GB2536576A

GB2536576A - Buried object detection system

Info

Publication number: GB2536576A
Application number: GB1604739.1A
Authority: GB
Inventors: Edward Martin James; Kiani Saeed; Tamadoni Reza; James Wilkinson Alexander; Griffiths Luke; Rebecca Sills Melanie; Ferber Ralf-Gunter
Original assignee: Reece Innovation Centre Ltd
Current assignee: Reece Innovation Centre Ltd
Priority date: 2015-03-20
Filing date: 2016-03-21
Publication date: 2016-09-21
Also published as: GB201604739D0; GB201504717D0

Abstract

A method and apparatus for detecting buried objects comprises generating an acoustic signal and using the signal to measure the mechanical impedance of the earth. The presence of buried objects results in a change in the measured mechanical impedance. Boundary waves, such as Rayleigh, Love or Sholte waves, may be measured to determine the mechanical impedance. A plurality of acoustic sources that generate multiple acoustic signals may be provided and there may be one or more acoustic sensor. Acoustic sources and receivers can be connected to signal generation and signal processing units and devices for measuring earth movement and chemical detection may also be provided. Uses include humanitarian mine clearance, environmental geophysics and highway construction and maintenance.

Description

BURIED OBJECT DETECTION SYSTEM

Introduction

There exist a number of non-intrusive methods for detecting buried, near-surface objects. The methods used depend primarily on the application. The construction industries use seismic and electromagnetic mapping to find foreign objects. Metal detectors (using electric field disturbance) are also used to find objects near to the surface, and are used often in military and humanitarian mine clearing.

Military route clearance brings additional challenges of real time data analysis and decision-making as detection is only a short range ahead of a moving vehicle. Clearance rate is a prime factor in determining whether a particular technology/embodiment is suitable for route clearance applications. The technology is focused on detecting explosive devices (EDs) over other types of buried object.

Humanitarian area clearance covers a broad range of situations. Areas to be cleared may be large in 2 dimensions (as compared to a primarily I -dimensional road) and require repeated surveys as sweeps. Time constraints are less significant in this application, but the terrain to be covered may be more rugged with shrubbery and non-level ground prevalent The threat profile is also different, focusing more on standard military ordnance, which may be been buried for decades.

Prior Art

The technological landscape of subterranean mine detection is diverse. Technologies 25 can be primarily classified as removal, triggering or observational. Removal technologies clear mines to a 'safe' area without any explicit detection. These technologies are primarily mine ploughs, humanitarian rotary combs and suchlike. These technologies are primarily used to clear a safe lane. The ordnance removed is easier to detect once 'dug-up' in this fashion, but still requires careful searching by manual methods.

Triggering detection involves activating any buried explosive ordnance from a safe condition. Examples of this technology are mine roller systems and flails that activate pressure mines, or magnetic signal duplicators that activate magnetically fused mines.

These systems are controlled from a heavily armoured vehicle at a standoff distance from the suspect area. They can be effective for a known threat type, however they are ineffective against command operated EDs (operated by, for example, wire, radio or phone command).

Observational technologies involve detecting the mine without activating it. The old and most thorough form of this technique is a man armed with a probe and a trowel. This is still one of the major methods employed for humanitarian de-mining. However, the clearance rate is very slow and so its use in military route clearance is curtailed.

A relatively new technique employed in military route clearance involves ground penetrating radar. A number of military systems exist based on commercial hardware. These systems are also used in the construction industry. They use microwave-wavelength radar signals emitted into the ground; the reflected signals are then analysed for their time delay (indicating depth), strength (indicating density) and spatial profile (indicating the geometry of the detected object). These systems provide good resolution, however real time data processing is intensive, requiring a large amount of compute power. A key drawback of this technology is that it offers almost nil performance over water impregnated ground conditions, where most of the incident microwave radar signal is absorbed by the liquid water molecules. Ground penetrating radar is only useful in arid desert or frozen permafrost terrains.

Acoustic mine detection offers the possibility of avoiding this drawback as it works as effectively in water impregnated soils as elsewhere. A large body of research exists into this as a technique, which has resulted in one commercial system in the military market.

The university of Mississippi and elsewhere has conducted an extensive amount of public research (ref I) (ref 2) into the use of acoustic and other mine detection methods. Other research has been carried out around the world at other universities.

However, practical inventions arising from this research are rare. This research has focused on the use of Laser Doppler Vibrometry to measure the surface resonance induced by the passing of a Rayleigh-type seismic wave over a buried mine. An available military system using this technique is Raytheon's Soteria Product (ref 3). This system has limitations in that the vehicle must remain stationary while an area is observed, and this process can take minutes at a time. This discounts such a system as an effective route clearance tool.

Research in India has been conducted using Ultrasonic Doppler Vibrometry (UDV) as a detector rather than a more conventional LDV system. They have also designed a wheel with included exciters that act as an acoustic source. However, this wheel is not able to act as a receiver and is missing features we believe are required to turn it into a workable solution (primarily a contacting layer for ensuring good acoustic coupling between the wheel and the ground) (ref 4).

