WO2007099054A1 - Method for infrared detection of buried unexploded objects in humanitarian demining operations - Google Patents

Abstract

A method for the infrared detection of buried unexploded objects in the sub-surface layer of soils at small or moderate depths, in humanitarian demining operations, characterized by the fact that the target soil is locally heated by means of a microwave apparatus, in order to increase the thermal discontinuity between the unexploded objects and the materials surrounding them. Such a thermal discontinuity allows the individuation of the detonator and thus of the UXO. The unexploded objects can be localized at sub-soil depths ranging from some millimeters up to about 150 cm. The frequencies involved in the microwave apparatuses range from 2.45 Ghz up to 10 GHz depending on soil composition and the depth of at which the unexploded objects are buried, and the heating times of the soil range from 2 to 20 minutes, and preferably from 5 to 10 minutes. The apparatus for performing the above method comprises at least one infrared camera and at least one microwave antenna (1).

Description

TITLE: "Method for infrared detection of buried unexploded objects in humanitarian demining operations" DESCRIPTION STATE OF ART 1. Humanitarian demining operations

The demining activity is usually distinguished between "operating activity" and "humanitarian activity". The operating activity is concerned with military operations, whose target is removing (or

reducing at inertia) a percentage > 70% of contaminative war material.

Following international standards, in humanitarian activity the minimum target is 99.6% with a residual risk of 0.4%. To this aim, specialists for detecting and defusing bombs so far perform manual operations. To improve both effectiveness and safety during these manual operations, metal detectors, bulldozers, and trained dogs are also used.

The actual number of Unexploded Objects (so called UXOs) is generally underestimated. It must be stressed that, due to their economical difficulties, resident populations are often interested in UXOs from which some metals (copper, aluminum etc ..) and even explosive charge can be recovered and commercialized. 2. Technical aspects of UXOs 2.1 Mines

Mines, so far the most feared post-war danger, are composed by the envelope, the explosive charge, and the lighter. The envelope is made using a variety of materials such as metals, plasties, wood or even cardboard. Sophisticated envelopes have been also constructed to protect mines against detection or environmental agents. Modern bakelite envelopes, as well as synthetic-resin envelopes, keep mines uncorrupted against chemical agents, and seriously reduce the effectiveness of the electronic instruments of detection.

The lighter is the component that mostly affects mine production techniques and costs. Both reliability and handling safety of a mine are dependent on the lighter. The lighter can be an anti-tank mechanism or an anti-man mechanism. The former one is generally a pressure-mechanism, which can be activated from loads greater than 120 -150 kg, according to the international standards. More recently, high-cost electronically activated mechanisms have been constructed in some of the most advanced countries. Concerning anti-man mechanisms, either traction or pressure mechanisms are available, but activating loads are generally lower, within the range 5 -12 kg. The explosive charge does not exceed few hundred grams in weight for a typical landmine. In anti-tank mines, up to some kilograms of explosive charge can be present.

An anti-tank mine can be either a sub-body mine or an anti-track mine. These are all high-potential bombs, most of them equipped with suitable anti-removal devices that can cause their explosion even in case of very small collisions. During the most recent conflicts, anti-removal or 'trapping' techniques have been actually adopted also with landmines. Landmines can be buried, hidden within bushes, positioned on stakes and connected with hindrance wires. Landmines can have either long-range or short- range effects, the former ones being the most dangerous, since explosions can cause mortal effects up to distances of 30-40 m, and splinters can be projected up to distances of 100 m. Directional landmines also exist which are usually electrically activated by remote systems. These mines are mortally effective within a radius of about 100 m, and their splinters are emitted within a cone whose aperture is 60° about.

Landmines can be distinguished also in fragmentation landmines and circumstance landmines. These latter ones are frequently used and are very dangerous for both civil population and professional mine removers, because their construction does not follow standards. Explosive charges often exceed standardized weights of landmines, moreover lighters are often "manipulated" to increase their sensitivity and thus their lethal effects. 2.2 Clusters bombs Cluster bombs are generally contained within a dispenser dropped from an aircraft. The dropping causes the opening of the dispenser and the activation of its sub-munitions, whose subsequent dispersion on the soil depends on several parameters such as flying velocity, height from soil and angle of throw. The sub-munitions released from the dispenser can explode even up to 24 hours after reaching the soil. However, a percentage of about 20-25% of these cluster bombs do not explode but, conversely, they can remain activated for a long period without any auto-destroying mechanism being adopted. Thus cluster bombs always need to be destroyed in place. Several kinds of cluster bombs exist, such as fragmentation bombs or hollow-charge bombs, in any case very wasting effects can be caused to both people and things. The most powerful cluster bombs can perforate a steel plate of 125 mm in thickness; moreover their effects are lethal within a radius of 150 m about. Some of these cluster bombs are mechanically activated, in other cases activation is obtained by means of piezoelectric devices. As above said, there are no auto-destroying mechanisms in case of defects preventing from their explosion. Cluster bombs can capture curiosity and interest from unadvised people; moreover experts must act very carefully, especially when cluster bombs are hidden within brushwood or within excavations due to explosions.

