Technical Field

The present disclosure relates to methods for water ingress determination, in particular for water ingress determination in a ground overburden using particle detection.

Background

A wide variety of infrastructure, mining and similar projects involve in excavating or tunnelling into the ground, thereby generating a ground overburden. Ground overburdens may also result from natural features, for example, due to cave systems. A ground overburden comprises the natural and manmade materials (rock, soil, sand, concrete, brick and so on) which lies between a void (that is, a volume containing gas or fluid, such as air) in the ground and the surface of the Earth. The ground overburden comprises both the materials located between the void and the surface along a direct line between the centre of the planet and the surface, and also materials located proximate to such a line. The ground overburden may therefore comprise all materials which may be structurally impacted by the presence of the void.

The structural integrity of a void may degrade over time. Factors including weathering, application or removal of load, vibration and so on can lead to structural degradation. Structural degradation may result in dangerous weaknesses, for example, potential collapse of rail or road tunnel infrastructure. Accordingly, where structural weakness may lead to a risk to life or may cause economic loss, it is advisable to periodically assess the structural integrity of voids such that structural degradation may be identified and addressed.

When assessing the structural integrity of a void in the ground, it is typically necessary to determine the properties of the ground overburden. In particular, variations in the properties of the ground overburden may be indicative of weaknesses that may compromise the void. An example of this, from the field of tunnel engineering, is a situation in which movement in the material forming the ground overburden of a tunnel can indicate a risk of the tunnel structure becoming compromised and potentially collapsing. One of the key factors influencing the structural integrity of voids, particularly manmade voids such as bore holes, vehicle or pedestrian tunnels, and so on, is the ingress of water into ground overburdens. Where water penetrates into the ground overburden it can reduce the stability of the overburden, and may subsequently cause damage to the structure of the void. Continuing with the example voids of bore holes and tunnels, water penetration may cause: deterioration and/or erosion of mortar in masonry linings; corrosion of reinforcement and/or internal fittings; degradation/reduction in strength of concrete; lifting/damage to part of the floor of a tunnel; frost damage and other icing effects; loss of support due to small particulate matter movement; and so on. Determining a change in the amount of water in a ground overburden may therefore support rapid detection and remediation of potential damage causes.

There are several existing techniques for detecting water ingress into voids and overburdens. Measurements of rainfall onto the surface above the overburden may be taken using tipping bucket measurement systems, rain gauges, and so on. Measurements of water content near the surface of the ground may use time domain reflectometry, in which a number of probes are inserted into the ground and pulses transmitted between the probes to allow the ground capacitance to be measured. Water detection within voids (such as bore holes and tunnels) typically relies on visual inspection for signs of water penetration by a human expert, although water collection pots may also be used.

There are some issues with existing water ingress detection techniques. Measurement of rainfall onto the surface is not necessarily proportional to the amount of water (if any) in the ground overburden/void. Near surface measurements are also not necessarily representative of the water content of the entire ground overburden; typically the near surface measurement techniques obtain measurements within approximately 1 metre of the surface, while the ground overburden may be tens or hundreds of metres thick. Near surface measurements also provide very localised data (concerning only the ground between the probes, typically only a few metres); for large voids such as rail tunnels several hundred separate measurements may be required to cover the entire surface overlying the ground overburden. Visual inspections and water collection pots are both time consuming and typically require lengthy periods of access to the voids, which can be both difficult and economically damaging (for example, where the void is an active rail tunnel).

Summary It is desirable to provide methods for determining water ingress into ground overburdens that address one or more of the issues discussed above. The summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. The summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. For the avoidance of doubt, the scope of the claimed subject matter is defined by the claims

Embodiments of the disclosure provide methods for water ingress determination in ground overburdens. A method comprises positioning a particle detector directed towards the ground overburden and measuring a first flux of particles through the ground overburden, using the particle detector. The method further comprises determining the presence of water incident upon the ground overburden and, in response to determining the presence of water, measuring a second flux of particles through the ground overburden using the particle detector. The method also comprises comparing the first flux of particles and second flux of particles and deriving a water ingress measurement based on the comparison. The method may allow accurate estimation of water ingress into a ground overburden without requiring substantive invasive measurements of the overburden.

