WO2005045466A1

WO2005045466A1 - Cavity monitoring system

Info

Publication number: WO2005045466A1
Application number: PCT/AU2004/001530
Authority: WO
Inventors: Graham Michael Brooker
Original assignee: Cmte Development Limited
Priority date: 2003-11-04
Filing date: 2004-11-04
Publication date: 2005-05-19

Abstract

The invention provides an underground cavity monitoring system (CMS). The CMS uses a radar system operating in the millimetre wavelength band. The radar system includes a transmitter for generating a modulated electromagnetic waveform, transmit and receive antennas housed in and antenna module (10) for respectively radiating the waveform into space toward a distant object and receiving the waveform reflected from the distant object, and a receiver for measuring the amplitude of the reflected waveform and determining the time delay between the transmitting of the waveform and receiving the reflected waveform. A control system collates information received from the receiver and controls the antennas. The antenna module (10) is transformable between a stored mode, where the antenna is collapsed and an operational mode, where the antenna is unfurled and deployed.

Description

TITLE: CAVITY MONITORING SYSTEM Field of the Invention The present invention relates to underground mining and in particular, to the use

of underground radar systems.

Background of the Invention The invention has been developed primarily for use in monitoring underground voids or stopes formed as a result of removal of ore during mining and will be described hereinafter with reference to this application. However, it will be

appreciated that the invention is not limited to this particular field of use and may be applied to other areas either above or below ground. Once the mining process is completed, the stopes are filled with crushed rock and concrete to stabilize the surrounding rock. The concrete, is actually a cemented

aggregate fill, or CAF and is normally poured or pumped down through bore-holes drilled from the surface to strengthen the walls of the stope. The ratio of CAF to crushed rock is critical in providing a safe environment below the surface, however, because there is no method to accurately monitor the ratio, a large safety factor, and excess of CAF is used. This is uneconomical because

CAF is significantly more costly than crushed rock. A number of cavity monitoring systems (CMS) already exist that produce images of stopes. These present CMS's use a time-of-flight laser range finder integrated onto a motorised scanning head to scan the interior surface of the void from which an image can be constructed. Laser radar, generally operating in the infrared band, is used because a very

narrow beam can be produced from a small aperture, which results in high angular

resolution measurements. However this technology is limited in its application -

because laser wavelengths are strongly absorbed and scattered by suspended

particulates in the air. These particulates include dust, smoke and water droplets, or

combinations of these. This is particularly a problem during filling of stopes when an accurate real time

image of the stope is required to regulate the filling of the stope and ratio of CAF to

crushed rock. It is an object of the present invention to overcome or ameliorate at least one of

the disadvantages of the prior art, or to provide a useful alternative.

Disclosure of the Invention h a first aspect, the invention provides an underground cavity monitoring

system including a radar having an operating wavelength in the millimetre band. Preferably, the wavelength is in the range of about 1mm to 1cm.

Preferably, the radar uses a waveform with an operational frequency of between

about 30 GHz and 300 GHz.

Preferably, the radar has uses a waveform with operational frequency of between

35 GHz and 225 GHz. Preferably, the radar has uses a waveform with operational frequency selected

form the frequencies of about 35 GHz, 77GHz, 94 GHz, 140 GHz and 225 GHz.

More preferably, the radar uses a waveform with an operational frequency of

77 GHz or approximately 94 GHz which corresponds to an atmospheric window.. In a second aspect the invention provides an underground cavity monitoring

system including a radar system having: a transmitter for generating a modulated electromagnetic waveform; a transmit antenna for radiating the waveform into space toward a distant

object; a receive antenna for receiving the waveform reflected from the distant object; a receiver for measuring the amplitude of the reflected waveform and

determining the time delay between the transmitting of the waveform and receiving

the reflected waveform; and a control system for collating information received from the receiver and

controlling the antennas.

In another aspect, the invention provides a deployable antenna module for a

cavity monitoring radar system.

Preferably, the antenna is transformable between a stored mode, where the

antenna is collapsed and disposed within a housing, and an operational mode, where

the antenna is deployed outside the housing.

Preferably the antenna is transformed between stored and operational modes by

respectively deflating or inflating a bladder.

The selection of the operating frequency and the size of the deployed antenna

offer a good compromise between atmospheric attenuation and the angular resolution

required to produce clear images of the interior of the cavity.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described, by way of

example only, with reference to the accompanying drawings, in which: Fig. 1 is a graph displaying signal frequency vs. signal absorption;

Fig 2 is a schematic representation of a cavity monitoring system in operation; Figs 3 A and 3B are examples of images produced by a cavity monitoring system in accordance with the invention; Figs 4A - 4D are a sequence of schematic representations of a deployable antenna in accordance with the invention; and Figs 5 A and 5B are representations of an alternative antenna deployment.