None of the prior art incorporates advanced techniques for processing multiple signal paths received via different detectors in order to improve the signal to noise ratio of the received acoustic signals, or to provide images of the locations of buried objects.

None of the prior art describes the correct physics relevant to the acoustic observations. The prior art states that the acoustic signal propagating from the source excites a resonance at the object that is observed by the sensor. The principal acoustic modes that are useful in imaging buried objects are: reflections, scattering and diffractions of the various incident acoustic signals from the buried object and changes in mechanical impedance of the earth caused by the inclusion of the buried object The present invention solves the problems of improved signal to noise ratio, improved scanning rate, source-ground acoustic coupling and packaging and usage considerations. It enables more detailed scanning with the ability to work in a wider range of environments than present systems, and increased robustness and ease of use and maintenance during its operational life.

According to an aspect of the present invention there is provided a method of detecting buried objects, comprising the steps of: generating an acoustic signal; measuring back scattered incident acoustic signals; and applying a migration algorithm to the back scattered signals.

In some embodiments scattered surface waves may be measured and migrated, for example Rayleigh waves, Love waves or Sholte waves.

A further aspect provides a method for detecting buried objects comprising the steps of: generating an acoustic signal; and using the signal to measure mechanical impedance of the earth, buried objects causing changes in the measured mechanical impedance. Boundary waves are measured to measure mechanical impedance, for example Rayleigh waves, Love waves or Sholte waves.

A further aspect provides a method for detecting buried objects comprising the steps of: generating an acoustic signal so as to cause movement of the ground; and measuring generated acoustic signals, whereby the presence of an object affects ground movement and hence affects the generated acoustic signals.

In some embodiments higher or lower ground movement is mapped.

A further aspect provides a buried object detection method comprising the steps of: emitting an acoustic signal; and measuring the emitted signal to determine changes in movement of the earth caused by inclusion of buried objects.

A further aspect provides an acoustic subterranean ordnance detection method comprising the steps of: emitting an acoustic signal; and measuring the emitted signal to determine changes in movement of the earth caused by inclusion of buried objects. Earth movement may be determined by measuring displacement and/or velocity and/or acceleration and/or pressure and/or pressure gradient.

The present invention also provides apparatus for performing the method of any preceding claim.

The apparatus may comprise one or more acoustic sources and one or more acoustic sensors.

The apparatus may comprise means for measuring earth movement.

The apparatus may comprise means for measuring boundary waves.

In some aspects and embodiments a plurality of acoustic sources may be used to generate a plurality of acoustic signals.

A plurality of spatially separated acoustic receivers may be used to receive incident acoustic signals.

In some aspects and embodiments an acoustic source may be co-located with an acoustic receiver.

Alternatively or additionally an acoustic source is spaced from an acoustic receiver.

Acoustic sources and receivers may be connected to signal generation and signal processing units.

Acoustic sources and/or acoustic receivers may be rigidly mounted. Alternatively or additionally acoustic sources and/or acoustic receivers may be flexibly mounted.

In some aspects and embodiments an acoustic source may be the same component as an acoustic sensor.

Methods and apparatus may use irregular spatial sampling of acoustic source positions.

Methods and apparatus may use irregular spatial sampling of acoustic detector positions.

Methods and apparatus may include a multidimensional array (ID, 2D, 3D) of one or more acoustic sensors and one or more acoustic sources in a multidimensional array (ID, 2D, 3D).

The or each source and receiver may be connected to signal generation and processing units.

One or more acoustic sensors or detectors may be used.

Sensors or detectors may be: piezo-electric transceivers; loudspeakers, subwoofers, laser Doppler vibrometer sensors, ultrasonic Doppler vibrometer sensors, geophones, hydrophones, microphones, or accelerometers.

The direction or sensitivity of the or at least one of the sensors may be: omnidirectional, single axis, dual axis, triple axis, rotational axes.

Two or more sensors with different directional sensitivity may be used.

The acoustic source may generate a multitude of elastic signals that propagate from the source location and are either scattered or reflected from sub-surface interfaces or buried objects, or induce resonances within the buried object.

The elastic signals emitted from the acoustic source may include: reflections of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); refractions (alternatively known as diving waves) of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); and surface waves that propagate along the air/earth interface in the case of Rayleigh waves and Love waves on land, or along the water/earth interface in the case of Scholte waves in marine environments.

Multiple acoustic sources may be provided and may be activated sequentially.

Multiple acoustic sources may be provided and may be activated absolutely simultaneously.

Multiple acoustic sources may be provided and may be activated substantially simultaneously.

Multiple acoustic sources may be provided and activation of sources may be near simultaneous.

Multiple acoustic signals may be generated that contain the property of being orthogonal to each other.

Multiple receivers may be provided in a two-dimensional arrangement to form a three-dimensional array.

In some aspects and embodiments a Rayleigh wave, a Love wave or a Scholte wave may be excited at the acoustic source.

The apparatus, method or system may comprise on or more further detection systems, for example physical and/or chemical detection systems. For example, a metal detection means, a neutron detection means, or a volatile organic chemical detection means. Combinations of two or more different types of detector may be used to increase reliability of the system and to reduce false positives.

