WO2007038940A1

WO2007038940A1 - A boring head, a method and an apparatus for accomplishing a conveyance line boring operation

Info

Publication number: WO2007038940A1
Application number: PCT/EP2005/010137
Authority: WO
Inventors: Susanna Apone; Federico Ballesio; Flavio Cucchietti; Giuseppe Galliano; Marco Infantino; Francesco Muoio
Original assignee: Telecom Italia S.P.A.
Priority date: 2005-09-20
Filing date: 2005-09-20
Publication date: 2007-04-12

Abstract

The invention refers to a boring head (1) for accomplishing a boring operation, said boring head (1) comprising a drill (3) and a sensor unit (5), wherein said sensor unit (5) comprises two sensors (17, 19) extending longitudinally on a support (20) and lying on respective axes (12, 14), the sensor unit (5) being mounted with respect to the drill (3) so that the respective axes of said sensors (17, 19) are arranged, in use, at different predetermined angles (θ) with respect to a horizontal plane (41). The invention further refer to an apparatus and to a method for accurately monitoring the position of a boring head (1) relative to a buried utility pilot conveyance line (45) during a boring operation in the vicinity of said pilot conveyance line (45).

Description

A BORING HEAD, A METHOD AND AN APPARATUS FOR ACCOMPLISHING A CONVEYANCE LINE BORING OPERATION

Technical field

The invention relates to a boring head, to a method and to an apparatus for accomplishing a conveyance line boring operation.

In particular, the invention relates to a boring head, to a method and to an apparatus for accomplishing a conveyance line boring operation in order to lay a conveyance line substantially parallel to an existing underground utility conveyance line.

Background art Utilities, such as those providing electric, gas, water, and telephone services, often bury their conveyance lines, e.g., cables and pipes, underground for reasons of safety and aesthetics.

When it is necessary to intervene for maintaining or developing an existing conveyance line infrastructure, such utilities face relevant operative problems and high costs. An example for this is the need for the telecom operators of promoting the. development of broadband communications by improving their access networks, currently in copper, through the gradual introduction of fibre optic cables. The development of a conveyance line infrastructure is particularly critical in urban environments, because it requires digging operations which may cause inconvenience.

Basically, two main techniques for burying conveyance lines underground are known: the "trench" technique and the "trenchless" technique.

The "trench" technique consists in realising open-cut excavations. This technique has a very negative social impact as it causes great inconvenience to the population and to the traffic. Besides that, the costs connected with the urban regulations and the limitations imposed by this technique, e.g. the long times needed for obtaining the authorisations and the prohibition of digging in protected areas, further discourage its use. An alternative conveyance line burying technique is the so called "trenchless" technique. This technique consists in digging two pits only, a launch pit and a reception pit. An apparatus able to boring horizontally is placed in the launch pit and, opportunely driven by an operator, is made to advance into the ground until it reaches the reception pit. The "trenchless" technique exhibits high advantages as far as the environmental and social impact is concerned, and its use, in particular in urban environments, is therefore preferable.

Among the "trenchless" techniques, "Directional Drilling" and "Microtunnelling" are the techniques allowing a control of the boring head which are mainly used by service operators (e,g, telecom operators). These techniques allow to perform a controlled boring operation, directly starting from the surface ground or from a launch pit, by pushing hollow rods in the ground, wherein the first of said hollow rods is associated to an orientable boring head. The rods advance in the ground thanks to the independent actions of a ram and of an hydraulic machine. These actions can occur either under dry conditions or by using a jet of water.

Within the "trenchless" techniques, the most common system for driving and locating a boring head is the system known as "walk over". This system is based on a probe housed in the boring head and emitting an electromagnetic signal. The electromagnetic signal is detected by a receiver located on the ground surface so as to determine the relative position of the boring head and to allow an operator to direct its path accordingly.

However, this system is not reliable enough, as the signal emitted by the probe may interfere with other utilities passing in the vicinity of the boring path, so providing the ground surface receiver with erroneous data or data difficult to be evaluated.

