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Contribution to classification of buried objects based on acoustic impedance matching
Ultrasonics 41 (2003) 115–123 www.elsevier.com/locate/ultras
Contribution to classification of buried objects based on acoustic impedance matching J. Stepanic Jr.
a,b,* ,
H. W€ ustenberg a, V. Krstelj b, H. Mrasek
a
a
Department VIII.4––Nondestructive Testing: Acoustical and Electrical Methods, BAM––Federal Institute for Material Characterization and Testing, Unter den Eichen 87, D-12205 Berlin, Germany b Department of Quality, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lu ci ca 5, HR-10000 Zagreb, Croatia Received 24 January 2002; accepted 14 September 2002
Abstract Determination of material the buried objects are made of could contribute significantly to their recognition, or classification. This is important in detecting buried antipersonnel landmines within the context of humanitarian demining, as well as in a variety of other applications. In this article the concept has been formulated of the approach to buried objectÕs material determination starting with ultrasonic impulse propagation analysis in a particular testing set configuration. The impulse propagates through a characterized transfer material in such a way that a part of it, a reflected wave, carries the information about the buried objectÕs surface material acoustic impedance. The limit of resolution capability is theoretically analyzed and experimentally evaluated and the influencing factors described. Among these, the contact between clean surfaces of the transfer material and buried object is emphasized. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 43.35.Zc; 43.58.Bh Keywords: Ultrasound; Acoustic impedance; Reflected wave; Buried objects; Antipersonnel landmines
1. Introduction Humanitarian demining is a time-consuming process accompanied by great risk, in which mine contaminated regions are transformed into mine non-contaminated regions in accordance with the requirements [1]. The important part of the process is antipersonnel landmines detection (APLD), which usually consists of two parts, the first one being detection of buried objects and the second one the classification of detected objects into antipersonnel landmines or other objects [2–4]. Principal problems related to the APLD are how to assure high enough probability of detection of low-metal content mines having plastic bodies, and how to reliably dis-
*
Corresponding author. Address: Department of Quality, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lucica 5, HR-10000 Zagreb, Croatia. Tel.: +385-1-6168497/406; fax: +385-1-6118-710. E-mail addresses: [email protected] (J. Stepanic Jr.), [email protected] (H. W€ ustenberg), [email protected] (V. Krstelj), [email protected] (H. Mrasek).
criminate landmines among a much larger number of other buried objects having some characteristics rather similar to those of the landmines. These problems have been intensified in the last several years owing to the tremendously large number of mines buried during the last decade. The estimates say that more than 80 million landmines contaminate 90 countries worldwide and cause 15 000–20 000 casualties per year [5]. The present rate of humanitarian demining is too low to solve the landmines problem within admissible time. The slowness of humanitarian demining motivated engagement of scientists and experts from different scientific and technological disciplines in APLD R&D work [3]. Additional contribution to the motivation for that work is the significant difference between the reached general scientific and technology level and the level of humanitarian demining equipment [6]. Presently, there are no proofs that any of the techniques considered could eventually bring about the wanted improvement in humanitarian demining [7]. There is even lack of data for establishing the theoretical limits for all the techniques considered so far in APLD R&D work [8]. Directions of
0041-624X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 2 ) 0 0 4 3 1 - 6
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APLD R&D work may be divided into three categories: the first one includes techniques which represent improvement of existing techniques, while the second one comprises techniques based on the known principles which are at present not exploited in APLD. The third, sensor fusion category includes combinations of techniques from other two categories. The first category work is oriented towards partial, yet relatively soon to be realized improvement in APLD. The second category includes methods for standoff APLD, prevalently exploitations of different parts of electromagnetic spectrum. Work in the third category was motivated by the realization that it is not necessary to develop a single technique which will fulfill the requirements of APLD, but that it may be sufficient to formulate a combination of techniques which will fulfill the requirements. In principle, carefully formulated combination may exploit relative complementarity of different techniques in order to bring about APLD improvement. It should be mentioned here that in the present humanitarian demining at least two APLD techniques are used in a particular demining operation, hence representing technically relatively low, sensor fusion level. In our previous work we concentrated on the approach of the first category, the manual probing [2,9]. In this technique a deminer perpetually probes the soil with a suitable probe, usually thin metallic rod. The probe enables the deminer to establish contact with a buried object, which is considered as detection. Using multiple subsequent probing in the close vicinity of the position where the first contact was established, it is in principle possible to determine the shape of the buried object. However, because of the high risk accompanying each contact with the buried landmine, regular probe application usually ends after the first contact has been established and excavation of the object takes place. Having in mind that landmines are relatively rare objects compared to other natural or artificial buried objects, e.g. stones and different wooden objects, such a lack of confirmation capability is a serious drawback of probing. Besides, rather small dimensions of modern landmines require several thousands of probings per 1 m2 . Generally, when standard operating procedures are followed, the average demining rate with manual probing is 25 m2 per day per deminer. It is to be compared with the mechanical demining rate of up to 30 000 m2 per day per machine. However, different conditions significantly reduce the average mechanical demining rate, or make its usage impossible. Mechanical demining is useful in case of flat, almost horizontal soil profiles, which had been previously used for agricultural purposes, in periods when soil is sufficiently dry. On the contrary, it is not useful in case of rocky soils, woods and other soils with dense vegetation, in regions which are not flat and horizontal enough, in urban regions, in wet soil regions, during raining periods etc.