The present invention may be suitable for use in one or more of the following 20 applications: military; humanitarian mine clearance; highway construction and maintenance; utility infrastructure mapping; and environmental geophysics.

The present invention also provides a real-time detection and alert system comprising a method or apparatus as described herein.

The present invention also provides a vehicle capable of performing the method as described herein, or fitted with apparatus or a system as described herein.

The vehicle may comprise an automatic braking function linked to detection of a buried object The present invention also provides a vehicle having means for detection of hazardous objects, the vehicle including means for automatically stopping the vehicle and/or preventing motion if a hazardous object or a potentially hazardous object is detected.

The present invention also provides a vehicle having means for on-move, real-time detection of hazardous objects, the vehicle including means for automatically stopping the vehicle and/or preventing motion if a hazardous object or a potentially hazardous object is detected.

The vehicle may be unmanned.

The vehicle may be manned.

The vehicle may be a mine detector / mine clearance vehicle with military and/or humanitarian utility.

The present invention also provides a convoy of vehicles including one or more vehicles as described herein. I0

In some embodiments all vehicles in the convoy may be fitted with an automatic breaking system operable collectively if a hazardous object is detected by one or more vehicles in the convoy.

In some aspects and embodiments the system may generally consists of a multidimensional array (I D, 2D, 3D) of one or more acoustic sensors. It also consists of one or more acoustic sources in a multidimensional array (I D, 2D, 3D), which may either be the same component as the sensor (e.g. a piezo-electric transceiver), substantially co-located (e.g. within the same sub-assembly) or at locations offset from one another. These sources and receivers are connected to signal generation and processing units and ultimately to a controlling computer, in a manner known to those in the art. The mounting arrangement for these receivers may either be a rigid frame, or a mechanical mounting arrangement that allows relative motion between the components, for example on different spring arms or axles. The use of a flexible mounting arrangement mandates several inventive steps in the method of collecting and processing data and are laid out below. Systems conforming to the basic configurations outlined above can be constructed for use in both land and marine environments.

The acoustic source generates a multitude of elastic signals that propagate from the source location and are either scattered or reflected from sub-surface interfaces or buried objects, or induce resonances within the buried object The principal elastic signals emitted from the acoustic source that are of most utility include: reflections of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); refractions (alternatively known as diving waves) of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); and surface waves that propagate along the air/earth interface in the case of Rayleigh waves and Love waves on land, or along the water/earth interface in the case of Scholte waves in marine environments.

A preferred embodiment of the apparatus includes Human Machine Interface (HMI) components to communicate the detection or lack thereof of a buried object back to a user. However in other embodiments this equipment may be handled outside of the system and is not a required part of the invention. Furthermore, another embodiment of the system may include an interface to other computer systems to signal the detection of a buried object, for example to apply vehicle brakes or active countermeasures. This is also not a required part of the invention.

The sensor or detector used at any given position may be of a number of types, including but not limited to piezo-electric transceivers, loudspeakers (including subwoofers), Laser Doppler Vibrometer (LDV) sensors, Ultrasonic Doppler Vibrometer (UDV) sensors, geophones, hydrophones immersed in either a fluid or a gel, microphones or accelerometers. Each of these devices may have a direction of sensitivity that is: omni-directional, single axis, dual axis or triple axis, or even rotational axes sensors. Combinations of different such sensors can be used to measure signals that can then be combined to achieve wavefield separation and a better measurement of the desired elastic signal.

The invention covers a method of activating the apparatus described above in a specific sequence and recording and processing specific properties of the signals generated.

Firstly one of the transmitters is fired, emitting a defined or controlled signal. Recordings are then taken at a number of the receiver points over a defined time range. This is called a "shot", with the resulting data being termed a shot record. These are then processed to isolate each of the various acoustic propagation modes or signals that can be utilised to indicate buried objects within the ground.

If additional acoustic sources at different locations are used in the system, then this procedure is repeated in a defined cyclic pattern. After each cycle the processed signals from each shot can be combined used a spatio-temporal filtering algorithm. This constructively combines signals that represent the same physical location.

Reflected, refracted, scattered or diffracted signals in these data can then compared to profiles of known mine types, and/or passed through a heuristic algorithm to determine potential threats. These data are then recorded and passed to the HMI components, if present. The signal may also be outputted in other ways, for example as a simple yes/no binary signal representing a given confidence threshold, or as either I D, 2D or 3D images offering the possibility of more detailed interpretation.

As an optional enhancement to the method described above, the signal used may be defined in order to contain a number of preferential properties. It may be designed to contain a range of signal frequencies in a short signal (for example a 3 octave pulse) to allow for analysis of non-linear frequency responses too. Acoustic signal may either be impulses or swept signals.

Another optional enhancement involves generating multiple signals that contain the property of being orthogonal to each other. Alternatively, firing time dithering between the initiation of each acoustic transmitter can be employed. This allows multiple acoustic transmitters to be fired simultaneously and the signals emitted by each acoustic transmitter separated according to the transmitter that generated them.