Moreover, the "trenchless" techniques require either the availability of an accurate map showing the position and the path of all the existing underground conveyance lines, or a previous survey made along the whole estimated boring path by means of echographic/georadar techniques. It should be noted that the above mentioned map is in practice never available, and that the echografic/georadar techniques are quite expensive and the information they provide is not reliable enough to allow a boring operation without risks. Other techniques for controlling the boring head during a boring operation so as to avoid the collision with an existing underground conveyance line have been described in patents and patent applications.

United States patents no. US 6,297,638 and no. US 5,994,904 describe methods for generating an alert when the boring head is near an existing underground conveyance.

United States patent application no. US 2004/0008030 discloses a sensor group constituted by inductive coils on the planes x, y and z, for providing the operator with an indication of the position of the boring head in order to avoid a collision with existing buried lines. The Applicant remarks that the methods described in US 6,297,638, US 5,994,904 and US 2004/0008030 allow to avoid physical damages by stopping the boring operation and manually defining a different path. Typically, the different path is obtained by drawing back the boring head, by modifying either its depth or its side direction and then by resuming the boring operation.

US 5,929,758 describe a method exploiting the presence of a previously buried cable to be used as a reference guide cable when it is necessary to bury a further cable along the same direction. However, the Applicant has observed that this method does not allow a precise control of the distance between the boring head and the reference guide cable as it does not allow to know the precise relative position of the boring head, since the boring head might be situated in any point around the periphery of the reference guide cable.

Disclosure of the present invention

It is therefore a first object of the present invention to provide a boring head, an apparatus and a method for accurately monitoring the position of the boring head during a boring operation in order to determine its position relatively to an existing underground utility conveyance line.

A second object of the present invention is to provide a boring head, an apparatus and a method for driving a boring head in a boring path at a reduced distance from an existing underground utility conveyance line. A third object of the present invention is to provide a boring head, an apparatus and a method for driving a boring head in a boring path in order to favour the extension of an existing conveyance line infrastructure.

A fourth object of the present invention is to provide a boring head, an apparatus and a method for driving a boring head in a boring path without resorting to expensive surveys and therefore at low cost.

A fifth object of the' present invention is to provide a boring head, an apparatus and a method for driving a boring head in a boring path with a simple and accurate control, while minimising at the same time the risk of damaging other underground conveyance lines and the relative services they transport.

In particular, said control is obtained by exploiting as a guide means an existing conveyance line, hereinafter referred to as "pilot conveyance line", and a probe associated to the boring head and able to precisely evaluate its position with respect to the pilot conveyance line.

According to the invention, the boring operation is performed in parallel to an existing conveyance line at a distance of few centimetres, typically 15 to 20 cm, in order to minimise the risk of colliding with other conveyance lines and with the pilot conveyance line. In particular, the invention allows maintaining the boring path at the same depth of the pilot conveyance line, thereby minimising the risk of interrupting the path of other transversal conveyance lines as said transversal conveyance lines are positioned parallel to the floor surface and the boring head would be covered by the safe area of the pilot conveyance line.

According to the invention, it is proposed a method which is based on the injection of an electromagnetic signal on the armor or metallic sheath/part of the pilot conveyance line in order to generate a magnetic field around it. The metallic part of the pilot conveyance line is normally accessible in correspondence with pits or small chambers already existing or pits made locally on purpose, and can be directly connected with an already existing ground terminal by opening a cable connection or by direct connection with the pilot conveyance line. The signal irradiated by the pilot conveyance line is received and analysed by a receiving unit located on the boring head. The boring head comprises, besides a sensor unit, a processing signal unit, fed for instance with local batteries.

The sensor unit comprises two sensors extending longitudinally on a support and lying on respective axes arranged at a predetermined angle (θ) with respect to a horizontal plane.

In a preferred embodiment, the sensor unit comprises two magnetic field sensors of the inductive coil type.

The invention exploits the different coupling of the magnetic field obtained in the various angular positions between the sensor unit and the pilot conveyance line.

It should be taken into account that, during the boring operation, the boring head does not maintain a fixed inclination; on the contrary, the boring head is normally subjected to a rotation in order to better control the boring path. This rotation is stopped when it is necessary to modify the path direction.

