PERFORMING TIME-ACCURATE MEASUREMENTS IN A MINE

TECHNICAL FIELD

The present invention relates to a mining and mineral exploration system for performing a temporary, independent and a distributed time-accurate measurements in a mine, the system comprising a plurality of measuring units configured to receive a simulated GPS signal in order to perform time- synchronized measurements in deep mining and mineral exploration applications. The invention also relates to a method for performing time- accurate measurements in underground spaces.

BACKGROUND

A measuring system generally comprises a plurality of measuring units that are each capable of performing measurements in a vicinity of the location where they are placed. In order to synchronize measurements and record in each measuring unit when particular measurements are taken, the

measuring units each comprise a receiver that is able to receive GPS signals. When performing measurements in GPS-denied environments, an outdoor external antenna connected to a central GPS unit can be used in order to receive-or re-transmit the GPS signal to the measuring units by a smaller antenna placed on the central GPS unit inside the actual environment. This enables the measuring system to be adapted to the location where

measurements are to be performed although the distance to the actual environment will be the limiting factor. In an outdoor application the measuring units can act independent of a central GPS unit and each receives the GPS signal from satellites

Within the field of mining and mineral exploration, measurements need today more and more to be performed in deep underground spaces. One of the main aims is to detect and quantify mineral deposit in terms of geometry and location as well as their quality so that decisions for extending the mine in relevant directions can be taken. Measuring units are of course not able to detect GPS signals from satellites when placed underground, but a central GPS unit can still be used as long as it can be connected to an antenna at the ground surface level. For deep mining and mineral exploration

applications and when mining infrastructures such as tunnels to be utilized, however due to long distance to surface, modifications to the GPS-timing system must be made.

There are some GPS-timing systems that comprise a highly accurate clock and a GPS unit that uses clock signals to generate simulated GPS signals within a closed space. Even though this allows for simulated GPS signals to be created in the underground, environmental factors and the large distances in the mine still make it difficult to connect to the measuring units and to synchronize an array of measuring units distanced from each other in order to acquire reliable data. Especially where measuring units need to be placed in different tunnels or even on different levels or synchronized with measuring units on the surface in order for measurements to cover the relevant area, there are no systems today that are able to provide accurate time synchronization and thus measurements for these purposes.

Deep mining is often carried out at a depth of 1- 1.5 km below the surface of the Earth, but in some mines the mining operation may extend to a depth of 2-4.5 km or even more. In some deep mining operations it is desirable to measure properties of material between tunnels or between a tunnel and the surface. Such measurements require time-accurate and synchronized measuring units placed both inside tunnels deep underground and on the surface.

There is therefore a need for an improved measuring system that enables measurements in varying locations within a deep mine in a time- synchronized manner. There is also a need for an easy and efficient way of modifying an existing measuring system to make it suitable for such deep mining applications and at the same time avoiding costly modifications to each commonly available measuring unit.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate or at least to minimize the problems mentioned above. This is achieved through a mining and mineral exploration system and a mining and mineral exploration method for performing distributed time-accurate measurements in a mine according to the appended independent claims.

The system and method according to the present invention allows for time- synchronized measurements being taken in deep mining applications so that a plurality of measuring units suitable for use in applications on ground surface can also be used in deep mining applications.

Thus, mining and mineral exploration system comprises a local GPS unit configured to generate a simulated GPS signal, a remote unit for placing at a remote site, said remote unit being operatively connected to the local GPS unit for receiving the simulated GPS signal, at least one radiating cable connected to the remote unit and configured to extend from the remote unit in a first direction at the remote site, the radiating cable further being configured to transmit the simulated GPS signal in a vicinity of the radiating cable for transmitting the simulated GPS signal to a plurality of measuring units.

