Overburden positioning method

Abstract

A first particle detector 60A is positioned on an upper surface and a second particle detector 60B is positioned on a lower surface. The upper and lower surfaces are separated by an overburden. The position of the second particle detector relative to the first is based on detecting a flux of particles 69 through the first particle detector and the second particle detector. The particles may be cosmic ray muons. The measurements at the two detectors may be caused by interactions at each detector from the same particle. Determining the position of the second detector may involve measuring time interval between the detected flux at the two detectors and the trajectory of the flux of particles. The magnitude of the flux may indicate an anomalous region 67 e.g. void in the overburden. The method may be used to determine a map of a void or tunnel in which the second particle detector is located. The second particle detector is moved inside the void or tunnel. The location of the second particle detector in the void / tunnel is determined relative to the first particle detector on the surface in order to generate the map.

Description

Technical Field The present disclosure relates to methods for positioning through an overburden, in particular for positioning particle detectors and anomalous regions through an overburden using particle detection. Background The detection, location, and mapping of underground assets and / or subterranean objects is important across construction, archaeology, infrastructure, exploration, military and other sectors concerned with subterranean profiling. Underground assets may include, for example, existing tunnel networks and structures. In cases where the full shape and size of a tunnel network is unknown, for example excavation projects, it may be beneficial to generate a map or image of one or more tunnels in the network to avoid possible risk to the safety of workers and / or damage to any present assets. This problem is exacerbated for locations and types of services that have poor documentation or have an older date of installation. In addition, to avoid asset strikes and the like of any present subterranean objects, said subterranean objects may need to be detected before significant excavation activities take place. A wide variety of infrastructure, mining and similar projects involve excavating or tunnelling into the ground, thereby generating a ground overburden. Ground overburdens may also result from natural features, for example, due to cave or tunnel systems. A ground overburden comprises the natural and manmade materials (rock, soil, sand, concrete, brick and so on) which lies between a void (which may be 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. This void may for example be an underground asset, for example a tunnel network or cave system. The structural integrity of a void or tunnel may degrade over time, from factors including weathering, ingress of water, application or removal of load, and vibration from activity in the tunnel or on the above surface. Structural degradation may result in dangerous weaknesses, for example, potential collapse of rail or road tunnel infrastructure which in turn may result in older tunnel network mappings being inaccurate or outdated. Accordingly, where structural weakness are likely, it is advisable to periodically remap void and / or tunnel networks such that any structural degradation may be identified and updated maps generated. When assessing the structural integrity of a void in the ground, it may be beneficial to determine if any anomalous regions are present in the overburden which may in turn impact the integrity of the tunnel or void underneath. In particular, the presence of anomalous regions in the ground overburden may be indicative of possible threats to the structural integrity of the tunnel such as pockets of water or slurry in the overburden. Anomalous regions may also be indicative of further tunnels or man-made features, such as rooms comprising equipment or other objects in a wider tunnel network. There are several existing techniques for subterranean profiling that can be used for the purposes detailed above. For example, existing techniques include the use of probes embedded into the surface of a volume of ground. However, such near surface measurements are not necessarily representative of the subterranean object or underground asset 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 or volume of ground to be considered 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. Alternatively, subterranean objects may be detected and / or underground assets may be mapped using visual inspections (potentially in conjunction with excavations). For example, for uncharted tunnel and void systems may be mapped by visual inspection, by entering the tunnel system and map the system dragging a wire or using a laser rangefinder. However, visual inspections (and potential associated excavations) are both time consuming and (for example, where the void is an active rail tunnel) may require lengthy periods of access to the voids, which can be both difficult and economically damaging and may further pose a health risk to those entering the tunnel for inspection purposes. Excavations themselves can also be costly and inconvenient especially if excavation is done in an unnecessary place. Summary It is desirable to provide methods for positioning and mapping underground assets through an overburden that addresses one or more of the issues discussed above. For example, it is desirable to provide methods for positioning and mapping underground assets through an overburden that allows for easier, quicker, and / or more efficient detection and map generation. In addition, it is desirable to provide methods for positioning and mapping underground assets through an overburden that permit deeper / wider / more extensive probing of a surface and / or improve the probability of detection and reduce false alarms. 