One of the fundamental aspects of the present invention is the information about the inclination of the sensor unit with respect to the roadway. This information is obtained by maintaining a fixed position, independently from the head rotation, said fixed position being obtainable by using ball bearings and by unbalancing the centre of gravity of the internal components of the receiving unit.

Further features and advantages of the present invention will be made clearer by the following detailed description of some examples thereof, provided purely by way of example and without restrictive intent. The detailed description will refer to the following Figures, wherein:

- Figure 1 shows a typical underground infrastructure comprising a pilot conveyance line and a boring path for a further conveyance line to be buried; - Figure 2 shows a side view of a boring head according to the invention;

- Figure 3 shows a perspective view of the boring head of Figure 2;

- Figure 4 shows a cross section view of a sensor unit of the boring head of Figures 2 and 3;

- Figures 5a and 5b respectively show two block diagrams of an electronic unit housed in the boring head of Figures 2 and 3 and of a central control unit for driving the boring head; - Figures 6a, 6b and 6c shows three possible spatial relationships between the boring head and the pilot conveyance line of Figure 1;

- Figure 7 shows a diagram illustrating the working area in which an operator should maintain the boring head; - Figure 8 shows a graphical representation of the central control unit displayed on a monitor of a boring head with respect to a pilot conveyance line.

Best mode for carrying out the invention

With reference to Figure 1, it is represented a typical underground infrastructure wherein there are illustrated: a roadway 41, a transversal conveyance line 43, a pilot conveyance line 45, a boring path 47, a further conveyance line 49, a safe area 51 generated by the pilot conveyance line 45 and the normative distances 53 between two underground conveyance lines.

It is the task of an operator to drive a boring head 1, visible in Figure 2, in order to bore a boring path 47 which is close and parallel to the pilot conveyance line 45.

With reference to Figures 2 and 3, a boring head 1 comprises a boring drill 3 fixed to an housing 21, inside which a receiving unit 8, comprising a sensor unit 5, an electronic processing unit 7 able to receive signals detected by said sensor unit 5, and a battery compartment 16 are housed.

The housing 21 comprises an external hollow cylindrical tube made of a non-ferromagnetic material and provided with openings 4 filled with material transparent to radio frequency signals (e.g. Teflon), in order to allow a radio-frequency signal, generated by the electronic processing unit 7, to be emitted outside the housing 21.

The sensor unit 5 and the electronic processing unit 7 are coaxially mounted around an internal metallic hollow tube 11, made of a non-ferromagnetic material as well. The electronic processing unit 7 and the sensor unit 5 are mounted on the internal hollow metallic tube 11 through ball bearings 9 having a low friction coefficient. The internal metallic hollow tube 11 is mechanically connected with the housing 21 and with at least one boring rod 15 used for pushing from backwards the boring head 1 into the ground. To this purpose, the boring head 1 exhibits a rear portion 13a opportunely shaped in order to be mechanically coupled with a corresponding front portion 13b of the boring rod 15. As a consequence of this arrangement, the housing 21 rotates at the same speed of the boring rod 15 during the boring operation.

The receiving unit 8 is mechanically realised so that its centre of gravity is translated downwards with respect to a central axis 18 of the housing 21. This can be obtained by housing the electronic processing unit 7, mechanically integral with the sensor unit 5, the battery compartment 16, and possible (not shown) ballasts in the lower part of the housing 21.

The centre of gravity of the receiving unit 8 translated downwards with respect to the central axis 18 allows the sensor unit 5 to always remain in the same position, in particular without rotating, with respect to a horizontal plane, even if the boring head 1 rotates continuously.

Possible deviations with respect to the central axis 18 due for instance to damaged bearings, oscillations or other inconvenience, may be controlled through a (not shown) tilt sensor, in particular an inclinometer, integral with the sensor unit 5, which is able to generate an alert in case the inclination of the sensor unit 5 deviates with respect to the zenith over a predetermined limit.

Figure 4 shows a cross section of the sensor unit 5.