2.3 "Depauperated Uranium" (DU) The Isotopes U²³⁸ (99,3 %), U²³⁵ (0,7 %), plus small quantities of U²³⁴

are involved in DU. Only α e β particles are emitted, thus radioactivity

risks are relatively limited, whereas chemical toxicity can be elevated.

A penetrator and a sabot, this latter containing the DU, form a DU projectile. The shooting of the projectile causes the separation of the penetrator from the sabot. The penetrator first reaches and perforates the target, whereas the sabot transforms into a gel before reaching the target and exploding. The sabot during its explosion, causing a large wasting to everything encountered, releases many incandescent drops. Unexploded DU projectiles can cause a large number of small fragments and can penetrate soils up to depths of 2 m about.

It is known that infrared sensors can provide a satisfactory imaging of UXOs, because such sensors allow individuating the discontinuity zone between the explosive charge and the lighter. Such a discontinuity zone can be seen as a dielectric-metal interface. The crucial point is that, during the acquisition phase, a sufficient thermal discontinuity must be present between an UXO (or its components) and the surrounding materials.

According to the present invention, we have discovered that such a thermal discontinuity allows the individuation of the lighter (or detonator) that actually allows detecting the UXO. This result can be obtained by a local heating of soil, since heat absorption varies from one material to another if heating is fast and sufficiently intensive. Surprisingly, we have discovered that it is possible to obtain the said object by utilizing a microwave apparatus as the heating system to be used in combination with infrared sensors.

Microwave heating is now available from technologies largely diffused from the industrial point of view. Thus costs of microwave heating apparatuses are now relatively low. Moreover, iperthermal techniques for medical applications as well as telecommunications techniques have provided in recent years a large number of antennas or applicators widely differing in size, shape and cost, so it appears possible the adoption of a suitable microwave heating system in demining operations. Principles of microwave heating

The theoretical aspects of microwave heating are summarized below under some simplifying assumptions that allow for a first evaluation of the method according to the invention. In the following description :

1 ) The magnetic permeability of a material is assumed to be independent on the frequency, thus magnetic losses are not considered.

2) Diffraction phenomena and multiple reflections from materials are not considered.

Though condition 1) is quite acceptable, nevertheless condition 2) is quite restrictive, because it means in practice that only homogeneous materials having regular surfaces are considered. However, we are here mainly concerned with the order of magnitude of the electromagnetic power needed for heating a given material, thus condition 2) can be acceptable at least from such a point of view. The mean electromagnetic power P_av dissipated within a volume V of a homogeneous material in presence of an electric field E, with frequency f, can be expressed as:

⁽¹⁾ In Eq. (1 ) we have put ω =2πf, moreover the complex dielectric

permittivity έ(ω) has been introduced, which characterizes the

material with volume V:

έ (ω ) = ε_o (ε '- jε " ) (2)

In Eq. (1) σ is the ohmic conductivity; in Eq. (2) ε' and ε" are real and

>0; εo = 8.85x10^"12 F/m is the vacuum permittivity.

The absorption of electromagnetic energy within the material causes an increase of temperature at a rate depending on several parameters: the mass M, the specific heat c, the thermal conductivity and the time-interval t. We just recall that c is the amount of heat for increasing of 1 °C the temperature of a mass of 1 Kg. Rigorously, one should distinguish between Cv, the specific heat at constant volume V, and Cp, the specific heat at constant pressure p. However cy and c_p are significantly different only in gases, whereas they are substantially the same in liquids and solids, thus we do not consider further such a distinction. For solids, if Qh is the amount of heat from the exterior and T the temperature, one has:

δCL

Cp = (3)

^ ' p=constant

(Actually, in Eq. (3) one should speak about thermal capacity, which is equal to c_p for a unitary mass).