The system preferably also comprises a plurality of measuring units for performing measurements in the mine, each of the measuring units comprising a GPS receiver that is configured to receive the simulated GPS signal when the measuring unit is in the vicinity of the radiating cable for the measuring unit to be able to perform time-accurate measurements and synchronization based on the simulated GPS signal. Thereby, a simulated GPS signal can be generated in the mine and be transmitted to each of the measuring units by the radiating cable. This allows for using existing measuring units configured for use on the ground surface without

modifications, resulting in an efficient and convenient measuring system that can be transported into the mine and be placed where measurements are to be taken without requiring access or disturbance to local systems already in place inside the mine.

According to an aspect of the invention, the system further comprises at least two radiating cables connected to the remote unit and configured to extend in different directions from the remote unit, and further comprising a power divider (preferably an RF-splitter) arranged to couple the simulated GPS signal to each of the radiating cables. Thereby, measuring units can be placed in two different vicinities extending from the remote unit, allowing for time-synchronized measurements in at least two locations simultaneously.

According to another aspect of the invention, the at least one radiating cable is less than 200 m long, preferably less than 100 m long. Thereby, the simulated GPS signals is amplified within certain limits, preferably also by using a a line amplifier in the remote unit and transmitted through the radiating cable at a power that enables the measuring units to receive the GPS signals along the entire length of the radiating cable. This eliminates the need for line amplifiers along the radiating cable and thereby avoids the problem of maintaining a sufficient signal to noise ratio despite

amplification. The vicinity is defined as an area surrounding the radiating cable where the GPS signal is strong enough to be received by the measuring units, i.e. where the GPS signal is within a dynamic range of an internal GPS receiver of the measuring units. It is advantageous to have a large vicinity to allow spreading the measuring units over a larger area in order to increase the quantity as well as quality of measuring data.

According to yet another aspect of the invention, the system comprises at least one additional remote unit that is operatively connected to the remote unit or to the local GPS unit, and also comprises at least one additional radiating cable connected to the additional remote unit and configured to extend from the additional remote unit at the remote site, the additional radiating cable further being configured to transmit the simulated GPS signal in a vicinity of the radiating cable, and a plurality of additional measuring units for performing measurements in the mine, each of the additional measuring units comprising a receiver that is configured to receive the simulated GPS signal when the additional measuring unit is in the vicinity of the radiating cable for the additional measuring unit to be able to perform time-accurate measurements based on the simulated GPS signal. Thereby, the measurements can be performed over a much larger area inside the mine, allowing for measuring units to be placed in different locations but still operate in a time-synchronized manner.

According to a further aspect of the invention, the remote unit and the additional remote unit are configured to be placed in different tunnels, preferably on different levels of a mine. Thereby, the area where

measurements can be performed is rendered even larger, and it is also possible for direct measurements to at the same level of the mine but from different directions, such as for detecting a presence and extent of a mineralization from above the known mineralization as well as from below, or from either sides on the same level. This is advantageous since it provides greater details and higher quality data for these purposes that are still reliable. Since only the remote units need to be connected by cable to the local GPS unit and / or to each other, it is also possible to perform cost effective measurements since the amount of cable needed to cover a very large area is greatly decreased compared to existing systems.

According to yet another aspect of the invention, the vicinity of the radiating cable is defined as an area in which a power of a signal emitted from the radiating cable is at least a predetermined minimum signal power. The predetermined minimal signal power is preferably at a lower end of a dynamic range of the measuring units or slightly above the lower end, to ensure that each measuring unit placed anywhere in the vicinity is able to receive the simulated GPS signals. Thereby, it can be ensured that the entire length of the radiating cable gives rise to a vicinity along the radiating cable and that good quality measurements can be performed.