In some embodiments, a first surface flux of particles and/or a second surface flux of particles may be measured, prior and post the determination of the presence of water incident on the ground overburden respectively. The first surface flux and/or second surface flux may be used to improve the accuracy of the water ingress measurement, to reduce the impact of background noise, to identify anomalous results, and so on.

In some embodiments, at least one additional measurement of the flux of particles through the ground overburden may be made subsequent to the step of measuring the second flux of particles through the ground overburden. The at least one additional measurement of the flux of particles through the ground overburden may be used to estimate the variation in the water ingress measurement with time. Accordingly, the drainage of water from the ground overburden may be monitored and the effectiveness of any drainage solutions used in the ground overburden evaluated.

In some embodiments, the ground overburden may be sampled such that the composition of the ground overburden may be determined. The determined composition may be utilised in deriving the water ingress measurement. Where the ground overburden composition is determined, this information may improve the accuracy of the water ingress measurement.

Brief Description of

For a better understanding of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 is a flowchart of a method in accordance with embodiments;

Figure 2A is a schematic diagram of a particle detector that may be used in embodiments; Figures 2 Bi and 2Bii are schematic diagrams of a further particle detector that may be used in embodiments;

Figure 3 is a diagram of an example simulated void and overburden geometry; and Figure 4 plots results obtained from the simulated geometry of Figure 3.

Detailed Description

Embodiments disclosed herein may utilise particle detection when determining water ingress in ground overburdens. As will be appreciated by those skilled in the art, fluxes of particles may be generated by a wide range of natural and manmade sources; examples of the former include naturally occurring radioactive elements such as radon and thoron, while examples of the latter include particle generators. Although various different particle flux sources may be utilised, embodiments may be particularly well suited to utilising the flux of secondary particles generated by cosmic ray interactions with the atmosphere of the Earth. The flux of secondary particles may be particularly suitable as it is a natural source of particles (therefore no manmade source is required), the secondary particles are sufficiently penetrating to allow a measurable flux to be detected in a void under a ground overburden several tens of metres thick, and established techniques for monitoring secondary particles generated by cosmic ray interactions may be utilised. In some embodiments, the secondary particles are charged particles, typically electrons and/or muons, and the relative trajectories of the particles may also be detected.

Figure 1 is a flowchart showing a method in accordance with embodiments. The method may be performed using any suitable particle detector. Examples of suitable particle detectors for performing the method shown in Figure 1 are the particle detectors 20A and 20B shown schematically in Figure 2A and Figure 2B respectively; the particle detectors 20A and 20B may collectively be referred to using reference sign 20.

Figure 2A is a side view of particle detector 20A. As shown in Figure 2A, the particle detectors may comprise a plurality of detector elements 20Ai to 20Axx. The detector elements may be arranged in two arrays or layers 21, 22, each of which forms a position sensitive detector. Other forms of detector element arrangements, such as nested helices, may also be used in embodiments. The arrays 21, 22 in Figure 2A each comprise 10 detector elements; larger or smaller numbers of detector elements may be used in other arrays. Larger numbers of arrays may also be used, although typically use of a single array is avoided if possible as use of a single array can preclude the use of some background noise filtering techniques. Where two arrays 21, 22 are used, typically the arrays 21, 22 are parallel to each other and spaced apart vertically such that, in use, one of the arrays is located further from the centre of the Earth (and closer to the surface) than the other.

Where the arrays are spaced apart in the vertical direction, the array furthest from the centre of the Earth may be referred to as an upper detector array 21 and the other array may be referred to as a lower detector array 22. Where larger numbers of arrays are used, the arrays typically form a stack configuration with upper, middle and lower arrays. In Figure 2A the upper detector array 21 comprises detector elements 20Ai to 20Ax (labelled using roman numerals), and the lower detector array 22 comprises detector elements 20Axi to 20Axx. The detector elements shown in Figure 2A are in the form of bars, extending into the plane of the figure. In Figure 2A the detector elements in the upper and lower detector arrays 21, 22 extend parallel to one another; in an alternative configuration the bars of the upper detector array may extend in a direction substantially perpendicular to those of the lower detector array.