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 appended claims. Embodiments of the present disclosure provide methods for positioning and mapping underground assets through an overburden. The method comprises positioning a first particle detector on an upper surface, positioning a second particle detector on a lower surface, wherein the upper surface and the lower surface are separated by the overburden, detecting a flux of particles through the first particle detector and the second particle detector, and determining a position of the second particle detector using the detected flux of particles. In some embodiments, determining a position of the second particle detector comprises determining a time interval between a first measurement of the detected flux of particles at the first particle detector and a second measurement of the detected flux of particles at the second particle detector. The first measurement of the detected flux and the second measurement of the detected flux may be associated with a particle among the flux of particles. Alternatively or additionally, in some embodiments determining a position of the second particle detector comprises determining a first trajectory of the detected flux of particles through the first particle detector and a second trajectory of the detected flux of particles through the second particle detector, wherein the first trajectory and the second trajectory are aligned with one another. The first trajectory of the detected flux and the second trajectory of the detected flux may be associated with a particle among the flux of particles. In some embodiments, the method further comprises determining a magnitude of the flux of particles at the second particle detector, and determining whether an anomalous region is present in the overburden based on the determined magnitude of the flux of particles. The method may further comprise comparing the determined magnitude of the flux of particles with an expected magnitude of the flux of particles. Alternatively or additionally, the method may further comprise determining a magnitude of the flux of particles at a plurality of positions on the lower surface, determining a variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface, and upon determining the presence of an anomalous region in the overburden, imaging the anomalous region based on the variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface. The anomalous region may be a region of anomalous density or a void. In some embodiments, the particles are muons. In more specific embodiments, the muons are cosmic ray muons. In some embodiments, the method further comprises positioning a plurality of second particle detectors on the lower surface, detecting the flux of particles through each of the second particle detectors and the first particle detector, and determining a position of each second particle detector using the detected flux of particles associated with said second particle detector. Alternatively or additionally, in some embodiments the method further comprises positioning a plurality of first particle detectors on the upper surface, detecting the flux of particles through the second particle detector and at least one of the first particle detectors, and determining a position of the second particle detector using the detected flux of particles. Further alternatively or additionally, in some embodiments the method further comprises positioning a plurality of first particle detectors on the upper surface, positioning a plurality of second particle detectors on the lower surface, detecting the flux of particles through each of the second particle detectors and at least one of the first particle detectors, and determining a position of each second particle detector using the detected flux of particles associated with said second particle detector. The method may further comprise synchronising each of the first particle detectors with one another. Alternatively or additionally, the method may further comprise synchronising each of the second particle detectors with one another. In some embodiments, the method further comprises generating a map of the position of the second detector relative to the first particle detector. In more specific embodiments, the method further comprises determining a position of the first particle detectors using a Global Navigation Satellite System (GNSS) network, and generating a map of the position of the second detector relative to the first particle detector using the GNSS location of the first particle detector. In some embodiments, the method further comprises positioning a third particle detector on a further lower surface, wherein the lower surface and the further lower surface are separated by a further overburden, detecting the presence of a further flux of particles at the third particle detector using the second particle detector, and determining a position of the third particle detector using the detected flux of particles. In more specific embodiments, the method further comprises positioning a plurality of third particle detectors on a further lower surface, detecting the presence of a further flux of particles at each of the third particle detectors using the second particle detector, and determining a position of each third particle detector using the detected flux of particles. The further lower surface may be closer to the central point of the Earth than the lower surface. In some embodiments, the lower surface may be closer to the central point of the Earth than the upper surface. Alternatively or additionally, the overburden may be one of: a volume of water, a volume of earth, or a man-made structure. Brief Description of Drawings 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 of the present disclosure; Figure 2A is a schematic diagram of a primary particle detector that may be used in embodiments; Figures 2Bi and 2Bii are schematic diagrams of a further primary particle detector that may be used in embodiments; Figure 3 is a diagram of an example first particle detector interacting with a particle; Figure 4 is a diagram of a further example particle detector interacting with a particle; Figure 5 is a diagram of an example second particle detector interacting with a particle; Figure 6 is a diagram of an apparatus performing a method in accordance with embodiments of the present disclosure in a first geometry; and Figure 7 is a diagram of an apparatus performing a method in accordance with embodiments of the present disclosure in a second geometry. Detailed Description Embodiments disclosed herein may utilise particle detection when positioning through an overburden. For example, an overburden may be one of: a volume of water, a volume of earth, or a man-made structure. 