The sensor unit 5 comprises a first magnetic field sensor 17 and a second magnetic field sensor 19, in particular two inductive coils 17,19 extending longitudinally on a support 20 and in the same direction of central axis 18. Said support 20 is preferably made of a non-ferromagnetic material. Moreover, the inductive coils 17,19 lie on respective axes 12,14 having a mutual inclination of +/- 45 degrees with respect to the horizontal plane represented by the roadway 41. The inclination of 45 degrees allows to maximize the voltage induced in both sensors 17,19 when the sensor unit 5 and the pilot conveyance line 45 lay in a plane parallel to the roadway 41. However, any other mutual inclination between the two sensors 17,19 can be chosen, since this would only determine a variation of the mathematical relationships describing the magnetic coupling between the two sensors 17,19, as it will be explained in the following. The sensors 17,19 can be for instance realised by winding a certain number of turns of enamelled wire. A lightening area 22 is provided for making an upper part of the sensor unit 5 lighter.

With reference to Figure 5a and 5b, there are schematically shown the electronic components housed in the electronic processing unit 7 and their interrelationship. An electronic switch 26 receives analog signals from the two sensors 17,19, said analog signals being then amplified by an amplifier 23. An analog/digital converter 25 converts the analog signals received by the amplifier 23 into digital signals which are opportunely filtered by a digital signal processor 27 and transmitted through a transmitter 29. The transmitted signals carry the effective value of the voltage induced in the sensors 17,19 by a pilot signal injected on a conductor of the pilot conveyance line 45, e.g., its armor or sheath.

Depending upon the section of the pilot conveyance line, the pilot signal can be injected on its armor or metallic sheath by means of a current clamp or by a direct electric coupling with the output of a signal amplifier controlled by a current generator.

The pilot signal generation system is referred to a first earth point which can be present in a small chamber located upstream of the small chamber from which the boring operation will be started. Along the path of the boring direction, typically at a distance of one hundred meters in correspondence with a further access point on the pilot conveyance line, the injection current loop is closed to a second earth point.

The pilot signal generation system may further comprise a current sensor which is positioned on the branch of the pilot conveyance line in the same boring direction. The current sensor allows to obtain an indication of the effective current injected in the armor/sheath. The frequency of the pilot signal is relatively low and can be modified by the operator. From an operative point of view, said frequency should be comprised between the upper limit of the frequencies occupied by the POTS (Plane Old Telephone Service) and the lower limit of the frequencies occupied by the xDSL services, both for avoiding an excessive attenuation by the ground crossed by the boring head 1, and for avoiding to generate noise in the radio receivers and undesired couplings with the telecom cables transporting active services. A typical value range for the pilot signal frequency is in the range between 4 kHz and 20 kHz, while a typical value range for the injected current is between 10 mA and 10 A depending mainly on the interference environment.

The signal transmitted by the transmitter 29 is then received by a receiver 31 of a central control unit 30 which further comprises a digital processing system 33 and a monitor 35 where the position of the boring head 1 and that of the pilot conveyance line 45 are displayed to the operator which is responsible for driving the boring head 1 into the boring path 47. The use of at least two magnetic field sensors 17,19 allows, by reciprocally comparing the amplitudes of the signals detected by the sensors 17,19, to know precisely the exact position of the boring head 1 with respect to the pilot conveyance line 45.

Again with reference to Figure 1, when the boring operation begins, the boring head 1 is positioned at a reference distance, e.g. 15 cm, with respect to the pilot conveyance line 45 and at the same depth of the pilot conveyance line 45 in order to exploit the safe area 51 generated by the pilot conveyance line 45, thereby minimising the risk of colliding with other transversally arranged conveyance lines 43. All information obtained by the sensors 17,19 during the boring operation and the number of boring rods 15 employed is recorded. Since the length of the rods 15 is fixed, the operator has an indication of the longitudinal advancement of the boring head 1 into the ground as well.

The receiving unit 8 of the boring head 1 is realised in such a way that when an increase of the voltage induced in the sensors 17,19 indicates that the boring head 1 is approaching the pilot conveyance line 45 in a same horizontal plane 55, while a decrease of the voltage induced in the sensors 17,19 indicates that the boring head 1 is moving away from the same horizontal plane 55.

When the pilot conveyance line 45 and the boring head 1 are not at the same depth any more, i.e. they do not lie on the same horizontal plane 55, then the voltage induced in the first sensor 17 is different than the voltage induced in the second sensor 19.

