Subterranean object determination method

Abstract

Embodiments provide methods for detecting and characterising subterranean objects. A method comprises positioning a particle detector comprising primary and secondary particle detection portions directed towards a volume of ground. The method further comprises measuring a flux of primary particles towards the volume of ground using the primary particle detection portion, wherein the primary particles are muons, and measuring a flux of secondary particles emitted from the volume of ground using the secondary particle detection portion, wherein the secondary particles are generated through interactions between the primary particles and the volume of ground. The method also comprises determining the presence of a subterranean object at least partially comprised within the volume of ground; and characterising the subterranean object. The characterisation of the subterranean object is based on analysis of the measurements of the flux of primary particles and flux of secondary particles.

Description

[0001] SUBTERRANEAN OBJECT DETERMINATION METHOD

[0002] Technical Field

[0003] The present disclosure relates to methods for subterranean object determination, in particular for subterranean object detection and characterisation in a volume of ground using particle detection.

[0004] Background

[0005] The detection, location, and characterisation of underground assets and / or subterranean objects is important across construction, archaeology, farming, and other sectors concerned with subterranean profiling. In the construction sector, for example, asset strikes on (at least partially) subsurface assets by excavation activities endanger workers and public services such as telecommunications, sewage, and electricity. This problem is exacerbated for locations and types of services that have poor documentation or have an older date of installation. To avoid asset strikes and the like, subterranean objects may need to be detected before significant excavation activities take place.

[0006] For sectors such as farming, subterranean profiling may also be needed to determine the composition of the ground in areas of farmland. For example, the composition of the ground of farmland may need to be known in order to determine for what agricultural activities the farmland is suitable for, and whether the composition of the ground will impact the quality of any grown crops.

[0007] Furthermore, 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 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.

[0008] The structural integrity of a void may degrade over time. Factors including weathering, ingress of water, 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.

[0009] 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 also not necessarily representative of the subterranean object 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.

[0010] Alternatively, subterranean objects may be detected using visual inspections (potentially in conjunction with excavations). 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. Excavations themselves can also be costly and inconvenient especially if excavation is done in an unnecessary place.

[0011] Summary

[0012] It is desirable to provide methods for detecting and characterising subterranean objects that address one or more of the issues discussed above. For example, it is desirable to provide methods for detecting and characterising subterranean objects that permit deeper probing of a surface and / or improve the probability of detection and reduce false alarms. In addition, it is desirable to provide a method for detecting and characterising subterranean objects that allows for the characterisation of a subsurface through detection of underground assets and / or strata of material for agricultural and construction purposes.

[0013] 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.

[0014] Embodiments of the present disclosure provide methods for detecting and characterising subterranean objects. The method comprises positioning a particle detector comprising primary and secondary particle detection portions directed towards a volume of ground. The method further comprises measuring a flux of primary particles towards the volume of ground using the primary particle detection portion, wherein the primary particles are muons and measuring a flux of secondary particles emitted from the volume of ground using the secondary particle detection portion, wherein the secondary particles are generated through interactions between the primary particles and the volume of ground. The method also comprises determining the presence of a subterranean object at least partially comprised within the volume of ground, and characterising the subterranean object, wherein the characterisation of the subterranean object is based on analysis of the measurements of the flux of primary particles and flux of secondary particles.

[0015] In some embodiments, characterising the subterranean object comprises determining the location of the subterranean object. The determination of the location of the subterranean object may utilise at least one of: measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, and measurements of the trajectory of the primary particle among the flux of primary particles and trajectories of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle. Alternatively or additionally, the determination of the location of the subterranean object may comprise measurements of the flux of primary particles and measurements of the flux of secondary particles at several locations relative to the volume of ground, and triangulation of the location of the subterranean object using the measurements of the flux of primary particles and the measurements of the flux of secondary particles. In some embodiments, characterising the subterranean object comprises determining the composition of the subterranean object. The determination of the composition of the subterranean object may utilise at least one of: measurements of multiplicity of second particles among the flux of second particles, measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, and estimates of the energy of second particles among the flux of second particles.