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 thorium, while examples of the latter include particle generators. In some embodiments, the particle flux may comprise primary particles that are cosmic ray muons, typically generated by interactions between high energy cosmic rays (for example, protons) with the upper atmosphere of the Earth. Embodiments may also utilise measurements of the flux of secondary particles generated by primary particle interactions with the Earth and subterranean objects. The use of cosmic ray muons (as primary particles) in conjunction with measurements of the flux of primary and secondary particles may be particularly suitable as cosmic ray muons are a natural source of particles (therefore no manmade source is required), the primary particles are sufficiently penetrating to allow subterranean object detection across a volume of ground several tens of metres thick, and established techniques for monitoring primary and secondary particles may be utilised. In some embodiments, 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 detectors, in particular any suitable combination of first particle detectors and second particle detectors. Examples of suitable first particle detectors for performing the method shown in Figure 1 are the particle detectors 31 / 32 as shown in Figure 3 (which may collectively be referred to using reference sign 30) and particle detectors 41 / 42 shown in Figure 4 (which may collectively be referred to using reference sign 40). Examples of suitable second particle detectors for performing the method shown in Figure 1 are the particle detectors 51 / 52 as shown in Figure 5 (which may collectively be referred to using reference sign 50). Further 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. In embodiments of the present invention, the method S100 comprises positioning a first particle detector on an upper surface in Step S101. Figure 3 demonstrates a side view of a first particle detector positioned on an upper surface. As shown in Figure 3, Figure 4, and Figure 5, a particle detector suitable for the method of the present invention may comprise a primary particle detection portion 31 / 41 / 51 and a secondary particle detection portion 32 / 42 / 52. Alternatively, detection portions 31 / 41 / 51 and 32 / 42 / 52 may both be configured to detect primary particles and secondary particles. The primary and / or secondary particles may form a part of the flux of particles being measured through the first and second particle detectors. Other forms of detector element arrangements, such as nested helices, may also be used in embodiments. As shown in Figure 2A, the primary particle detection portion 31 / 41 / 51 may comprise two arrays 21, 22. In some embodiments, the secondary particle detection portion may have the same configuration, or alternatively may have a different configuration. Each array 21, 22 may comprise a plurality of detector elements (the detector elements are labelled 20Ai to 20Axx in Figure 2A). The detector elements may be arranged in two arrays or layers 21, 22, each of which forms a position sensitive detector. 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 or smaller numbers of arrays may also be used, although use of a single array may be avoided if possible as use of a single array may preclude the use of some background noise filtering techniques. Where two arrays 21, 22 are used, the arrays 21,22 may be 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 may 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 primary particle detection portion may be arranged to output a detection signal when it detects a cosmic ray particle, which may be a primary particle originating from a cosmic ray interaction with the atmosphere of the Earth, passing through it. In some embodiments, the primary particle may be a muon. Any suitable form of detector element may be used. Typical examples of detector elements utilise scintillators; scintillators are 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, which may be 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-d I phenyl oxazole (PPO) and 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP). Other forms of detection medium which may be used depending on availability and the particular requirements of a detector 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 detection rate of primary particles is relatively infrequent (for example, of the order of tens of particles per minute, with the exact rate dependent on the size of the detector elements, thickness of any volume of ground overlying the detector, density of any ground overlying the detector, and so on), if a detection in the lower array of the primary particle detection portion occurs within a short time period of a detection in the upper array of the primary particle detection portion, 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, may be 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 q> (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 particles through a first particle detector and a second particle detector may be used to determine the position of the secondary particle detector with reference to the first particle detector. That is, a determination that a detected trajectory at a second particle detector corresponds to a detected trajectory at a first particle detector may be used to determine the position of the secondary particle detector with reference to the first particle detector. As an example of this, the processing system may be configured to exclude particles having trajectories outside a given range of 0 and (p values from the particle flux measurements. Selecting the angular ranges in this way allows the field of vision of the first particle detector to be directed towards the estimated or assumed position of the second particle detector, for example in a case where the second particle detector is not directly below the first particle 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 methods of positioning through an overburden 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, or wherein the detector is to be positioned in a remote location where a mains power connection cannot practically be provided. A further example of a situation in which a compact and battery powered detector may be of use is in a case where the detector is intended to be mobile for the purposes of generating a map of a void or tunnel network, while reducing the need for additional equipment to move the detector. 