地震地壳形变InSAR测量中的关键技术分析

Key technical issues in seismic crustal deformation measurement by InSAR

InSAR技术是监测地震地壳形变的有力手段,对于研究地震机理、防震减灾具有重要的意义。InSAR测量结果可以作为分析地震断层几何学特征和动力学机制的研究资料,为进一步研究断层活动的时空特征以及大陆岩石圈动力学规律,建立断层运动模型,获取地震的地球物理参数及其演化过程提供参考依据。随着InSAR技术研究的不断深入,提升InSAR技术的应用水平,高效、高可靠性地获取地壳形变信息是行业用户和国内外学者关心的问题。本文从地震地壳形变监测需求出发,结合InSAR技术特点,归纳和概述了地震地壳形变InSAR测量中更为关心的技术问题及其研究现状,重点分析并提出了InSAR技术在地震地壳形变监测中需要加强重视与进一步深化研究的一些关键技术问题,包括大气效应、视线向模糊、大范围监测、高相干目标选取、大数据高效能计算以及InSAR技术自身质量评价与控制体系的构建,并对此提出了一些解决思路,旨在提高InSAR技术在地震领域中的应用水平。

Interferometric Synthetic Aperture Radar(InSAR) has been proved to be a high-precision geodetic approach for monitoring the crustal deformation of the earth. Comparison with other methods, such as leveling, GPS, Very-Long-Baseline Interferometry(VLBI), Satellite Laser Ranging(SLR) and so on, InSAR has the remarkable advantages of continuous large-area coverage and centimeter to millimeterlevel measurement accuracy, its measuring results are the significant geophysical parameters needed by seismic research, thereby becoming an important means of seismic crustal deformation monitoring.The occurrence and development of earthquakes have periodic characteristics. A complete seismic cycle can be divided into four phases: pre-seismic, co-seismic, post-seismic and inter-seismic. Different seismic processes have their own crustal deformation characteristics, so we should to adopt different methods for the deformation monitoring at different stages. At present, D-InSAR technology is mainly used to monitor the co-seismic deformation, while time-series analysis of InSAR is in the inter-seismic deformation monitoring. In recent years, some new satellite platforms, namely, Sentinel-1 A/1 B RADARSAT-2, ALOS-2, TerraSAR/TanDEM-X and COSMOSkyMed, have enhanced the ability to obtain SAR data globally with short revisit cycles and possibility for monitoring crustal deformations worldwide and routinely. In the meantime, the number of InSAR users, including governments, research institutes, and commercial companies, is expanding year by year, then the demands to improve the accuracy and reliability of InSAR results are also increasing. This paper first summarizes the landmark works and up-to-date research status of InSAR technology in seismic crustal deformation monitoring, then focuses on major limitations hindering the technique deeper applications, such as Atmospheric Phase Screen(APS), LineOf-Sight(LOS) ambiguity, and imaged swath width. The existing methods to remove APS have been summarized, and an emerging method based on Numerical Weather Prediction(NWP) model has been discussed especially. The problems on swath width limitation and LOS ambiguity have been analyzed and the possible solutions are introduced. InSAR time-series analysis is an advanced method for monitoring inter-seismic and post-seismic displacement. In the paper, the most important four methods of InSAR time-series, which are PS-InSAR,SBAS, StaMPS, and SqueeSAR respectively, are described in detail. The paper concludes with a discussion of the key technical issues on InSAR applications and the associated ways toward to the solutions. The conclusions are given as follows:(1) APS is a main source of error in InSAR processing, which could be eliminated or mitigated by APS correction with the output from NWP models.(2) One-dimensional measurement along the LOS direction has greatly limited the capability of InSAR technique in the investigation of crustal deformations which demands three dimensional deformation components. To obtain accurate 3-D surface deformation not only need to incorporate descending InSAR result together with ascending one, or with MultiAperture Interferometry(MAI), but also SAR satellite in large oblique orbit should be considered.(3) High-coherence target selection is an important prerequisite for InSAR time series analysis. The problems, such as unevenly distribution of PS and low coherence of the natural surface, are main reasons for poor performance of InSAR time-series analysis application in Earthquake research. Reasonable choice of the thresholds and optimal high-coherence target selection strategies can improve the accuracy of InSAR results.(4) Wide area mapping is urgently needed in the crustal deformation monitoring, since the coverage of the great earthquakes faults usually extended over several hundreds or even up to thousands kilometers. Strategies to merge multi-track InSAR results together with ScanSAR interferometry and its time series analysis should be took into consideration in future.(5) Time-series analysis algorithms must be adapted to incorporate each new image in an efficient and optimal manner without starting the processing from scratch. New approaches should be proposed as efficient processing schemes to exploit the unprecedented Big Data for high-precision near-real-time processing.(6) Now the validation of InSAR outputs or its accuracy estimation relies heavily on GPS, leveling, and other external data. Here, we propose an idea to introduce the 'Totally Quality Control' into InSAR processing chain that goes through every step of InSAR time series processing to indentify the possible artifacts in the processing and correcting them to ensure the quality of the outputs.