GB2326713A

GB2326713A - Mole position location

Info

Publication number: GB2326713A
Application number: GB9713498A
Authority: GB
Inventors: Neil Jeffery Tucker
Original assignee: COGENT TECHNOLOGY Ltd; University of Hertfordshire
Current assignee: COGENT TECHNOLOGY Ltd; University of Hertfordshire
Priority date: 1997-06-27
Filing date: 1997-06-27
Publication date: 1998-12-30
Also published as: GB9713498D0

Description

Mole Position Location The present invention relates to an apparatus and a method for locating the position of an underground tunnel boring percussive mole.

Percussive impact moles are used as a convenient means of boring tunnels underground, for example for the laying of cables, pipes or other conduits through soil. Such moles may bore a hole through tightly compacted soil or gravel, they are not suitable for the boring of holes through rock such as sandstone or limestone.

In most cases a hole or trench is dug in the ground at the planned start and end points of the tunnel, and a pneumatically or hydraulically driven mole of generally cylindrical shape is inserted into a cradle which prepositions the mole at the start of boring of the tunnel.

Such percussive moles may have a diameter of about 50 mm to 250 mm and have a percussive hammer which operates at about 8 Hz. An example of this type of mole is the 2" (50.8 mm) diameter hammer head mole made by the Vermeer Manufacturing Company of Palla, Iowa, USA. Such a mole is intended to travel in a straight line, but if the mole hits an obstacle or discontinuity underground, such as a rock comparable in size to the diameter of the tunnel bore, then the mole may be deflected from its intended path. In this case, the mole may miss the exit hole or trench. This may result in much time consumed digging in an attempt to locate the mole, or the tunnel may have to be rebored, if the end location of the tunnel is too far from the intended end location.

Some types of percussive mole may have a non-symmetric rotatable head, which permits the mole to be steered by rotating the head, and this allows curved tunnels to be bored, for example under a body of water, or for the start and end points of the tunnel to be near or at ground level. Again, unless the position of the mole may be monitored during boring of the tunnel, the mole may come back to the surface in the wrong location. Although such a steerable mole permits inadvertent deflection to be corrected, again this is only possible if the position of the mole underground may be determined. Consequently, such steerable moles are normally provided with radio position location apparatus, which is relatively expensive.

One type of position location relies on a radio signal transmitted by a so-called "sonde" at, for example, about 8 kHz, which is then detected by a directional receiver carried about at ground level. The receiver then indicates to the user which direction the mole is in relative to the receiver. Whilst this type of equipment is effective, it is necessary to incorporate the sonde with the mole, and also to provide a power supply, either as an on board battery or by an electrical cable trailed behind the mole with the pneumatic supply cable. Although such a sonde could be incorporated during manufacture with a mole, it is impractical to incorporate this type of equipment as an add-on to an existing mole. Furthermore, the necessity to provide a transmitting sonde with every mole means that every mole is more expensive, even though it would be necessary to have only one directional receiver for a set of moles, for example of different bore diameter.

It is an object of the present invention to provide a more convenient apparatus and method for locating the position, and optionally also the depth, of an underground tunnel boring percussive mole.

Accordingly, the invention provides an apparatus for locating the position of an underground percussive tunnel boring mole, comprising four or more acoustic transducers; a supporting frame holding the transducers in a spaced apart arrangement such that there are four transducers amongst which no three transducers are linearly aligned and so that these transducers may be placed in contact with the ground to detect the sound of mole percussion in order to produce output signals from the transducers representative of the percussive sound; reception means to receive the output signals from the transducers; timer means to determine any differences in times of arrival of the output signals at the reception means; and calculation means to calculate from the time differences the direction from which the percussive sound reached the transducers.

Also according to the invention, there is provided a method for locating the position of an underground percussive tunnel boring mole, comprising the steps of: i) placing four or more acoustic transducers spaced apart on the ground such that there are four transducers amongst which no three transducers are linearly aligned; ii) using the transducers to detect the sound of mole percussion in order to produce output signals from the transducers representative of the percussive sound; iii) receiving with receiver means the output signals from the transducers; iv) timing with timer means the arrival of the output signals at the receiver means to determine any differences in times of arrival of the output signals at the reception means; and v) calculating with calculation means from the time differences the direction from which the percussive sound reached the transducers.

