Chapter 3: Vehicle-Mounted GPR System for Landmine Detection

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40 3. Vehicle-Mounted GPR System for Landmine Detection Table 3.2: Definition of the confidence ratings. Ratings Criteria 0 I am 100 % sure that there is nothing here 25 It seems that there is something here 50 I am 100 % sure that there is something here 75 I am not totally sure, but I would say the detected object seems to be a landmine 100 I am 100 % sure that the detected object is a landmine 3.5 Experimental Results An example of the experimental result is shown here. The measurement was carried out at a sand pit in the GPR Lab., Tohoku University, Japan. The targets were landmine models of Type72 and PMN2 shown in Fig. 3.12, and a mine-like 3 stone buried as Fig. 3.13. They are buried at a depth of 2 cm, and the landmine model PMN2 is buried vertically. The soil is dry sand mixed with gravels whose diameters are around 2 cm. The GPR antennas were scanned by an x-y stage (Device Co., Ltd., Japan) with a velocity of 100 mm/s as shown in Fig. 3.14. The measurement interval is 30 mm in both x and y directions. The raw time domain data acquired by VNA #1 (inner pair) is shown in Fig. 3.15 as a horizontal slice at a time of 2.4 ns, which corresponds at a depth of about 2 cm. The three targets are visible in this profile, however also clutter can be seen and the responses from the targets are not so clear. Thus, they cannot be recognized without a priori knowledge. Fig. 3.16 shows vertical and horizontal slices of the processed data at x =110 cm and at a depth of 2 cm. The three targets are clearly imaged and they have obviously different shapes and dimensions to clutter imaged at left on the horizontal slice. Note that the depth in the vertical slice is defined from the tip of the antennas, thus the ground surface is imaged at a depth of 10 cm. The measurement was a blind test, i.e. the operator did not know the types, positions, depths, and numbers of buried objects prior to the measurement. From the image, the operator 3 The term mine-like object is defined as an object whose shape and dimension resemble that of a landmine.
3.5 Experimental Results 41 could detect all of the objects including the stone on site. The quick detection algorithm is applied to this data. The window width w in Eq. 3.11 has to be defined first from the wavelength. Fig. 3.17(a) shows traces at ( x, y ) = (1050, 1000) mm (at Type72 buried), and (800, 1000) mm (at nothing buried). Since the semi-period seems 20 mm, the width w is defined as 20 mm. In these traces, the reflection from the target cannot be seen at the depth of the targets buried, 120 mm, since it is masked by the reflection from the ground surface. Whereas, it can be seen below a depth of 200 mm as a tail of the reflection. The envelope and moving averaged envelope of the traces are shown in Figs. 3.17(b) and (c). By moving average, the envelope is smoothened. Fig. 3.17(d) shows the normalized envelope. The tail of the reflection at a depth of 200 mm is enhanced. The histogram of the value pˆ( xy , ) in Eq. 3.13 is shown in Fig. 3.18. Here the thresholds to rate the confidence are simply defined as (a) (b) Fig. 3.12: Landmine models of Type72 (a), and PMN2 (b).
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Chapter 3: Vehicle-Mounted GPR System for Landmine Detection

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