# Post-Earthquake Damage Inspection of Wood-Frame Buildings by a Polarimetric GB-SAR System
## 1 Introduction  
Destructive earthquakes cause different degrees of damage to buildings all over the world each year. The rate of collapsed buildings, the grade of damage in the affected area, and the types of damages to each building are essential information for successful rescue and reconstruction in the disaster area [1; 2]. In the stage of post-earthquake reconstruction, there is a great demand for efficient and accurate assessment of the safety of in situ residential buildings. Recently, space-borne and air-borne remote sensing techniques, including optical sensors [3] and synthetic aperture radar (SAR) [4; 5], have been popularly used for the overall damage evaluation in a vast area. These remote sensing techniques can detect collapsed or obviously unsafe buildings. For the moderately and/or slightly damaged buildings, visual observation by experienced engineers is still the main method to evaluate their safety [6; 7]. However, a visual inspection is restricted to check the superficial damages--e.g., crack, spalling, deformation, etc.--and the evaluation results highly depend on the engineer's experience. To deal with concealed earthquake damage, invasive techniques based on drilling have been popularlyapplied [8]. To avoid the secondary damage to building structures, many non-destructive testing (NDT) methods have been developed for inspection and evaluation of earthquake-induced structural damages [9]. Stress wave measurements--which include elastic, acoustic, and ultrasonic waves--can identify the internal deterioration [10] and crack [11], and can determine the mechanical properties of a structural member, such as modulus of elasticity and strength. While the stress wave measurements require the access to both sides of the structural member. Acoustic emission measurement has been used to predict earthquake damage in a concrete core under uniaxial compressive loading [12], and to detect biodegradation in wooden buildings [13]. Wood-frame residential buildings are popularly constructed in earthquake-prone countries, due to its good seismic performance [14]. For instance, over two million housing units are built annually in North America and Japan. The general range of the fraction of wood structures to total structures seems to be between 80% and 90% in all regions of the USA [15]. Recently, wood-frame construction is not only used for single- and multi-family dwellings, but also for low-rise commercial and industrial buildings [16]. Unlike steel or concrete structures, earthquake damage to wood-frame buildings cannot be readily observed by naked eyes, since wooden structures are commonly covered by wall finishes made of gypsum or other materials. Thus, development of NDT techniques without removal of wall finishes is of significant meaning for damage inspection of the invisible wooden structures. Thermography, X-ray radiography, nuclear magnetic resonance, and radar imaging have been tentatively studied [17; 18]. Among these techniques, radar imaging is one of the most promising approaches, because it allows for quick data acquisition and imaging of non-accessible structures inside the wall. In addition, polarimetric radar can provide richer information of embedded and/or buried objects [19], and can enhance the diagnostic power required for building inspection [20; 21]. Up to now, the major limitation of microwave sensing techniques for the real use of inspecting wooden structures is its insufficient resolution for detecting small defects inside a damaged wooden structure, of which the common types are cracks, loosening or failing of connections, large deformation, etc. [14]. To detect the subsurface damage of the wooden structure with sufficient resolution, we developed a ground-based synthetic aperture radar (GB-SAR) system operating in an ultra-wideband (UWB) from 1 to 20 GHz. The image resolution can be greatly improved as a result of the UWB operation and SAR processing. In addition, the full-polarimetric data analysis can provide complementary information on the targets.