The use of at least four transducers arranged non-linearly along the surface of the ground then results in at least three mutually independent time differences, and from these it is possible to solve three independent equations formulated to determine the direction in three dimensions from which the percussive sound, or vibration, reached the transducers. In general, the acoustic signal may reach each of the transducers at different times according to the distance each signal has to travel to reach the transducer.

The transducers may each be an acoustic sensor, such as a microphone with an electrical output, although the output signals may be optical. In a preferred embodiment of the invention, the acoustic sensor is an accelerometer, which detects by contact with the ground the movement of the surface of the ground owing to the passage of the vibrations (ie acoustic waves). The transducers may, of course, be sensitive to acoustic vibrations in frequency ranges or amplitudes which are inaudible to human ears, and the terms " sound" and "acoustic" as used herein therefore include such inaudible vibrations.

Preferably, the apparatus comprises a memory, the memory storing data representative of the speed of sound through the ground, in which the calculation means calculates from the speed of sound data a distance along the surface of the ground to a point nominally above the mole. The speed of sound in the ground does vary depending on a number of factors, such as the composition of the soil, the presence of ground water, and the compaction or depth of the soil.

In a basic embodiment of the invention, the data may comprise a single fixed value, representative of typical soil for example 1.0 km/s. In more refined embodiments, the value may be input or selected by a user, for example from a computer menu. In practice the underground speed of sound may be between 250 m/s and 2.5 km/s.

In one embodiment, an initial calibration routine may be run when the mole is first forming the tunnel from the cradle. At this point the distance and depth to the mole is known, and so the speed of sound value may be varied to achieve an optimal fit between the calculated and known values for distance and depth.

Using the speed of sound data the calculation means may then also calculate from the speed of sound data the nominal depth of the mole below the surface of the ground.

Because the apparatus may be taken by a user to a remote location, such as in the middle of a field, it is convenient if the apparatus comprises a display means to display the information calculated by the calculation means. Preferably, the display means is adapted to plot the movement of the mole in two dimensions representative of the surface of the ground, when the apparatus is left stationary in place with the transducers on the ground.

Such a two-dimensional display would then have coordinates oriented in a fixed relationship with the transducers.

Preferably, the transducers are arranged in a fixed, predetermined relationship to each other. The calculation means therefore performs calculation with values representative of a known predetermined and fixed orientation and spacing between pairs of transducers.

In a preferred embodiment of the invention, there are just four transducers. Three of the transducers may then be arranged in an essentially equilateral triangular arrangement, with the fourth transducer in a central location between the three triangular transducers. This arrangement provides a more consistent performance than an asymmetric arrangement, in terms of the percentage accuracy of direction, distance and depth calculations.

The transducers may conveniently be arranged on a set of arms extending between pairs of transducers, the arms being in use substantially parallel with the ground, that is in most situations substantially horizontal. Then, the transducers may be arranged as feet of the horizontal arms.

Because the apparatus may need to be used across a wide variety of ground conditions, from flat and level to sloping and rough, the feet may advantageously be spring biased or loaded by spring biasing means so that each transducer may locate with the surface of uneven ground.

For example, each transducer may be housed in a cylindrical body, slidably retained within and projecting from a cylindrical housing. The spring may then be contained within the housing, providing a biassing force against a back surface of the transducer. The strength of the spring biasing may then be selected according to the weight of the apparatus, and the degree of unevenness of ground to be tolerated, so that if one or more feet remain suspended above the ground when two or more other feet are firmly in contact with the ground, the suspended feet may be pushed into contact with the ground by the spring biassing means.

Particularly on sloping or slippery ground, it may be advantageous to provide the feet have a downwardly projecting cleat, spike, or other such ground gripping means, which may engage within the surface of the ground to help keep the apparatus in place.