Each of the detector elements in the detector may be arranged to output a detection signal when it detects a cosmic ray particle, which is a secondary particle originating from a cosmic ray interaction with the atmosphere of the Earth, passing through it. Any suitable form of detector element may be used. Typical examples of detector elements utilise scintillators; materials that absorb energy from incident charged particles and then emit the absorbed energy as electromagnetic radiation (often in the visible region of the electromagnetic spectrum). Example detectors may comprise a scintillation material connected to a light detector such as a photomultiplier tube (PMT), silicon photomultiplier (SiPM) or photodiode via a waveguide (such as a fibreoptic or wavelength shifting fibre); using such a configuration, an incident charged particle (such as a secondary particle, typically an electron or muon, originating from a cosmic ray interaction) may cause the scintillation material to scintillate, and the resulting pulse of electromagnetic radiation may then be carried by the waveguide to the light detector and detected. Any suitable scintillator may be used, for example, polystyrene doped with one or more fluors such as 2,5-diphenyloxazole (PPO) and l,4-bis(5-phenyloxazol-2-yl)benzene (POPOP). Other forms of detector which may be used depending on availability and the particular requirements of a water ingress detection instance include radiochemical detectors, gas drift chambers and so on. Typically, the light detectors will indicate the detection of the pulse of electromagnetic radiation via an electrical signal. The detector may be configured such that the specific detector element in which an electromagnetic pulse originates can be determined; this may help improve the resolution of the detector. The electrical detection signal may then be processed using a processing unit 23.

As shown in Figure 2A, the processing unit 23 may be connected to the arrays 21, 22 and configured to receive the detection signals from the detector. Typically, the processing unit 23 functions as a coincidence detector that is configured to detect a particle passing through both arrays 21, 22. The timing of the detection signals and the positions of the detector elements which detect the particle may be used to estimate the direction, and hence the trajectory of the particle. Figure 2A shows a situation in which detector element 20Aiv in the upper array 21 and detector element 20Axvii in the lower array 22 have detected a particle. Using this information and the timing of the detection signals, the trajectory of the incident particle as shown by the dashed line 24 in Figure 2A may be estimated. As the arrival of particles is relatively infrequent (typically of the order of tens of particles per minute, with the exact rate dependent on the size of the detector elements, thickness of the ground overburden, density of the ground overburden, and so on), if a detection in the lower array occurs within a short time period of a detection in the upper array, the two detections can be assumed to be of the same particle. The use of coincidence detection allows false triggers due to background noise to be identified and excluded from particle flux measurements; this is one reason why use of a single planar array, excluding the possibility of coincidence detection use, is typically avoided.

The direction of the trajectory, that is, the incidence angle of the particle at the detector, may be measured as an angle 0 to the vertical (zenith) direction, along with an azimuthal angle cp (not shown in Figure 2A). The accuracy with which the trajectory can be estimated (the angular resolution) depends on the particular configuration of the detector; contributing factors include the dimensions of the detector elements, the relative spacings and number of the arrays, the accuracy with which the particle incidence time can be determined and so on. In general terms: the smaller the number of detector elements is; the closer the arrays are to one another; the smaller the number of arrays; and the lower the accuracy with which the particle incidence timings can be determined, the lower the angular resolution of the detector.

The trajectories of the particles may be used to allow water ingress measurements for ground overburdens not directly above the detector. As an example of this, the processing system may be configured to exclude particles having trajectories outside a given range of 0 and cp values from the particle flux measurements. Selecting the angular ranges in this way allows the field of vision of the detector to be directed towards the desired ground overburden, including where this ground overburden is not directly above the detector.

A typical processing unit, such as processing unit 23 of Figure 2A, may include a processor 25, a memory 26, a clock source 27 and a positioning system 28. The processing unit 23 may be configured to connect to the array readouts and to further systems as may be required. The processing system 23 may be configured to record and store trajectory information for detected particles (both particles identified as forming part of a particle flux of interest and potentially also particles identified as background noise), or the processing system 23 may be configured to store a particle count without storing trajectory information. Typically the time at which particles are detected is recorded; this information may be of particular use in subsequent analyses of data. The positioning system 28 may be a Global Navigational Satellite System (GNSS), or any other suitable positioning system. Where use of a satellite-based system is impractical due to the depth below ground of the intended measurement site, an alternative means for locating the detector (potentially including manual measurements input into the processing unit) may be used.

