GPR Signal Processing with Geography Adaptive Scanning using Vector Radar for Antipersonal Landmine Detection

Ground Penetrating Radar (GPR) is a promising sensor for landmine detection, however there are two major problems to overcome. One is the rough ground surface. The other problem is the distance between the antennas of GPR. It remains irremovable clutters on a sub-surface image output from GPR by first problem. Geography adaptive scanning is useful to image objects beneath rough ground surface. Second problem makes larger the nonlinearity of the relationship between the time for propagation and the depth of a buried object, imaging the small objects such as an antipersonnel landmine closer to the antennas. In this paper, we modify Kirchhoff migration so as to account for not only the variation of position of the sensor head, but also the antennas alignment of the vector radar. The validity of this method is discussed through application to the signals acquired in experiments.


Introduction
Over seventy million landmines are said to be untreated. So, it is big social problem for safety life environment after the conflict. Currently, there are two major methods of landmine detection. One is search using metal detector and other is search using an exploratory needle, however, those method have problems. An exploratory needle method requires operator to come close to landmine and to contact physically with land mine. So, dangerousness of exploratory needle method can not be denied. A metal detector is one of the most major sensors for a current humanitarian demining. It has been relied for many years because there is no better sensor in viewpoint of cost and simplicity. It hardly misses an antipersonnel landmine (APM) in shallow subsurface. However, it has a serious problem. It responses all objects including metal, even though most of them are entirely safe. This feature of metal detector brings tremendous false alarms, and it reduces efficiency of demining. On the other hand, GPR (Ground Penetrating Radar) is a sensor expected as an alternative of metal detector or support device of it (Shimoi, N, 2002), (Orifici, D, 2004). GPR system measures the response time of electromagnetic wave reflected by buried object, and it is originally used for archeological digging, detection of underground pipe and detection of lack in reinforced concrete. GPR can seek plastic landmine unlike in the case of metal detector. And GPR can obtain the information concerned with buried objects such as depth and figuration. It is considered extremely effective for landmine detection, and some researchers have been studied (Feng, X., Sato, M. 2004), (Daniels, D. J., 2004). Though GPR is expected high performance as a sensor system for landmine detection, there are many problems under stand-alone operation. At first (1) decrement of an electromagnetic wave becomes large, and performance turns worse by a water state of the soil where a mine is laid underground. In addition, (2) reliability of a detection result deteriorates when operation is conducted with non-homogeneous soil. As a more serious problem, (3) when there is the irregularities or a slant in a ground surface, they are projected onto an image of the underground, and it becomes difficult to distinguish a shallow undergrounding object. For the first problem, it can be moderated influence by choosing the electromagnetic wave frequency, which is hard to be absorbed water. And combination with a metal detector is effective on the second problem. However, because the last problem is a reliability fall at the depth that synergy with a metal detector can expect, it cannot be overlooked. As influence of rough ground surface, (i) electromagnetic wave reflection by the small irregularities on ground surface and (ii) heterogeneity of velocity field generated by fluctuation of air layers with inclination of ground surface, are included. For the enhancement of mine detection system, we have studied about methodology that vector type grand penetrating radar with three antenna elements (Kimura, N. et al., 1992), (Murasawa, K. et al., 1992) is adaptively scanned for ground surface (Fukuda, T. et al., 2005), (Yabushita, H., 2005), (Hasegawa, Y. et al, 2004), (Yabushita, H. et al. 2004), (Hasegawa, Y. et al, 2005) Above metioned problems are improved by geography adaptive scanning. Because, when antenna plane faces ground surface in parallelism with constant distance, (i) effect of reflection by the small irregularities on ground surface can be reduced on computer, and (ii) velocity field can be calculated correctly on computer by keeping air layers constant. On the other hand, location error of antennas in migration, which is vanishingly small when target area is enogh deep, is not mentioned. When mesuaring shallow area with electromagnetic wave, position of antennas should be considered. Esspecially, because manipulator changes position of antennas on geography adaptive scanning method, effect of location error becomes bigger. In this paper, effectiveness of geography adaptive scanning method for searching buried object with rough ground condition is mentioned. And signal processing method, which is obtained by extension of pre-stack migration considering the feature of vector radar system, is proposed. Finally, effectiveness of vector radar system and proposed signal processing method is confirmed by experiment.

Ground penerating radar and its signal processing
In this section, fundamental principle of GPR and outline of signal processing is introduced. It is needed to explain effectiveness of our approach.