Similar considerations regarding applicability of other APLD techniques in different conditions, i.e. metal detection and detection using specially trained dogs, led independently several research groups to the conclusion that improvement of manual probing technique may bring about significant improvement in humanitarian demining [2,10–15]. The improvement would not be the development of a single, universal probing technique which would be a sufficient APLD technique for all conditions encountered in humanitarian demining, but it would rather be the development of faster, less risky probing technique with confirmation capability. Approaches made by different research groups may be divided into two categories. In the first category the goal is to make the probe movement in the probing process easier, more precise and with smaller interval of fluctuations in the force applied to the probe and in the speed of the probeÕs tip. In the second category, the researches are oriented toward the development of probe confirmation capability. In the first category, the combination of all the desired results is (i) reducing the force needed for the probe to penetrate through soil, followed by reducing the risk of activating the landmine trigger during probing [10]; (ii) raising the level of probe positioning control through equipping the probe with miniature automatic digging system [11], and (iii) probing process automation, the consequence of which would be the removal of the deminer from the probing position, i.e. drastic reduction of the probing risk [12]. Since the work regarding these problems has still not reached its limits, regarding probe confirmation capability development it is appropriate to consider that the contact of the probe with a buried object is an established, otherwise unspecified contact [2]. Although therefore, the confirmation capability development depends on the probe positioning development, postponing the confirmation-oriented work till positioning-oriented work ends may seem as rather time-inefficient solution. Certainly, such a simultaneous approach requires some interference between the categories. There are several independent approaches to probe confirmation capability development [9,13–15]. Basically, they use the contact region between the probe tip and the buried object surface as a source of an acoustic or elastic wave. In one approach, in the passive configuration, the acoustic impulse generated during probe impact onto the buried object surface is recorded and analyzed [13]. Using advanced signal processing algorithms the object material characteristic signature is looked for in the frequency domain. The results show significant classification capability of the approach in the controlled environment. However, the limitations are in the allowable impact speed which influences intensity of the emitted impulse, and generally uncharacterized local soil conditions. In other approaches the
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active configuration is set, realized as the ultrasound transducer installed in the probe body for sensoring, which takes place after the contact with the buried object has been established [14,15]. In one approach [14], landmines and other buried objects are regarded as acoustic resonant cavities. Using broadband transducer their eigenspectra are determined. From it, the shape of the object is reproduced. However, in this approach the boundary conditions are generally unknown thus making the interpretation of the spectrum obtained relatively difficult. In another approach [15], ultrasonic impulse generates the probe tip axial oscillations. As the tip is in contact with the buried object, the oscillations are influenced by the buried object material elastic characteristics. This influence serves as a measure of the buried object material. There is in principle a significant difference between the response of a mine plastic body, compared to the response of a stone, or some metal object. The problems associated with this approach are that tip axial oscillations generally have larger amplitude for lower ultrasonic forcing impulse frequency. In the low frequency limit of tip oscillations the problem is to determine the ratio of the tip momentum transferred to the buried object surface and to the buried object neighboring soil. The problem is more significant in wet soils, in particular because of the mine induced changes in the water content of local soil [16]. Somewhat complementary to these approaches, the usage of relatively high frequency transducer was considered in the concept of buried objects materials characterization oriented approach [9], as one realization of the OCULAR technique [2]. In this approach it is assumed that ultrasonic transducer is mounted near the probe tip, requiring furthermore rather small transducerÕs dimensions. The result is its low-power operation. Because of that, there is no significant relative motion of tip and buried object centers of mass, resulting in efficient ultrasound impulse transfer through static, non-homogeneous system. Because of the nonhomogeneity in the tip–surface interface, the reflected part of the incidence ultrasonic impulse propagates back to the transducer and serves as the information carrier of the buried object surface characteristics. In particular, an important characteristic about the buried object surface is its acoustic impedance, as it differs significantly for different classes of buried objects. While one may argue that the complexity of the buried object surface geometry and surface conditions, as well as of the neighboring soil precise state certainly influence the suitable realizations of the approach, the analysis of theoretical limit is needed as the first step towards realization. The theoretical limit of the impedance matching based approach to the probe confirmation capability development is the topic of this article. It is performed starting with one realization of the equipment applying
Piezoelectric slab
117
Probe
Two-material slab Buried object
Fig. 1. Schematic representation of the possible impedance matching configuration for buried object materials characterization using ultrasound. The width of the tip for field equipment should be less than several millimeters.
the described approach. It is shown magnified in Fig. 1. The slab inserted between the piezoelectric transducer and the tested object has two functions. First, it makes possible the sensitivity of the equipment to a particular type of material, in this case plastics. This is realized using the matching of impedances. Secondly, the slab brings about wear resistance of the equipment by mechanically protecting the transducer, which is important especially during probe penetration, in order to make possible reliable application. For this technique, the limits are analyzed on the resolution of the determination of the buried object acoustic impedance, in the idealized situation corresponding to the particular scenario in which antipersonnel landmines with plastic bodies and low content of metal should be differentiated among the collection of other objects; stones, pieces of glass, or metal objects. Tip and surface geometries as well as relative positions of the testing device and the tested object are considered reduced, and the contact layer is not considered. It is shown that in ideal conditions there is sufficient confirmation capability of the approach proposed. In the following section theoretical base for the ultrasonic propagation through the two-material slab is given, and the structure of the developed model described. In addition, the results obtained from the model are presented. This approach is useful in order to obtain insight about processes occurring during impulse propagation. The detailed, real-time modeling is possible using a quadrupole model, which is described and used in Section 2. Section 3 describes the experimental set-up and results obtained in experiments. Interpretation of the results obtained both from the theoretical and experimental part is given in Section 4. The main results and lines of future development are presented in Section 5.
2. Theoretical modeling In the model, we start with the characteristic quantities in case of normal incidence of the plane, monochromatic ultrasonic wave on the planar slab that in the
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idealized case adopted for the moment consists of two plane parallel layers [17]. Quantities describing material through which the incident wave propagates have subscript n ¼ 1. Similarly, layers and the material through which the transmitted wave propagates have subscripts n ¼ 2, 3 and 4, respectively. All four materials are considered homogeneous, linear, elastic solids. Relevant quantities are waveÕs angular frequency x, layersÕ thicknesses––d2;3 , acoustic impedance of nth material–– Zn , and ultrasonic velocities in materials vn . Wave vector in nth material is kn ¼ x=vn . For normal incidence on smooth interfaces there is no conversion of ultrasonic waves, hence the ultrasonic waveÕs polarization is not considered explicitly in the modeling part. The waves are in the modeling described through a scalar field uðz; tÞ a magnitude of a displacement field, as shown in Fig. 2. Here z-axis is normal to the interfaces, pointing in the direction of the incident wave. The plane xy of the accompanied rectangular coordinate system coincides with the interface between the materials 1 and 2. Boundary conditions applied on the interfaces z ¼ 0, d2 and d2 þ d3 are a continuity of u, and ou=oz. The quantity we are interested in is the transmission coefficient T ðxÞ, 2 Z1 GðxÞ T ðxÞ ; ð1Þ A Z4 which gives the transmitted part of the energy. Here j j denotes the absolute value. The reflection coefficient is R ¼ 1 T and will be important in the experimental part. After solving the set of equations given by boundary conditions for G=A, one obtains T ðxÞ ¼ ð4Z1 =Z4 Þjð1 þ Z1 =Z4 Þ cos k2 d2 cos k3 d3 ½Z2 =Z3 þ Z1 Z3 =ðZ2 Z4 Þ sin k2 d2 sin k3 d3 i½ðZ1 =Z2 þ Z2 =Z4 Þ sin k2 d2 cos k3 d3 2
þ ðZ1 =Z3 þ Z3 =Z4 Þ cos k2 d2 sin k3 d3 j : ð2Þ pffiffiffiffiffiffiffi Here i ¼ 1. For further considerations the limit of one-material slab is of interest. In that case the expression for transmission coefficient is
A⋅exp[i(k1z-ωt)] C⋅exp[i(k2z-ωt)]
E⋅exp[i(k3z-ωt)]
B⋅exp[i(-k1z-ωt)] D⋅exp[i(-k2z-ωt)]
F⋅exp[i(-k3z-ωt)]
G⋅exp[i(k4z-ωt)]
z Z2 v2 d2
Z1 v1
Z3 v3 d3
Z4 v4
z=0
Fig. 2. Schematic representation of the ultrasonic wave components in different materials. Dependence of the amplitudes B; . . . ; G on other quantities is suppressed.