According to a further aspect of the invention, each remote unit further comprises an amplifier for amplifying the simulated GPS signal inserted into each radiating cable. Thereby, the simulated GPS signal can be amplified to a preferred maximum signal power that is preferably selected as an upper end of the dynamic range of the measuring units or slightly below the upper end. This ensures that even measuring units placed directly adjacent to an end of the radiating cable that is connected to the remote unit are able to receive the simulated GPS signal in the desired way. It is also highly advantageous that the amplification is adjustable so that the maximum signal power can be selected depending on the measuring units that are to be used with the system. In some embodiments, it would also be

advantageous to set the maximum signal power to a limit above the dynamic range of the measuring units to allow for the vicinity of the radiating cable where the signals can be received to be extended outwards so that the measuring units can be placed further away, even if this means that an area immediately adjacent to the radiating cable near the remote unit will not be included in the vicinity since the signal power in that area will be too large to be received by the measuring units.

According to yet another aspect of the invention, the at least one radiating cable comprises a plurality of radiating segments, said radiating segments being connected to each other to form the radiating cable, the radiating segments preferably being connected by coaxial cables for allowing the simulated GPS signal to be transmitted along the radiating cable. Thereby the radiating segments can be stored and transported in an easy and convenient way and be joined together by connectors such as coaxial cables or other suitable connecting means that are able to transfer the simulated GPS signal from one radiating segment to another. It also has the advantage of minimizing the risk of damage to the radiating cable when placed inside the mine, since the connectors can be made more robust and also since replacing a damaged radiating segment or connector is far easier and more cost effective than repairing or replacing a radiating cable that does not comprise segments.

According to the present invention, there is also provided a mining and mineral exploration method for performing distributed time-accurate measurements in a mine, the method comprising providing a simulated GPS signal in a local GPS unit, a remote unit operationally connected to the local GPS unit, and a plurality of measuring units, transmitting the simulated GPS signal from the local GPS unit to the remote unit; transmitting the simulated GPS signal from the remote unit in at least one radiating cable, wherein the radiating cable is configured to transmit the simulated GPS signal in a vicinity of the radiating cable, and receiving the simulated GPS signal in each of the measuring units for performing time-accurate

measurements, wherein the measuring units are each placed in the vicinity of the at least one radiating cable.

According to an aspect of the invention, the method further comprises transmitting the simulated GPS signal from the remote unit in at least two radiating cables, wherein the radiating cables extend in different directions from the remote unit. Preferably, the method also comprises transmitting the simulated GPS signal from the local GPS unit or from the remote unit to an additional remote unit through an optical fiber; transmitting the simulated GPS signal from the additional remote unit in at least one additional radiating cable, wherein the additional radiating cable is configured to transmit the simulated GPS signal in a vicinity of the additional radiating cable, receiving the simulated GPS signal in each of a plurality of additional measuring units for performing time-accurate measurements, wherein the additional measuring units are each placed in the vicinity of the at least one additional radiating cable.

Preferably, the method further comprises placing the remote unit and the additional remote unit on different levels of a mine.

The present invention also provides a method for adapting a measuring system to enable distributed time-accurate measurements in a mine, the method comprising providing a plurality of measuring units, wherein each of the measuring units comprises a receiver that is configured to receive a GPS signal, providing a remote unit that is operatively connected to a local GPS unit for receiving a simulated GPS signal that is generated by the local GPS unit, providing a radiating cable that is operatively connected to the remote unit and configured to transmit the simulated GPS signal in a vicinity of the radiating cable, and placing each of the measuring units in the vicinity of the radiating cable for receiving the simulated GPS signal. Thereby, a plurality of measuring units configured for use above ground can be transported into the mine and placed in the vicinity of a radiating cable that is able to transmit simulated GPS signals. This gives the significant advantage that measuring units need no modification to be able to be used with the measuring system in the mine, resulting in time and cost savings while also providing the time-synchronized measurements that are needed for the mining and exploration applications.

Many additional benefits and advantages of the invention will become readily apparent to the person skilled in the art in view of the detailed description below.

DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses a schematic view of a mine;

Fig. 2 discloses a schematic view of a system according to a preferred embodiment of the present invention; Fig. 3 discloses a schematic view of a system according to a second embodiment of the present invention;

Fig. 4 discloses a schematic view in more detail of the system according to the second embodiment;

Fig. 5a discloses a schematic view of a vicinity of a radiating cable according to the preferred embodiment or the second embodiment; and

Fig. 5b discloses a schematic view of another vicinity of a radiating cable according to the preferred embodiment or the second embodiment.

DETAILED DESCRIPTION

A GPS signal as defined herein is a signal from a global navigation satellite system such as the Global Positioning System (GPS), the GLONASS or a similar system in which signals are transmitted from a satellite and

receivable in a unit having a suitable receiver. When simulated GPS signals are described in the following, these are to be understood as signals that are similar to those signals that are transmitted from any of the satellites that form part of a global navigation satellite system.

For the present invention, the GPS signal or simulated GPS signal is used mainly for ensuring time accuracy and synchronization of a plurality or array of measuring units. There is also the possibility of using the GPS signal for positioning and this may be contemplated in some embodiments where it is deemed suitable. However, in most embodiments the GPS signal would be used only for timing and synchronization.

Fig. 1 discloses a schematic view of a mine 100 for exploiting a

mineralization 130 that extends below a ground level 140. A mineshaft 1 10 extends essentially vertically into the subsurface and a plurality of tunnels 150 are connected to the mineshaft 1 10 for exploration and mining and for extracting material removed from the mine 100. In some cases, access to the mining tunnels can also be possible via spiral-shaped tunnel ramps.

Some of the tunnels 124, 125, 123, 122, 121, 129 extend in a generally horizontal direction while other tunnels 126, 120, 127, 128 extend to connect the generally horizontal tunnels or to connect tunnels to the mineshaft 1 10. The mineralization 130 has an extension that is only partly covered by the tunnels for mining or exploration purposes and in order to determine the quality and extension of the mineralization 130 measurements need to be performed. Generally, measuring units act by emitting for example seismic signals that are transmitted through and / or reflected from the mineralization and depending on the reflection traveltime and properties of the reflected signals (amplitude) conclusions can be drawn regarding composition, shape and location of the mineralization. This is well known within the art and will not be described in detail herein, and it is also to be noted that what is said above regarding the mine is also general information that is aimed at an improved understanding of the present invention.

In order to perform the desired measurements, some measuring systems exist that rely in most cases on a GPS antenna at ground level for receiving GPS signals from satellites and transferring the received GPS signals through cable to a desired location within the mine bearing in mind only a limited cabling can be done in this case. In other systems, relative timing can be obtained using an accurate clock in deep mines and the measuring units are modified to be able to obtain time information and synchronization from this clock. This results in the need both to adapt the existing

measuring units and to apply a large quantity of cables to provide the GPS signal to each measuring unit.

However, mines are generally very large and complex in terms of shape and structures and in order to accurately detect the presence and extension of a mineralized body, measurements over a large area are needed, preferably also on separate levels of the mine or at least in different tunnels on the same levels. Using a modified measuring system as outlined above is expensive and cumbersome and can result in a low accuracy of

measurements. At the same time, building custom made measuring units for mining applications is very expensive and time consuming since

modifications depending on the precise circumstances of the location where the measuring units are to be placed are necessary.

Fig. 2 discloses a preferred embodiment of the present invention, having a local GPS unit 1 that generates a simulated GPS signal for use with a plurality of measuring units 4. The local GPS unit 1 is operatively connected to a remote unit 2, preferably through a fiber optic cable 5 but alternatively through a coaxial cable or by a transmitter at the local GPS unit 1 ,

preferably an antenna, and a receiver at the remote unit 2 if they are close enough that the simulated GPS signals can be received with a sufficient quality by the remote unit 2. In the preferred embodiment, at least one radiating cable 3 is operatively connected to the remote unit 2 for

transmitting the simulated GPS signal in a vicinity 6 of the radiating cable 3. Preferably, the radiating cable 3 is connected directly to an output of the remote unit 2 but there may also be intermediate components that transmit the simulated GPS signal from the remote unit 2 to an input end of the radiating cable 3. The measuring units 4 each comprise an internal GPS receiver for receiving the simulated GPS signal. Also, the measuring units 4 are placed in the vicinity of the radiating cable 3 and therefore able to receive the simulated GPS signal and to perform time-accurate and synchronized measurements because each measuring unit 4 receives the same simulated GPS signal at the same time.

