GB2105135A

GB2105135A - Method for locating objects

Info

Publication number: GB2105135A
Application number: GB08116600A
Authority: GB
Inventors: Norman Brian Wilcock
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-05-30
Filing date: 1981-05-30
Publication date: 1983-03-16

Abstract

A method for locating a locus on or in a body, e.g. a buried pipeline or building foundations, which comprises establishing a datum point on or in the body; mounting a magnetic field transmitter on said datum point; and locating said datum point by means of the magnetic field transmitted by the said transmitter; and a transmitter for use in the method.

Description

SPECIFICATION Method for locating objects The present invention relates to a surveying method and to a device for use therein.

In many instances it is desired to monitor land, tips, pipelines or other structures to determine whether there is any movement thereof or to re-discover the location of a body which has become buried or obscured since it was last used, e.g. a survey borehole or an entry point to a drainage system. To achieve this, it is customary to establish a marker stake or a series of datum points. These serve to locate the object or as reference points in a survey to monitor these from some fixed point.

Typically the datum points are provided as concrete or other blocks set into the structure to be monitored. These often become buried by further material, e.g. waste or infill, and are difficult to locate.

Location of the datum points may often require prolonged digging and searching before a survey or other operation can be carried out.

We have now devised a method which reduces the problems caused by burial of the datum points.

According, the present invention provides a method for locating a locus on or in a structure, which comprises establishing a datum point on or in the structure; mounting a magnetic field transmitter on said datum point; and locating said datum point by means of the magnetic field transmitted by the said Transmitter. Preferably, the structure is surveyed by means of a plurality of datum points located as above.

The invention also provides a structure having a datum point thereon or therein and having a magnetic field transmitter mounted on said datum point.

The invention is of particular use where the datum points have become obscured or buried, notably where they are underground. The method of the invention finds widespread use in monitoring the movement of structures, e.g. of underground pipelines, of foundations of buildings and the like or of locating buried objects, e.g. the entry points to drainage systems on new building sites or water table borehole heads. The term structure is used herein in a general sense to denote items which have been fabricated, e.g. bridges, pipelines or buildings, and the land, e.g. waste tips, land infill sites and fields or roads. However, it is of especial use in monitoring the movement of soil or waste at a site, e.g. of a spoil tip or at a land infill site on which buildings are to be built. For convenience the invention will be described with reference to monitoring movement at a land infill site.

In carrying out the method of the invention a series of datum points are established using conventional means at points on the site. The datum points can take any suitable form, e.g. concrete blocks or slabs set into the site and can incorporate means, e.g. post holes, to receive surveying rods for carrying out subsequent surveys of the site.

The invention is characterised in that the datum points are provided with magnetic field transmitters which enable the datum points to be readily located.

Preferably, each datum point is provided with a transmitter although this need not be the case, e.g.

where the datum points are laid out in a regular pattern and, say, only every other one carries a transmitter. Also, if desired, some or all of the datum points may carry more than one transmitter each operating at a different frequency, e.g. where it is desired to survey the site in several different manners.

The magnetic field transmitters conveniently take the form of a rod aerial directed upwardly having a coil around the rod which is energised by a suitable oscillator circuit. Conveniently the oscillator circuit is a solid state circuit powered by a suitable battery and the whole device is potted up in a suitable water resistance resin or casing. Atypical transmitter assembly circuit diagram is shown diagrammatically in Figure 1 of the accompanying drawings and comprises an aerial rod element 1, an aerial coil 2 around the rod 1, an oscillator circuit 3 and a battery or other power source 4. The rod 1 is of circular or other cross-section and is made from a magnetisable material, e.g. a ferrite material, and is mounted with its longitudinal axis substantially vertically. Coil 2 is wound around the rod at any suitable point, e.g.

on a cylindrical former which is a sliding fit upon the rod and can be moved axially up or down the rod. It is preferred to locate the coil 2 around the lower portion, e.g. within the bottom 10 to 40% of the length of the rod 1 so that the areas of maximum flux density are located predominantly above the aerial, that is the resultant perceivable signal from the aerial is directed predominantly upwards.

Coil 2 isenergised via an oscillator circuit which is adopted to feed pulses of electrical energy through the coil 2 from power source 4, preferably at a frequency of from 50 to 300, notably 100 to 150, kHz.

Preferably, there is provided a second oscillator circuit or free running pulse generator, which triggers the first oscillator circuit on and off so that the magnetic signal from the aerial is a pulsed signal, e.g. pulsed on for from 500 to 2000 microseconds with rest periods of say 50 to 200 milleseconds. The oscillator circuits can be of any suitable form but a typical example comprises a Wein Bridge oscillator whose load is a tuned capacitor/inductor where the inductor is the aerial coil, as is shown diagrammatically in Figure 1.