Fundamental principle of GPR
GPR(Ground Penetrating Radar) works as follow: A transmitting antenna radiates electromagnetic waves in the underground, and a receiving antenna receives reflected waves from objects under the ground, and finally radar system estimates structure under the ground based on electromagnetic contrasts. Multistatic GPR have several measure types such as common source, common receiver, common midpoint, common offset (Daniels, D. J., 2004). And in this paper, common offset measurement using vector GPR is adopted. It is difficult to judge buried objects by just displaying a time-series signal of GPR to a graph. Therefore, a time-series signal is expressed by colored line, and a chronological order section is expressed by arranging colored line in horizontal direction. Otherwise, distribution of reflection strength of under ground space is calculated by signal processing, and it is displayed by a cross-section surface and volume rendering (Daniels, D. J., 2004). In this paper, it is a research purpose to make reflection strength distribution on three-dimensional space which can make distinguish of undergrounding objects easy. An operator will select appropriate visualization method from time crosssection surface, level cross-section surface and perpendicular cross-section surface depending on the situation.

Signal Processing
Signal processing of GPR consists of some processes. Firstly, a reflection from a ground surface and coupling between transmission and reception antennas are removed by a method of average signal subtraction. As a result, reflected signal from underground object is amplified. Secondly, those signals are changed to threedimensional distribution map of reflection rate by using deconvuluion, scattering matrix process, inverse Fourier transform, time offset adjustment, normal move out correction, migration and recovery of gain. In those processes, migration process requires special consideration at adaptive scanning with vector GPR. In this paper, we foused on enhancement of the migration process. Figure 1 shows vector GPR adopted for this research. This GPR system have three antenna elements and realize measurement with three polarization characteristic at one time as shown in Fig. 2. It has ability to be able to calculate a measurement signal in arbitrary polarization characteristics by using scattering matrix process for acquired measurement signals (Kimura, N. et al., 1992), (Murasawa, K. et al., 1992).  ( , ).

Time-domain migration with adaptive scanning
Normally, ) , , ( t y x φ distribution of reflection rate in three-dimensional space is acquired by scanning ground surface.However, because GPR antenna does not have so strong directivity, when there is a small object in space φ measured signal appears with spatially-extended condition. In other words, when reflected wave is simply converted to distance from antenna, high resolution images is not to be acquired. The process that

Kirchhoff migration
Firstly, the condition that a radar which have transmitting antenna and receiving one on same point scans on horizontal plane is considered. More specifically, following equations are able to be approved. . where, (8) z y x  , (Schneider, W. A., 1978).

Migration with geography adaptive scanning with vector GPR
Kirchhoff migration on preceding section is assumed flat surface scanning as manipulation of radar system. Originally, it can not be adopted for signals obtained by geography adaptive scanning. Therefore, Kirchhoff migration considering geography adaptive scanning is proposed. And, when distance between antenna and buried object is short, effect of distance between transmitting antenna and receiving antenna can not be neglected : this effect is neglected in Eq. (7). Therefore, prestack migration method is applied to solve this problem. However, if each method is used independently, enhanced image is not able to be obtain. In this paper, methodology which combine idea of geography adaptive scanning Kirchhoff migration and one of prestack migraion, is extended for vector GPR.

Kirchhoff migration with geography adaptive scanning
Kirchhoff migration in previous section is conducted with assumption that height of radar ) , ( y x z g stays constant. When geography adaptive scanning is conducted, height of each antenna should be considered in migration process with Eq. (7) in order to obtain chiseled image (Hasegawa, Y. et al, 2004), (Yabushita, H. et al. 2004).

Prestack Kirchhoff migration
Migration explained in previous subsection is conducted with assumption that transmitting antenna and receiving antenna are on the same point. Many radar systems have transmitting antenna and receiving antenna are on the different point. Traveling time of electromagnetic wave is not proportional to depth of buried object on such radar system. When radar system which have antenna distance h measures buried object located immediately below with depth d , p t : traveling time of electromagnetic wave is described in Eq. (17) (Sato, M., 2002).
Virtual antenna is set on center of transmitting and receiving antenna for the purpose of avoiding this problem as shown in Fig. 4 acquired with assumption that electromagnetic wave is transmitted and received by virtual antenna, and migration is conducted using φ with Eq. (7) (Feng, X., Sato, M., 2004) .
where, p r is traveling distance from virtual antenna to buried object, and p rˆ is traveling distance calculated by normal moveout correction. Figure 6 shows relation of p râ nd actual distance p r against for x : horizontal distance from antenna to buried object, and this data is normalize by 2 / h . It can be understood from this figure that when 0 = x , p rˆis equal to p r , and the larger x becomes, the further p rˆ differes with p r . This effect is remarkable when an antenna set gets close to buried object, and it causes degradation of precision at imaging of reflecition rate. Additionally, common midpoint measurement have problem that inclined surface can not be imaged precisely because stacking process accentuates only reflection components perpendicular to antenna plane. Therefore, stacking with migration method is proposed. It called "prestack migration" and expressed by Eq. (20) Kirchhoff integral is changed in Eq. (20), in order to interweave precise traveling time when electromagnetic wave passes through marked points toward antenna. And, stacking process is conducted at one time by integrating along with h : displacement on direction between the antennas. It is known that precise imaging is realized by those methods in order to avoid above mentioned problems (Feng, X., Sato, M., 2004).. Prestack migration is suitable for the imaging a mine like object which is small and buried close to ground surface.