This allows each data acquisition cycle to take a fraction of the time otherwise required if single source shot records were recorded, allowing for faster data collection and a wider acoustic surveying geometry to be sampled.

The invention covers a method of using the above apparatus and algorithms in sequence as part of a real-time detection and alerting system that can be used to detect buried objects in close proximity to the apparatus.

Another optional enhancement of the system is to use multiple receivers in a two-dimensional arrangement, creating a 3D array. This allows measurement of the direction of received signals by comparison of the time delay of received signals with the wave velocity. This information can be added into the spatial algorithm to improve its effectiveness.

One preferred embodiment involves exciting a Rayleigh wave at the acoustic source that propagates in every radial direction from the source. Measurements of the ground movement are recorded. The Rayleigh wave is a boundary wave that propagates along the air/earth interface. The short wavelength components only penetrate to a shallow depth within the earth. As the wavelength increases, then the depth of the Rayleigh wave's penetration and oscillation within the earth increases. If a measurement is made above a buried object that is captured by the oscillating movement of the surrounding earth, then the presence of the object will affect the signal measured above it on the surface. For instance, if the buried object was less dense than the surrounding soil, then the acceleration of the ground measured above the object would be greater than at a nearby location unaffected the presence of the object. Similarly, if the buried object were more dense than the surrounding soil, then the acceleration of the ground measured above the object would be less than at a nearby location unaffected the presence of the object This method is effectively measuring the mechanical impedance of the near surface of the earth. Mapping the higher or lower ground movement (accelerations, velocities, displacements or pressures) can identify buried objects that are either less dense or more dense than the surrounding soil respectively. The depth of penetration of the incident Rayleigh wave can be inverted to provide approximate depth information of the identified buried objects. A similar approach can be achieved through the excitement of a horizontally polarised Love wave at the surface. A similar approach can be achieved through the excitement of the water/earth interface in a marine environment by a Scholte wave.

Another preferred embodiment relies on recording scattering and diffraction of the incident Rayleigh wave off the surface of the buried object. The Rayleigh wave is excited at the acoustic source position. It propagates radially from the source position. When it is incident on the buried object, some of the Rayleigh wave will be scattered and/or diffracted off the object's surface. The acoustic detectors will record the scattered and diffracted Rayleigh wave energy. A class of algorithm know in the geophysical surveying industry as migration algorithms can be applied to these I5 scattered signals to focus the scattered and diffracted energy at the exact location of the scattering points, thus providing the location of the buried object. This approach enables a look-ahead, or more accurately, a look-around option for buried object location. The illumination of the buried object can be improved by using multiple acoustic sources at different azimuthal angles with respect to the buried object's position and / or 2D or 3D distributions of acoustic sensors. Indeed, this approach can detect the location of buried objects at any azimuth angle surrounding the source and measurement locations. As the source and detectors move to their next survey locations, the mapping of the positions of the buried objects is reinforced. The velocity of the acoustic signals propagating through the medium can be estimated from the data, and knowledge of this velocity is key to accurate location of the buried object. The depth of the buried object can be determined by using knowledge of the depth of penetration of the incident Rayleigh wave. The same approach can be used with Love waves on land and Scholte waves in the marine environment.

Similar imaging techniques can be applied to compressional and shear wave acoustic signals that are either reflected or refracted from the surface of the buried object.

Both the mechanical impedance and migration of scattered or diffracted acoustic energy approaches can be used in combination to locate buried object locations and the depths of burial. Both provide subtly different information that can eliminate false positives, where say a buried stone is mis-identified as a buried mine or IED. Both methods not only provide direct information about a buried object, but can also identify that an object has been buried. When an object is buried, a hole is usually dug in soil that has previously become naturally consolidated. When the object is buried, the hole is back-filled. The back-filled soil will be less dense and consolidated than the surrounding media. This unconsolidated soil volume will usually, although not exclusively, be in the shape of a column above the buried object. The detection of unconsolidated soil above a buried object can be used to further eliminate false positive object identifications.

Both approaches can be applied on the sea-floor, riverbed, estuary or lake bottom. The marine system would excite a Scholte-wave that propagates along the water/earth boundary. The mechanical impedance and migration of scattered energy approaches would be applied to the marine data as described above.

The data processing and interpretation approaches described in this invention can also be used in time-lapse mode. If previous data are available from an earlier survey, then these legacy data can be scaled and matched before being subtracted from the data currently being acquired. Such time-lapse data highlight changes between surveys.

Thus, the signals from buried objects and structures that were present during the legacy survey will be subtracted from the new survey data. Objects that have been buried after the legacy data were acquired will thus stand out and be much easier to identify. This approach largely eliminates false positive object identification, which is of particular interest to military applications.