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 primary particle detection portion arrangement, here using an upper detector array 21 and lower detector array 22 having two detector elements each. Figure 2Bii 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 2Bi 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 may allow for the detector to be easily protected from damage using a protective enclosure. Alternatively or additionally, detectors may be mounted on trolly systems or within vehicles (potentially operating using power supplied by the vehicle) such that the detectors are mobile; 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 the methods of positioning through an overburden disclosed herein, and as indicated in step S101 of Figure 1, a first particle detector is positioned on an upper surface. This upper surface may for example be an upper surface of an overburden. The first particle detector may be positioned in such a way as to be directed towards a volume of ground to be monitored, wherein it is estimated or predicted that a second particle detector is present underneath the overburden forming a part of the volume of ground. The primary particle detection portion of the first particle detector may be a particle detector as shown in Figure 2 or another particle detector. Possible geometries for the first particle detector relative to the overburden and / or volume of ground to be monitored are shown in Figure 3 and Figure 4. A possible geometry for the second particle detector relative to the overburden is shown in Figure 5. It should be understood that the teachings above regarding Figure 2, which are provided with reference to the geometry of Figure 3, are also applicable to the second particle detector of present embodiments and in particular are applicable to the geometries depicted in Figure 4 and Figure 5. The first particle detector may be considered to be directed towards a volume of ground to be monitored when the volume of ground to be monitored is in the field of view of the first particle detector. Figure 3 demonstrates a first particle detector performing a method in accordance with present embodiments, for example, for the purposes of positioning a second particle detector through an overburden wherein the first particle detector is positioned above the volume of ground 33 to be monitored. An example primary particle forming a part of the flux passing through the first particle detector travels along example primary particle trajectory 34, which intersects primary particle detection portion 31 and secondary particle detection portion 32. The primary particle may be a muon, and the primary particle detection portion 31 may be a muon detector. The secondary or correlated particles may be neutrons and / or gamma rays, and the secondary particle detection portion 32 may be a neutron and / or gamma ray detector. When the first particle detector has been positioned towards the volume of ground, the primary particle detection portion of the first particle detector may be used to measure a flux of primary particles towards the volume of ground using the primary particle detection portion, wherein the primary particles may be muons. The flux of primary 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. The measurements of the flux of particles through the first particle detector may comprise measurements of the trajectory of at least one primary particle through the primary particle detection portion, and / or a timing / time stamp of at least one primary particle passing through the primary particle detection portion. Alternatively or additionally, the measurements of the flux of particles through the first particle detector may comprise measurements of the trajectory of at least one secondary particle through the secondary particle detection portion and / or a timing / time stamp of at least one secondary particle passing through the secondary particle detection portion as discussed below. 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 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 flux. The flux of particles may be 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. The secondary particle detection portion of the first particle detector may be used to measure a flux of secondary particles emitted from the volume of ground, wherein the secondary particles are generated through interactions between the primary particles and the volume of ground. With reference to Figure 3, the primary particle may interact with the volume of ground 33 at an interaction point 36 to produce one or more secondary particles. Alternatively, as demonstrated in Figure 4, the primary particle may interact with a subterranean object 47 at an interaction point 36 to produce one or more secondary particles. The measurements of the flux of secondary particles may comprise measurements of at least one of the trajectory of the secondary particles through the secondary particle detection portion and energy deposited in the secondary particle detection portion by the secondary particles. As detailed above, the measurement of flux of particles through the first particle detector may comprise detections of primary particles and / or the detection of secondary particles. As shown in Figure 3 and Figure 4, the secondary particle(s) may travel in a direction which intersects secondary particle detection portion 32 such as example secondary particle trajectory 35. The secondary particles produced by the interaction between the primary particle and the ground at the interaction point 36 and the properties associated with the secondary particles may be dependent on the material of the volume of ground. Such properties include multiplicity, energy and / or timing properties of the secondary particles. Figure 4 demonstrates a further first particle detector