[0016] In specific embodiments, at least one of the primary and secondary particle detection portions utilises scintillation-based detection. The scintillation-based detection may utilise a liquid scintillator and / or a plastic or solid scintillator.

[0017] In specific embodiments, the measurements of the flux of primary particles may comprise measurements of at least one of the trajectory of the primary particles through the primary particle detection portion and energy deposited in the primary particle detection portion by the primary particles. Additionally or alternatively, 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.

[0018] In some embodiments, the flux of secondary particles detected by the secondary particle detection portion comprises one or more of: neutrons; and gamma rays.

[0019] In specific embodiments, at least one of the determination of the presence of the subterranean object and characterisation of the subterranean object utilises a trained neural network to analyse measurements of the flux of primary particles and flux of secondary particles.

[0020] The subterranean object may be any of: a pipe, a cable, a natural void, a human-generated void, or a stratum of material. Where the object is a stratum of material, the boundaries of the stratum / between strata may be defined by interfaces between materials (such as rock / soil interfaces, for example) or variations in density of a given material. The method may further comprise mapping the route of the subterranean object through the volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles. Alternatively, the method may further comprise mapping the extent of the stratum of material in the volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles. The plurality of measurements of the flux of primary particles and the plurality of measurements of the flux of secondary particles may be measured with the particle detector in a plurality of locations.

[0021] Brief Description of Drawings

[0022] 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:

[0023] Figure 1 is a flowchart of a method in accordance with embodiments of the present disclosure;

[0024] Figure 2A is a schematic diagram of a primary particle detector that may be used in embodiments;

[0025] Figures 2 Bi and 2Bii are schematic diagrams of a further primary particle detector that may be used in embodiments;

[0026] Figure 3 is a diagram of an example particle detector performing a method in accordance with embodiments of the present disclosure in a first geometry;

[0027] Figure 4 is a diagram of an example particle detector performing a method in accordance with embodiments of the present disclosure in a second geometry; and

[0028] Figure 5 is a diagram of an example particle detector performing a method in accordance with embodiments of the present disclosure in a third geometry.

[0029] Detailed Description

[0030] Embodiments disclosed herein may utilise particle detection when detecting and characterising subterranean objects. 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.

[0031] 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 31 and 32 as shown in Figure 3. 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.

[0032] In embodiments of the present invention, the method comprises positioning (S101) a particle detector comprising primary and secondary particle detection portions directed towards a volume of ground. Figure 3 demonstrates a side view of a particle detector for characterising a subsurface. As shown in Figure 3, a particle detector suitable for the method of the present invention may comprise a primary particle detection portion 31 and a secondary particle detection portion 32. Alternatively, detection portions 31 and 32 may both be configured to detect primary particles and secondary particles. Other forms of detector element arrangements, such as nested helices, may also be used in embodiments.

[0033] As shown in Figure 2A, the primary particle detection portion 31 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 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.

[0034] 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.

[0035] 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.

[0036] 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 is 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; 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 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.

[0037] 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 (typically 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, is typically avoided.

[0038] 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.

[0039] The trajectories of the particles may be used to allow subterranean object detection and characterisation for objects not directly above or below 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 object or volume of ground to be analysed , including where this object or volume of ground is not directly above the detector.

[0040] 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.

[0041] For some implementations of subterranean object detection and characterisation 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, or wherein the detector is to be positioned in a remote location where a mains power connection cannot practically be provided. 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 2 Bi 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 allows the detector to be left in situ in a void (such as a tunnel, as discussed above) and / or remote location, 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.

[0042] In accordance with the subterranean object 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 volume of ground to be monitored. The primary particle detection portion may be a particle detector as shown in Figure 2 or another particle detector. Possible geometries for the particle detector relative to the volume of ground to be monitored are shown in Figure 3, Figure 4, and Figure 5. The 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 particle detector.