T0 ðxÞ ¼ ð4Z1 =Z4 Þjð1 þ Z1 =Z4 Þ cos k23 d23 iðZ1 =Z23 þ Z23 =Z4 Þ sin k23 d23 j2 ;
ð3Þ
where k23 is the wave vector in the slab of thickness d23 consisting of a single material having acoustic impedance Z23 . The accompanied reflection coefficient is R 0 ¼ 1 T0 . The impedances of the slab are matched when the transmission coefficient is maximized as a function of the slabÕs acoustic impedance and thickness. In case of one-material slab impedance matching is achieved when the following two conditions are fulfilled pffiffiffiffiffiffiffiffiffi Z23 ¼ Z1 Z4 ; k23 d23 ¼ p=2: ð4Þ If one wants to match the impedances in case of twomaterial slab, then the correct procedure is to maximize (2) as a function of quantities Z2;3 and d2;3 . In general case this results in rather non-tractable expressions. This is the reason why such an approach is not suitable in general, for realistic case modeling. The ultimate objective of implementing the impedance matching method in humanitarian demining imposes additional conditions on materials chosen, as the slab should provide additionally the mechanical protection for the piezoelectric crystal, i.e. make the system wear resistant. This requires material 3 to be rather stiff for the practice, so its acoustic impedance is taken to be relatively very large compared to other impedances. For laboratory considerations it is sufficient to take a material having rather large acoustic impedance, in our case copper. In this limit, two-material slab has a simple physical interpretation. Because then the inertial contributions are concentrated in the stiff material, and elastic contributions in the other material, it resembles the mass on the spring. Relevant quantity in the experiment is the reflected wave amplitude. Figs. 3–6 show transmission coefficients of the energy for the systems composed of a piezoelectric crystal and the matching layers. They could be measured in case when a broad-band ultrasonic impulse is incident on z ¼ 0 interface. In order to simulate the realistic probes, the model should include frequency response of the electrically excited piezoelectric crystal, two layers of glue, realistic backing, finitely thick contact layer, e.g. of water and relevant components of the electrical circuit. The inclusion of these elements changes the values of parameters in which the impedances are matched. In the approach this is direct generalization of the aforementioned equation solving that would bring about rather intractable equations. The model that was developed before in this context, that makes possible the realistic situation, real-time modeling is the quadrupole model [18]. In it, all the included elements are treated as variables, hence the relevant spectrum and pulse shape
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Z23, MRayl
5
4
6 7
1 0.95 T0 0.9 0.85
T
0.75 0.5 0.25
0.3 0.25
0.3
0.05
0.25 d3/v3, µs
0.1 d23/ v23 , µs
(a)
119
0.15
(a)
0.2
d2/v2, µs
0.2
T0 1
Z2, MRayl
T
4
5
6
7
Z23 = 4,8 MRayl A
0.8
0.5
B
Z23 = 1,5 MRayl
C 0.6
1
(b)
2 ω/(2π), MHz
3
4
Fig. 3. Transmission coefficient T0 for Z1 ¼ 7:2 MRayl and Z4 ¼ 3:2 MRayl; (a) x ¼ 4p MHz. Impedances are matched for d23 =v23 ¼ 0:125 ls and Z23 ¼ 4:8 MRayl; (b) d23 =v23 ¼ 0:125 ls and two different values of Z23 .
T0
(b) Fig. 5. Transmission coefficient T for Z1 ¼ 7:2 MRayl, Z4 ¼ 3:2 MRayl. (a) x ¼ 4p MHz, Z2 ¼ 6:8 MRayl and Z3 ¼ 45 MRayl, (b) d2 =v2 ¼ 0:31 ls and d3 =v3 ¼ 0:254 ls. A x ¼ 4p MHz, Z3 ¼ 42 MRayl; B x ¼ 4p MHz, Z3 ¼ 45 MRayl and C x ¼ 4:1p MHz, Z3 ¼ 45 MRayl.
1 T
1
0.5 0.5
10
20
30
40
Z4, MRayl Fig. 4. Transmission coefficient T0 for Z1 ¼ 7:2 MRayl, Z23 ¼ 4:8 MRayl, d23 =v23 ¼ 0:125 ls and x ¼ 4p MHz.
are easily calculated in all possible cases, which makes that approach suitable for our purpose. The principle of the program is shown in Fig. 7. The underlying principles are the same in quadrupole model and two-material slab approach, which is why we use explicit solutions of the later approach where appropriate. 3. Experiment The experimental part of the work was performed in Department VIII.4 at BAM––Berlin using USIP-11 and the longitudinal probes, shown in Fig. 8a, manufactured
10
20
30
40
Z4, MRayl
(a) 1
T
0.5
(b)
1
2
3
4
ω/(2π) MHz
Fig. 6. Transmission coefficient T for Z1 ¼ 7:2 MRayl, Z2 ¼ 3:7 MRayl, Z3 ¼ 45 MRayl, d2 =v2 ¼ 0:31 ls and d3 =v3 ¼ 0:254 ls with (a) x ¼ 4p MHz, (b) Z4 ¼ 3:2 MRayl.
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Fig. 7. Quadrupole model: (a) schematic representation of the input parameters, (b) principle of calculations.
for matching the acoustic impedances between the piezoelectric ceramic specimen ðZ1 ¼ 7:2 MRayl, natural frequency 2 MHz) and Plexiglas ðZ4 ¼ 3:2 MRayl), as well as conventional probes of central frequencies 2.25 and 3.5 MHz, shown in Fig. 8b. Using contact technique, the amplitude and spectral characteristics of the first echo from the probe–object interface were analyzed. The manufactured probesÕ composition includes twomaterial slab composed of plastic (Z2 ¼ 3:2 MRayl, d2 ¼ 50 lm) and copper (Z3 ¼ 42 MRayl, d3 ¼ 50 lm) layers of 10 10 mm2 area, accompanied by two layers of glue (Z ¼ 3:1 MRayl) each of 5 lm thickness. Acoustic impedance of the backing layer realized from glue and PbO was 7.0 MRayl. The conventional probes were designed for other purposes, and were chosen as a measure of results achievable with non-matched probes [19].
The samples used represented materials of objects that could be found in the soil, with the acoustic impedances given in Table 1. Additionally, measurements included air and water as the tested materials. SamplesÕ surfaces were smooth enough, and their thickness large enough to separate various possible echoes. The typical A-scans obtained are shown in Fig. 9. Results collected with probes having impedance matching layer are shown in Fig. 10, while results obtained with probes without impedance matching layer are shown in Fig. 11. The quantity Z in graphs equals the quantity Z4 in Fig. 2.
4. Discussion From the theoretical results for T0 the effect of the impedance matching can be seen from Figs. 3 and 4.
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121
Fig. 8. Probes (a) with impedance matching layer, and (b) without it.
Fig. 9. Amplitude of the reflected ultrasonic impulse with (a) Z4 ¼ 3:2 MRayl (Plexiglas), (b) Z4 ¼ 45 MRayl (steel). Arrows refer to reflected signals of interest.