a first particle detector positioned on an upper surface with reference to a void or subterranean object 47. As shown in Figure 4, a primary particle travels along example primary particle trajectory 44; the primary particle trajectory may pass through the primary particle detecting portion 41 and / or the secondary particle detecting portion 42. The primary particle may then travel through and penetrate volume of ground 43 until interacting with the volume of ground 43 at interaction point 46, producing secondary particles. The interaction point 46 may be on the surface of, or inside the void or subterranean object 47. These secondary particles may then travel along example secondary particle trajectory 45, which passes through the secondary particle detecting portion 42 as shown in Figure 4. As previously detailed, the first particle detector (which may be formed from the combined detector system of the primary particle detection portion and the secondary particle detection portion) may be placed on the ground, directly above the region of interest or volume of ground to be monitored. As shown in Step S102 of Figure 1, method S100 comprises positioning the second particle detector (which also may be formed from the combined detector system as discussed above) on a lower surface, for example through underground access. The upper surface and the lower surface are separated by the overburden or volume of ground 53. Figure 5 demonstrates a second particle detector performing a method in accordance with present embodiment for the purposes of positioning the second particle detector through the overburden 53 wherein the second particle detector is positioned underneath the volume of ground or overburden 53. The underground access 58 in which the combined particle detector 51, 52 is placed may be an adit, tunnel or borehole. As shown in Figure 5, a primary particle travels along example primary particle trajectory 54; the primary particle trajectory may pass through the primary particle detecting portion 51 as depicted in Figure 5. Before reaching the combined detector system 51, 52 the primary particle interact with the volume of ground 53, for example at interaction point 56. The interaction point 56 may be on the surface of, or inside the void or subterranean object 57, generating secondary particles. These secondary particles may then travel along example secondary particle trajectory 55, which passes through the secondary particle detecting portion 52. As shown in Figure 5, the second particle detector may be located directly beneath (on a line extending from the volume of ground to be monitored to the centre of the planet) the volume of ground to be monitored, potentially the second particle detector may be located in a void (such as a road, rail or pedestrian tunnel, adit or bore hole) which the volume of ground to be monitored or overburden overlies. That is, the second particle detector may be positioned directly beneath the first particle detector, which is placed on the overburden over where the second particle detector is estimated to be positioned. Where the first particle detector is of a type having a field of view which can be directed, the first particle detector may be positioned so as to not be directly above the overburden over where the second particle detector is estimated to be located but so as to be directed towards the overburden. As shown in Step S103 of Figure 1, the method S100 further comprises detecting a flux of particles through the first particle detector and the second particle detector. As depicted in Figure 6, a flux of particles 69 may pass through a first particle detector 60A and a second particle detector 60B. Following this, the method S100 further comprises determining a position of the second particle detector using the detected flux of particles, as shown in Step S104 of Figure 1. In some embodiments, determining a position of the second particle detector may comprise determining a time interval between a first measurement of the detected flux of particles at the first particle detector and a second measurement of the detected flux of particles at the second particle detector. In such embodiments, the first measurement of the detected flux and the second measurement of the detected flux may be associated with a particle among the flux of particles. That is, although the flux of particles passing through each detector is formed of multiple particles, the first measurement taken at the first particle detector and the second measurement taken at the second particle detector may be caused by interactions at each detector from the same particle. In embodiments where the speed of the particle is known, for example, the depth of the overburden can be calculated using the time interval between the first and second measurements. As detailed below, the direction of trajectory 0 may also be measured, and the relative position of the second particle detector with reference to the first particle detector may be determined. Alternatively or additionally, this method may be used to monitor the location of the second particle detector relative to the first particle detector, in order to generate a map of the void or tunnel in which the second particle detector is located. Alternatively or additionally, determining a position of the second particle detector may comprise determining a first trajectory of the detected flux of particles through the first particle detector and a second trajectory of the detected flux of particles through the second particle detector, wherein the first trajectory and the second trajectory are aligned with one another. In such embodiments, the first trajectory of the detected flux and the second trajectory of the detected flux are associated with a particle among the flux of particles. That is, although the flux of particles passing through each detector is formed of multiple particles, the first measurement taken at the first particle detector and the second measurement taken at the second particle detector may be caused by interactions at each detector from the same particle. Once the first trajectory and second trajectory have been determined, the trajectories can be compared and aligned in order to for example calculate the depth of the overburden. Alternatively