An alternative embodiment uses the ambient Rayleigh wave energy that is propagating at the air/earth interface as the acoustic source. This ambient Rayleigh wave energy is generated by vehicles, industrial plant, wind coupling into the ground through tree roots and many other sources that couple their energy into the earth. We can create a spatial interferometer sensor by locating a pair of detectors on the ground and cross-correlating the signals recorded on each sensor with that of the other. The interferometric approach effectively generates a trace that is a measure of signal propagating from one sensor to the other. This pair of sensors replaces a single sensor from previous embodiments that included a controlled and active acoustic source. Such spatial interferometric sensors are excellent at measuring surface wave energy propagating in-line with the pair of detectors. Such measurements can be used with both the mechanical impedance and migration of scattered energy approaches to map and characterise buried objects. This approach is described in detail for the characterisation of earthquake signals in Benson et al (2007) (reference 5).

The present invention also provides an acoustic ground coupling device comprising a compartment filled with particulate material, the compartment housing means for generating an acoustic signal and sensor means for detecting back scattered acoustic signals from the ground.

The particulate material may, for example, be a granular material such as sand.

The present invention also provides a plurality of compartments as described herein arranged as a streamer.

The present invention also provides an object detection system based on power spectrum density. The system may comprise one or more of the steps illustrated in Figure 8.

The present invention also provides a method for detecting buried objects using waveform correlation, comprising the steps of: providing one or more acoustic sensors; determining a reference signal for the or each sensor; emitting a swept frequency acoustic signal; and determining the cross-correlation coefficient between resulting recorded signal and the respective reference signal.

Different aspects and embodiments of the invention may be used separately or together.

Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with the features of the independent claims as appropriate, and in combination other than those explicitly set out in the claims.

Other aspects, objectives and advantages of the present invention will appear more clearly on reading the following description of several embodiments thereof, given by way of non-limiting examples and with reference to the appended drawings. The figures are not necessarily to scale for all the elements represented so as to improve the readability thereof. In the remainder of the description, for the sake of simplicity, identical, similar or equivalent elements of the various embodiments bear the same numerical references.

The present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure I shows a number of transceivers arranged in a I D line array.

Figure 2 gives a layout of the transceiver array attached to a vehicle.

Figure 3A shows the paths taken by acoustic signals transmitted from a station I and received at stations 2. Figure 3B then shows how signals transmitted from station 2 and reflected from the same objects would be received at stations I, 8 and 16.

Figure 4A shows how a signal transmitted from a station are diffracted when reflected from a buried object This signal is then picked up at multiple receivers. This produces generated signals as shown in the stream plot in Figure 4B. The migration algorithm of the invention converts this into a single received signal at the correct location, as shown in Figure 4C.

Figure 5 shows one alternative arrangement of transmitters and receivers.

Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

The terminology used herein to describe embodiments is not intended to limit the scope. The articles "a," "an," and "the" are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including," when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein.

A preferred embodiment of this invention is shown in Figure I. This shows a number of transceivers arranged in a I D line array. There are 17 transceivers (I) at equal 0.25m spacing, giving a total width of 4m.

Figure 2 gives a layout of the transceiver array attached to a vehicle. The transceivers are mounted on a transverse beam (2). This is in turn attached to a boom (8), which can pivot on a mounting plate (3). This steering arrangement is power using actuator(s) (7), and allows the sensor array to cover the tracks of the host vehicle during cornering. The mounting plate is attached to the host vehicle (4). Within the vehicle is contained the power and data processing unit (5), connected to the transceiver array by a communications cable (6).

Figure 3A shows the paths taken by acoustic signals transmitted from station I (9) and received at stations 2 (10), 9 (11) and 17 (12). Figure 3B then shows how signals transmitted from station 2 (10) and reflected from the same objects would be received at stations I (9), 8 (13) and 16 (14). The spatial algorithm combines these separate traces from different shots and combines those that represent the same spatial location.

Figure 4A shows how a signal transmitted from a station (15) are diffracted when reflected from a buried object. This signal is then picked up at multiple receivers. This produces generated signals as shown in the stream plot in Figure 4B. The migration algorithm of the invention converts this into a single received signal at the correct location, as shown in Figure 4C.

These algorithms are processed in the data processing module in Figure 2. An embodiment of a visualisation technique is to display the strength of the reflected signal as a colour to an operator. Another embodiment is to emit a warning tone should the reflected strength of the received signal be above a set threshold, once the effect of the ground and other known objects is removed.

Figure 5 shows one alternative arrangement of transmitters (17) and receivers (16).

Here the transmitters (17) would be mounted on the boom (8) of Figure 2. This arrangement provides an alternative area of coverage based around a triangle formed by the outer point This arrangement could be potentially advantageous for certain applications.

The embodiment(s) described above would be suitable for providing land mine or IED threat detection in front of a moving vehicle. This provides self-protection for the vehicle, assuming it is moving at a speed that would allow it to stop before activating any detected threat. It would also be suitable in a route-proving role to establish there are no detected threats along a given lane driven by the host vehicle at a certain time.

A marine version of the equipment, exploiting exactly the same data acquisition distribution geometries described in earlier embodiments can be applied to sea-floor, estuary or riverbed marine applications.

Further developments, for example for mine or IED detection, are described below.

Ground coupling development: The latest technological advancement includes a practical means of coupling both the source and sensors to the ground surface such that it allows stable and even contact points for the source and sensors without altering the mechanical impedance of the transfer medium too much.