It has to be noted that for most materials c_p does not greatly change over a wide range of temperatures. This allows us to say that the

amount ΔQh of heat per unitary mass, which is needed to obtain a temperature variation AT=T-T₀ (T₀ is the reference temperature) is

approximately given by:

ΔQ_h=c_pΔT (4)

To obtain ΔT for a material with mass M, the needed amount of heat is

MΔQ_h. This means that P_av is, in the time interval t:

MΔα Mc ΔT Pav= — p =— — (5)

Rigorously, Eq. (5) is valid if thermal conduction through S is negligible, where S is the surface including V. This is a frequently adopted hypothesis, which requires in practice a small thermal conductivity (this is generally true for soils) and a sufficiently short heating time. To proceed, in Eq. (1) we put:

Most experimental data are reported in literature in terms of ε' and

tan(δ), where:

tan(£) = ^- (7) ε

Let us consider the case where the electromagnetic field propagates as a TEM wave along the z axis direction. Propagation is also affected by attenuation, that is, E can be expressed as [29]:

E = e^JϋXE , where E = E_m e^~]βrZe^~β'^z , with βi and β_r real and >0(8)

The term βi represents the attenuation coefficient of the wave, and its

mathematical expression is: βi= -imaginary part of ψ^ε_ϋμ_ϋ{ε'-jε" _eff )\ (9)

In Eq. (9) μ₀ is the magnetic permeability of vacuum, (μ_r « 1 for soils).

Let L and A be, respectively, the height and the base area of the volume V, where A is perpendicular to the z axis, then Eq. (1 ) becomes:

P_*Ao₃ε₀ε_eff "J(E * .E)dV

(10)

-I B i

The attenuation of the transmitted power is proportional to e thus let the penetration factor be:

D= Jj- (11)

From Eq. (11) at z=D the power attenuation factor is Me. The penetration factor D can give an estimate of a practical depth that the transmitted power can reach. More precisely, from z=0 to z=D 63% of transmitted power results to be dissipated, whereas the remaining 37% is approximately dissipated from z=D up to z=2D. Thus, if L >D then heating of V will occur only partially. Conversely, if L << D the transmitted power will reach too large depths preventing from a localized heating within V. Accordingly, the best operating condition will be given by: L « D (12) Thus, from V =AL «AD, Eq. (10) becomes:

In order to evaluate the incident power P_in at the area A, for a normal incidence at the air-soil interface, that is, at z=0, the transmission coefficient is:

_^ c_t — = ^'v » ^{~ J £} « (14)

, = + 1

Thus:

From the Poynting Theorem, in terms of the rms value of the electric field (E_rms) P_in is given by:

In Eq. (16) Z₀ «377 Ω is the characteristic impedance of vacuum

(approximately that of air). From previous formulae it appears that microwave range is preferable to lower frequencies (e.g., radio frequencies) because shorter D values can be obtained. However

these considerations are strongly limited by the behavior of έ(ώ) . The

determination of such a parameter is a crucial point, and so far only experimental measurements appear to be useful. The complex permittivity έ(ω) strongly depends on the specific material considered.

The parameters affecting έ(ω) are the composition and the geometric texture of the material, but also the percentage of water, which plays an important role in microwave heating.

Table 1 collects some values of ε' and tan(δ) for materials of interest in

humanitarian demining actions. The reference T₀ temperature is 25°C;

frequencies range from 100 Hz to 10 Ghz. The mean accuracy for ε¹ is

±2%, whereas it is ±1 % for low-losses materials (tan(δ) < 0.005), and

±5% for high-losses materials (tan(δ) >1 ); the mean accuracy for

tan(δ) is ±5%, up to ±10% for high-losses materials.