With reference to Figure 6a, it is shown a first possible spatial relationship, in which the boring head 1 is aligned with the pilot conveyance line 45. In this case, the first sensor 17 and the second sensor 19 have the same effective area with respect to the pilot conveyance line 45. In case the sensors 17,19 are inductive coils, the effective area is defined as the area of a turn multiplied by the cosine of the angle formed by the magnetic induction vector and the vector normal to the turn. Therefore, the voltage induced in the first sensor 17 is the same as the voltage induced in the second sensor 19. This information indicates to the operator that the boring head 1 and the pilot conveyance line 45 are parallel to the roadway 41.

With reference to Figure 6b, it is depicted a second possible spatial relationship, in which the boring head 1 is upperly misaligned with respect to the pilot conveyance line 45. In this case, the first sensor 17 exhibits an effective area greater than the effective area of the second sensor 19; therefore, the voltage induced in the first sensor 17 is greater than the voltage induced in the second sensor 19. This information indicates to the operator that the boring head 1 is closer to the roadway 41 than the pilot conveyance line 45.

With reference to Figure 6c, it is depicted a third possible spatial relationship, in which the boring head 1 is lowerly misaligned with respect to the pilot conveyance line 45. In this case, the second sensor 19 exhibits an effective area greater than the effective area of the first sensor 17 and therefore the voltage induced in the second sensor 19 is greater than the voltage induced in the first sensor 17. This information indicates to the operator that the boring head 1 is farther to the roadway 41 than the pilot conveyance line 45.

The theoretical principles lying behind the invention and the information obtainable from the voltages induced in the sensors 17,19 for assisting the operator in driving the boring head 1 parallel to the pilot conveyance line 45 will now be explained.

The Faraday's law states that voltage V induced in a turn is proportional to the variation of magnetic flux Φ in a time unit:

_Λ_ dΦ

V = n dt wherein n is the number of turns of an inductive coil.

By assuming that the wavelength of the signal is much greater than the development of the inductive coils 17,19, in the condition of maximum coupling, the following relationship is satisfied:

V₀ = n^- ω = nBSω

wherein V₀ is the effective value of the induced voltage, B is the strength of the magnetic induction acting on the coils 17,19, S is the area of a single turn and ω-2τϊ, where f is the frequency of the pilot signal injected on the armor / sheath of the pilot conveyance line 45.

According to the Ampere's law, the magnetic induction B at a distance R from the pilot conveyance line 45 is given by: . B = μ₀-?— wherein μ₀ is the magnetic permeability of the vacuum. It is therefore possible to measure the distance R from an inductive coil with respect to the pilot conveyance line 45 by adopting the following relation:

/

R = nμ₀Sω

2πV_n

With reference again to Figure 6a, when the pilot conveyance line 45 and the sensor unit 5 have the same depth, the axis of the sensor unit 5 and the axis of the pilot conveyance line 45 define the horizontal plane 55 which is parallel to the roadway 41. For this reason, the voltages Vi₅V₂ induced on the coils 17 and 19 should be reduced by a factor proportional to cos (θ), wherein θ is by construction equal to 45 degrees, since this is the inclination of the coils 17,19 with respect to the horizontal plane 55.

When, on the contrary, the depth of the sensor unit 5 is different than the depth of the pilot conveyance line 45, the plane defined by the axis of the pilot conveyance line 45 and by the sensor unit 5 is characterised by an inclination of a degrees (figures 6b and 6c) with respect to the horizontal plane represented by the roadway 41.

In particular, it is possible to obtain the following relation between the inclination angle a and the values of magnetic induction Bi and B₂, which are in turn proportional to the induced voltages Vi and V₂:

As it can be seen in Figure 7, the ratio between Bi and B₂ is equal to 1 only when the sensor unit 5 and the pilot conveyance line 45 move on the same horizontal plane 55, while said ratio increases or decreases according to whether, in the example of fig. 6, the sensor unit 5 or the pilot conveyance line 45 deviate with respect to the horizontal plane 55.