8 |Amplitude|/2, dB
However, the effect is not pronounced, as the 50% change in slab impedance Z23 results in only about 10% change in transmission coefficient, Fig. 3a. Additionally, the combining influence of the layer thickness and acoustic properties is seen from the dependence of T0 on d23 =v23 . In the case shown, the magnitudes of both influences are similar. From frequency dependence of T0 , Fig. 3b, it can be seen that when impedances are matched there is no significant reduction in different components of initially broadband spectrum. For manufacturing, it is suitable that the region of interest does not represent rather sharply defined region in the parametric space spanned by Z23 and d23 =v23 . The capability of such a layer is a different topic. Fig. 4 shows that the matching slab with parameters assuring maximized transmission in case of tested object made from Plexiglas, gives 50% smaller value of T0 for different tested object material, which could be in the case shown a stone, glass or aluminum. By increasing the frequency of the wave, the condition for matching the impedances tends toward smaller values of d23 =v23 , while there are no changes of the position of maximum in the direction of Z23 , in accordance with (4). On the basis of the facts presented, one can say that one-material matching slab differentiates various mate-
6 4 2 0 0
10
20
30
40
50
Z , MRayl
Fig. 10. Reflected wave amplitude in case of probes with impedance matching layer.
rials as a consequence of impedance matching, although the effect is not pronounced.
Table 1 Acoustic impedances of the samples used Material
Air
Water
Plexiglas
Glass
Granite
Aluminum
Bronze
Steel
Z4 , MRayl
0
1.5
3.2
15
16
17
42
45
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|Amplitude|/2, dB
14 12 10 8 6 4 2 0
0
10
|Amplitude|/2, dB
(a)
20
30
40
Z , MRayl
10,00 8,00 6,00 4,00 2,00 0,00 0
(b)
10
20
30
40
Z , MRayl
Fig. 11. Reflected wave amplitude in case of probes without impedance matching layer for probesÕ frequencies (a) 2.25 MHz and (b) 3.5 MHz.
The two-material matching layer, besides enabling a mechanically durable testing system, provides also a larger parameter set, spanned by Z2 , Z3 , d2 =v2 and d3 =v3 . In general, therefore, one expects regions of parameter values that provide a rather selective system, yet preserving manufacturing suitability. Fig. 5a shows a transmission coefficient T of a twomaterial slab for a particular combination of Z2 and Z3 . The resonance transmission occurs also for several other values of Z2 for a given, relatively large value of Z3 , fixed by mechanical requirements. Contrary to the case of one-material layer, the frequency dependence of the resonance transmission region is more pronounced, as the frequency change of 2.5% induces transmission coefficient change of about 20%, as can be seen from curves B and C in Fig. 5b. The dependence of the resonance transmission characteristics on the slight changes in value of Z3 about the fixed value of 45 MRayl is relatively negligible, as can be seen from curves A and B in Fig. 5b. This contributes to making it possible to put the copper layer instead of the steel layer in the experimental part. Differentiation property of the two-material matching layer can be seen from Fig. 6a, where the transmission coefficient for the matched case (Z4 ¼ 3:2 MRayl) is two or three times larger than for other materials of tested objects. Typical frequency characteristic of the two-material slab is shown in Fig. 6b, in case of one set of matching parameters. Frequency characteristics look like a filter of a basic frequency and its approximate higher har-
monics (here only 3.94 MHz peak is seen). Interval of transmitted frequencies has relatively narrow full width half maximum of 0:2 2p MHz. The low frequency transmission maximum is exploited in other approaches to characterization of buried objects [13,15]. Here care should be taken in order to take properly into account the non-negligible tip oscillation amplitude [15]. Simulated results are confirmed experimentally. Fig. 10 shows that there is a differentiation between plastic materials, represented here through Plexiglas. Compared to the case when there is no impedance matching, shown in Fig. 11, there is certain improvement However, it is not clear to what amount this difference would be degraded in realistic conditions, with unknown contact and with unknown surface condition on buried object. Some factors, the influence of which could be seen in the experimental results, have not been included in the theoretical part. These are the ultrasound attenuation and the surface roughness. They were not included in laboratory conditions because suitable preparation of samples enabled their rather low influence of the results. However, these have to be included in the description of the general case. The finite attenuation lowers the transmission, and therefore enhances the reflection. This effect is seen from the expression for reflection coefficient in case of normal incidence of a wave onto a single interface between two half spaces R¼
ðZ4 Z1 Þ2 þ X42 2
ðZ4 þ Z1 Þ þ X42
;
ð5Þ
where X4 represents the imaginary part of the total acoustic impedance of the half space, i.e. tested object material. In order to simplify the analysis of the finite attenuation, the attenuation of the material 1 is still neglected. As the attenuation is larger for larger frequencies, reflection enhancement is more pronounced for larger frequencies. This could be seen from Fig. 11a and b, where the plastic material does achieve the smallest amplitude value because of the impedance matching, but at the same time does not achieve negligible value owing to the finite attenuation. The effect of attenuation is more pronounced for higher frequencies, as raising the frequency by 50% in case of Fig. 11b compared to Fig. 11a results in degradation of the reflected wave amplitudeÕs difference for Plexiglas and materials with impedances larger than 15 MRayl from 5 dB to less than 2 dB. The finite surface roughness will result in a different frequency dependence of the transmission, as shown in detail elsewhere [20]. The possibility to use the piezoelectric sensor to characterize the surface roughness already exists, e.g. [21]. Although designed for application in a different environment, that technique belongs to local surface characterization techniques.
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5. Summary and conclusions It can be seen that two-material matching layer realization of the buried object materials characterization makes possible satisfying the mechanical conditions and still provide the designer with a possibility to obtain good sensitivity in a specific region of parametric space available. The obtainable resolution is satisfactory in laboratory conditions, hence an open topic is the amount of its degradation in more realistic models, in particular in field conditions. In addition, in further work slight differences in the response should be properly combined and provide additional pieces of information about the buried object material by means of neural networks or other complex signal processing algorithms. Acknowledgements J.S. Jr. greatly acknowledges the hospitality and help of the employees in Department VIII.4 of BAM––Berlin, where part of this work was done. This work was funded through the contracts SEQUA no. 247, and CRO-MoST 120 098. References [1] UN Mine Clearance Policy Unit, International Standards for Humanitarian Mine Clearance Operations. Available from. [2] V. Krstelj, J. Stepanic Jr., Humanitarian de-mining detection equipment and working group for antipersonnel landmines detection, Insight 42 (3) (2000) 187–190. [3] C. Bruschini, B. Gros, A survey of current sensor technology research for the detection of landmines, in: Int. Workshop SusDem Õ97, Proceedings, Zagreb, 1997, pp. S6.18–S6.27. [4] Various authors, in: Mine Identification Novelties Euroconference Õ99, Proceedings, Milano, 1999. Available from . [5] International Campaign to Ban Landmines, Landmines Monitor Report 2001, Landmine Monitor Core Group, USA, 2001, pp. 26 and 37.