For some implementations of water ingress determination methods as discussed herein, it may be desirable to utilise a compact detector that is capable of operating under battery power. An example of a situation in which a compact and battery powered detector may be of use is where the detector is to be located in a void that is a road, pedestrian or rail tunnel (without access to mains power) for an extended period of time, and it is desired to continue utilising the tunnel (that is, vehicles or pedestrians may pass through the tunnel) while the detector is in position. In order to provide a compact detector with low power requirements that may be satisfied by a battery source, detectors having smaller numbers of arrays and numbers of detector elements within arrays may be used. Figure 2Bi shows a side view schematic of a compact detector arrangement, here using an upper detector array 21 and lower detector array 22 having two detector elements each. Figure 2 Bii shows a plan view of the same detector arrangement. In the configuration shown in Figure 2B, each detector element has dimensions of 200mm x 200mm x 10mm, and the gap between the upper and lower detector arrays is 150mm. In the configuration shown in Figure 2B, channels in the detector elements allow the positioning of wavelength shifting fibres 29 (4 wavelength shifting fibres per detector element are used), which act as waveguides to convey generated electromagnetic radiation to light detectors (here, SiPM ). A light detector may be used to monitor a single waveguide, or a plurality of waveguides, depending on the specific detector configuration used. The wavelength shifting fibres are shown using circles in Figure 2 Bi and dashed lines in Figure 2Bii. The detector elements may be formed, for example, from injection moulded slabs of scintillation material, where each element may be formed from plural slabs.

The compact configuration of the Figure 2B system allows the detector to be left in situ in a void (such as a tunnel, as discussed above), operating on battery power. The detector may then be collected after an extended period of operation, potentially of several days or weeks, and the data collected retrieved for subsequent analysis. The compact detector may also be easily protected from damage using a protective enclosure. In alternative configurations, detectors may be mounted on trolly systems or within vehicles (potentially operating using power supplied by the vehicle); this may be of particular use when it is desired to take a number of readings of short duration in a single measuring session, for example, along the length of a tunnel.

In accordance with water ingress detection methods disclosed herein, and as indicated in step S101 of Figure 1, a particle detector is positioned in such a way as to be directed towards a ground overburden to be monitored. The particle detector may be a particle detector 20 as shown in Figure 2 or another particle detector. The particle detector may be considered to be directed towards a ground overburden, or portion of the same, when the ground overburden is in the field of view of the particle detector. The particle detector may be located directly beneath (on a line extending from the ground overburden to the centre of the planet) the ground overburden, potentially the particle detector may be located in the void (such as a road, rail or pedestrian tunnel or bore hole) which the ground overburden overlies. Where the particle detector is of a type having a field of view which can be directed, the particle detector may be positioned so as to not be directly beneath the ground overburden but so as to be directed towards the ground overburden; in such a situation the detector may be located in the void or in another suitable location. As an illustrative example of another suitable location, where the overburden forms all or part of a hill and the void is a tunnel through the hill, the particle detector may be located adjacent to and directed towards the hill. When the particle detector has been positioned to be directed towards the ground overburden, the particle detector is then used to measure a first flux of particles through the ground overburden, as shown in step S102. The first flux of particles may be measured using a single measurement over a suitable time frame; the time frame used may be determined by the period required by the particle detector being used to obtain an accurate reading of the particle flux. Typically, a single measurement time frame is of the order of 1 hour to 24 hours. As an alternative to using a single measurement, a plurality of measurement may be taken and then averaged in order to obtain a measurement of the first flux of particles; this option has the advantage relative to use of a single measurement of reducing the impact of fluctuations in the particle flux, and the drawback of increasing the time required to obtain the measurement of the first flux. The flux of particles is measured as a number of particles per unit area of detection surface (of the detection elements) and unit time, for example particles per square meter per second. A typical measured flux may be of the order of 0.01 particle per square centimetre of detection surface per second.

In some embodiments a further detector, which may be substantially the same as the particle detector directed towards the ground overburden or may be of a different type, may be used to measure a first surface flux of particles. Where a further detector is used, the first surface flux of particles is typically measured at the surface of the Earth above the ground overburden, in a location which would lie in the field of view of the particle detector directed towards the ground overburden. Measurement of the first surface flux may allow an estimate of the attenuation of the flux due to the ground overburden to be obtained, may assist in the discrimination of the particle flux from background noise sources (for example, due to the presence of radioactive elements in the surrounding environment), may allow anomalous results from the measurement of the first flux of particles through the ground overburden to be identified, and so on. The first surface flux measurements may therefore be of use when deriving the water ingress estimate.