The oscillator circuits and coil are powered by any suitable power source 4, e.g. a conventional battery or cell. The device can incorporate a receiver adapted to receive a signal from an external source for switching on and off the power source so as to prolong its life. Thus, for example the device could incorporate a second ferrite rod adapted to receive a signal generated by a transmitter operated by the person carrying out the survey which actuates a relay or other switch which switches on the power source 4. Alternatively, the second signal could be emitted by a transmitter on an adjacent datum point whereby actuation of one transmitter at one datum point initiates a "domino effect" actuation of all other transmitters on a site.When the second signal is terminated, it is preferred that the switch of the power source 4 automatically relapses to the off position, optionally after a suitable time delay, so as to prolong the life of the power source.

In place of a battery or cell, it is also possible to energise the oscillator circuits and coil in the transmitters on the datum points by a high energy signal generated by a suitable external transmitter operating on a second aerial within each transmitter. The high energy signal received by the second aerial is then converted to a direct current which powers the transmitter on the datum point. This high energy signal should operate at a frequency sufficientiy far removed from that generated by the transmitters on the datum points to reduce interference problems.

Whilst the invention has been described above in terms of a simple ferrite rod aerial in the transmitter, we prefer to shape the signal from the aerial so that it becomes more unidirectional thus aiding accurate location of the transmitter on the datum point. The shaping is conveniently achieved by the use of one or more metal reflectors or shapers arranged substantially symmetrically radially around the longitudinal axis of the coil 2. For example, rod 1 can be provided with a split ring shaped 5 made from a material which can be magnetised, e.g. a split ferrite ring, adjacent to the lower end of the rod 1 as shown in Figure 2.

The relative dimensions of rod 1, the ring 5 and the coil 2 and their relative positions will affect the shape of the magnetic field signal emitted (shows dotted).

In general, the presence of the ring 5 will enhance the shape of the central lobe A of the signal and will form a peripheral dish component B to the signal. If desired more than one ring 5 can be used to shape lobe A into a pencil beam and/or to modify the shape ofdish B.

Alternatively, the rod 1 can be provided with one or more reflector members which have an effect on the shape of the signal. Thus, rod 1 can be provided with a transverse reflector below coil 2, e.g. as a disc or dish of metal 10 located at or adjacent the lower end of the rod 1 as shown in Figure 3. The reflector can also take the form of a discontinuous annular ring or skirt of metal 20 located around the upper end of the rod 1 as shown in Figure 4. This skirt can be provided by a series of rods arranged axially around rod 1. The reflectors are made from a non-magnetic material, notably copper or aluminium. If desired, a shaper ring 5 can be used in combination with a reflector plate or ring.

By shaping the signal it is possible to provide a borad signal (dish B) by which the general position ofthe transmitter can be located and a narrow signal (lobe A) for accurate location of the transmitter.

We believe that magnetic field transmitters having such means for shaping the field of the signal produced are new. The invention therefore further provides a magnetic field transmitter comprising an aerial having a rod of a magnetisable material and a coil encircling part of the rod, and oscillator means for generating a pulsed magnetic field signal from the aerial, characterised in that there is provided means for shaping the signal from the aerial to enhance the longitudinal beamed signal from the aerial and to form a bowl shaped signal field substantially co-axial with the beamed signal. Preferably the shaping means comprises a discontinuous ring of magnetisable material substantially co-axial with the rod and/or a member made from nonmagnetic material located substantially symmetrically co-axial with the rod.

The magnetic field signals generated by the transmitters on the datum points can be received by any suitable apparatus incorporating a directional aerial. Thus, for example a receiver incorporating a directional ferrite rod aerial feeding the signals it receives via suitable amplification and detection circuits to a loudspeaker can be used. when the receiver is within the magnetic field emitted by a transmitter on a datum point and receiving sufficient field strength, an audible signal will be heard, the strength of the signal received indicating the proximity of the receiver to the transmitter.

We have found that the magnetic field signals at the frequencies mentioned above are more readily transmitted through soil than many other forms of signal. The present invention therefore provides a simple means for locating objects and datum points buried underground where other forms of detection are ineffective. In operating the method of the invention the transmitter on the datum point is activated and emits a signal. The receiver is activated and detects that signal. Movement of the receiver indicates the direction from which that signal is being generated and movement along that direction will cause the signal emitted by the receiver to rise or fall. It is thus possible to plot the circle of maximum signal strength surrounding the transmitter corresponding to the bowl signal field B.Approximately central of the circle will be lobe signal A whose position can be accurately determined using the receiver. The transmitter lies below that point and has been located without the need to excavate speculatively over a wide range of possible locations. Where the receiver has been suitable calibrated it may also be possible to determined from the strength of the signal at lobe A, the depth at which the transmitter has been buried.

CLAIMS (Filed 1.6.82.) 1. A method for locating a locus on or in a body, which comprises establishing a datum point on or in the body; mounting a magnetic field trasmitter on said datum point; and locating said datum point by means of the magnetic field transmitted by the said transmitter.