or additionally, this method may be used to monitor the location of the second particle detector relative to the first particle detector, in order to generate a map of the void or tunnel in which the second particle detector is located. In some embodiments, one first particle detector 60A and one second particle detector 60B may be used. Alternatively, and as depicted in Figure 6, in some embodiments a plurality of first particle detectors 60A and / or a plurality of second particle detectors 60B may be used. Accordingly, the method S100 may comprise positioning a plurality of second particle detectors on the lower surface, detecting the flux of particles through each of the second particle detectors and the first particle detector, and determining a position of each second particle detector using the detected flux of particles associated with said second particle detector. Alternatively or additionally, the method S100 may comprise positioning a plurality of first particle detectors on the upper surface, detecting the flux of particles through the second particle detector and at least one of the first particle detectors, and determining a position of the second particle detector using the detected flux of particles. In an embodiment where both a plurality of first particle detectors and a plurality of second particle detectors are used, the method S100 may comprise positioning a plurality of first particle detectors on the upper surface, positioning a plurality of second particle detectors on the lower surface, detecting the flux of particles through each of the second particle detectors and at least one of the first particle detectors, and determining a position of each second particle detector using the detected flux of particles associated with said second particle detector. In such embodiments, he method S100 may comprise synchronising each of the first particle detectors with one another. Alternatively or additionally, in such embodiments the method S100 may comprise synchronising each of the second particle detectors with one another. In an embodiment where a plurality of first particle detectors 60A, the position of one or more second particle detectors 60B may be determined using a triangulation method. That is, the flux of particles 69 may be measured through multiple first particle detectors 60A and the second particle detector 60B. Accordingly, the position of a single second particle detector 60B can be determined using multiple first particle detectors 60A in order to improve the accuracy of the determination of the second particle detector 60B location. As shown in Figure 6, an anomalous region 67 may be present or partially comprised in the overburden. The anomalous region 67 may be a subterranean object, for example a rock. Alternatively, the anomalous region 67 may be a void. The void may be filled with fluid, for example air, water, or slurry. An example of an anomalous region is a flooded tunnel in a tunnel network. The measured flux particles through the first particle detector 60A and / or the second particle detector 60B may be used to determine the presence of a subterranean object at least partially comprised within the overburden. In some embodiments, the method may further comprise determining a magnitude of the flux of particles at the second particle detector, and determining whether an anomalous region is present in the overburden based on the determined magnitude of the flux of particles. The method may additionally comprise comparing the determined magnitude of the flux of particles with an expected magnitude of the flux of particles. The particle flux through the overburden is proportional to the density of the overburden and the depth of the overburden; accordingly, an expected magnitude of the flux of particles can be predicted when the depth of the positioning of the second particle detector(s) 60B is known. When there is a discrepancy between the expected magnitude of the flux of particles at the second particle detector 60B and the determined magnitude of the flux of particles at the second particle detector 60B, this may indicate that an anomalous region 67 is present in the overburden. Furthermore, the density of the anomalous region 67 may be calculated based on the discrepancy between expected magnitude of the flux of particles at the second particle detector 60B and the determined magnitude of the flux of particles at the second particle detector 60B. The calculated density may then be used to further characterize the anomalous region 67. The variance of the flux of particles across multiple second particle detectors 60B may also be used to image an anomalous region 67. That is, in order to improve the accuracy of the determination of location of anomalous region 67, measurements of the flux of particles may be taken at several locations relative to the overburden. Triangulation of the location of the anomalous region using the measurements of the flux of particles can then be undertaken. Alternatively, mapping the route of the anomalous region (for example, when the anomalous region is a part of a tunnel network such as a flooded tunnel) through the overburden using the measurements of the flux of particles can then be undertaken. In further specific embodiments, the method may comprise determining a magnitude of the flux of particles at a plurality of positions on the lower surface, determining a variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface, and upon determining the presence of an anomalous region in the overburden, imaging the anomalous region based on the variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface. For example, the anomalous region may be a region of anomalous density or a void. The determination of a presence of an anomalous region 67 in the overburden may utilise machine learning to analyse measurements of the flux of particles. In specific embodiments, a neural network may be used to analyse measurements of the flux of particles. Alternatively, a random forest, support vector machine, or K-nearest neighbours algorithm may be used. A neural network may be preferrable, because it may be more finely tuned and typically trained to provide improved differentiation between materials. Training data may be provided to