the band-pass filters is the Mth upper harmonic of the basic frequency when a separate generator is used to generate the alternating current in the conductor, and the resonant frequency of each of the band-pass filters is Nth upper harmonic of the basic frequency when no separate generator is used, where N is equal to (cp+l), preferably (cp-l), or (2c+1), preferably (2c-1), M is equal to cp or 2c, where c is a properly selected integer and p is 1, 2, 3 or 6 as required, cp being an even number. Preferably a trace-localizing unit comprising a logarithmic amplifier is applicable to localize the layout, and an exponential amplifier is connected to the logarithmic amplifier in order to determine the depth of the conductor.
If an alternating current has to be generated in the conductor to be located, then a current generator is preferably applied for this purpose, as matching of the generator is not required in this case, the loading im-pedance is low and therefore a portable instrument canbe constructed. Tile current generator is preferably built ~' ~z~o~

of a well known transEormer regenerative circuitO

sr.ief description of the Drawings The method and apparatus corresponding to the presen-t invention is examplified by the embodiments and implementations described in detail referring to the drawings, where FIG. 1 is an illustration of the method pro-vided by the invention;
FIG. 2 is a schematic block diagram of the appara-tus provided by the invention;
FIG. 3 is the block diagram of the measuring receive-r provided by the invention;
FIG. 4 is the circuit diagram of the band-pass filter in the measuring receiver;
FIG. 5 is the circuit diagram the active recti-fier circui-t and logarithmic amplifier built of operational amplifiers; and FIG. 6 is a preferred circuit accomplishment of the current generator developed for the generator made of operation of the apparatus provided by the present invention.
Mode for Carrying out the Invention When implementing the method provided by -the present invention an easily available point of the con-ductor to be located is f.irst connected to a generator inducing alternating current, if required. Either gal-vanic or inductive coupling may be applied. When a current is flowing in the ~207024 9 .

conductor then the strength of the magnetic field induced may be sufficient to effect the measurements. ~y applying a reference voltage it can be determined whether the magnetic field strength is sufficient or not, if required.
Two ferrite antennas are used for the measurements, two band-pass filters are built of them, one of each. The resonance frequency of the band-pass filters is selected according as a generator is or is not applied for the measurement. In the latter case an odd, preferably 47-th, and in the former case an even, preferably 48-th upper harmonic of the basic frequency is adjusted. These values for the upper harmonics are obtained from theoretical considerations relating to the determination of the layout of said conductors.
For the proper measuring the upper ferrite antenna 16 is first set in vertical position and below it the lower ferrite antenna 16' is set in horizontal position.
Naturally a reverse arrangement can also be applied. The measurement is theoretically based on the fact the vari-ations of the horizontal (x-axis) component 4 and the vertical (y-axis) component 5 of the magnetic field gen-erated around a conductor 6 positioned in a depth in the relation to the lower, in this case horizontal ferrite antenna 16' are different ~see Fig. 1). The horizontal (x-axis) components 4 form a bell curve, while the verti-cal ty-axis) components 5 form an S-curve, and consider-ing the difference of height d between the two positions where the two different components are measured, the two curves can be illustrated as seen in Fig. 1. The two curves are intersecting in point S, the peak of the com-~., ~7~