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[6] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 1. [7] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 11. [8] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 16. [9] J. Stepanic Jr., Material characterization by mechanical point contact impact emitted ultrasound, in: 15th WCNDT, Proceedings, Rome, Italy, 2000. Available from. [10] J. Trevelyan. Available from . [12] Humanitarian Demining Technology Development Programme. Available from . [15] M.A. Borza, F.G. De Witt, Acoustic landmine prodding instrument with force feedback, US Patent #6 109 112, 2000. [16] O. Dullum, Migration of explosives in soil, in: Nordic Demining Research Forum Summer Conference 2001, Ebeltoft, Denmark. Available from . [17] J.H. Rose, Ultrasonic Waves in Solid Media, Cambridge University Press, New York, 1999 (Chapter 4). [18] H. W€ ustenberg, Phased array probes with delay time control for the ultrasonic inspection of nuclear components, GRS Nr. 1500 730 9, 1990. [19] H. W€ ustenberg, A. Erhard, N. Brekow, Evaluation of different ultrasonic probe concepts for the inspection of claddings, Materials Evaluation 54 (10) (1996) 1077. [20] D. Markucic, Possibilities of material classification by means of ultrasound, in: 15th WCNDT, Proceedings, Rome, Italy, 2000. Available from . [21] P. Kielczynski, W. Pajewski, M. Szalewski, Piezoelectric sensor applied in ultrasonic contact microscopy for the investigation of material surfaces, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 46 (1) (1999) 233–238.
Contribution to classification of buried objects based on acoustic impedance matching J. Stepanic Jr.
a,b,* ,
H. W€ ustenberg a, V. Krstelj b, H. Mrasek
a
a
Department VIII.4––Nondestructive Testing: Acoustical and Electrical Methods, BAM––Federal Institute for Material Characterization and Testing, Unter den Eichen 87, D-12205 Berlin, Germany b Department of Quality, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lu ci ca 5, HR-10000 Zagreb, Croatia Received 24 January 2002; accepted 14 September 2002
Abstract Determination of material the buried objects are made of could contribute significantly to their recognition, or classification. This is important in detecting buried antipersonnel landmines within the context of humanitarian demining, as well as in a variety of other applications. In this article the concept has been formulated of the approach to buried objectÕs material determination starting with ultrasonic impulse propagation analysis in a particular testing set configuration. The impulse propagates through a characterized transfer material in such a way that a part of it, a reflected wave, carries the information about the buried objectÕs surface material acoustic impedance. The limit of resolution capability is theoretically analyzed and experimentally evaluated and the influencing factors described. Among these, the contact between clean surfaces of the transfer material and buried object is emphasized. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 43.35.Zc; 43.58.Bh Keywords: Ultrasound; Acoustic impedance; Reflected wave; Buried objects; Antipersonnel landmines
1. Introduction Humanitarian demining is a time-consuming process accompanied by great risk, in which mine contaminated regions are transformed into mine non-contaminated regions in accordance with the requirements [1]. The important part of the process is antipersonnel landmines detection (APLD), which usually consists of two parts, the first one being detection of buried objects and the second one the classification of detected objects into antipersonnel landmines or other objects [2–4]. Principal problems related to the APLD are how to assure high enough probability of detection of low-metal content mines having plastic bodies, and how to reliably dis-
*
Corresponding author. Address: Department of Quality, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lucica 5, HR-10000 Zagreb, Croatia. Tel.: +385-1-6168497/406; fax: +385-1-6118-710. E-mail addresses: [email protected] (J. Stepanic Jr.), [email protected] (H. W€ ustenberg), [email protected] (V. Krstelj), [email protected] (H. Mrasek).
criminate landmines among a much larger number of other buried objects having some characteristics rather similar to those of the landmines. These problems have been intensified in the last several years owing to the tremendously large number of mines buried during the last decade. The estimates say that more than 80 million landmines contaminate 90 countries worldwide and cause 15 000–20 000 casualties per year [5]. The present rate of humanitarian demining is too low to solve the landmines problem within admissible time. The slowness of humanitarian demining motivated engagement of scientists and experts from different scientific and technological disciplines in APLD R&D work [3]. Additional contribution to the motivation for that work is the significant difference between the reached general scientific and technology level and the level of humanitarian demining equipment [6]. Presently, there are no proofs that any of the techniques considered could eventually bring about the wanted improvement in humanitarian demining [7]. There is even lack of data for establishing the theoretical limits for all the techniques considered so far in APLD R&D work [8]. Directions of
0041-624X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 2 ) 0 0 4 3 1 - 6
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APLD R&D work may be divided into three categories: the first one includes techniques which represent improvement of existing techniques, while the second one comprises techniques based on the known principles which are at present not exploited in APLD. The third, sensor fusion category includes combinations of techniques from other two categories. The first category work is oriented towards partial, yet relatively soon to be realized improvement in APLD. The second category includes methods for standoff APLD, prevalently exploitations of different parts of electromagnetic spectrum. Work in the third category was motivated by the realization that it is not necessary to develop a single technique which will fulfill the requirements of APLD, but that it may be sufficient to formulate a combination of techniques which will fulfill the requirements. In principle, carefully formulated combination may exploit relative complementarity of different techniques in order to bring about APLD improvement. It should be mentioned here that in the present humanitarian demining at least two APLD techniques are used in a particular demining operation, hence representing technically relatively low, sensor fusion level. In our previous work we concentrated on the approach of the first category, the manual probing [2,9]. In this technique a deminer perpetually probes the soil with a suitable probe, usually thin metallic rod. The probe enables the deminer to establish contact with a buried object, which is considered as detection. Using multiple subsequent probing in the close vicinity of the position where the first contact was established, it is in principle possible to determine the shape of the buried object. However, because of the high risk accompanying each contact with the buried landmine, regular probe application usually ends after the first contact has been established and excavation of the object takes place. Having in mind that landmines are relatively rare objects compared to other natural or artificial buried objects, e.g. stones and different wooden objects, such a lack of confirmation capability is a serious drawback of probing. Besides, rather small dimensions of modern landmines require several thousands of probings per 1 m2 . Generally, when standard operating procedures are followed, the average demining rate with manual probing is 25 m2 per day per deminer. It is to be compared with the mechanical demining rate of up to 30 000 m2 per day per machine. However, different conditions significantly reduce the average mechanical demining rate, or make its usage impossible. Mechanical demining is useful in case of flat, almost horizontal soil profiles, which had been previously used for agricultural purposes, in periods when soil is sufficiently dry. On the contrary, it is not useful in case of rocky soils, woods and other soils with dense vegetation, in regions which are not flat and horizontal enough, in urban regions, in wet soil regions, during raining periods etc.