Table 1

Frequency [ Hz]

Material T⁰C 10" 10^J 10⁴ 10^b 10" 10' 3 x10⁸ 3x10⁹ 10^lu

Sandy soil 25 ε¹ 342 2 91 2 75 2 65 2 59 2 55 2 55 2 55 2 53 dry tan δ 0 196 0 08 0 034 0 02 0 017 0 016 0 01 0 0062 0 0036

Sandy soil 25 ε¹ 3 23 2 72 2 50 2 50 2 50 2 50 2 50 2 50 2 50 ÷water 2 2% tan δ 0 64 0 13 0 056 0 030 0 025 0 025 0 026 0 03 0065

Sandy soil 25 ε¹ 5 0 4 70 4 50 4 50 4 40 3 60 ÷water 3 9% tan δ 1500 19 1 75 0 3 003 0 046 0 12

Sandy soil 25 ε¹ 20 20 20 20 13 ÷water 16 9% tan δ 3425 367 4 0 35 0 03 0 13 0 29

Loamy soil 25 ε¹ 3 06 2 83 2 69 2 60 2 53 2 48 247 2 44 2 44 dry tan δ 007 0 05 0 035 0 03 0 018 0 014 0 0065 00011 0 0014

Loamy soil 25 ε¹ 18 6 9 4 3 5 3 5 3 50 + water 2 2% tan δ 2 1 1 6 0 65 0 45 0 06 0 04 0 03

Loamy soil 25 ε¹ 14 5 20 20 13 8 ÷wateri 3 8% tan δ 1 3 0 16 0 12 0 18

Clay soil 25 ε¹ 4 73 3 94 3 27 2 79 2 57 2 44 2 38 2 27 2 16 dry tan δ 0 12 0 12 0 12 0 1 0 065 0 04 0 02 0 015 0 013

Clay soil 25 ε¹ 21 6 20 1 1 3 + water 20 1 % tan δ 7800 1000 1 7 0 52 0 25

25 ε¹ 4 3 4 32 4 3 4 3 4 3 4 29

Bentonite soil 0 0031 tan δ 4 0 0022 0 0019 0 0017 0 0017 0 0018

In Table 2 are reported experimental measurements on concrete and mortar, with frequencies from 100 MHz up to18 GHz. Table 2

Frequency [GHz]

Material 01 1 15 2 4 6 8 10 12 14 16 18

Concrete ε¹ 48 48 48 48 48 47 46 45 45 45 45 45 dry tan δ <0005 0005 001 0015 002 003 004 005 006 007 007 008

Concrete ε¹ 15 15 148 145 14 138 135 13 128 12 115 11 wet tan δ 011 0115 0118 012 0125 013 013 0135 014 015 0155016

Mortar ε¹ 45 45 45 44 44 44 44 44 43 42 41 41 Dry tan δ 001 001 001 001 0015 0015 0015 002 0025 0028 0030035

Mortar ε¹ 127 125 123 12 118 115 112 11 109 105 101 99 Dry tan δ 01 011 011 011 0118 012 0125 0128 0130 01350138014

In Table 3 some examples are reported using data reported in Tables 1 and 2. Interpolation of available data is done for frequencies not explicitly considered in experiments. Throughout we have used L=D. It

has to be noted that for an increase ΔT of temperature from 0 to L, we

have an increase ΔT/2 from L to 2L, which can be useful in practice. In

Table 3 the reflected power P_r is also reported.

Table 3

In Table 3 frequencies have been set to obtain heating times of 5-10 minutes over the reported areas A. Concerning f =2.45 Ghz, this is the typical working frequency of microwave ovens. To comment results of Table 3, for example we see that for a sandy soil the best case is

given by: water percentage 2-4% at 25°C, frequencies 6 - 7 GHz; ΔT=

3° C up to 4-5 cm, ΔT= 2⁰C from 4 cm to 10 cm. The incident power P_in on the area A is the power at the antenna output, which is related to the power at the antenna input through the antenna parameters (directivity, gain). Highly directional horn- parabolic antennas (high gain antennas) can be useful to our aims. Antennas are connected with the microwaves signal generator + amplifier block (or transmitting block) by means of a set of cables and connectors. Cable-losses must also be considered, e.g., for 3 dB cable-losses the transmitting block must supply twice the power needed at the antenna input. As an example, signal generators (based on magnetron technology) are available supplying 700-800 W at 6 Ghz. Moreover, from either klystron or magnetron technologies microwave generators with a reduced size are now available. Concerning the energy supply required by the overall system, we see from Table 3 that a common 3 kW-6 kW electricity-generating group will be sufficient. Moreover, concerning safety of operators in humanitarian demining actions, using highly directional applicators (or antennas) allows us setting suitable safety rules - such as the minimum distance between the operator and the antenna output - without conditioning neither reliability nor effectiveness of the microwave system.