ponents 5 is at x = h ~ d, while the peak of the com-ponents 4 is at x = 0, that is at the trace of the con~
ductor to be located. In relation to the point S in one direction, here in the direction of increasing x values the componen-t 5 is higher, and in the other direction the component 4 is the higher one. As for the absolute value of -the component 5, in -the other branch of the component 4 point S' is obtained as a ~imilar limit.
When locating the conductor 6, approximating the trace 6' of the conductor a measurement is made at point 7. Here the component 5 is higher than component 4 (if the measurement is made approximating from point S', then the absolute value of the component 5 is to be considered). The quotient of the two positive values, preferably the logarithm formed from the quotient is followed. Out of the range between S' and S -the value of this logarithm is either positive or negative depen-ding on the formation of the quotient. At points S' and S the value of the quotient equals 1, that of the logarithm equals 0, and following this the sign of the logarithm will reverse in the range from S' to S. Thereby the limits are assigned within which the locating is to be made with an increased accuracy. The next measurement is made at point 8 and is continued so that the results of the measurements should be of higher and higher value with the required sign. In the vicinity of the trace 6' of the conduc-tor a corridor of D half-~idth is pointed out depending on the depth h, the limits of which are indicated by the value of the quotient or the lo~arithm exceedi~g a given value. Theoretically the half-width .~...
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~L2(370Z4 D may be selected as small as required, but because of the disturbing fields always existing and resulting in a high distortion of the vertlcal (y-axis) component 5 here, measuring of the component 5 and thereby form-ing the quotient and its logarithm provides perfectlyreliable results within the specified range, as it has been already mentioned in connection with the known methods, too. With applying the method provided by the present invention this range has been experienced to be smaller than 2 to 3 percent of the depth, which means that in the case of a depth of 1 m the trace 6' can be localized with an accuracy of 2 to 3 cm with respect to the cen-terline of the conductor and this is ~uite sufficient.
At the limi-t of the corridor of half-width D the value of the quotient and the logarithm, respectively, attains a given limit value specified for example by the overflow of a visual display and then the measuring is continued at point 9 by setting also the upper ferrite antenna 16 in horizontal position. Then, on the basis of the two horizontal (x-axis) components and the well known relations of the physics the depth of the conduc-tor is determined and indicated preferably by analog computing methods based on the electronic circuits app-lied for locating the trace 6'.
Several me-thods can be applied for indicating:
visual~ e.g. with arrows and/or digits, possibly display-ing additional information, too, or acoustic, e.g. vary-ing pitch of the -tone and/or tone modulation, or both visual and acoustic, etc.
An appara-tus suitable to implement the method ~20~V~L

provided by the presént invention is comprised of a measuring receiver and a generator applie~ when required.
The schematic block diagram of the measuring receiver is shown in Fig. 2 and more detailed in Fig. 3. In the measuring receiver the transducer 1 comprises band-pass filters 23 and 23' built of active and passive filter elements. Their output is c~nnected to the tracelocal-izing unit A and the depth measuring unit B being con-nected to a display unit 14 (Fig.2~. The ba~d-pass filters 23 and 23' provide voltage signals corresponding to the field strength detected and this voltage signal is the basis of the measurements.
The upper 16 and lower 16' ferrite antennas for-- ming the inductive element sensing the magnetic field strength in the band-pass filters 23 and 23' accomplished with coupled parallel oscillating circuits are connected to the unit C determining the position of the measuring receiver in relation to the conductor 6 to be located via the filter stages 22 and 22' and operational ampli-fiers 12 and 12' formed as rectifiers (see Fig.3) being thereafter connected to the logic unit 27. The output of the logic unit 27 is connected to the indica-tor 14.
The indicator 14 may be for example a unit providing acous~
tic and/or visual signals. One output of the operational amplifiers 12 and 12' each is connected to a logarithmic amplifier 13 being a fundamental part of the trace-local-ising unit A. An alternately switchable modified expon-ential amplier 15 and serially an analog divider 24 is connec~ed to the logarithmic amplifier 13, the output of said divider being also connected to the indicator 14.
The function of the modified exponential amplifier 15 is lZ~7~