[0043] Figure 3 demonstrates a particle detector performing a method in accordance with present embodiments, for example, for the purposes of characterisation of a subsurface wherein the detector is positioned above the volume of ground 33 to be monitored. An example primary particle 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. As shown in Figure 5, the 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 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 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 volume of ground to be monitored but so as to be directed towards the volume of ground to be monitored; in such a situation the detector may be located in the void or in another suitable location. As indicated in Step S102 of Figure 1, when the particle detector has been positioned towards the volume of ground, the primary particle detection portion is used to measure a flux of primary particles towards the volume of ground using the primary particle detection portion, wherein the primary particles are 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. Typically, a single measurement time frame is of the order of 1 hour to 24 hours. The measurements of the flux of primary particles may comprise measurements of at least one of the trajectory of the primary particles through the primary particle detection portion and energy deposited in the primary particle detection portion by the primary particles.

[0044] 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.

[0045] As indicated in Step S103 of Figure 1, once a flux of primary particles towards the volume of ground has been measured, the secondary particle detection portion is used to measure a flux of secondary particles emitted from the volume of ground using the secondary particle detection portion, 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.

[0046] As shown in Figure 3, 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. Thus, the material of the volume of ground at the interaction point 36 can be characterised by determining the secondary particles produced by the interaction and the properties associated with said secondary particles. Such properties include multiplicity, energy and / or timing properties of the secondary particles. Embodiments of the present disclosure may also provide improved ground material characterisation through gamma ray detection, which allows for the detection of spectral readouts from elements present in the soil, for example in agricultural settings.

[0047] Embodiments of the present disclosure may also be used for the detection of water in a volume of ground. In such cases, water acts as an attenuator for neutron flux and not for gamma flux. Accordingly, the detection of both neutron flux and gamma flux may allow for a determination of moisture in the volume of ground. The combination of correlated neutron and gamma information permits deeper probing of the subsurface than otherwise to improve the probability of detection and reduce false alarms; neutron flux may be attenuated by water and depth of the volume of ground being monitored, which may result in neutron detection of subterranean objects being less viable at increased depths. For example, neutron detection of subterranean objects may no longer be considered viable at depths of over 70cm. Gamma flux suffers from reduced attenuation with depth in comparison to neutron flux, and in embodiments of the present disclosure wherein both neutron and gamma flux is measured, gamma flux can be substituted for neutron flux at depths where neutron flux is less viable in order to provide an increased range of depths at which subterranean object detection can take place.

[0048] As indicated in Step S104 of Figure 1, the measured flux of primary particles and measured flux of secondary particles may be used to determine the presence of a subterranean object at least partially comprised within the volume of ground. For example, the time taken between the primary particle passing through the primary particle detector and the associated secondary particle or particles passing through the secondary particle detector may be measured to determine the presence of a subterranean object. The energy deposition within the primary and / or secondary particle detectors may also be measured. The induced voltage pulse shape of the primary and / or secondary particle detectors may additionally be measured, because the induced voltage pulse shape provides information on the type of particle passing through the detector.

[0049] Figure 4 demonstrates a particle detector performing a method in accordance with present embodiment 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, allowing for the detection of 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.

[0050] A simple binary determination (subterranean object or no subterranean object) can be made by detection of a variation between the flux of primary particles through the volume of ground in the field of view of the particle detector and flux of secondary particles through the volume of ground in the field of view of the detector. However, for more complex determinations such as the presence of water in the volume of ground or composition of the ground, more analysis is required to provide accurate determinations. In particular, for the determination of water ingress in a volume of ground, where the modelling uses the composition of the volume of ground, the change in water content of the volume of ground (as an estimate of the volume of water per unit volume of the ground) can be directly related to the variation in secondary particle flux. Typically, the presence of water in a ground volume increases the density, and therefore increases the opacity and decreases the secondary particle flux penetrating the volume of ground. For the determination of composition of a volume of ground, a change in the material comprising the volume of ground can be related to the characteristics of the secondary particles produced by the reaction between the primary particle and the material. Varying characteristics may include for example varying multiplicity, energy and / or timing properties of the secondary particles.