It is the operator's task to maintain the reference condition existing at the beginning of the boring operation by driving appropriately the boring head 1 in the working area 52.

The induced voltages V₁, V₂, which are therefore proportional to the only field magnetic components Bi and B₂, are amplified by the amplifier 23 and then filtered by the digital signal processor 27 in order to obtain the only signal component isofrequential with the pilot signal injected on the pilot conveyance line 45. If the pilot conveyance line 45 has been laid in an open-cut way, it will be advantageously possible to benefit from the characteristics of the backfill, typically sand or ground without stones, located within a few tens of centimetres, said backfill having originally been inserted in the reinstatement operations after having laid the pilot conveyance line 45. The presence of backfill will make the boring operation easier. Moreover, the magnetic permeability of backfill can be considered negligible and uniform and therefore it does not have any influence on the above mentioned formulae.

A possible graphical representation of the spatial relationship between the pilot conveyance line 45 and the boring head 1 is shown in Figure 8. The graphical representation is displayed on the monitor 35 of the operator's central control unit 30.

Obviously, while the principle of the invention remains unchanged, the details of the implementation of the invention and its embodiments might be varied considerably with respect to what has been herein described and illustrated, without departing from the spirit and scope of the invention as defined by the appended claims.

As a variant of the invention, it is possible to employ a third sensor (not represented), in particular a third inductive coil, arranged in a plane which is transversal with respect to the planes of the other two sensors 17,19 thus transversal to the axis of the boring head and so of the boring direction in order to improve the boring precision of the boring head 1 during the boring operation. In particular, a third sensor may allow to further reduce, up to a few centimetres, the distance between the boring head 1 and the pilot conveyance line 45. The third sensor avoids that the boring head might approach the pilot conveyance line 45 obliquely. When the axis of the third sensor is parallel to the pilot conveyance line 45, no voltage is induced on the third sensor as the magnetic induction vector is parallel to the pilot conveyance line 45. On the contrary, if the boring head 1 approaches to or moves away from the pilot conveyance line 45, a magnetic induction is generated and therefore a corresponding voltage is induced on the third sensor, said induced voltage indicating whether the boring head 1 is approaching to or moving away from the pilot conveyance line 45.

A further variant consists in avoiding the use of the transmitter 29 and of the receiver 31 by employing a dedicated cable preventively inserted into the boring rods 15 in order to feed the whole unit 5,7 remotely. This solution allows to reduce at a minimum the electronic components present in the boring head 1 which are subjected to not negligible mechanical stresses.

Claims

1. A boring head for accomplishing a boring operation, said boring head (1) comprising a drill (3) and a sensor unit (5), characterised in that said sensor unit (5) comprises two sensors (17,19) extending longitudinally on a support (20) and lying on respective axes (12,14), the sensor unit (5) being mounted with respect to the drill (3) so that the respective axes of said sensors (17,19) are arranged, in use, at different predetermined angles (θ) with respect to a horizontal plane (41).

2. A boring head according to claim 1, characterised in that said two sensors (17,19) are magnetic field sensors.

3. A boring head according to claim 2, characterised in that said two sensors (17,19) are inductive coils.

4. A boring head according to claim 1, characterised in that said predetermined angles (θ) are +/- 45°.

5. A boring head according to claim 1, characterised in that said support (20) is cylindrical and made of a non-ferromagnetic material.

6. A boring head according to claim 1, characterised in that said sensor unit (5) is coupled to said drill (3) by means of elements (9) having a low friction coefficient.

7. A boring head according to claim 1, characterised in that said boring head (1) further comprises an electronic unit (7) able to receive signals detected by said sensor unit (5), said electronic unit (7) comprising: an electronic switch (26) for receiving the analog signals output by said sensors (17,19), an amplifier (23) for amplifying said analog signals, an analog/digital converter (25) for converting said analog signals into digital signals, a digital signal processor (27) for filtering said digital signals, and a transmitter (29) for transmitting said digital signals outside said boring head

(I)-

8. A boring head according to claim 7, characterised in that said sensor unit (5) and said electronic unit (7) are coaxially mounted around a tube (11) through respective ball bearings (9).