2. A datum point on a body having a magnetic field transmitter mounted on said datum point.

3. A magnetic field transmitter comprising an aerial having a rod of magnetisable material and a coil encircling part of the rod, and oscillator means for generating a pulsed magnetic field signal from the aerial to enhance the longitudinal beamed signal from the aerial and to form a bowl shaped signal field substantially co-axial with the beamed signal.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. position, optionally after a suitable time delay, so as to prolong the life of the power source. In place of a battery or cell, it is also possible to energise the oscillator circuits and coil in the transmitters on the datum points by a high energy signal generated by a suitable external transmitter operating on a second aerial within each transmitter. The high energy signal received by the second aerial is then converted to a direct current which powers the transmitter on the datum point. This high energy signal should operate at a frequency sufficientiy far removed from that generated by the transmitters on the datum points to reduce interference problems. Whilst the invention has been described above in terms of a simple ferrite rod aerial in the transmitter, we prefer to shape the signal from the aerial so that it becomes more unidirectional thus aiding accurate location of the transmitter on the datum point. The shaping is conveniently achieved by the use of one or more metal reflectors or shapers arranged substantially symmetrically radially around the longitudinal axis of the coil 2. For example, rod 1 can be provided with a split ring shaped 5 made from a material which can be magnetised, e.g. a split ferrite ring, adjacent to the lower end of the rod 1 as shown in Figure 2. The relative dimensions of rod 1, the ring 5 and the coil 2 and their relative positions will affect the shape of the magnetic field signal emitted (shows dotted). In general, the presence of the ring 5 will enhance the shape of the central lobe A of the signal and will form a peripheral dish component B to the signal. If desired more than one ring 5 can be used to shape lobe A into a pencil beam and/or to modify the shape ofdish B. Alternatively, the rod 1 can be provided with one or more reflector members which have an effect on the shape of the signal. Thus, rod 1 can be provided with a transverse reflector below coil 2, e.g. as a disc or dish of metal 10 located at or adjacent the lower end of the rod 1 as shown in Figure 3. The reflector can also take the form of a discontinuous annular ring or skirt of metal 20 located around the upper end of the rod 1 as shown in Figure 4. This skirt can be provided by a series of rods arranged axially around rod 1. The reflectors are made from a non-magnetic material, notably copper or aluminium. If desired, a shaper ring 5 can be used in combination with a reflector plate or ring. By shaping the signal it is possible to provide a borad signal (dish B) by which the general position ofthe transmitter can be located and a narrow signal (lobe A) for accurate location of the transmitter. We believe that magnetic field transmitters having such means for shaping the field of the signal produced are new. The invention therefore further provides a magnetic field transmitter comprising an aerial having a rod of a magnetisable material and a coil encircling part of the rod, and oscillator means for generating a pulsed magnetic field signal from the aerial, characterised in that there is provided means for shaping the signal from the aerial to enhance the longitudinal beamed signal from the aerial and to form a bowl shaped signal field substantially co-axial with the beamed signal. Preferably the shaping means comprises a discontinuous ring of magnetisable material substantially co-axial with the rod and/or a member made from nonmagnetic material located substantially symmetrically co-axial with the rod. The magnetic field signals generated by the transmitters on the datum points can be received by any suitable apparatus incorporating a directional aerial. Thus, for example a receiver incorporating a directional ferrite rod aerial feeding the signals it receives via suitable amplification and detection circuits to a loudspeaker can be used. when the receiver is within the magnetic field emitted by a transmitter on a datum point and receiving sufficient field strength, an audible signal will be heard, the strength of the signal received indicating the proximity of the receiver to the transmitter. We have found that the magnetic field signals at the frequencies mentioned above are more readily transmitted through soil than many other forms of signal. The present invention therefore provides a simple means for locating objects and datum points buried underground where other forms of detection are ineffective. In operating the method of the invention the transmitter on the datum point is activated and emits a signal. The receiver is activated and detects that signal. Movement of the receiver indicates the direction from which that signal is being generated and movement along that direction will cause the signal emitted by the receiver to rise or fall. It is thus possible to plot the circle of maximum signal strength surrounding the transmitter corresponding to the bowl signal field B.Approximately central of the circle will be lobe signal A whose position can be accurately determined using the receiver. The transmitter lies below that point and has been located without the need to excavate speculatively over a wide range of possible locations. Where the receiver has been suitable calibrated it may also be possible to determined from the strength of the signal at lobe A, the depth at which the transmitter has been buried. CLAIMS (Filed 1.6.82.)

1. A method for locating a locus on or in a body, which comprises establishing a datum point on or in the body; mounting a magnetic field trasmitter on said datum point; and locating said datum point by means of the magnetic field transmitted by the said transmitter.