the neural network in order to train the neural network. For example, histograms of measured quantities may be presented as training data to the neural network to provide a detection metric or performance metric. The measured quantities may be previously obtained data, associating primary and secondary particle characteristics with known subterranean objects. For example, measured quantities may include measurements of the time interval between the detection of a particle among the flux of particles at the first particle detector 60A and the detection of the same particle among the flux of particles at the second particle detector 60B, trajectory of a particle among the flux of particles at the first particle detector 60A and the trajectory of the same particle among the flux of particles at the second particle detector 60B, and / or magnitude of the flux of particles at the first particle detector 60A and magnitude of the flux of particles at the second particle detector 60B. In specific embodiments, training the neural network may end once the performance metric reaches a certain value such as 90%, that is, when the neural network correctly classifies input data for 90% of inputted data. In other embodiments, other thresholds may be used. Training data may be provided to the neural network until this performance metric has been reached. While training data is being provided to the neural network, weighting of nodes and edges forming the neural network may be altered so as to increase the performance metric of the neural network. Once the performance metric of the neural network has reached the desired value, the weighting of nodes and edges forming the neural network may be fixed, that is, the neural network may be considered fully trained. In embodiments of the present invention, the particles may be muons. More specifically, the muons may be cosmic ray muons. By repeatedly determining a position of a second particle detector across the lower surface, a map may be generated of the lower surface using the determined positions as reference data. That is, in some specific embodiments the method may further comprise generating a map of the position of the second detector relative to the first particle detector. The map may be generated with reference to one or more first particle detectors positioned on the upper surface. Alternatively or additionally, the method may further comprise determining a position of the first particle detectors using a Global Navigation Satellite System (GNSS) network, and generating a map of the position of the second detector relative to the first particle detector using the GNSS location of the first particle detector. Use of a GNSS network may allow for the map to be generated using absolute positioning, rather than relative positioning with reference to one or more first particle detectors on the upper surface. As shown in Figure 7, one or more second particle detectors 70B may be used to improve the accuracy of measurements for deeper overburdens by providing an intermediate measurement within the depth of the overburden. Accordingly, the second particle detector 70B may be positioned between one or more first particle detectors 70A and one or more third particle detectors 70C. Accordingly, the flux of particles 79 may pass through the first particle detector 70A, the second particle detector 70B and the third particle detector 70C to facilitate more accurate determination of the position of the third particle detectors and the presence of anomalous regions for deeper overburdens. That is, in some embodiments the method may further comprise positioning a third particle detector on a further lower surface, wherein the lower surface and the further lower surface are separated by a further overburden, detecting the presence of a further flux of particles at the third particle detector using the second particle detector, and determining a position of the third particle detector using the detected flux of particles. In more specific embodiments, the method may further comprise positioning a plurality of third particle detectors on a further lower surface, detecting the presence of a further flux of particles at each of the third particle detectors using the second particle detector, and determining a position of each third particle detector using the detected flux of particles. Having a plurality of third particle detectors may allow for a map of the further lower surface to be generated, without requiring a mobile third particle detector. In such embodiments, the further lower surface may be closer to the central point of the Earth than the lower surface. Additionally, the lower surface may be closer to the central point of the Earth than the upper surface. In further embodiments, there may be more than the three layers of particle detectors as depicted in Figure 7. For example, the arrangement disclosed above may be extended to comprise a fourth layer of fourth particle detectors located on a surface lower than the further lower surface. The fourth layer of fourth particle detectors would then operate in a manner corresponding to the third particle detectors as depicted in Figure 7, with the second and third particle detectors acting as intermediate layers that facilitate more accurate determination of the positioning of the fourth particle detectors. Alternatively or additionally, additional layers of lower particle detectors may be added below the fourth layer, in accordance with the principles detailed above. Embodiments of the present invention allow for the generation of maps and surveys of underground surfaces by detecting and positioning second particle detectors. That is, by positioning second particle detectors on the lower surface, the lower surface being located below an overburden, the position of the second particle detectors may be used as reference points to map out the lower surface. A map or survey of the underground surface (e.g. forming a tunnel network) may then be generated without the need for a direct connection between the underground surface and the overground surface, or a line of sight within the void. Further, the existence of anomalous regions within the overburden may be detected and mapped. 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.