Preferred Embodiments of the Invention The invention provides an underground cavity monitoring system including an electro-magnetic radar utilising an operating wavelength selected to provide an

increased penetration of smoke, dust, water droplets and other airborne particles while still maintaining reasonably good angular resolution from a small antenna aperture. The selected wavelength of the emitted radiation is in the millimetre range. That is, waveforms having a wavelength from about 1cm to 1mm corresponding to a frequency range of about 30GHz to 300GHz. The cavity monitoring system includes a time-of-flight electromagnetic radar

system having a transmitter for generating a modulated waveform of a predetermined wavelength; transmit and receive antennas for respectively radiating the waveform into space as a constrained beam electromagnetic signal and receiving the signal reflected from any distant objects; and a receiver, for detecting small values of radiation received by the receiver antenna and discriminating the time delay between

the radiated and returned signal to calculate the distance of the object from the radar. The magnitude of the reflected signal is also important and used to provide an indication of the nature of the reflecting surface. A control system collates the distance to the object with the horizontal and

vertical rotational direction to provide a relative coordinate location of the object from

the system, hi this way, by scanning the surface of the stope, a detailed picture of its

surface can be attained. This is presented in a plot extractor or graphical interface. When selecting the appropriate frequency it is important to take into account the

penetration of the waveform through the medium it travels. Generally, absorption

increases with increasing waveform frequency, thus the lower the frequency the better

penetration.

Higher frequency systems operating in the infrared or visible bands do not

penetrate the vapour and airborne particles particularly well and so cannot be used to

perform this monitoring function at all.

Lower frequency systems operating in the microwave band exhibit good

penetration characteristics, but do not allow for a sufficiently narrow beam to give the

required angular resolution to generate a contour map of the surface. This is because

the width of a radiated electromagnetic beam is a function of the ratio of its

wavelength and the antenna aperture (disk diameter). In many applications the size of

the antenna is not an important consideration. However, in this application, operating

in confined spaces underground, it is particularly preferred to have an aperture of

under 300mm. Whilst still producing a beam width of less than 1°. It is not possible

to obtain the same narrow beam in the millimetre wave band as is possible in the

infrared band without resorting to a large aperture.

Accordingly, a compromise must be reached between the dust penetration

capability, the required angular resolution and the size of the aperture that can be

accommodated for a specific task. Figure 1 is an example of a graph showing absorption against increasing frequency. It will be appreciated that the penetration of a waveform may be effected by airborne particles. Different frequencies will be affected differently by different particles. That is to say, an atmosphere containing one type of particle, for instance dust or water droplets of a certain size, will often exhibit a reduction in penetration at

a particular, confined range of frequencies. Trace line 1 provides an indication of the penetration of an increasing frequency waveform through an atmosphere having a number of different types of airborne particles. The peaks show frequencies with relative increased absorption characteristics while the troughs indicate frequencies with better penetration.

Trace line 2 provides an indication of the penetration of fog which is indicative of the atmosphere (smoke, dust, water droplets and other airborne particles) expected within the slope cavity. By studying these peaks and troughs, waveform frequencies are selected which

provide a reduced tendency to be absorbed. Specific frequencies of around 35, 77, 94, 140 and 225 GHz have been identified as providing good, relative penetration in clear

air and also through smoke, dust and water particles. In this embodiment, frequencies of about 77 GHz and 94 GHz have been selected as being most suitable. However in other embodiments, other frequencies in

the millimetre band may be used, particularly those having frequencies of 35, 140 and

225 GHz. Advantageously, using these frequencies the CMS according to the invention can scan the stope surface and build up a contour map of the surface so that the position of the rock fill and the CAF can de determined (and controlled) in real time. This allows the appropriate fill ratios to be maintained and the CAF wall thickness

reduced. This may be accomplished in two ways. The angle of the reflecting surface indicates the lay of the fill. Referring to Figure 2, the CMS, 3 is positioned to survey

the stope 4, the CAF, 5 is substantially liquid and therefore tends to self level and lie

generally horizontally whilst the crushed rock 6 will tend to heap in a pile under the

point at which it enters the stope. Thus by analyzing the shape of the bottom floor of the stope as it is filled, the interface between the CAF and crushed rock can be

estimated as the stope is filled.