Similar considerations regarding applicability of other APLD techniques in different conditions, i.e. metal detection and detection using specially trained dogs, led independently several research groups to the conclusion that improvement of manual probing technique may bring about significant improvement in humanitarian demining [2,10–15]. The improvement would not be the development of a single, universal probing technique which would be a sufficient APLD technique for all conditions encountered in humanitarian demining, but it would rather be the development of faster, less risky probing technique with confirmation capability. Approaches made by different research groups may be divided into two categories. In the first category the goal is to make the probe movement in the probing process easier, more precise and with smaller interval of fluctuations in the force applied to the probe and in the speed of the probeÕs tip. In the second category, the researches are oriented toward the development of probe confirmation capability. In the first category, the combination of all the desired results is (i) reducing the force needed for the probe to penetrate through soil, followed by reducing the risk of activating the landmine trigger during probing [10]; (ii) raising the level of probe positioning control through equipping the probe with miniature automatic digging system [11], and (iii) probing process automation, the consequence of which would be the removal of the deminer from the probing position, i.e. drastic reduction of the probing risk [12]. Since the work regarding these problems has still not reached its limits, regarding probe confirmation capability development it is appropriate to consider that the contact of the probe with a buried object is an established, otherwise unspecified contact [2]. Although therefore, the confirmation capability development depends on the probe positioning development, postponing the confirmation-oriented work till positioning-oriented work ends may seem as rather time-inefficient solution. Certainly, such a simultaneous approach requires some interference between the categories. There are several independent approaches to probe confirmation capability development [9,13–15]. Basically, they use the contact region between the probe tip and the buried object surface as a source of an acoustic or elastic wave. In one approach, in the passive configuration, the acoustic impulse generated during probe impact onto the buried object surface is recorded and analyzed [13]. Using advanced signal processing algorithms the object material characteristic signature is looked for in the frequency domain. The results show significant classification capability of the approach in the controlled environment. However, the limitations are in the allowable impact speed which influences intensity of the emitted impulse, and generally uncharacterized local soil conditions. In other approaches the
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active configuration is set, realized as the ultrasound transducer installed in the probe body for sensoring, which takes place after the contact with the buried object has been established [14,15]. In one approach [14], landmines and other buried objects are regarded as acoustic resonant cavities. Using broadband transducer their eigenspectra are determined. From it, the shape of the object is reproduced. However, in this approach the boundary conditions are generally unknown thus making the interpretation of the spectrum obtained relatively difficult. In another approach [15], ultrasonic impulse generates the probe tip axial oscillations. As the tip is in contact with the buried object, the oscillations are influenced by the buried object material elastic characteristics. This influence serves as a measure of the buried object material. There is in principle a significant difference between the response of a mine plastic body, compared to the response of a stone, or some metal object. The problems associated with this approach are that tip axial oscillations generally have larger amplitude for lower ultrasonic forcing impulse frequency. In the low frequency limit of tip oscillations the problem is to determine the ratio of the tip momentum transferred to the buried object surface and to the buried object neighboring soil. The problem is more significant in wet soils, in particular because of the mine induced changes in the water content of local soil [16]. Somewhat complementary to these approaches, the usage of relatively high frequency transducer was considered in the concept of buried objects materials characterization oriented approach [9], as one realization of the OCULAR technique [2]. In this approach it is assumed that ultrasonic transducer is mounted near the probe tip, requiring furthermore rather small transducerÕs dimensions. The result is its low-power operation. Because of that, there is no significant relative motion of tip and buried object centers of mass, resulting in efficient ultrasound impulse transfer through static, non-homogeneous system. Because of the nonhomogeneity in the tip–surface interface, the reflected part of the incidence ultrasonic impulse propagates back to the transducer and serves as the information carrier of the buried object surface characteristics. In particular, an important characteristic about the buried object surface is its acoustic impedance, as it differs significantly for different classes of buried objects. While one may argue that the complexity of the buried object surface geometry and surface conditions, as well as of the neighboring soil precise state certainly influence the suitable realizations of the approach, the analysis of theoretical limit is needed as the first step towards realization. The theoretical limit of the impedance matching based approach to the probe confirmation capability development is the topic of this article. It is performed starting with one realization of the equipment applying
Piezoelectric slab
117
Probe
Two-material slab Buried object
Fig. 1. Schematic representation of the possible impedance matching configuration for buried object materials characterization using ultrasound. The width of the tip for field equipment should be less than several millimeters.
the described approach. It is shown magnified in Fig. 1. The slab inserted between the piezoelectric transducer and the tested object has two functions. First, it makes possible the sensitivity of the equipment to a particular type of material, in this case plastics. This is realized using the matching of impedances. Secondly, the slab brings about wear resistance of the equipment by mechanically protecting the transducer, which is important especially during probe penetration, in order to make possible reliable application. For this technique, the limits are analyzed on the resolution of the determination of the buried object acoustic impedance, in the idealized situation corresponding to the particular scenario in which antipersonnel landmines with plastic bodies and low content of metal should be differentiated among the collection of other objects; stones, pieces of glass, or metal objects. Tip and surface geometries as well as relative positions of the testing device and the tested object are considered reduced, and the contact layer is not considered. It is shown that in ideal conditions there is sufficient confirmation capability of the approach proposed. In the following section theoretical base for the ultrasonic propagation through the two-material slab is given, and the structure of the developed model described. In addition, the results obtained from the model are presented. This approach is useful in order to obtain insight about processes occurring during impulse propagation. The detailed, real-time modeling is possible using a quadrupole model, which is described and used in Section 2. Section 3 describes the experimental set-up and results obtained in experiments. Interpretation of the results obtained both from the theoretical and experimental part is given in Section 4. The main results and lines of future development are presented in Section 5.
2. Theoretical modeling In the model, we start with the characteristic quantities in case of normal incidence of the plane, monochromatic ultrasonic wave on the planar slab that in the
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idealized case adopted for the moment consists of two plane parallel layers [17]. Quantities describing material through which the incident wave propagates have subscript n ¼ 1. Similarly, layers and the material through which the transmitted wave propagates have subscripts n ¼ 2, 3 and 4, respectively. All four materials are considered homogeneous, linear, elastic solids. Relevant quantities are waveÕs angular frequency x, layersÕ thicknesses––d2;3 , acoustic impedance of nth material–– Zn , and ultrasonic velocities in materials vn . Wave vector in nth material is kn ¼ x=vn . For normal incidence on smooth interfaces there is no conversion of ultrasonic waves, hence the ultrasonic waveÕs polarization is not considered explicitly in the modeling part. The waves are in the modeling described through a scalar field uðz; tÞ a magnitude of a displacement field, as shown in Fig. 2. Here z-axis is normal to the interfaces, pointing in the direction of the incident wave. The plane xy of the accompanied rectangular coordinate system coincides with the interface between the materials 1 and 2. Boundary conditions applied on the interfaces z ¼ 0, d2 and d2 þ d3 are a continuity of u, and ou=oz. The quantity we are interested in is the transmission coefficient T ðxÞ, 2 Z1 GðxÞ T ðxÞ ; ð1Þ A Z4 which gives the transmitted part of the energy. Here j j denotes the absolute value. The reflection coefficient is R ¼ 1 T and will be important in the experimental part. After solving the set of equations given by boundary conditions for G=A, one obtains T ðxÞ ¼ ð4Z1 =Z4 Þjð1 þ Z1 =Z4 Þ cos k2 d2 cos k3 d3 ½Z2 =Z3 þ Z1 Z3 =ðZ2 Z4 Þ sin k2 d2 sin k3 d3 i½ðZ1 =Z2 þ Z2 =Z4 Þ sin k2 d2 cos k3 d3 2
þ ðZ1 =Z3 þ Z3 =Z4 Þ cos k2 d2 sin k3 d3 j : ð2Þ pffiffiffiffiffiffiffi Here i ¼ 1. For further considerations the limit of one-material slab is of interest. In that case the expression for transmission coefficient is
A⋅exp[i(k1z-ωt)] C⋅exp[i(k2z-ωt)]
E⋅exp[i(k3z-ωt)]
B⋅exp[i(-k1z-ωt)] D⋅exp[i(-k2z-ωt)]
F⋅exp[i(-k3z-ωt)]
G⋅exp[i(k4z-ωt)]
z Z2 v2 d2
Z1 v1
Z3 v3 d3
Z4 v4
z=0
Fig. 2. Schematic representation of the ultrasonic wave components in different materials. Dependence of the amplitudes B; . . . ; G on other quantities is suppressed.