Kirchhoff migration with vector GPR
In section 3.2.1 and 3.2.2, geography adaptive scanning and distance of antennas are considered at migration process respectively. However, both of them are needed for enhancement of migration when geography adaptive scanning is conducted with vector GPR. Those methods have common idea to obtain traveling time with accuracy. Therefore, traveling time of electromagnetic wave from transmitting antenna to reciving antenna is calculated according to position of antennas, and migration process specialized for geography adaptive scanning with vector GPR is formulated. Figure 4 shows coordinate system of GPR. Here, position of focused point under ground is indecated as  The procedure to average signals on each polarization mode corresponds to abstract isotropic reflective component (Kimura, N. et al. 1992). So, it is possible to treat this procedure that both of migration and polar processing is conduted coinstantaneously by Eq. (21).

Experimental environment
Measurement experiment using vector GPR for detection of imitation mine is conducted to confirm the effectiveness of proposed reconstruction algorithm of image. A buried object used for experiment is plastic imitation of Type 72 antipersonnel landmine made in China φ 78mm 40mm . Appearance of buried object is shown in Fig. 8. Dry sand with 3 % water content proportion is set in large bucket, and its surface is formed with inclined plane. Then, imitation of mine is buried horizontally with condition that its upper side is same depth for ground surface. Firstly, Ground for experiment after burial of imitation of mine is shown in Fig. 9. And, Secondly, ground surface data measured by laser range finder is shown in Fig. 10. Finally, appearance of geography adaptive scanning manipulated by PA10 manipulator is shown in Fig. 11.

Flat scanning and geography adaptive scanning
Firstly, the effectiveness of geography adaptive scanning against for flat surface is described. Migration process expressed by equation (21) is conducted for GPR signal obtained under the both condition of flat scanning and geography adaptive scanning as shown in Fig. 12. Each horizontal slice is shown in Fig. 13. Figure 13(a) is for flat scanning, and figure 13(b) is for geography adaptive scanning. It is diffcult to distinguish a buried object in horizontal slice (a) by the influence of reflection wave from ground surface. On the other hand, a form of circular buried object can be found in horizontal slice (b).
Geography adaptive scanning Flat scanning

Effect of polarization process with vector GPR
Next, the effect to integral three polarized electromagnetic signals obtained by vector GPR is explained. Figure 14 shows result of conduction of proposed migration process for signal obtained by geography adaptive scanning. Each horizontal slice obtained with single polarization mode skipping addition concerned with m in Eq. (21) are shown in Fig. 14 (a), (b) and (c). On the other hand, a cross section obtained in accordance with Eq. (21) inclusive integration of three polarization modes is shown in Fig. 14 (d). All horizontal slices are in the depth where imitation mine is buried.
It can be confirmed that there is a buried object in the position where actually imitation mine is buried (x=200mm, y=175mm), even if in the case where migration process is conducted with single polarization mode. However, image of buried object is distorted by polarization charactoristic of ground surface. Especially, distortion is remarkable in the case with m = 2. And, image of clutter on upper side of screen is more clear than that of imitation mine. On the other hand, image which is obtained by integrating three images of individual polarization mode have distinct circular object share. Furthermore, clutter is lower reduced than that in the case of single polarization mode.

Migration specified for vector GPR
In this subsection, the effect of migration considering positions of transmitting and receiving antennas on generation proper image of underground is confirmed. Image calculated with assumption that all transmitting and receiving antennas are on center of GPR is shown in Fig. 15(a). In this case, migration process is conducted for each single polarization mode using Eq. (11), and then those are integrated. And image calculated by proposed method is shown in Fig. 15(b). Proposed method realizes distinct image. However, image which does not have strong reaction compared with proposed one is generated with assumption that transmitting antenna and receiving one are on same position. Because, center of reaction for buried object on each image are misaligned by the influence of displacement of the antennas.

Conclusion
Vector GPR system with geograpy adaptive scanning has ability to seek antipersonal landmine burid in gournd, which have rough surface. Because both influence of rough ground can be reduced. One is the reflection wave by small irregularities on ground surface. Other is heterogeneity of velocity field generated by fluctuation of air layers. However, signal processing also has to be improved considering location of antenna elements. Therefore, in this paper, signal processing method specialized for vector GPR based on prestack migration is proposed. Finally, effectiveness of vector radar system and proposed signal processing method was confirmed by experiment.