Christian N. Koyama

Post-Earthquake Damage Inspection of Wood-Frame Buildings by a Polarimetric GB-SAR System

Structural damage inspection after an earthquake is essential for safety assessment of the affected wood-frame buildings and for making knowledgeable decision regarding their repair, renovation, or replacement. We present a polarimetric radar system for sensing the concealed wood-frames damaged by earthquakes. This system employs an antenna array consisting of four linearly polarized Vivaldi antennas recording full-polarimetric radar echoes in an ultra-wideband ranging from 1 to 20 GHz. The detailed design of the system and the signal processing algorithms for high-resolution 3D imaging are introduced. We conducted a number of surveys on damaged wooden wall specimens in laboratory. The experiment results indicate that the high-frequency radar waves can penetrate the wooden walls. Deformations of wooden structures (about 2 cm displacement) inside the wall, as well as the concealed small metal nails (about 3 mm in diameter and less than 2 cm in length) and bolts can be clearly imaged. The shape and orientation of the wooden members have shown a great sensitivity to the radar polarization. It is concluded that radar polarimetry can provide much richer information on the condition of concealed building structures than the conventional single-polarization subsurface penetrating radar.

Development of a biomass corrected soil moisture retrieval model for dual-polarization ALOS-2 data based on ALOS/PALSAR and PI-SAR-L2 observations

An empirical soil moisture retrieval model for dual-polarimetric L-band SAR imaging, the ALOS-2 standard observation, is being developed. The model, based on empirical relations between ALOS/PALSAR and Pi-SAR-L2 observations and ground measurements, can correct for the disturbing effects caused by surface roughness or vegetation cover. In this paper we focus on airborne campaigns carried out over the Sendai area, Japan, in 2013 and 2014. In situ surface parameters (soil moisture and surface roughness) were measured on bare soil and under rice canopy. Using roughness and biomass corrections the SAR derived soil moisture estimates yield an accuracy of 5.9 Vol.-% RMSE. The results indicate that the proposed approach, which was mainly parameterized in test sites in Germany, can also work in different geographic regions.

Full polarimetric UWB GB-SAR for damage assessment of wooden building structures

A full polarimetric ground-based SAR system for subsurface damage assessment of wooden building structures has been developed. The system is based on a 4-channel VNA connected to a polarimetric antenna array. The array elements consist of 4 newly developed circular polarization cavity-backed spiral antennas, which have very good wideband characteristics and excellent polarimetric performance. By 2 dimensional scanning the system can achieve polarimetric 3D imaging with super high-resolution of 1 cm in x-/y-direction and 2 cm in z-direction. Using the polarimetric phase information, deformations of internal wooden structures like beams and bars with inclination angles <;1 deg. can be detected, which is not possible with any other non-destructive testing technique.

Design of wideband small spiral antenna for SAR

A design of a wideband small spiral antenna for SAR is proposed. First, to miniaturize the spiral size, characteristics of input impedance and axial ratio for equiangular and Archimedean spiral antennas are analyzed regarding the effects of substrate and spiral turn. It is found that the miniaturized 6-turn Archimedean spiral antenna on a FR-4 substrate which is 4.4 cm in diameter has the good characteristics over a frequency range of 3 GHz to 20 GHz. Second, by experimentally observing time domain radar characteristics, it is confirmed that a cavity 6.1 cm deep sufficiently filled with an EM absorber is effective to obtain clear radar signals. It is also found that leaving a space between Tx and Rx antennas is important to improve the power ratio of a reflected wave to a direct wave.

High-resolution imaging of damaged wooden structures for building inspection by polarimetric radar

A full-polarimetric radar system for non-contact, non-destructive high-resolution imaging and inspection of wooden buildings damaged by earthquakes is developed. The stepped frequency continuous wave system is based on a network analyzer and employs a multi-channel antenna array consisting of four linearly polarized Vivaldi antennas operating in an ultra-wideband ranging from 1 GHz to 20 GHz. The signal processing methods for high-resolution 3D imaging and polarimetric analysis are introduced in this paper. We conducted two-dimensional scans on damaged wooden wall specimens in the laboratory. The measurement is carried out at a five mm step. The results indicate that the microwaves up to 20 GHz can penetrate the wooden walls. Cracks and deformations of wooden structures inside the wall, as well as of metal nails and bolts can be clearly imaged. The observed radar signatures related to the damaged wooden structures are discussed. Our findings demonstrate that the polarimetric radar technique is able to provide richer information than conventional single-polarization subsurface penetrating radar.