Because the apparatus need not be heavy, and may be readily transportable by a user with the need for a A trolley or wheels, the apparatus need not have any sort of housing, but may have may have alternatively a substantially vertical post which rises from the plane of the horizontal arms to a control head for a user to control the apparatus. The control head may contain the receiver means, calculation means, and display, a control panel and, for example also an associated battery power supply and other electronics. Particularly the control panel and display may then be at waist height for convenient operation.

In a preferred embodiment, the horizontal arms form a Yshaped frame, the vertical post then rising from a base proximate the junction Y-shaped arms of the frame.

To facilitate the moving of the apparatus about from place to place, a handle may extend laterally from the post by which a user may move the apparatus.

The invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view of an apparatus according to the invention for locating the position of an underground percussive tunnel boring mole, having four feet with transducers therein; Figure 2 is an enlarged fragmentary partial cross section through one foot and transducer of Figure 1; Figures 3 and 4 are a schematic views, respectively in two and three dimensions, of the apparatus of Figure 1 being used to detect vibrations from a percussive mole boring a tunnel through the ground; Figure 5 is a plot of four output signals from the transducers, showing voltage against time, and extrapolations back to an initial rise from zero volts; Figure 6 is a flow chart showing the steps taken handling the four transducer output signals; and Figure 7 is a simplified representation of a two dimensional display of calculated results showing the location and movement of the percussive mole.

Referring first to Figure 1, an apparatus for locating the position of an underground percussive mole 1 comprises four accelerometer transducers 2,3,4,5, three of which 2,3,4 are arranged spaced apart at the ends of horizontal steel arms 6,7,8, of hollow square box cross-section. The three accelerometers 2,3,4 are thereby located on vertexes of an equilateral triangle of about 500 mm on each side, the fourth accelerometer 5 being centred between the other three.

An example of a suitable type of accelerometer is the "QZL Series" sold by Sensonics Limited of Chesham, Buckinghamshire, England.

Each accelerometer is in contact with the ground 10, and is part of a corresponding foot 12,13,14,15, one of which 12 is shown in more detail in the fragmentary view of Figure 2. The foot 12 comprises a cylindrical hollow steel housing 21 with a coil spring 23 therein which is under compression to bias one outwards end of the accelerometer 2, which also has a cylindrical form. The accelerometer 2 is slidable within a sleeve formed by the housing 21, and biased by the spring 23 to project at least partially out of an open end 25 of the housing 21.

The cylindrical body of the accelerometer 2 is retained within the housing 21 by a pair of collinear pins 27,28 extending perpendicularly away from the axis of the cylindrical accelerometer 21 on opposite sides of the accelerometer. The pins move between end stops 30,31 of a pair of corresponding longitudinal slots 32,33 in opposite sides of the housing 21.

As drawn in Figure 2, the cylindrical body of the accelerometer 2 is fully recessed within the sleeve of the housing 21, as would be case if the foot 2 were resting on a relatively high point of ground 10 compared with others of the feet 13,14,15.

Centered in the projecting end of the accelerometer body 2 is an axial aligned pin 35. Each of the other feet has a similar pin. This pin 35 projects into the ground, which can help to stabilize the apparatus 1 against inadvertent movement. Importantly, the pin 35, also helps to transmit to the accelerometer 2 vertical components of movement of the ground 10 (the accelerometers are insensitive to lateral movement).

The output signal from the accelerometer 2 is transmitted through a twisted pair wire 37, which runs through the centre of the spring 23, and into the hollow arm 6 towards the junction 38 of the Y-shaped arms 6,7,8, and from there similar wires run up a vertical hollow steel post 40 of square box cross-section, to a control head 42 atop the post 40. The post 40 does not join the arms exactly at the junction, but just to one side of the junction, where the post is welded a hollow cross member 44. This cross member serves to brace two of the arms 7,8.

The control head 42 is therefore not exactly above the Yjuction 38. The centre of mass of the control head 42 and post 40, together with the relatively minor weight of a handle 46 extending transversely from a side of the post 40 away from the Y-junction 38, is therefore centred to one side of the Y-junction 38. The weight from the apparatus 1 on the feet 12,13,14,15 is therefore not evenly spread, but concentrated on two feet 13,14, which are here the rear feet, as defined by the orientation of a control panel 48 and display 50 on an uppermost surface of the control head 42.