Typically the first surface flux is measured prior to determining the presence of water incident upon the ground overburden (see step S103), and preferably the measurement of the first surface flux is at least partially concurrent with the measurement of the first flux of particles through the ground overburden; this is useful where the first surface flux is to be used for background noise discrimination and anomalous result identification in particular. In some embodiments the same detector may be used to measure the first surface flux and the first flux. Although use of the same detector avoids potential disparities in the measured flux due to detector differences, use of the same detector also precludes simultaneous measurements of the first surface flux and first flux.

After the first flux of particles through the ground overburden (and potentially the first surface flux) have been measured, the presence of water incident upon the ground overburden is then determined as shown in step S103. The particle detector(s) used to determine the first flux and first surface flux are not necessarily deactivated once the first flux and first surface flux have been measured. Instead, the particle detector(s) may continue to take measurements and the measurements may subsequently (in an analysis after the measurements have been taken) be differentiated into those obtained before the presence of water incident upon the ground overburden is determined and those after the presence of water incident upon the ground overburden is determined. Typically all of the particle flux measurements and the presence of water incident upon the ground overburden determination are given a time stamp by the particle detector relative to a universally applied clock source (for example, relative to Greenwich Mean Time, GMT), so differentiating between flux measurements before and after surface water determination in subsequent analyses may be achieved.

The presence of water incident upon the ground overburden may result from natural events, such as rain or snowfall. However, in some situations it may not be suitable to use natural sources of water, either because such sources are not available or because it is necessary to complete measurements during a restricted time frame. Where natural sources of water are not suitable for use, water may be applied to the surface of the ground overburden using any suitable water delivery system such as a sprinkler system, water tank, and so on.

The presence of water incident upon the ground overburden may be determined by human observation, using weather reports, using instrumentation located on or near (either above or below) the surface of the ground overburden, and so on. The means used to determine the presence of water may be tailored based on the requirements of a given water ingress determination, for example, where more precise results are required an instrumental measurement of the water may be favoured over human observation. In some embodiments, the presence of water incident upon the ground overburden may be determined when the estimated volume of water per square meter (of ground overburden surface) incident on the surface of the ground overburden is above a predetermined threshold, said threshold being set based on the requirements for a given water ingress determination. The predetermined threshold may be satisfied when, for example, a given amount of rain (as may be measured using a rain gauge) has fallen upon the ground overburden. The predetermined threshold may also incorporate a time component, that is, may be satisfied only if a given volume of water is incident upon each square meter of the ground overburden within a given time frame. Thresholds may help ensure the robustness of the ground overburden. If natural sources of water (as discussed above) are insufficient to satisfy the predetermined threshold for a given water ingress determination, the natural sources may be supplemented using a sprinkler system or similar as discussed above. Where particle detectors are used to take continuous measurements as discussed above, the time at which the predetermined threshold is reached may be used as the point at which it is determined that sufficient water has come into contact with the ground overburden to make a water ingress determination. Further, measurements from the continuous measurement series that are within a given interval before the point at which the presence of water incident upon the ground overburden is determined may be discarded, as these measurements may also show evidence of water ingress and may therefore reduce the accuracy of subsequent water ingress derivations. The given interval may be based on the situation of the experiment (for example, may be set as the time period between the start of rainfall and the point at which the presence of water incident upon the ground overburden is determined), or may be set as a given interval (for example, 30 minutes).

Once the presence of water incident upon the ground overburden has been determined, a second flux of particles through the ground overburden is then measured, as shown in step S104. The second flux of particles through the ground overburden is typically, although not always, measured using the same particle detector as was used to measure the first flux of particles through the ground overburden to reduce the possibility for the introduction of errors due to changes in measurement setups. If possible, the particle detector may remain static during and between the measurements of the first flux and second flux of particles through the ground overburden. Further, to provide more detailed results (as may be used to generate a topological profile) and to allow detection of erroneous results, one or more additional detectors may be used to monitor the same ground overburden, that is, to measure a first flux and/or second flux of particles through the ground overburden.