Inappropriate ground coupling can hamper the transmission of the waves or act as a filtering process where the frequency spectra of the recorded signals are not the true representation of the propagated acoustic waves through the medium.

The intermediary coupling solution between the source/sensor and the ground surface should have a similar elastic property as the ground underneath the source/sensor where it allows the acoustic waves (useful frequency band) to propagate and reach the sensors. For this purpose a ground coupling device is constructed as follows: 1.1 The metallic shaker base plate is not suitable for coupling to hard surfaces such as gravel. The coupling device is constructed as shown in Fig. 6 where it surrounds the base plate of the electromagnetic shaker and provide a deformed coupling to any hard surface such as gravel.

1.2 The coupling technique shown in Fig. 6 is comprised of a water resistant fabric that is made into a pseudo cubic enclosure that is filled with fine-grain sand and encloses the electromagnetic base plate.

I. Water resistant fabric 2. Sand compartment 3. Electromagnetic shaker 4. Base plate 1.3 This ground coupling device or enclosure can easily deform to any surface condition, soft or hard such as soil or gravel.

1.4 It also allows the shaker to freely move since the shaker's main body (magnet) needs to be free from any obstruction.

1.5 The sensors e.g. accelerometers' base plates either plastic or metallic are not suitable for coupling to hard surfaces such as gravel. A streamer device was constructed to address the sensor-ground coupling problem.

1.6 The streamer (Fig. 7) is constructed using a water resistant fabric that is made into 20 pseudo cubic compartments. Each compartment is filled with fine grain sand and encloses a sensor's base plate.

I. Water resistant fabric 2. Sand compartment 3. Sensor # I 4. Sensor # 20 5. Sensor base unit 6. Distance between sensors 100 mm 1.7 The sensors are secured by the top end of each compartment such that the distance between two adjacent sensors remains at about 100 mm. Therefore using the streamer with 20 compartments 2 meters of ground at a time can be investigated.

2 Object Detection based on Power Spectrum Density (PSD) 2.1 A flow chart for this detection methodology is shown in Fig 8.

2.2 A swept frequency signal of 50 to 1000 Hertz is played using the electromagnetic shaker shown in Fig 6.

2.3 The streamer apparatus, shown in Fig 7, with an offset of 500 mm from the shaker Fig 6, records the propagated mechanical waves. The streamer apparatus covers 2 m of ground at a time and then shaker-streamer system is positioned with 500 mm intervals as far as ground investigation is required.

2.4 The method of Welch power spectrum (ref. I) is employed to transform the signals from the time domain into frequency domain.

2.5 The centre frequency of the baseband spectrum for each signal is found as shown in Fig. 9 by examining the maxima for the lowest frequency band. This is computed analytically where the frequency power maxima (peaks) of the entire spectrum are compared against their frequency band. The maximum power at the lowest frequency band (baseband) is selected.

I. Power spectrum on top of a buried object 2. Power spectrum, no buried object 2.6 The centre frequency is then used as a parameter to indicate the centre of the bandwidth of interest A narrow bandpass filter centred at the computed centre frequency is used to filter the signals for all sensors.

2.7 The energies of the filtered signals are computed using the root mean square RMS technique. The energies of the bandpass spectra for all signals are turned into a colormap schema where the buried objects are highlighted with higher contrast compared to their surrounding regions.

3 Waveform Correlation 3.1 A swept frequency signal of 50 to 1000 Hertz is played using the electromagnetic shaker shown in Fig 6.

3.2 The streamer apparatus, shown in Fig 7, with an offset of 500 mm from the shaker Fig 6, records the propagated mechanical waves. The streamer apparatus covers 2 m of ground at a time and then shaker-streamer system is positioned with 500 mm intervals as far as ground investigation is required.

3.3 Over a homogeneous medium all waveforms, for a given receiver, should be very similar. The method of waveform correlation exploits the lack of homogeneity in the regions of a medium under investigation as a cursor to a disturbed region or a buried object.

3.4 The median trace for all the recorded signals of a single sensor is selected. This is referred to as the reference signal.

3.5 For each sensor the cross-correlation coefficient between the recorded signal and the reference signal is computed.

3.6 The higher the cross-correlation coefficient (between 0 and I) the better similarity between the reference and the signal. A value closer to zero indicates anomalous behaviour of the actual waveform.

3.7 The computed coefficients for all traces are then mapped against the sensors' locations using a colormap schema where the disturbed regions, e.g. buried objects, has different colour to their surrounding regions.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiments shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Applications: This invention may aid detection and classification of buried objects in the ground. This capability is using within the Military world to detect mines and improvised explosive devices (IEDs) lain by an enemy, so that they can be detected as a threat, classified as to the type of threat, and defeated before activation.

In humanitarian mine clearance this technology will help detect buried anti-tank and anti-personnel munitions so that they can be made safe and removed to allow safe civilian activity in the area.

In highway construction and maintenance this invention will allow analysis of a proposed highway route to determine the density of foreign objects buried along the proposed path and so allow a more informed plan of construction.