9. A boring head according to claim 8, characterised in that said tube (11) is made of a non-ferromagnetic material.

10. A boring head according to claim 8 or 9, characterised in that said tube (11) is mechanically connected with a housing (21), said housing (21) comprising a further tube made of a non- ferromagnetic material and provided with openings (4) filled with a material transparent to radio-frequency signals, in order to allow a radio-frequency signal, generated by said electronic unit (7), to be emitted outside said housing (21).

11. A boring head according to claim 10, characterised in that said sensor unit (5) and said electronic unit (7) are arranged in the lower part of said housing (21) so that their centre of gravity is translated downwards with respect to a central axis (18) of said housing (21).

12. A boring head according to claim 11, characterised in that said boring head (1) further comprises a battery compartment (16) and at least one ballast, both housed in said lower part of said housing (21).

13. A boring head according to claim 1, characterised in that said boring head (1) further comprises a tilt sensor able to generate an alert in case said sensor unit (5) deviates with respect to the zenith over a predetermined limit, said tilt sensor being integral with said sensor unit (5).

14. A boring head according to claim 1, characterised that said sensor unit (5) further comprises a third sensor arranged in a plane which is transversal with respect to the planes of said two sensors (17,19).

15. An apparatus for accomplishing a boring operation comprising:

- a boring head (1) as claimed in any of claims 1 to 14; - a central control unit (30) for controlling said boring head (1), and

- a pilot signal generator for impressing a pilot signal on the armor or sheath of a pilot conveyance line (45).

16. An apparatus according to claim 15, characterised in that said central control unit (30) comprises a receiver (31) for receiving a signal transmitted by said boring head (1), a digital processing system (33) for processing said signal and a monitor (35) for displaying the position of the boring head (1) and the position of said pilot conveyance line (45).

17. An apparatus according to claim 15, characterised in that said boring head (1) is electrically fed through a dedicated cable.

18. An apparatus according to claim 15, characterised in that said pilot signal is a current signal.

19. An apparatus according to claim 15, characterised in that said pilot signal generator comprises a current sensor.

20. An apparatus according to claim 18, characterised in that said current signal has an amplitude in the range of 10 mA to 10 A.

21. An apparatus according to claim 18, characterised in that the frequency of said pilot signal is in the range of 4 kHz to 20 kHz.

22. A method for accurately monitoring the position of a boring head (1) relative to a buried utility pilot conveyance line (45) during a boring operation in the vicinity of said pilot conveyance line (45), comprising the steps of: a) impressing a pilot signal on said' conveyance line (45); b) detecting, via at least a first (17) and a second (19) magnetic field sensor arranged in the boring head (1), the magnetic fields emitted by said conveyance line (45) and the corresponding voltages induced on said magnetic field sensors (17,19); c) comparing the amplitudes of the voltage signals induced on said at least two magnetic field sensors (17,19).

23. A method according to claim 22, characterised in that, between said step b) and said step c), it is further provided the step of filtering the component isofrequential with said pilot signal of said voltage signals induced in said magnetic sensors (17,19).

24. A method according to claim 22 or 23, characterised in that, in said step c), an increase of the voltage induced in said magnetic field sensors provides an information that the boring head (1) is approaching the pilot conveyance line (45) in a same horizontal plane (55), while a decrease of the voltage induced in said two magnetic field sensors provides information that the boring head (1) is moving away from the same horizontal plane (55).

25. A method according to claim 22 or 23, characterised in that, in said step c), if the voltage induced in one (17,19) of said magnetic field sensors is different than the voltage induced in the other (19,17) of said magnetic field sensors, than information is provided according to which said boring head (1) and said pilot conveyance line (45) are at different depths.

26. A method according to claim 25, characterised in that, in said step c), if the voltage induced in . said first sensor (17) is greater than the voltage induced in said second sensor (19), then information is provided according to which said boring head (1) is upperly misaligned with respect to said pilot conveyance line (45).

27. A method according to claim 25, characterised in that, in said step c), if the voltage induced in the second sensor (19) is greater than the voltage induced in the first sensor (17), then information is provided according to which said boring head (1) is lowerly misaligned with respect to said pilot conveyance line (45).