T0 ðxÞ ¼ ð4Z1 =Z4 Þjð1 þ Z1 =Z4 Þ cos k23 d23 iðZ1 =Z23 þ Z23 =Z4 Þ sin k23 d23 j2 ;
ð3Þ
where k23 is the wave vector in the slab of thickness d23 consisting of a single material having acoustic impedance Z23 . The accompanied reflection coefficient is R 0 ¼ 1 T0 . The impedances of the slab are matched when the transmission coefficient is maximized as a function of the slabÕs acoustic impedance and thickness. In case of one-material slab impedance matching is achieved when the following two conditions are fulfilled pffiffiffiffiffiffiffiffiffi Z23 ¼ Z1 Z4 ; k23 d23 ¼ p=2: ð4Þ If one wants to match the impedances in case of twomaterial slab, then the correct procedure is to maximize (2) as a function of quantities Z2;3 and d2;3 . In general case this results in rather non-tractable expressions. This is the reason why such an approach is not suitable in general, for realistic case modeling. The ultimate objective of implementing the impedance matching method in humanitarian demining imposes additional conditions on materials chosen, as the slab should provide additionally the mechanical protection for the piezoelectric crystal, i.e. make the system wear resistant. This requires material 3 to be rather stiff for the practice, so its acoustic impedance is taken to be relatively very large compared to other impedances. For laboratory considerations it is sufficient to take a material having rather large acoustic impedance, in our case copper. In this limit, two-material slab has a simple physical interpretation. Because then the inertial contributions are concentrated in the stiff material, and elastic contributions in the other material, it resembles the mass on the spring. Relevant quantity in the experiment is the reflected wave amplitude. Figs. 3–6 show transmission coefficients of the energy for the systems composed of a piezoelectric crystal and the matching layers. They could be measured in case when a broad-band ultrasonic impulse is incident on z ¼ 0 interface. In order to simulate the realistic probes, the model should include frequency response of the electrically excited piezoelectric crystal, two layers of glue, realistic backing, finitely thick contact layer, e.g. of water and relevant components of the electrical circuit. The inclusion of these elements changes the values of parameters in which the impedances are matched. In the approach this is direct generalization of the aforementioned equation solving that would bring about rather intractable equations. The model that was developed before in this context, that makes possible the realistic situation, real-time modeling is the quadrupole model [18]. In it, all the included elements are treated as variables, hence the relevant spectrum and pulse shape
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Z23, MRayl
5
4
6 7
1 0.95 T0 0.9 0.85
T
0.75 0.5 0.25
0.3 0.25
0.3
0.05
0.25 d3/v3, µs
0.1 d23/ v23 , µs
(a)
119
0.15
(a)
0.2
d2/v2, µs
0.2
T0 1
Z2, MRayl
T
4
5
6
7
Z23 = 4,8 MRayl A
0.8
0.5
B
Z23 = 1,5 MRayl
C 0.6
1
(b)
2 ω/(2π), MHz
3
4
Fig. 3. Transmission coefficient T0 for Z1 ¼ 7:2 MRayl and Z4 ¼ 3:2 MRayl; (a) x ¼ 4p MHz. Impedances are matched for d23 =v23 ¼ 0:125 ls and Z23 ¼ 4:8 MRayl; (b) d23 =v23 ¼ 0:125 ls and two different values of Z23 .
T0
(b) Fig. 5. Transmission coefficient T for Z1 ¼ 7:2 MRayl, Z4 ¼ 3:2 MRayl. (a) x ¼ 4p MHz, Z2 ¼ 6:8 MRayl and Z3 ¼ 45 MRayl, (b) d2 =v2 ¼ 0:31 ls and d3 =v3 ¼ 0:254 ls. A x ¼ 4p MHz, Z3 ¼ 42 MRayl; B x ¼ 4p MHz, Z3 ¼ 45 MRayl and C x ¼ 4:1p MHz, Z3 ¼ 45 MRayl.
1 T
1
0.5 0.5
10
20
30
40
Z4, MRayl Fig. 4. Transmission coefficient T0 for Z1 ¼ 7:2 MRayl, Z23 ¼ 4:8 MRayl, d23 =v23 ¼ 0:125 ls and x ¼ 4p MHz.
are easily calculated in all possible cases, which makes that approach suitable for our purpose. The principle of the program is shown in Fig. 7. The underlying principles are the same in quadrupole model and two-material slab approach, which is why we use explicit solutions of the later approach where appropriate. 3. Experiment The experimental part of the work was performed in Department VIII.4 at BAM––Berlin using USIP-11 and the longitudinal probes, shown in Fig. 8a, manufactured
10
20
30
40
Z4, MRayl
(a) 1
T
0.5
(b)
1
2
3
4
ω/(2π) MHz
Fig. 6. Transmission coefficient T for Z1 ¼ 7:2 MRayl, Z2 ¼ 3:7 MRayl, Z3 ¼ 45 MRayl, d2 =v2 ¼ 0:31 ls and d3 =v3 ¼ 0:254 ls with (a) x ¼ 4p MHz, (b) Z4 ¼ 3:2 MRayl.
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Fig. 7. Quadrupole model: (a) schematic representation of the input parameters, (b) principle of calculations.
for matching the acoustic impedances between the piezoelectric ceramic specimen ðZ1 ¼ 7:2 MRayl, natural frequency 2 MHz) and Plexiglas ðZ4 ¼ 3:2 MRayl), as well as conventional probes of central frequencies 2.25 and 3.5 MHz, shown in Fig. 8b. Using contact technique, the amplitude and spectral characteristics of the first echo from the probe–object interface were analyzed. The manufactured probesÕ composition includes twomaterial slab composed of plastic (Z2 ¼ 3:2 MRayl, d2 ¼ 50 lm) and copper (Z3 ¼ 42 MRayl, d3 ¼ 50 lm) layers of 10 10 mm2 area, accompanied by two layers of glue (Z ¼ 3:1 MRayl) each of 5 lm thickness. Acoustic impedance of the backing layer realized from glue and PbO was 7.0 MRayl. The conventional probes were designed for other purposes, and were chosen as a measure of results achievable with non-matched probes [19].
The samples used represented materials of objects that could be found in the soil, with the acoustic impedances given in Table 1. Additionally, measurements included air and water as the tested materials. SamplesÕ surfaces were smooth enough, and their thickness large enough to separate various possible echoes. The typical A-scans obtained are shown in Fig. 9. Results collected with probes having impedance matching layer are shown in Fig. 10, while results obtained with probes without impedance matching layer are shown in Fig. 11. The quantity Z in graphs equals the quantity Z4 in Fig. 2.