[0051] As indicated in Step S105 of Figure 1, analysis of the measurements of the flux of primary particles and flux of secondary particles can be used to characterise the subterranean object or void 47. For example, characterising the subterranean object or void 47 may comprise determining the location of the subterranean object. The location of the subterranean object or void 47 may be determined using measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle. Alternatively or additionally, the location of the subterranean object or void 47 may be determined using measurements of the trajectory of the primary particle among the flux of primary particles and trajectories of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle. In order to improve the accuracy of the determination of location of the subterranean object or void 47, measurements of the flux of primary particles and measurements of the flux of secondary particles may be taken at several locations relative to the volume of ground. Triangulation of the location of the subterranean object using the measurements of the flux of primary particles and the measurements of the flux of secondary particles can then be undertaken. Alternatively, mapping the route of the subterranean object through a volume of ground using the measurements of the flux of primary particles and the measurements of the flux of secondary particles can then be undertaken.

[0052] Alternatively or additionally, characterising the subterranean object may comprise determining the composition of the subterranean object. For example, the secondary particle(s) generated by the primary particle interacting with the void of subterranean object 47 and the characteristics thereof may be determined by the properties of the void of subterranean object 47. Therefore, determination of the composition of the subterranean object may utilise at least one of: measurements of multiplicity of secondary particles among the flux of secondary particles, measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, and estimates of the energy of secondary particles among the flux of secondary particles. For the purposes of this disclosure, multiplicity is taken to mean the number and types of secondary particles resulting from a single incident primary particle. For example, the energy deposition within the secondary particle detector and / or induced voltage pulse shape of secondary particle detector may be used to identify the type of particle of the secondary particles and / or the characteristics thereof. The characteristics of the secondary particles when known may also allow for detected secondary particles to be associated with the specific (detected) primary particles that formed said detected secondary particles in some embodiments.

[0053] At least one of the determination of the presence of the subterranean object and characterisation of the subterranean object may utilise machine learning to analyse measurements of the flux of primary particles and flux of secondary particles. In specific embodiments, a neural network may be used to analyse measurements of the flux of primary particles and flux of secondary particles. Alternatively, a random forest, support vector machine, or K-nearest neighbours algorithm may be used. A neural network is preferrable, because it can 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 primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, trajectory of the primary particle among the flux of primary particles and trajectories of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, measurements of multiplicity of secondary particles among the flux of secondary particles, measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle, and / or estimates of the energy of secondary particles among the flux of secondary particles.

[0054] 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.

[0055] In some embodiments the subterranean object 47 may be any of a pipe, a cable, a natural void, or a human-generated void. For example, the subterranean object 47 may be a pipe for transporting potable water, gas, oil, or sewage. The subterranean object 47 may be a cable for transporting power or data. Alternatively, the subterranean object may be a void formed by old construction or mining works, or a void formed by a natural cave or karst. For these examples, the subterranean object 47 may not fall wholly within the volume of ground being analysed. Therefore, in specific examples mapping the route of the subterranean object through a volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles may be undertaken. This may allow for the subterranean object 47 to be mapped across several volumes of ground, across which the subterranean object 47 might span. Therefore, the plurality of measurements of the flux of primary particles and the plurality of measurements of the flux of secondary particles may be measured with the particle detector in a plurality of locations.

[0056] Alternatively, the subterranean object 47 may be a stratum of material. The stratum of material may not fall wholly within the volume of ground being analysed. Therefore, in specific examples mapping the extent of the stratum of material in the volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles may be undertaken. This may allow for the stratum of material to be mapped across several volumes of ground, across which the stratum of material might span. Accordingly, the plurality of measurements of the flux of primary particles and the plurality of measurements of the flux of secondary particles may be measured with the particle detector in a plurality of locations.

[0057] In some embodiments, the combined detector system may be used to analyse the composition of the volume of ground. Different materials and elements present in the volume of ground may produce secondary particles with different characteristics, for example varying multiplicity, energy and / or timing properties of the secondary particles. In specific embodiments, a neural network can be trained to analyse the characteristics of the secondary particles in order to determine the composition of the ground producing the secondary particles.