to restore the value logarithmized by the logarithmic amplifier 13 and to reduce the value obtained by 1.
Breaking the interconnection between the opera-tional amplifier 12 and the logarithmic amplifier 13 a reference voltage source 37 can be interposed applying a voltage of Ur to the logarithmic amplifiex 13. The reference voltage source 37 provides a basiS for evalu-ating the measured magnetic field strength when neces-sary and makes possible to decide about the possible appli-cation of a generator (i-t is required for example when the conductors are densely laid side by side, the current flow-ing in the conductor to be located is small, etc.1 The unit C comprises in series connection a phase shifter 25 and a multiplier unit 26, whëre the phase shif-ter 26 is connected to -the operational amplifier 12 and one output of the multiplier unit 26 is connected to the operational amplifier 12'.
The band-pass filter 23 (see Fig. 4) comprises following the ferrite antenna 16 a tuning stage 17 inclu-ding parallel capacitors switchable as required, a coup-ling stage 18 provided with an inductance 19 and a capa-citor unit 20. The active filter stage 22 comprises active filter elements 21. The switchable capacitors of the coupling stage 18 provide fox adjusting the tuning frequencies rela-ted to the operating modes with or with out generator. The band-pass filter 23l has a similar construction.
The circuit realization of the operational ampli-fier 12 applied as a rectifier (Fig. 5) is well known in ttself and requires no comments. The band~pass filter 23' is connec-ted to the operational amplifier while the band-pass ..i :~\
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filter 23 is connected to the operational amplifier 12 being applied as a rectifier and having a similar construc-tion, and ~oth band-pass filters are so connected to the logarithmic amplifier 13. The logarithmic amplifier 13 is built of operational amplifiers 2 and 3, where the output of -the operational amplifier 3 is fed back to its in-verting input and the output of the operational amplifier is fed back to its noninverting output by a transistor each, the emitters of the two transistors being connected.
Thereby the processing of the voltage signal is facilitated from the measuring point of view, and measured signal re-tains its sign relating to its natural value given in dBs.
The units described herein can also be accomplished in different ways by utilizing known circuits and elements.
When developing the apparatus and method provided by the present invention one of the fundamental realiza-tions was the fact well known from the theory of the rec-tifying circuits that the n = c.p + 1 order upper harmonics exist in the current I of a p cycle rectifying circuit.
20 Here the value of c is 1, 2, 3,....... , while in the case of the practicable circuits p is 1, 2, 3, and 6. Con-sidering these for example at the frequencies of the 24-th and 48-th upper harmonics no upper harmonic exists at any value of p, therefore these are preferable to be selected when 25 a generator is applied. Because of the application of ferrite antennas and band-pass filters the 48-th upper harmonic is more favourable. However, if a generator is not applied the 47-th upper harmonic is always present and therefore the band-pass filters ~3 and 23' can be easily retuned to the 47-th upper harmonics with small auxiliary capacitors.

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~z(37~6~24 The current obtained ls only one 47-th part of that obtained at the basic frequency, but it is measure-able quite well.
For the apparatus provided by the invention 5 . the generator is preferred to be realized AS a cur-rent generator exe.mplified by the circuit diagram shown in Fig. 6. In this case a transformer current regenerative circuit 29 is connected to the output of a voltage-controlled oscillator 28 to drive the rec-tifier unit 30 and controlled switches 31. On pole of the loading 34 is connected to one pole of said switches, while the other pole is connected to the in-put of the voltage-controlled oscillator 28 via an operational amplifier 38 as a rectifier and an inte-grator 36. The controlled switch 31 is connected to a frequency divider 33 and a quartz-controlled oscil-lator 32. The inverting input of the integrator 36 is connected to a source providing a reference volt-age UR.
The method and apparatus corresponding to the present invention provides for determining the trace and depth of conductors conducting an alternating cur-rent with a high selectivity and accuracy.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for determining the trace and depth of an underground metallic conductor in which there exists an alternating current of a basic frequency which generates a corresponding electromagnetic field having horizontal and vertical components, said method comprising:
a) detecting the electromagnetic field by ferrite antennas at an upper and a lower level, wherein the ferrite antennas are matched to first and second coupled parallel oscillating circuits, the capacitance of such circuits being selected such that each of the circuits has a resonant frequency equal to the Nth upper harmonic of the basic frequency when a separate generator is not used to generate the alternating current in the conductor, and each of the circuits has a resonant frequency equal to the Mth upper harmonic of the basic frequency when a separate generator is used to generate the alternating current in the con-ductor, wherein (i) N is equal to (CP?1) or (2C?1) (ii) M
is equal to CP or 2C, (iii) C is a properly selected integer greater than 1, (iv) P is 1, 2, 3, or 6, and (v) the product CP is an even number;
b) determining the trace of the conductor from sensing the vertical component of the magnetic field at either one of the upper and the lower levels, sensing the horizontal component of the magnetic field at the other of the levels, and forming the ratio of the two sensed com-ponents; and c) determining the depth of the conductor from sensing the horizontal component of the magnetic field at both the upper and lower levels.

2. The method of claim 1 wherein the resonant fre-quency of each of the circuits is the Nth upper harmonic of the basic frequency when a separate generator is not used to generate the alternating current in the conductor, where N is equal to (CP-1) or (2C-1).