In some embodiments a second surface flux of particles may also be measured. As in embodiments wherein a first surface flux of particles is determined, the second surface flux of particles may be used to estimate attenuation due to the ground overburden, assist in the discrimination of the particle flux from background noise, allow anomalous results to be identified, and so on. The measurement of the second surface flux of particles is preferably at least partially concurrent with the measurement of the second flux of particles through the ground overburden. In some embodiments the measurements for the second surface flux and second flux may be substantially simultaneous (as for the first surface flux and first flux). The second surface flux may be measured in situations where a first surface flux has not been measured, although typically where one of the first surface flux and second surface flux is measured the other is also measured (often using the same detector).

Once the second flux of particles through the ground overburden (and potentially also the second surface flux) have been measured, the measurements may then be analysed. A comparison between the fluxes of particles may then be made as shown in step S105. As explained above, the analysis of the measurements of the fluxes may be performed at any time after the measurements have been taken, either at the measurement site or at a different location (such as a laboratory). The comparison between the measurements of the first flux and second flux through the ground overburden is typically a numerical comparison. The analysis then continues as the results of the comparison are used to derive a water ingress measurement, as shown in step S106. The water ingress measurement may be simple binary result, that is, water has or has not penetrated the ground overburden. Typically the water ingress measurement is more detailed and provides an estimate of the amount of water per unit volume in the ground overburden.

In some embodiments the accuracy of the water ingress measurement may be improved by reducing the impact of factors which may introduce errors; first and/or second surface flux measurements may be used to help reduce the impact of background sources, fluctuations in the particle source, and so on. The water ingress measurement also typically relies on modelling of the ground overburden; it is useful to know the composition of the ground overburden so that the density of the ground overburden (and how that density may vary with water content) can be used in the water ingress measurement. Accordingly, the accuracy of the water ingress measurement may be improved by sampling the ground overburden and determining the composition of the overburden (soil type where the overburden is formed from soil, porosity, and so on). Where the ground overburden is sampled, this may be achieved using physical sampling, for example, by drilling a core sample. Additionally or alternatively the sampling may comprise electronic sampling based on ground penetrating radar or the like. To obtain the water ingress measurement the ground overburden may be modelled in some embodiments, either using a machine learning system such as a neural network or using empirical fitting of data. The modelling of the ground overburden may not be used in particular where the water ingress measurement is a simple binary result as discussed above, in which case the comparison between the first flux and second flux may provide all the information necessary to determine whether the flux has changed due to water ingress into the overburden. Where the ground overburden is modelled, the model may take as inputs information on the composition of the ground overburden that may be obtained through sampling as discussed above.

The modelling of the ground overburden may comprise utilising computer simulations to infer the presence, and potentially quantity, of water in the ground overburden that is the subject of the measurement. The flux of particles through a ground overburden is directly related to the opacity of the ground overburden to the particles, which in turn is directly related to the density and thickness of the ground overburden. The thickness of the ground overburden is very unlikely to substantially change over a measurement period, so any change in the opacity before and after the presence of water on the surface of the ground overburden is detected can be attributed to variations in the density of the ground overburden. The addition of water to a ground overburden has an impact on the density of the ground overburden, and therefore on the penetration of secondary particles originating from cosmic ray interactions (such as electrons and muons) through the ground overburden. Accordingly, measurements of the particle flux before and after surface water detection can enable water ingress determination.

A simple binary determination (water ingress or no water ingress) can be made by detection of a variation between the first flux of particles through the ground overburden in the field of view of the particle detector and second flux of particles through the ground overburden in the field of view of the detector, however to accurately estimate the quantity of water present more analysis is required. Where the modelling uses the composition of the ground overburden, the change in water content of the overburden (as an estimate of the volume of water per unit volume of the overburden) can be directly related to the variation in particle flux. Typically, the presence of water in a ground overburden increases the density, and therefore increases the opacity and decreases the particle flux penetrating the ground overburden.