The radiating cable 3 is also known as a leaky cable or leaky feeder and essentially comprises a coaxial cable that has gaps or slots in its outer conductor to allow a signal to leak out of the cable along its length. Thus, the radiating cable is essentially an extended antenna that is easy and convenient to handle and that is very suitable for the present application in transmitting a simulated GPS signal to a plurality of measuring units 4.

In the preferred embodiment disclosed in Fig. 2, two radiating cables 3 are connected to the remote unit 2 and provide two different vicinities in which measuring units 4 can be placed. There may be an overlap between these two vicinities but it is advantageous to keep them separated in order to cover as large area as possible. The vicinity 6 is discussed in detail below with reference to Fig. 5a-5b.

Fig. 3 discloses a second embodiment of the invention that differs from the preferred embodiment in that an additional remote unit 2’ is connected to the remote unit 2 and has at least one but preferably at least two additional radiating cables 3’ that each transmits the simulated GPS signal to a plurality of additional measuring units 4’. The additional remote unit 2’ is in this second embodiment connected to the remote unit 2 by a fiber optic cable 5 but here also a connection through an antenna and receiver is possible as described above with reference to Fig. 2. Alternatively, the additional remote unit 2’ can be connected to the local GPS unit 1, and this is especially advantageous when measurements are to be performed in a large area since the local GPS unit 1 can be placed in the middle and remote units 2, 2’ can be placed on either side of the local GPS unit 1. Of course, further remote units can also be connected in the same manner as the remote unit 2 and the additional remote unit 2’ so that the measuring system comprises any suitable number of units that each comprises a remote unit 2, at least one radiating cable 3 and a plurality of measuring units 4.

Due to the modular nature of the present invention, the local GPS unit 1 can be placed at a central location such as the tunnel 121 shown in Fig. 1. The remote unit 2 can be placed further along the same tunnel 121, whereas the additional remote unit 2’ can be placed in another part of that tunnel 121 or at a tunnel on a different level, such as tunnels 122 or 129. With a very large measuring system, remote units can be placed in any number of tunnels or tunnel portions on a plurality of levels and thereby enable time-accurate measurements over a very large area on multiple levels of the mine. This gives the opportunity for measurements of a much higher accuracy and data quality than previously possible with very low cost of modifying existing measuring systems.

Fig. 4 discloses the mining and mineral exploration system in more detail, and although the second embodiment is shown in this Figure, it is to be noted that what is said of each component also applies to the preferred embodiment and to other embodiments of the present invention.

Thus, the local GPS unit 1 comprises a highly accurate clock 1 1 that provides clock signals to a GPS simulator 12 where the simulated GPS signal is created as is well known within the art. The simulated GPS signal may be transmitted to a GPS master unit 13 before being transformed into an optical signal by a converter 14. The GPS master unit 13 may be connected to an antenna 15 that can be used where a remote unit 2 is placed close enough to receive the simulated GPS signal in this way. In some

embodiments, the GPS master unit 13 can be left out so that the GPS simulator 12 is connected directly to the converter 14 and/or the antenna 15.

A fiber optic cable 5 is connected to the converter 14 and transmits the simulated GPS signals to the remote unit 2 where they are received in an optical splitter 21 that splits the signals so that they can also be transmitted to at least one additional remote unit 2’. Alternatively, the optical splitter 21 can be left out if only one remote unit 2 is to be used in the system or if each remote unit 2, 2’ is to be connected directly to the local GPS unit, as has been described briefly above.