Near range polarimetric SAR for non-destructive inspection of wooden buildings

We report our recent development in near rage radar systems and their applications to Non Destructive Inspection (NDI) for wooden buildings. We developed a radar system having 32 linearly aligned antenna elements operating at 10-20GHz. This system can acquire quite dense data on a wall. Then we discuss the possibility of 2-dimentional array system, based on the idea of sparse array. We demonstrated that the 8-ch array radar system can visualize the 3-dimentional structure without movement for radar scanning. Then we demonstrate that polarimetric information can image very small structure of buildings, which is under the limitation of radar resolution.

Estimation of soil moisture and debris pile volume from Pi-SAR2X and Pi-SAR-L2 square-flight data

Full polarimetric airborne SAR data acquired over the Tsunami affected coastal area in Northeastern Japan is used to estimate soil moisture and debris volumes. Using empirical surface roughness calibration of the I2EM soil moisture is estimated with a RMSE of 5.2 Vol.-%. The heights of Tsunami debris piles are estimated with a RMSE of 0.71 m by using a reference point aided stereo-SAR approach. Debris pile volumes are estimated with an overall RMSE of 2478 m3. The accuracy increases with increasing size of the piles. While the error ranges from 50% to 90% for the smallest piles, it is in the order of 20% to 30% for the large piles.

In-Situ Measurement of Soil Permittivity at Various Depths for the Calibration and Validation of Low-Frequency SAR Soil Moisture Models by Using GPR

At radar frequencies below 2 GHz, the mismatch between the 5 to 15 cm sensing depth of classical time domain reflectometry (TDR) probe soil moisture measurements and the radar penetration depth can easily lead to unreliable in situ data. Accurate quantitative measurements of soil water contents at various depths by classical methods are cumbersome and usually highly invasive. We propose an improved method for the estimation of vertical soil moisture profiles from multi-offset ground penetrating radar (GPR) data. A semi-automated data acquisition technique allows for very fast and robust measurements in the field. Advanced common mid-point (CMP) processing is applied to obtain quantitative estimates of the permittivity and depth of the reflecting soil layers. The method is validated against TDR measurements using data acquired in different environments. Depth and soil moisture contents of the reflecting layers were estimated with root mean square errors (RMSE) on the order of 5 cm and 1.9 Vol.-%, respectively. Application of the proposed technique for the validation of synthetic aperture radar (SAR) soil moisture estimates is demonstrated based on a case study using airborne L-band data and ground-based P-band data. For the L-band case we found good agreement between the near-surface GPR estimates and extended integral equation model (I2EM) based SAR retrievals, comparable to those obtained by TDR. At the P-band, the GPR based method significantly outperformed the TDR method when using soil moisture estimates at depths below 30 cm.

Urban damage mapping using scattering mechanism investigation technique for fully polarimetric SAR data

Mapping of the urban damage levels with synthetic aperture radar (SAR) is still challenging. Fully polarimetric SAR (PolSAR) has the potential to identify the type of scattering mechanism changes induced by urban damage. In radar polarimetry, dominant double-bounce scattering mechanism in urban areas is primarily induced by the ground-wall structures. Thereby, within an urban patch, the reduction of the dominant double-bounce scattering mechanism reflects the urban damage level in terms of destroyed ground-wall structures, which is the basis of this study. Based on our previous study, the proposed polarimetric index is further investigated. A rapid urban damage mapping technique including mainly two steps of urban area extraction and damage level index estimation is proposed. The 3.11 East Japan Earthquake and Tsunami is adopted as the study case using multi-temporal ALOS/PALSAR PolSAR data. Experimental studies demonstrate that the estimated damage levels are closely consistent to the ground-truth.

Investigation on near range SAR for inspecting inner structure of buildings

We are developing radar technology to inspect the inner structure of wooden buildings suffered from strong quake by earthquake. GB-SAR (Ground Based Synthetic Aperture Radar) for inspection of wooden and concrete walls and structures has been developed and the system was evaluated by test measurements. The GB-SAR system uses frequency bandwidth 1-20GHz, and it can acquire full polarimetric radar signal. We found that frequency up to 20GHz can be usefully used for inspection, and show that the radar could achieve the resolution about 1.5cm, and found that t can be used for detection of crack inside concrete structure. Then Synthetic Aperture (SAR) Radar signal processing is used to reconstruct 3-dimenatioal images of inner structure of the targets. We found that the radar polarimetry gives us very precise information of the damaged structures, and demonstrated that radar polarimetry is a useful tool for detecting fractures inside a concrete structures, and detection of small deformation of wooden structures.