4. Discussion From the theoretical results for T0 the effect of the impedance matching can be seen from Figs. 3 and 4.
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121
Fig. 8. Probes (a) with impedance matching layer, and (b) without it.
Fig. 9. Amplitude of the reflected ultrasonic impulse with (a) Z4 ¼ 3:2 MRayl (Plexiglas), (b) Z4 ¼ 45 MRayl (steel). Arrows refer to reflected signals of interest.
8 |Amplitude|/2, dB
However, the effect is not pronounced, as the 50% change in slab impedance Z23 results in only about 10% change in transmission coefficient, Fig. 3a. Additionally, the combining influence of the layer thickness and acoustic properties is seen from the dependence of T0 on d23 =v23 . In the case shown, the magnitudes of both influences are similar. From frequency dependence of T0 , Fig. 3b, it can be seen that when impedances are matched there is no significant reduction in different components of initially broadband spectrum. For manufacturing, it is suitable that the region of interest does not represent rather sharply defined region in the parametric space spanned by Z23 and d23 =v23 . The capability of such a layer is a different topic. Fig. 4 shows that the matching slab with parameters assuring maximized transmission in case of tested object made from Plexiglas, gives 50% smaller value of T0 for different tested object material, which could be in the case shown a stone, glass or aluminum. By increasing the frequency of the wave, the condition for matching the impedances tends toward smaller values of d23 =v23 , while there are no changes of the position of maximum in the direction of Z23 , in accordance with (4). On the basis of the facts presented, one can say that one-material matching slab differentiates various mate-
6 4 2 0 0
10
20
30
40
50
Z , MRayl
Fig. 10. Reflected wave amplitude in case of probes with impedance matching layer.
rials as a consequence of impedance matching, although the effect is not pronounced.
Table 1 Acoustic impedances of the samples used Material
Air
Water
Plexiglas
Glass
Granite
Aluminum
Bronze
Steel
Z4 , MRayl
0
1.5
3.2
15
16
17
42
45
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|Amplitude|/2, dB
14 12 10 8 6 4 2 0
0
10
|Amplitude|/2, dB
(a)
20
30
40
Z , MRayl
10,00 8,00 6,00 4,00 2,00 0,00 0
(b)
10
20
30
40
Z , MRayl
Fig. 11. Reflected wave amplitude in case of probes without impedance matching layer for probesÕ frequencies (a) 2.25 MHz and (b) 3.5 MHz.
The two-material matching layer, besides enabling a mechanically durable testing system, provides also a larger parameter set, spanned by Z2 , Z3 , d2 =v2 and d3 =v3 . In general, therefore, one expects regions of parameter values that provide a rather selective system, yet preserving manufacturing suitability. Fig. 5a shows a transmission coefficient T of a twomaterial slab for a particular combination of Z2 and Z3 . The resonance transmission occurs also for several other values of Z2 for a given, relatively large value of Z3 , fixed by mechanical requirements. Contrary to the case of one-material layer, the frequency dependence of the resonance transmission region is more pronounced, as the frequency change of 2.5% induces transmission coefficient change of about 20%, as can be seen from curves B and C in Fig. 5b. The dependence of the resonance transmission characteristics on the slight changes in value of Z3 about the fixed value of 45 MRayl is relatively negligible, as can be seen from curves A and B in Fig. 5b. This contributes to making it possible to put the copper layer instead of the steel layer in the experimental part. Differentiation property of the two-material matching layer can be seen from Fig. 6a, where the transmission coefficient for the matched case (Z4 ¼ 3:2 MRayl) is two or three times larger than for other materials of tested objects. Typical frequency characteristic of the two-material slab is shown in Fig. 6b, in case of one set of matching parameters. Frequency characteristics look like a filter of a basic frequency and its approximate higher har-
monics (here only 3.94 MHz peak is seen). Interval of transmitted frequencies has relatively narrow full width half maximum of 0:2 2p MHz. The low frequency transmission maximum is exploited in other approaches to characterization of buried objects [13,15]. Here care should be taken in order to take properly into account the non-negligible tip oscillation amplitude [15]. Simulated results are confirmed experimentally. Fig. 10 shows that there is a differentiation between plastic materials, represented here through Plexiglas. Compared to the case when there is no impedance matching, shown in Fig. 11, there is certain improvement However, it is not clear to what amount this difference would be degraded in realistic conditions, with unknown contact and with unknown surface condition on buried object. Some factors, the influence of which could be seen in the experimental results, have not been included in the theoretical part. These are the ultrasound attenuation and the surface roughness. They were not included in laboratory conditions because suitable preparation of samples enabled their rather low influence of the results. However, these have to be included in the description of the general case. The finite attenuation lowers the transmission, and therefore enhances the reflection. This effect is seen from the expression for reflection coefficient in case of normal incidence of a wave onto a single interface between two half spaces R¼
ðZ4 Z1 Þ2 þ X42 2
ðZ4 þ Z1 Þ þ X42
;
ð5Þ
where X4 represents the imaginary part of the total acoustic impedance of the half space, i.e. tested object material. In order to simplify the analysis of the finite attenuation, the attenuation of the material 1 is still neglected. As the attenuation is larger for larger frequencies, reflection enhancement is more pronounced for larger frequencies. This could be seen from Fig. 11a and b, where the plastic material does achieve the smallest amplitude value because of the impedance matching, but at the same time does not achieve negligible value owing to the finite attenuation. The effect of attenuation is more pronounced for higher frequencies, as raising the frequency by 50% in case of Fig. 11b compared to Fig. 11a results in degradation of the reflected wave amplitudeÕs difference for Plexiglas and materials with impedances larger than 15 MRayl from 5 dB to less than 2 dB. The finite surface roughness will result in a different frequency dependence of the transmission, as shown in detail elsewhere [20]. The possibility to use the piezoelectric sensor to characterize the surface roughness already exists, e.g. [21]. Although designed for application in a different environment, that technique belongs to local surface characterization techniques.
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5. Summary and conclusions It can be seen that two-material matching layer realization of the buried object materials characterization makes possible satisfying the mechanical conditions and still provide the designer with a possibility to obtain good sensitivity in a specific region of parametric space available. The obtainable resolution is satisfactory in laboratory conditions, hence an open topic is the amount of its degradation in more realistic models, in particular in field conditions. In addition, in further work slight differences in the response should be properly combined and provide additional pieces of information about the buried object material by means of neural networks or other complex signal processing algorithms. Acknowledgements J.S. Jr. greatly acknowledges the hospitality and help of the employees in Department VIII.4 of BAM––Berlin, where part of this work was done. This work was funded through the contracts SEQUA no. 247, and CRO-MoST 120 098. References [1] UN Mine Clearance Policy Unit, International Standards for Humanitarian Mine Clearance Operations. Available from
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[6] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 1. [7] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 11. [8] N. Kingsbury, Land Mine Detection DODÕs Research Program Needs a Comprehensive Evaluation Strategy, US GAO Report, GAO-01 239, 2001, p. 16. [9] J. Stepanic Jr., Material characterization by mechanical point contact impact emitted ultrasound, in: 15th WCNDT, Proceedings, Rome, Italy, 2000. Available from