A second converter 22 receives the simulated GPS signals from the optical splitter 21 or from the fiber optic cable 5 directly. In the second converter, the optical signals are converted into radio signals that are transmitted to a remote control unit 23 that controls operation of the remote unit 2. An amplifier 24 is preferably connected to the remote control unit 23 for amplifying the simulated GPS signals before they are given as input to the radiating cable 3. If more than one radiating cable 3 is used, an RF splitter 25 is provided for receiving the amplified simulated GPS signal and splitting it to provide input for the two radiating cables 3 as disclosed in Fig. 4. The simulated GPS signal is then transmitted in the radiating cable 3, and due to the gaps or openings in the cover of the radiating cable 3 the radiating cable 3 is able to act as an antenna and transmit the simulated GPS signal in the vicinity along its entire length.

The control unit 23 may be operated by manually controlling its operation but may preferably be operated remotely, such as by including a master control unit (not shown) in the local GPS unit 1 that is configured to control operation of each remote unit 2, 2’ and that communicates with control units 23 of each remote unit 2, 2’ through the same fiber optic cable 5 or wireless connection through an antenna and receiver as the simulated GPS signals are transmitted. In some embodiments, the control units 23 may follow a predetermined mode of operation but in other embodiments the operation of the control units 23 may instead be dynamically adjusted by a person or by the master control unit. For example, the amplification of the simulated GPS signal before input into each radiating cable 3 may be adjusted depending on the dynamic range of the measuring units 4 or depending on a distance from the radiating cable 3 where it is desirable to place the measuring units 4, so that the size and shape of the vicinity 6 are adjusted. There may also be a feedback function so that an information regarding signal power or reception of the simulated GPS signal in the measuring units 4 is transmitted back to the remote unit 2, 2’ to allow for the amplification being set depending on such information.

The control unit 23 may also be a distributed control unit whose functions are distributed throughout the system.

The vicinity is defined as an area surrounding the radiating cable 3 where the simulated GPS signal is within a dynamic range of the measuring units 4, i.e. where the measuring units 4 are able to receive the simulated GPS signal. If the simulated GPS signal has a power that is above a maximum or below a minimum of the dynamic range of the measuring units 4, the simulated GPS signal cannot be received and used by the measuring units 4 which prevent them from performing the desired time-accurate

measurements. It is therefore highly advantageous to be able to detect the power of the simulated GPS signal at various distances from the radiating cable and also to be able to select the amplification of the simulated GPS signal used as input to the radiating cables 3. It is therefore in some applications highly advantageous to select the input power of the simulated GPS signal to the maximum of the dynamic range of the measuring units 4, and in some applications advantageous instead to select the input power so that the signal power at a far end of the radiating cable 3 is above the minimum of the dynamic range of the measuring units 4 since this allows the vicinity to be extended along to the end of the radiating cable. In some embodiments, a signal-to-noise ratio or power of the GPS signal can be performed to determine if the GPS signal should be amplified or otherwise adjusted to be able to achieve a desired result of being received by the measuring units 4.

It is advantageous therefore to limit a length of the radiating cable 3, preferably to a maximum of 200 m or less and more preferably to a

maximum of 100 or less. Thereby, the vicinity of the radiating cable 3 can be made to extend along the entire length of the radiating cable 3. It is advantageous to avoid line amplifiers along the length of the radiating cable 3, since the signal to noise ratio is significantly decreased by amplification so that the signal quality is rendered too poor to be able to be received by the measuring units 4.

In some parts of a mine 100, there may be radiating cables already in place for use in communications systems. These radiating cables are highly unsuitable for use as antennas in measuring systems such as that disclosed by the present invention, since the amplifications needed at intervals of 300- 500 m render a simulated GPS signal useless for the measuring units 4. If only a shorter length of such a radiating cable were to be used for

transmitting simulated GPS signals, this would also prevent the use of the communications system while measurements are being performed in order to achieve sufficient signal quality to reach at least some of the measuring units 4. For security reasons and for reasons of practicality it is not permitted to make such interruptions of the communications system and the presence of other signals transmitted at the same time would interfere with the operation of the measuring units. It would also be a problem to rely on radiating cables that form part of existing communications system in the mine since exploration and assessment of mineralization quality is generally performed in parts of the mine and within a short period of time where mining does not yet take place and where communications system are therefore not necessarily available.