Typical diameters of horn-parabolic antennas range from 10 cm to 60 cm; moreover a battery set of these antennas can be adopted. Alternatively, other applicators, such as those recently used in iperthermal therapy, are available, which are formed by matrices of thin needle-antennas. In these cases, the transmitting block can be connected with the applicator by means of a suitable wave-guide where needle-antennas are screwed down. This configuration is suitable for clay soils where DU cones, or other devices not workable by contact, have to be detected. Magnetron generators connected with a conductive plate (by a wave-guide) provide another alternative solution. In practice a baking plate of an industrial microwave oven is used without the baking tunnel: the radiation heating is obtained by putting the plate near to the soil (at a vertical distance of about 5-10 cm).

Description of some embodiments of the invention. In the annexed drawings, a practical embodiment of the invention is shown. In the drawings:

Figure 1 is a side view of a battery of nine microwave pods mounted on a support. In said view the microwaves generating group has been diagrammatically shown. Figure 2 is a front view of the battery of nine microwave pods of Figure 1, and

Figure 3 is an example of an operative vehicle mounting the battery of microwave pods of Figures 1 and 2. With reference to the drawings, with reference numeral 1 the microwave pods are shown. Said pods 1 are bolted on a frame 2, either aluminum frame or steel frame, and connected through a set of coaxial cables 3 to the microwave generating group formed by the electric power generating group 4, the microwave signal generator 5 and the amplifier 6. With numeral 9 length-meters are designated which are used for a correct positioning of the frame 2 with respect to the soil. Since pods 1 are relatively low-cost devices, they can be easily substituted in case of accidental explosions of UXOs. For the soil-positioning of pods a hydraulic arm can be used (such as those used for maintenance of industrial or building aerial platforms). Alternatively, as shown in Figure 3, a steel trellis 7can be used, either anchored at a tracked vehicle 8 (caterpillar or tank). Microwave signals are generated by a microwave signal generator 5 and amplifier 6, which are block produced by several industries such as Ericsson, Nokia, Anritsu, Agilent, Linn High Term Technologies or Phamitech, with frequencies ranging from 2 GHz up to 18 GHz and with maximum transmitted power of the order of 1-2 kW. The overall energy supply is given by a 3kW-6 kW electric-power generating group 4; in case of emergency the system can be supplied from standard vehicle-batteries. The infrared cameras can be installed on several supports, though their natural positioning is over a telescopic arm, similar to those shown in Figure 3 for parabolas, over an airborne system, an UAV, an Ultra-Light Aircraft or an Ultra-Light Helicopter. Moreover, soil- recognition operators can handle infrared cameras. Concerning infrared cameras for recognition and detection operations even in desert areas, the FLIR System TermaCAM series P or series SC are available. Other suitable infrared cameras are: IR FlexCam PRO produced by InfraRed Solutions, which, in addition to features of previous cameras, is very compact so as to be easily handled, and the TVS-500 Model produced by AVIO.

Claims

1. A method for the infrared detection of buried unexploded objects in the sub-surface layer of soils at small or moderate depths, in humanitarian demining operations, characterized by the fact that the target soil is locally heated by means of a microwave apparatus (1 ), in order to increase the thermal discontinuity between the unexploded objects and the materials surrounding them.

2. The method according to claim 1 , characterized by the fact that unexploded objects can be localized at sub-soil depths ranging from some millimeters up to about 150 cm.

3. The method according to claims 1 or 2, characterized by the fact that involved frequencies in the microwave apparatuses range from 2.45 Ghz up to 10 GHz depending on soil composition and the depth at which the unexploded objects are buried.

4. The method according to any one of the preceding claims characterized by the fact that heating times of the soil range from 2 to 20 minutes, and preferably from 5 to 10 minutes.

5. Apparatus for performing the method according to the preceding claims, comprising at least one infrared camera and at least one microwave antenna (1 ).

6. Apparatus according to claim 5, in which said microwave antennas (1) are highly directional microwave antennas.

7. Apparatus according to claims 5 or 6, characterized by the fact that the generating and radiating microwave device involves one or more antennas (1) positioned on a suitable metallic supporting frame (2), an electric-power generating group (4), a microwave signal generator (5), an amplifier (6), and a set of cables and connectors (3) for linking the signal generator (5) and the amplifier (6) to the microwave antennas or applicators (1 ).

8. Apparatus according to claim 7, characterized by the fact that microwave antennas (1 ) are preferably parabolic or horn- parabolic antennas.

9. Apparatus according to claim 7, further comprising length- meters (9) for a correct positioning of the frame (2) with respect to the soil.