In addition, the magnitude of the reflected signal can be analyzed. Differences

in strength of the signal can be used to indicate the different surfaces of the crushed

rock pile and CAF fill.

An example of an image produced by a CMS is shown in Figs 3 A and 3B.

Normally a colour representation would be presented to the operator but in these

monochrome figures the stope floor 7 and tunnels 8 and 9 can clearly be seen. In certain applications, access to the stope may be limited. For instance, in some

cases access to the cavity is available only via a borehole or small passageway. In

such applications the antenna required for the above millimeter band CMS will not fit

through the passageway. Therefore in a second aspect, the invention provides a

deployable antenna for a cavity monitoring system. In this embodiment, the antenna is an inflatable structure. As shown in Fig 4A,

the deflated antenna in stored mode is housed in a tubular antenna module 10

dimensioned to be inserted into a bore hole 11 of reduced diameter. The module is

connected by an umbilical cord that is used to power and control the antenna. Other systems such as the receiver, transmitter and control system may be disposed within the module housing or located remotely. As seen in Fig 4B, once the module clears the borehole and enters the cavity, the antenna may be deployed by inflating a shaped bladder with air or another gas. The antenna, now in operational mode, can then be used to scan the stope as required, Fig 4C. A compact powered cradle holds the antenna and is able to swivel on at least two axes to direct and accurately aim the antenna. Once scanning has been completed the bladder is deflated, thereby collapsing the antenna. The collapsed antenna is then retracted into the housing and the module is withdrawn. This can be seen in Fig 4D. hi other embodiments the inflatable antenna may be replaced with a ribbed, folding or otherwise collapsible structure. For instance, Figs. 5A and 5B show a collapsible antenna 10 including four hinged panel sections 13 which fold out for deployment. The folded antenna in the stored mode is shown in Fig 5 A whilst the

deployed antenna in operational mode is shown in Fig. 5B. It will be appreciated that the structure shown in Figs. 5A and 5B would be located within a protective housing. As can be seen, the antenna is mounted for rotation on longitudinal and lateral axes. Specifically, the antenna portion, is able to yaw within cradle 14. The entire cradle can be longitudinally rotated around 15. It will be appreciated that the invention provides a cavity monitoring system

which is able to present a real time image of the surface of a stope through an atmosphere containing suspended particulates such as smoke, dust and water droplets. In all these respects, the invention represents practical and commercially significant

improvement over the prior art. The present invention has been described herein by way of example only. Skilled workers in this field will readily recognise many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.

Claims

1. An underground cavity monitoring system including a radar having an

operating wavelength in the millimetre band.

2. A system according to claim 1 wherein, the wavelength is in the range of about

10mm to 1cm.

3. A system according to claim 1 or 2 wherein, the radar uses a waveform with an

operational frequency of between about 30 GHz and 300 GHz.

4. A system according to claim 3 wherein, the radar has uses a waveform with

operational frequency of between 35 GHz and 225 GHz.

5. A system according to claim 4 wherein, the radar has uses a waveform with

operational frequency selected form the frequencies of about 35 GHz, 77GHz, 94

GHz, 140 GHz and 225 GHz.

6. A system according to claim 5 wherein, the radar uses a waveform with an

operational frequency of about 77 GHz.

7. A system according to claim 5 wherein, the radar uses a waveform with an

operational frequency of approximately 94 GHz.

8. A system according to any one of the preceding claim, including a radar system

having: a transmitter for generating a modulated electromagnetic waveform; a transmit antenna for radiating the waveform into space toward a distant

controlling the antennas.

9. An underground cavity monitoring system including a radar system having: a transmitter for generating a modulated electromagnetic waveform; a transmit antenna for radiating the waveform into space toward a distant

object; a receive antemia for receiving the waveform reflected from the distant object; a receiver for measuring the amplitude of the reflected waveform and

controlling the antennas.

10. A deployable antenna module for a cavity monitoring radar system.

11. An antenna according to claim 10 wherein the antenna is transformable

between a stored mode, where the antenna is collapsed and an operational mode,

where the antenna is unfurled and deployed.

12. An antenna according to claim 11 wherein the antenna is stored within a

housing, and deployed outside the housing.

13. An antenna according to claim 11 or 12 wherein the antenna is transformed between stored and operational modes by respectively inflating or inflating a bladder.

14. An antenna according to claim 11 or 12 wherein the antenna includes a plurality of hinged panel sections which are moveable between a closed position in the stored mode, and an open position in the operational mode.