Figure 3A and Figure 3B are diagrams of an example simulated void and overburden geometry, which may be used to determine measurement periods necessary to obtain accurate water ingress determinations in a given scenario. Figure 3A shows a side view of the simulated geometry, and Figure 3B shows an end view. In the example shown in Figure 3, the simulated geometry includes a void 31 20m below the surface of the ground overburden (not labelled in Figure 3; the ground overburden lies above the void 31). The simulated ground overburden is formed from silty soil of 50% porosity. The example simulation includes a series of volumes, specifically cubes, 32 (which are comprised in the ground overburden 31) having side lengths of 4m of varying water content located in the ground overburden above the void; the water content of the cubes has a cyclic pattern from 50% to 0% to 50%, in steps of 10%. The simulation also includes a compact detector system 33, modelling that shown in Figure 2B . The simulated geometry may be used in a Monte Carlo simulation; when the simulation is executed, the location of the detector system is adjusted such that it steps along the series of cubes (in Figure 3, the detector system is shown located below the centre cube of the 9 cubes). Each set of simulations comprises a single configuration of cyclic pattern in the cubes, and the muon rate is plotted as a function of the known water content of the cube directly above the muon detector system. Figure 4 plots the results obtained through the Monte Carlo simulation. The x axes of both plots in Figure 4 are the normalised volumetric water content in the simulated volumes (cubes) above the detector, wherein 0.0 indicates no water is present and 0.5 indicates that 50% of the content of the cube by mass is water. The top plot indicates the rate of particles detected (the flux) per second, while the bottom plot shows the error in the rate of particles. Figure 4 shows simulated results obtained using a short testing period and compact detector, however even taking these factors into consideration the variation in the detected flux with water content can clearly be seen. The error between the simulated particle flux and the simulated detected flux for the example shown in Figure 4 is in the region of 0.5% to 2%

Table 1 below shows an example of the relationship between water content, density and flux of muons for a situation in which the ground overburden in the field of view of the particle detector is 20m thick and has been determined to be silicate soil with a porosity of 50%. Use of information in a table such as Table 1, in combination with the first and second fluxes of particles through the ground overburden, allows an accurate measurement of water ingress to be made. Table 1

The water ingress determination result may be provided as a qualitative result relative to a known water content in addition to or alternatively to a quantitative estimate. As an example of this, where a number of readings are being taken in similar conditions (for example, at different locations in the same tunnel) it may be the case that the ground overburden is known to have an acceptable degree of water content at a certain location in the tunnel (for example, less than 5% water by volume). The second particle flux (after surface water determination) at this certain location may therefore be taken as an acceptable particle flux. If the second particle flux recorded at a further location is substantially equivalent to the acceptable particle flux, then a qualitative determination that the water ingress into the overburden at the further location is acceptable can be made. By contrast, if the second particle flux at the further location is significantly different (e.g. lower) than the acceptable particle flux, then the water ingress into the overburden at the further location may be qualitatively determined to not be acceptable.

One or more additional measurements of both the flux through the ground overburden and the surface flux may also be taken subsequent to the second measurement of the second flux of particles through the ground overburden. These additional measurements may be taken in the same way as discussed above in the context of the first and second measurements of the flux through the ground overburden and of the surface flux, and may also be taken with the particle detector(s) at the same locations as were used for the first and second measurements of the flux through the ground overburden and of the surface flux. The additional measurements may cover an extended period of time, of between a few hours and several days or weeks. When the additional measurements are used, the total period over which measurements are taken may encompass times before, during and after water is incident upon the ground overburden. In some embodiments, the additional measurements may be used to effectively monitor water ingress into the ground overburden, and to monitor the subsequent water egress from the ground overburden (via draining, evaporation, and so on). In this way, measurements of the effectiveness of any drainage solutions used to drain water from the ground overburden may be made. Additional measurements may also or alternatively be taken at different locations within a void (that is, wherein the ground overburden in the field of view of the detector is different for different measurements), thereby allowing the density of the ground overburden at a variety of locations to be measured and a topological profile of the ground overburden to be produced.

Methods in accordance with embodiments may allow accurate estimation of water ingress into ground overburden, and also measurements of the effectiveness of drainage solutions provided, without requiring substantial invasive measurements of the overburden. Methods may be implemented without requiring extended human activity, and can be implemented over extended periods of time if desired. Methods may be implemented without substantial disruption to human activity in voids (for example, without disrupting the use of pedestrian or vehicle tunnels), and can be tailored as required for accuracy and speed.

References in the present disclosure to "one embodiment", "an embodiment" and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It should be understood that, although the terms "first", "second" and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed terms.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "has", "having", "includes" and/or "including", when used herein, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components and/ or combinations thereof. The terms "connect", "connects", "connecting" and/or "connected" used herein cover the direct and/or indirect connection between two elements.

The present disclosure includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof. Various modifications and adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. For the avoidance of doubt, the scope of the disclosure is defined by the claims.