3. The method of claim 1 wherein the trace of the conductor is determined by moving in a direction determined by the sign of the logarithm of the absolute value of the quotient of the sensed vertical and horizontal components of the magnetic field to a corridor including the trace of the conductor, the width of the corridor being determined by the magnitude of said absolute value.

4. The method of claim 1 wherein analog circuits are used to determine the depth of the conductor.

5. The method of claim 1 wherein the resonant fre-quency of each of the oscillating circuits is equal to the 47th upper harmonic of the basic frequency when a separate generator is not used to generate the alternating current in the conductor.

6. The method of claim 1 wherein the resonant frequency of each of the oscillating circuits is equal to the 48th upper harmonic of the basic frequency when a separate generator is used to generate the alternating current in the conductor.

7. Apparatus for determining the trace and depth of an underground metallic conductor in which there exists an alternating current of a basic frequency which generates a corresponding electromagnetic field having vertical and horizontal components, comprising:
a) a measuring receiver comprising a transducer, which transducer comprises an upper and a lower ferrite antenna, at least one of which is rotatable in a vertical plane, for converting the magnetic field strength into a voltage signal;
b) a trace localizing unit connected to the transducer for sensing the magnetic field and for pointing out a corridor of a predetermined half-width depending on the depth of the conductor; and c) a depth measuring unit comprising analog cir-cuits connected to the transducer;
wherein the transducer comprises:
first and second band-pass filters, each comprising active and passive filter elements, wherein the upper ferrite antenna is an inductive element of the first filter and the lower ferrite antenna is an inductive element of the second filter and the capacitive elements of the filters comprise capacitors selected such that the resonant frequency of both filters is the Nth harmonic of the basic frequency when a separate generator is not used to generate the alternating current in the conductor, and the resonant frequency of both filters is the Mth harmonic of the basic frequency when a separate generator is used to generate the alternating current in the conductor, wherein (i) N is equal to (CP?1) or (2C?1), (ii) M is CP or 2C, (iii) C is a properly selected integer greater than 1, (iv) P is 1, 2, 3, or 6, and (v) the product CP is an even number.

8. The apparatus of claim 7 further comprising a first operational amplifier being applied as a rectifier connected to an active filter stage which is the output stage of the first band-pass filter, and a second operational amplifier being applied as a rectifier connected to an active filter stage which is the output stage of the second band-pass filter.

9. The apparatus of claim 8 further comprising a unit for determining the relative position of the measuring receiver with respect to the conductor, wherein the unit comprises a multiplier, one input of which is connected to the first operational amplifier and the other input of which is connected to the output of a phase shifter, the input of the phase shifter being connected to the second operational amplifier.

10. The apparatus of claim 9 wherein the trace localizing unit comprises:

a logarithmic amplifier connected to the outputs of both the first and second operational amplifiers being applied as active rectifiers; and an indicator, connected to the output of the logarithmic amplifier.

11. The apparatus of claim 10 further comprising a reference voltage source, wherein an input of the logarithmic amplifier is switchable between the reference voltage source and the output of the second operational amplifier.

12. The apparatus of claim 11 wherein the depth measuring unit further comprises an exponential amplifier the input of which is switchably connected to the output of the logarithmic amplifier, the output of the exponential amplifier being connected to an analog divider which is in turn serially connected to the indicator.

13. The apparatus of claim 12 wherein the logar-ithmic amplifier comprises a transistor and a third operational amplifier, wherein the output of the third operational amplifier is connected to the base of the transistor and the non-inverting input of the third operational amplifier is connected to the collector of the transistor.

14. The apparatus of claim 7 further comprising a current generator for generating an alternating current in the underground conductor.

15. The apparatus of claim 14 wherein the current generator comprises:
a) a voltage-controlled oscillator, the output of which is connected to b) a transformer current-regenerative circuit, which is in turn connected to c) a rectifier, which in turn is connected to d) controlled switches, the outputs of which are connected to e) one pole of a load; and wherein another pole of the load is connected to f) an operational amplifier applied as an active rectifier which in turn is connected to the inverting input of an integrator, which integrator has a reference voltage connected to its non-inverting input, the output of the integrator being connected to the input of the voltage-controlled oscillator.