This technology can detect and map underground tunnels.

This invention can be used to map utility infrastructure. Many locations have water, sewer, electricity and gas distribution systems that are poorly and inaccurately mapped. Surveying these areas with this technology will map each utility network system accurately and enable repair and intervention to be completed more effectively. This technology provides a useful tool for environmental applications. Environmental geophysics methods are often used to detect and map buried objects that are the source of pollution. This technology enables such mapping to be completed efficiently and accurately. The environmental applications can be extended to mapping subsidence risks, water table movement and old mine workings.

This technology can be applied to archaeology surveys to locate and map buried structures.

This invention will aid detection and classification of buried objects in the ground. This capability is used within the Military world to detect mines and improvised explosive devices (IEDs) lain by an enemy, so that they can be detected as a threat, classified as to the type of threat, and defeated before activation.

Utility networks are often poorly mapped, leading to major issues in effecting repairs and upgrades. The acoustic buried object detection approach will enable utility-related structures to be mapped accurately.

Environmental geophysical techniques are often used to locate sub-surface structures, locate buried objects that are sources of pollution and to identify mine shafts. The acoustic buried object approach described in this invention will provide an efficient means of using acoustic methods to provide environmental geophysical services.

References I. Sabatier, James M. [Online] http://www.dtic..rnilichic.inifulltext./u2.1a478728.pdf 2.[Online] I itip://www.rand.orgicontentida rnira ndipubsirnonograph reportsiM R I 6081MR1608.app g. pdf 3.[Online] http://w.r heon.co.ukirtnyvcrnigroupsigaileryidocum..'contentirtn 15780 I.pdf 4. Advanced Acousto-ultrasonic Landmine. Rajesh K.R, Murali.R, Mohanachandran R. s.l. : 201 I IEEE Global Humanitarian Technology Conference.

5. Processing seismic ambient noise data to obtain reliable broad-bandsurface wave dispersion measurements. Bensen, G.D., M.H. Ritzwoller, M.P. Barmin, A.L. Levshin, F. Lin, M.P. Moschetti, N.M. Shapiro, and Y. Yang, Geophys. J. Int., 169, 1239-1260 6. P. Welch. The use of fast Fourier transform for the estimation of power spectra: A method based on time averaging over short, modified periodograms. IEEE Transactions on Audio and Electroacoustics, Vol 15, P 70 -73, Jun 1967.