The unevenness of the weight is, however, insufficient to make any appreciable difference to the contact of the accelerometers 2,3,4,5 with the ground, or the stability of apparatus 1 against tipping, and so the apparatus 1 can be said to have a centre of mass substantially equally distributed on the four feet 12,13,14,15.

In any event, the positioning and arrangement of the orientation of the control panel 48 and display 50, along a centre line between two arms 7,8, and offset to one side of the Y-junction 38 has the advantage that a user's feet are less likely to come into contact with the pair of rearwardly directed arms 7,8 during the operation of the apparatus 1.

Considering now Figures 3 and 4, these show schematically how the vibrations or acoustic waves 52 radiating out from the head 54 of a percussive mole 56 will, in general, be detected at different times by two accelerometers 58,59 placed on the surface 10 of the ground, owing to the different distances 51,53 between the mole head 54 and the accelerometers 58,59. The mole 56 will have left a tunnel 60 behind the mole, through which the pneumatic lines 62 trail.

Figure 4 shows the situation with the four accelerometers 58,59,63,64, arranged equilaterally and centrally as before. Orthogonal x, y and z axes are drawn for clarity in perspective. Here there are, in general, four different distances 51,53,55,57, and hence vibration travel times, between the mole head 54 and the accelerometers 58, 59, 63, 64.

Figure 5 illustrates four output signals 71,72,73,74, expressed in volts against time in milliseconds. In general it is not desirable to measure the position of the detected vibrations by detecting peaks, or threshold crossings, because these can be affected by unpredictable variables, such as a difference in amplitude of the vibration due to variable ground contact of the accelerometers, or from vibrations reflected from underground objects and then superimposed at the accelerometers. Therefore, it is important that the time at which the vibration is first detected is calculated by extrapolating backwards from the initial upwards slope of each signal. Three accurate time delays may therefore be obtained from the four signals 71,72,73,74. In practice, it has been found that for most soil types, the time delays must be determined to an accuracy of +100 ns to produce an accuracy in calculated position (ie radial distance) and depth at a 5 m radius of +5W.

Figure 6 shows the steps involved in handling the output signals 81,82,83,84 from the four accelerometers 58,59,63,64. These are first amplified 85, and converted in a fast analogue-to-digital converter running at 10 MHz.

Then a microprocessor based circuit (not shown) calculates the initial times for the four output signals, the three associated time delays, and the data needed for display 88.

A simplified example of a suitable display 90 is shown in Figure 7. This shows an equilateral triangle 92 which is permanently displayed and is representative of the three peripheral transducers. Three detected and calculated points 94,95,96 representative of points on the ground below which the vibrations have been calculated to have originated, superimposed with an arrow 97 showing the direction of motion of the mole. An arrow 93 is displayed with a reading of 20 cm distance to a position on the ground above the mole. A calculated depth reading, here 3.4 m, is also displayed.

Once the mole has passed outside a particular radius 98, also permanently displayed, the user may then lift up the apparatus and move it along in the expected direction of the mole, so that the mole is now behind the user, and then set and leave the apparatus running until the mole is displayed crossing the display 90.

The apparatus described above may be used with any type of percussive mole boring a tunnel in soil, gravel, etc, and can result in a significant saving in time when the location of the mole would otherwise be uncertain. The apparatus may also be used with steerable moles, with or without radio detection apparatus. A particular advantage is that unlike systems which require a radio transmitter integrated with the mole, or synchronisation of the percussive action with an acoustic detector, there is no need to use a relatively expensive mole, or to run cables between the mole and the location detector or otherwise interconnect the operation of the location detection apparatus and the mole. It is therefore expected that the mole location detection apparatus and method according to the invention may be used to improve the efficiency and convenience of percussive mole tunnel boring.