However, the present invention as set out herein provides a highly

advantageous and suitable measurement system that overcomes all these drawbacks and challenges, and that provides time-accurate measurements of high quality as outlined above.

Fig. 5a discloses the vicinity 6 of a radiating cable 3 that has simulated GPS signal given as input from the remote unit 2. As shown in the Figure, the vicinity 6 has a shape that is wider at an input end where the signal is stronger and narrower at an output end where some of the power has been lost along the length of the radiating cable 3. In most applications, it is advantageous to provide a signal power at the output end that is at least at the minimum of the dynamic range of the measuring units 4, preferably above the minimum so that measuring units 4 can be placed at a distance from the radiating cable 3 and still receive the simulated GPS signal.

Fig. 5b discloses the vicinity 6 of the radiating cable where the input power of the simulated GPS signal is above the maximum of the dynamic range of the measuring units 4, so that the measuring units 4 have to be placed at a distance from the radiating cable 3. In this Figure, the vicinity 6 is wider at the output end since the signal power at this end is larger than in Fig. 5a. As is readily understood by the skilled person, it may be advantageous in different embodiments depending on the desired placement of the measuring units 4 to select the input power to adapt the width and extension of the vicinity to what is suitable at a particular time.

In one embodiment, the local GPS unit 1 may also comprise at least one radiating cable 3 and be configured to transmit the simulated GPS signal in the radiating cable 3. A plurality of measuring units 4 may in that

embodiment be placed in the vicinity of the radiating cable 3 and thus receive the simulated GPS signal in a similar manner as has already been described with reference to other embodiments. This has the benefit of creating a vicinity of the local GPS unit 1 where measuring units 4 can be placed in order to perform measurements without providing a remote unit 2 adjacent to the local GPS unit 1.

The measuring units 4 preferably comprise seismic receivers along with data acquisition systems, especially preferably in the form of Geophones (velocity- meter) or accelerometers. These geophysical receivers detect very small motions in the ground from active-seismic sources (controlled) and passive- seismic sources, including direct, refracted, reflected, diffracted and Rayleigh waves. A recording signal for a measuring unit for use with the present invention is a strength of vibration/motion, a so-called seismic amplitude, as a function of time (called a time-series signal). A time signal sampling is recorded in various ranges but can be as small as 0.25-2ms depending on the application. The system according to the present invention is preferably used to GPS time stamp a recorded signal with very high accuracy to be able to synchronize an array of measuring units, i.e. the plurality of measuring units 4 in a deep mine.

In some embodiments of the present invention, other types of measuring units can alternatively be used such as those used for electromagnetic surveys. In such embodiments the measuring units can measure electric and magnetic fields as a function of time. When time-synchronized

measuring units are used according to the present invention, models of resistivity or conductivity of subsurface materials can be provided.

