A new framework of building collapse extraction based on azimuth time-frequency analysis

Rapidly obtaining building damage information after the earthquake is vitally important to earthquake rescue and decision making. In this paper, we propose a new framework of building collapse extraction by combining polarimetric information and azimuth time-frequency analysis. The proposed framework consists of two main steps, i.e. open area removal and intact building identification. In the first step, we introduce the Shannon entropy to polarimetric parameters derived from H/A/α decomposition to remove open areas. Since intact buildings with orientation angles present a similar scattering mechanism to collapsed buildings, it seriously hampers the building collapse extraction accuracy. To solve this problem, in the second step we try to use azimuth time-frequency analysis to eliminate this deficiency. In the azimuth direction, sub-aperture images are obtained by spectrum splitting, which corresponds to observe the targets from different azimuth look angles. The nonstationary discrimination method is used combined with the SPAN parameter to identify intact buildings including intact buildings with orientation angles. The feasibility of the second step has been demonstrated by using E-SAR PolSAR data. The potential advantage of the proposed framework when we apply it to the earthquake-induced building collapse extraction includes that only one post-earthquake polarimetric SAR (PolSAR) image is needed. Moreover, orientated intact buildings can be identified and the final accuracy of building collapse extraction can be probably improved as a result.

1. Introduction

Earthquake is one of most catastrophic natural disasters which cause many severe damages like building collapse, landslides and mudslides. Unfortunately some areas in the world have been threatened by earthquakes for many years. The Synthetic Aperture Radar (SAR) has an advantage over optical satellite because of its all-time and all-weather observing capabilities, therefore it plays an increasing role in earthquake-induced building collapse extraction. Due to the unexpectedness and suddenness of earthquakes, suitable pre-earthquake SAR image is commonly unavailable. It is probably difficult to extract collapsed buildings accurately by using only post-earthquake single polarization SAR image due to the lack of enough target scattering information. The polarimetric SAR (PolSAR) measures scattering matrix which can describe the physical scattering mechanism of target. Moreover, it is sensitive to some characteristics of target like shape, size, material and orientation. These make it become a potential data source to researches on building collapse extraction by using only post-earthquake SAR image.