Claims

1. An apparatus for locating the position of an underground percussive tunnel boring mole, comprising: a) four or more acoustic transducers; b) a supporting frame holding the transducers in a spaced apart arrangement such that there are four transducers amongst which no three transducers are linearly aligned and so that these transducers may be placed in contact with the ground to detect the sound of mole percussion in order to produce output signals from the transducers representative of the percussive sound; c) reception means to receive the output signals from the transducers; d) timer means to determine any differences in times of arrival of the output signals at the reception means; and e) calculation means to calculate from the time differences the direction from which the percussive sound reached the transducers.

2. An apparatus as claimed in Claim 1, comprising a memory, the memory storing data representative of the speed of sound through the ground, in which the calculation means calculates from the speed of sound data a distance along the surface of the ground to a point nominally above the mole.

3. A apparatus as claimed in Claim 2, in which the calculation means calculates from the speed of sound data the nominal depth of the mole below the surface of the ground.

4. An apparatus as claimed in Claim 2 or Claim 3, in which the data representative of the speed of sound through the ground may be selected by a user.

5. An apparatus as claimed in any preceding claim, comprising display means to display the information calculated by the calculation means.

6. An apparatus as claimed in Claim 5, in which the display means is adapted to plot the movement of the mole in two dimensions representative of the surface of the ground, when the apparatus is left stationary with the transducers on the ground.

7. An apparatus as claimed in any preceding claim, in which the apparatus comprises just four acoustic transducers.

8. An apparatus as claimed in any preceding claim, in which the transducers are arranged in a fixed, predetermined relationship to each other.

9. An apparatus as claimed in any preceding claim, in which three of the transducers are arranged in an essentially equilateral triangular arrangement, with the fourth transducer in a central location between the three triangular transducers.

10. An apparatus as claimed in any preceding claim, comprising a set of substantially horizontal arms extending between pairs of transducers.

11. An apparatus as claimed in Claim 10, in which the transducers are arranged as feet of the horizontal arms.

12. An apparatus as claimed in Claim 11, in which the feet are spring loaded so that each transducer may locate with the surface of uneven ground.

13. An apparatus as claimed in Claim 11 or Claim 12, in which the feet have a downwardly projecting spike which may engage within the surface of the ground.

14. An apparatus as claimed in any one of Claims 10 to 13, in which a substantially vertical post rises from the plane of the horizontal arms to a control head for a user to control the apparatus.

15. An apparatus as claimed in Claim 14, in which the horizontal arms form a Y-shaped frame, the vertical post rising from a base proximate the junction of the frame.

16. An apparatus as claimed in Claim 15, in which a handle extends laterally from the post by which a user may move the apparatus.

17. An apparatus as claimed in any preceding claim, in which the acoustic transducers are accelerometers.

18. A method for locating the position of an underground percussive tunnel boring mole, comprising the steps of: i) placing four or more acoustic transducers spaced apart on the ground such that there are four transducers amongst which no three transducers are linearly aligned; ii) using the transducers to detect the sound of mole percussion in order to produce output signals from the transducers representative of the percussive sound; iii) receiving with receiver means the output signals from the transducers; iv) timing with timer means the arrival of the output signals at the receiver means to determine any differences in times of arrival of the output signals at the reception means; and v) calculating with calculation means from the time differences the direction from which the percussive sound reached the transducers.

19. A method as claim in Claim 18, comprising the step of calculating with the calculation means, using data representative of the speed of sound through the ground, a distance along the surface of the ground to a point nominally above the mole.

20. A method as claimed in Claim 19, comprising the step of calculating with the calculation means, using data representative of the speed of sound through the ground, the nominal depth of the mole below the surface of the ground.

21. A method as claimed in any one of Claims 18 to 20, comprising the step of displaying the information calculated by the calculation means.

22. A method as claimed in Claim 21, comprising the steps of: vi) leaving the transducers in place on the ground; and vii) displaying the information in two dimensions representative of the surface of the ground, as the mole moves beneath the ground.

23. An apparatus for locating the position of an underground percussive tunnel boring mole, substantially as herein described, with reference to and as shown in the accompanying drawings.

24. A method for locating the position of an underground percussive tunnel boring mole, substantially as herein described, with reference to the accompanying drawings.