Claims

1. A method of positioning through an overburden comprising:positioning a first particle detector on an upper surface;positioning a second particle detector on a lower surface, wherein the upper surface and the lower surface are separated by the overburden;detecting a flux of particles through the first particle detector and the second particle detector, anddetermining a position of the second particle detector using the detected flux of particles.

2. A method as claimed in Claim 1, wherein determining a position of the second particle detector comprises:determining a time interval between a first measurement of the detected flux of particles at the first particle detector and a second measurement of the detected flux of particles at the second particle detector.

3. A method as claimed in Claim 2, wherein the first measurement of the detected flux and the second measurement of the detected flux are associated with a particle among the flux of particles.

4. A method as claimed in any of Claim 1 to 3, wherein determining a position of the second particle detector comprises:determining a first trajectory of the detected flux of particles through the first particle detector and a second trajectory of the detected flux of particles through the second particle detector, wherein the first trajectory and the second trajectory are aligned with one another.

5. A method as claimed in Claim 4, wherein the first trajectory of the detected flux and the second trajectory of the detected flux are associated with a particle among the flux of particles.

6. A method as claimed in any preceding claim, wherein the method further comprises:determining a magnitude of the flux of particles at the second particle detector; anddetermining whether an anomalous region is present in the overburden based on the determined magnitude of the flux of particles.

7. A method as claimed in Claim 6, wherein the method further comprises:comparing the determined magnitude of the flux of particles with an expected magnitude of the flux of particles.

8. A method as claimed in any of Claims 6 and 7, wherein the method further comprises:determining a magnitude of the flux of particles at a plurality of positions on the lower surface;determining a variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface; andupon determining the presence of an anomalous region in the overburden, imaging the anomalous region based on the variance of the determined magnitude of the flux of particles across the plurality of positions on the lower surface.

9. A method as claimed in any of Claims 6 to 8, wherein the anomalous region is a region of anomalous density or a void.

10. A method as claimed in any preceding claim, wherein the particles are muons.

11. A method as claimed in Claim 10, wherein the muons are cosmic ray muons.

12. A method as claimed in any preceding claim, wherein the method further comprises:positioning a plurality of second particle detectors on the lower surface;detecting the flux of particles through each of the second particle detectors and the first particle detector, anddetermining a position of each second particle detector using the detected flux of particles associated with said second particle detector.

13. A method as claimed in any preceding claim, wherein the method further comprises:positioning a plurality of first particle detectors on the upper surface;detecting the flux of particles through the second particle detector and at least one of the first particle detectors, anddetermining a position of the second particle detector using the detected flux of particles.

14. A method as claimed in any preceding claim, wherein the method further comprises:positioning a plurality of first particle detectors on the upper surface;positioning a plurality of second particle detectors on the lower surface;detecting the flux of particles through each of the second particle detectors and at least one of the first particle detectors, anddetermining a position of each second particle detector using the detected flux of particles associated with said second particle detector.

15. A method as claimed in any of Claims 13 and 14, wherein the method further comprises: synchronising each of the first particle detectors with one another.

16. A method as claimed in any of Claims 12 and 14, wherein the method further comprises: synchronising each of the second particle detectors with one another.

17. A method as claimed in any preceding claim, wherein the method further comprises: generating a map of the position of the second detector relative to the first particle detector.

18. A method as claimed in Claim 17, wherein the method further comprises:determining a position of the first particle detectors using a Global Navigation Satellite System, GNSS, network; andgenerating a map of the position of the second detector relative to the first particle detector using the GNSS location of the first particle detector.

19. A method as claimed in any preceding claim, wherein the method further comprises:positioning a third particle detector on a further lower surface, wherein the lower surface and the further lower surface are separated by a further overburden;detecting the presence of a further flux of particles at the third particle detector using the second particle detector, anddetermining a position of the third particle detector using the detected flux of particles.

20. A method as claimed in Claim 19, wherein the method further comprises:positioning a plurality of third particle detectors on a further lower surface;detecting the presence of a further flux of particles at each of the third particledetectors using the second particle detector, anddetermining a position of each third particle detector using the detected flux of particles.

21. A method as claimed in any of Claims 18 and 19, wherein the further lower surface is closer to the central point of the Earth than the lower surface.

22. A method as claimed in any preceding claim, wherein the lower surface is closer to the central point of the Earth than the upper surface.

23. A method as claimed in any preceding claim, wherein the overburden is one of: a volume of water, a volume of earth, or a man-made structure.

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