[0058] As previously detailed, 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. Alternatively, the combined detector system can be placed underneath the volume of ground to be monitored, through underground access. Figure 5 demonstrates a particle detector performing a method in accordance with present embodiment for the purposes of characterisation of a subsurface wherein the detector is positioned underneath the volume of ground 53 to be monitored. 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. Embodiments of the present invention allow for subterranean object detection and subterranean profiling without penetration of the volume of ground to be analysed, which may avoid costly and inconvenient excavation. Embodiments of the present invention can also be placed on the surface of a volume of ground to be analysed or within a void, which may allow for more flexibility in the subterranean profiling process. Further, embodiments of the present invention may provide deeper and / or less localised data, and may avoid onerous set-up associated with existing techniques which allows for multiple measurements to be taken more easily.

[0059] 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.

[0060] 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.

[0061] 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

Claims1. A method for detecting and characterising subterranean objects, the method comprising: positioning a particle detector comprising primary and secondary particle detection portions directed towards a volume of ground; measuring a flux of primary particles towards the volume of ground using the primary particle detection portion, wherein the primary particles are muons; measuring a flux of secondary particles emitted from the volume of ground using the secondary particle detection portion, wherein the secondary particles are generated through interactions between the primary particles and the volume of ground; determining the presence of a subterranean object at least partially comprised within the volume of ground; and characterising the subterranean object, wherein the characterisation of the subterranean object is based on analysis of the measurements of the flux of primary particles and flux of secondary particles.

2. The method of claim 1, wherein characterising the subterranean object comprises determining the location of the subterranean object.

3. The method of claim 2, wherein the determination of the location of the subterranean object utilises at least one of: measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle; and measurements of the trajectory of the primary particle among the flux of primary particles and trajectories of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle.

4. The method of any of claims 2 and 3, wherein the determination of the location of the subterranean object comprises measurements of the flux of primary particles and measurements of the flux of secondary particles at several locations relative to the volume of ground, and triangulation of the location of the subterranean object using the measurements of the flux of primary particles and the measurements of the flux of secondary particles.

5. The method of any preceding claim, wherein characterising the subterranean object comprises determining the composition of the subterranean object.

6. The method of claim 6, wherein the determination of the composition of the subterranean object utilises at least one of: measurements of multiplicity of second particles among the flux of second particles; measurements of the time interval between the detection of a primary particle among the flux of primary particles and detection of one or more secondary particles among the flux of secondary particles that are correlated with the primary particle; and estimates of the energy of second particles among the flux of second particles.

7. The method of any preceding claim, wherein at least one of the primary and secondary particle detection portions utilises scintillation-based detection.

8. The method of claim 7, wherein the scintillation-based detection utilises a liquid scintillator and / or wherein the scintillation-based detection utilises a plastic scintillator.

9. The method of any preceding claim, wherein the measurements of the flux of primary particles comprise measurements of at least one of the trajectory of the primary particles through the primary particle detection portion and energy deposited in the primary particle detection portion by the primary particles.

10. The method of any preceding claim, wherein the measurements of the flux of secondary particles 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.

11. The method of any preceding claim, wherein the flux of secondary particles detected by the secondary particle detection portion comprises one or more of: neutrons; and gamma rays.

12. The method of any preceding claim, wherein at least one of the determination of the presence of the subterranean object and characterisation of the subterranean object utilises a trained neural network to analyse measurements of the flux of primary particles and flux of secondary particles.

13. The method of any preceding claim wherein the subterranean object is: a pipe; a cable; a natural void; or a human-generated void.

14. The method of claim 13, further comprising mapping the route of the subterranean object through the volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles.

15. The method of any of claims 1 to 12 wherein the subterranean object is a stratum of material.

16. The method of claim 15, further comprising mapping the extent of the stratum of material in the volume of ground utilising a plurality of measurements of the flux of primary particles and a plurality of measurements of the flux of secondary particles.

17. The method of any of claims 14 and 16, wherein the plurality of measurements of the flux of primary particles and the plurality of measurements of the flux of secondary particles are measured with the particle detector in a plurality of locations.

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