Claims

CLAIMSI. A method for detecting buried objects comprising the steps of: generating an acoustic signal; and using the signal to measure mechanical impedance of the earth, buried objects causing changes in the measured mechanical impedance.
2. A method as claimed in claim I, in which boundary waves are measured to measure mechanical impedance.
3. A method as claimed in claim 2, in which the boundary waves are Rayleigh waves, Love waves or Sholte waves.
4. A method of detecting buried objects, comprising the steps of: generating an acoustic signal; measuring back scattered incident acoustic signals; and applying a migration algorithm to the back scattered signals.
5. A method as claimed in claim 4, in which scattered surface waves are measured and migrated.
6. A method as claimed in claim 5, in which the scattered surface waves are Rayleigh waves, Love waves or Sholte waves.
7. A method for detecting buried objects comprising the steps of: -generating an acoustic signal so as to cause movement of the ground; and measuring generated acoustic signals, whereby the presence of an object affects ground movement and hence affects the generated acoustic signals.
8. A method as claimed in claim 7, in which higher or lower ground movement is mapped.
9. A buried object detection method comprising the steps of: emitting an acoustic signal; measuring the emitted signal to determine changes in movement of the earth caused by inclusion of buried objects.
10. An acoustic subterranean ordnance detection method comprising the steps of: emitting an acoustic signal; measuring the emitted signal to determine changes in movement of the earth caused by inclusion of buried objects.
I I. A method as claimed in claim 9 or claim 10, in which earth movement is determined by measuring displacement and/or velocity and/or acceleration and/or pressure and/or pressure gradient
12. Apparatus for performing the method of any preceding claim.
13. Apparatus as claimed in claim 12, comprising one or more acoustic sources and one or more acoustic sensors.
14. Apparatus as claimed in claim 12 or claim 13, comprising means for measuring earth movement.
15. Apparatus as claimed in any of claims 12 to 14, comprising means for measuring boundary waves.
16. A method or apparatus as claimed in any preceding claim, in which a plurality of acoustic sources are used to generate a plurality of acoustic signals.
17. A method or apparatus as claimed in any preceding claim, in which a plurality of spatially separated acoustic receivers are used to receive incident acoustic signals.
18. A method or apparatus as claimed in any preceding claim, in which an acoustic source is co-located with an acoustic receiver.
19. A method or apparatus as claimed in any preceding claim, in which an acoustic source is spaced from an acoustic receiver.
20. A method or apparatus as claimed in any preceding claim, in which acoustic sources and receivers are connected to signal generation and signal processing units.
21. A method or apparatus as claimed in any preceding claim, in which acoustic sources and/or acoustic receivers are rigidly mounted.
22. A method or apparatus as claimed in any preceding claim, in which acoustic sources and/or acoustic receivers are flexibly mounted.
23. A method or apparatus as claimed in any preceding claim, in which an acoustic source is the same component as an acoustic sensor.
24. A method or apparatus as claimed in any preceding claim, using irregular spatial sampling of acoustic source positions.
25. A method or apparatus as claimed in any preceding claim, using irregular spatial sampling of acoustic detector positions.
26. A method or apparatus as claimed in any preceding claim, comprising a multidimensional array (ID, 2D, 3D) of one or more acoustic sensors and one or more acoustic sources in a multidimensional array (1 D, 2D, 3D).
27. A method or apparatus as claimed in any preceding claim, in the or each source and receiver is connected to signal generation and processing units.
28. A method or apparatus as claimed in any preceding claim, in which one or more acoustic sensors or detectors are used.
29. A method or apparatus as claimed in claim 20, in which the sensors or detectors are: piezo-electric transceivers; loudspeakers, subwoofers, laser Doppler vibrometer sensors, ultrasonic Doppler vibrometer sensors, geophones, hydrophones, microphones, or accelerometers.
30. A method or apparatus as claimed in claim 18 or claim 19, in which the direction or sensitivity of the or at least one of the sensors is: omni-directional, single axis, dual axis, triple axis, rotational axes.
31. A method or apparatus as claimed in claim 19, in which two or more sensors with different directional sensitivity are used.
32. A method or apparatus as claimed in any preceding claim, in which the acoustic source generates a multitude of elastic signals that propagate from the source location and are either scattered or reflected from sub-surface interfaces or buried objects, or induce resonances within the buried object
33. A method or apparatus as claimed in claim 24, in which the elastic signals emitted from the acoustic source include: reflections of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); refractions (alternatively known as diving waves) of the compressional (P) and shear waves (vertically polarised Sv and horizontally polarised Sh); and surface waves that propagate along the air/earth interface in the case of Rayleigh waves and Love waves on land, or along the water/earth interface in the case of Scholte waves in marine environments.
34. A method or apparatus as claimed in any preceding claim, in which multiple acoustic sources are provided and are activated sequentially.
35. A method as claimed in any preceding claim, in which multiple acoustic sources are provided and are activated absolutely simultaneously.
36. A method as claimed in any preceding claim, in which multiple acoustic sources are provided and are activated substantially simultaneously.
37. A method as claimed in any preceding claim, in which multiple acoustic sources are provided and activation of sources is near simultaneous.
38. A method or apparatus as claimed in any preceding claim, in which multiple acoustic signals are generated that contain the property of being orthogonal to each other.
39. A method or apparatus as claimed in any preceding claim, in which multiple receivers are provided in a two-dimensional arrangement to form a three-dimensional array.
40. A method or apparatus as claimed in any preceding claim, in which a Rayleigh wave, a Love wave or a Scholte wave is excited at the acoustic source.
41. A method or apparatus as claimed in any preceding claim, comprising on or more further detection systems.
42. A method or apparatus as claimed in claim 41, comprising one or more further physical and/or chemical detection systems.
43. Apparatus or a method as claimed in any preceding claim, suitable for use in one or more of the following applications: military; humanitarian mine clearance; highway construction and maintenance; utility infrastructure mapping; and environmental geophysics.
44. A method substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
45. Apparatus substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
46. A real-time detection and alert system comprising a method or apparatus as claimed in any preceding claim.
47. A vehicle capable of performing the method of any preceding claim, or fitted with apparatus or a system according to any preceding claim.
48. A vehicle as claimed in claim 47, comprising an automatic braking function linked to detection of a buried object
49. A vehicle having means for detection of hazardous objects, the vehicle including means for automatically stopping the vehicle and/or preventing motion if a hazardous object or a potentially hazardous object is detected.
50. A vehicle having means for on-move, real-time detection of hazardous objects, the vehicle including means for automatically stopping the vehicle and/or preventing motion if a hazardous object or a potentially hazardous object is detected.
51. A vehicle as claimed in any of claims 47 to 50, in which the vehicle is unmanned.
52. A vehicle as claimed in any of claims 47 to 50, in which the vehicle is manned.
53. A convoy of vehicles including one or more vehicles as claimed in any of claims 47 to 52.
54. A convoy as claimed in claim 53, in which all vehicles in the convoy are fitted with an automatic breaking system operable collectively if a hazardous object is detected by one or more vehicles in the convoy.
55. An acoustic ground coupling device comprising a compartment filled with particulate material, the compartment housing means for generating an acoustic signal and sensor means for detecting back scattered acoustic signals from the ground.
56. An acoustic ground coupling device comprising a plurality of compartments as claims in claim 55 arranged as a streamer.
57. An object detection system based on power spectrum density comprising one or more of the steps illustrated in Figure 8.
58. A method for detecting buried objects using waveform correlation, comprising the steps of: providing one or more acoustic sensors; determining a reference signal for the or each sensor; emitting a swept frequency acoustic signal; and determining the cross-correlation coefficient between resulting recorded signal and the respective reference signal.