In one embodiment, the radiating cable 3 comprises a plurality of radiating segments that are connected to each other by a plurality of connectors to form the radiating cable 3. Each radiating segment is then connected to a subsequent radiating segment by a connector that connects one end of a first radiating segment to one end of a subsequent radiating segment so that the radiating segments are connected one after the other in an elongated cable that forms the radiating cable 3. The radiating segments are shorter segments of radiating cable and the connectors are preferably coaxial cables but alternatively other connectors or connecting means that are able to transmit the simulated GPS signal from one radiating segment to the next. This is advantageous since the storage, transport and arrangement of the radiating cable 3 in the mine is significantly facilitated by avoiding very long radiating cables 3 that need to be stored and transported in a rolled-up form and that are very sensitive to damages, especially during arranging or mounting the radiating cable 3 in the inventive system before measurements are to take place. During use, the risk of damages to the radiating cable 3 is also significantly lower than for other embodiments, since the radiating segments can more easily be protected by placing them where they are not subjected to mechanical forces by persons, objects or vehicles in the mine. Also, if a radiating segment is damaged it can easily be replaced and a new radiating segment connected in its place, whereas a radiating cable that does not comprise segments would need to be replaced in its entirety or be removed for repair. Using radiating segments is therefore especially robust, cost effective and reliable and also lowers the risk of measurements being interrupted or postponed. The same is of course true for the connectors that can also be individually replaced in an easy and cost effective way if they should be damaged or otherwise malfunction. The mining and mineral exploration method for performing distributed time-accurate measurements in a mine according to the present invention will now be described in more detail.

In order to transmit simulated GPS signals in a synchronized manner to the measurement units 4, a simulated GPS signals is provided in a local GPS unit 1 and transmitted to the remote unit 2, preferably through an optical fiber but alternatively by an antenna and receiver as described briefly above. From the remote unit 2, the simulated GPS signal is transmitted in at least one radiating cable 3 so that a vicinity 6 of the radiating cable 3 is formed in which the simulated GPS signal is within a dynamic range of at least some of the plurality of measuring units 4. The dynamic range is defined herein as a range in which the simulated GPS signal is strong enough to be received by a measuring unit 4. If the signal is below the dynamic range, the measuring unit 4 cannot detect it and if the signal is above the dynamic range it can also not be detected. For one suitable type of measuring units the dynamic range has a maximum of -5dBm and a minimum of - 144dBm.

Since each measuring unit 4 of the plurality of measuring units 4 is able to receive the same simulated GPS signal at the same time, synchronized and time-accurate measurements can be performed by the measuring units 4.

In embodiments where there are multiple remote units 2, 2’, each remote unit 2, 2’ receives the simulated GPS signal from the local GPS unit 1 as described above, preferably through an optical fiber 5 that is connected to both the local GPS unit 1 and the remote unit 2, 2’ or to the remote unit 2 and the additional remote unit 2’, but alternatively through transmission by means of antenna and receiver or through a combination of these means or through any other suitable way for transmitting signals.

When setting up a system for performing time-accurate measurements, the remote units 2, 2’ may be placed in different tunnels or even on different levels, and the radiating cables 3 may extend in different directions from the remote unit 2, 2’ or alternatively may extend in one direction and be placed in parallel or at an angle towards each other. The deciding factor for selecting placement and extension of the remote units 2, 2’ and the radiating cables 3 and measuring units 4 is the requirements of a particular

measurement or series of measurements that is to be performed. This is readily understood by the skilled person and will not be described in more detail herein.

In one full-scale application of the present invention, at least 100 but preferably at least 1000 measuring units can be employed at different levels in a mine and also some corresponding measuring units placed on the surface above. By using the distributed time-accurate measurements of the present invention, synchronized measurements can be made of a large volume of material that is between the surface and the deep mining tunnels. Such measurements can serve to identify and delineate extension of existing ore bodies, mapping fracture and fault systems, performing seismicity studies in deep stressed mines and calibrating existing mine seismometers. Other applications include anisotropy mapping due to mining induced fracturing, ore quality characterization, blasting optimization, mining performance, exploration ahead of mining and 4D imaging and

characterization (i.e. time-evolution of various mining blocks).

A full-scale test in a real condition of the present invention has been performed in a massive sulphide underground mine. The system and method of the invention were employed and allowed time-synchronization of 30 wireless recorders in an exploration tunnel at a depth of 600 meters below the surface as well as over 400 measuring units in four exploration tunnels combined with 300 units on the surface. This experiment enabled high- resolution imaging between surface and tunnels as well as between tunnel and tunnel.

It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.