利用星載SAR差分干涉測量改進(jìn)變形含水層系統(tǒng)的繪圖、監(jiān)測和分析

1、利用星載SAR差分干涉測量改進(jìn)變形含水層系統(tǒng)的繪圖、監(jiān)測和分析美國國家研究委員會(huì)地面沉降座談會(huì)（NRC；1991）就3種信息需求達(dá)成了共識：“第一，有關(guān)地面沉降大小和分布的基本的地球科學(xué)數(shù)據(jù)和信息要得到認(rèn)可并用來評價(jià)未來的問題。這些數(shù)據(jù)不僅能夠幫助研究局部地區(qū)的沉降問題，也能識別國家范圍內(nèi)的問題。第二，針對地面沉降開展沉降治理和工程方法的研究為了有效阻止或控制破壞第三，盡管美國現(xiàn)行的地面沉降減輕方法有很多種，但是對這些方法的成本效益進(jìn)行研究將有助于決策者做出更好的選擇?！庇懈鞣N基于地面和衛(wèi)星的方法可用來測量含水層系統(tǒng)的壓縮和地面沉降（表1）。SAR干涉測量理論上適合測量與含水層系統(tǒng)壓縮相關(guān)的地

2、面變形的空間范圍和大小。InSAR可以提供一個(gè)區(qū)域內(nèi)覆蓋整個(gè)含水層系統(tǒng)的數(shù)百萬個(gè)數(shù)據(jù)點(diǎn)，與使用大量人力而只能獲得有限個(gè)點(diǎn)測量數(shù)據(jù)的水準(zhǔn)測量，和GPS測量相比，通常而言，花費(fèi)要更低一些。通過識別研究區(qū)內(nèi)某一變形的特定區(qū)域，SAR干涉測量也可以用于定點(diǎn)測量并同時(shí)監(jiān)測局部和區(qū)域尺度上的地面沉降（如鉆孔伸長計(jì)、GPS監(jiān)測網(wǎng)絡(luò)、水準(zhǔn)路線；Bawden等，2003）。SAR干涉測量的這些優(yōu)勢，尤其是InSAR，能夠滿足NRC提出的每一種信息需求。SAR干涉測量的另一個(gè)重要優(yōu)勢就是SAR歷史數(shù)據(jù)的存檔文件越來越多。在很多地區(qū)，從上世紀(jì)90年代初開始，就已經(jīng)有了大量的數(shù)據(jù)集，因而這一時(shí)期的地面形變歷史測量數(shù)據(jù)

3、即可應(yīng)用。此外，為滿足新需求可以定制新數(shù)據(jù)。詳細(xì)的過程和費(fèi)用要依賴于使用的傳感器。Space-based Tectonic Modeling in Subduction Areas Using PSInSARR. M. W. Musson British Geological Survey M. Haynes NPA Group A. Ferretti TeleRilevamento EuropaINTRODUCTIONWhile the application of InSAR (INterferometric Synthetic Aperture Radar) techniques to

4、seismology has been well known since the mid-1990s (Massonnet et al., 1993; Massonnet et al., 1996), PSInSAR is generally unfamiliar to the Earth science community. The PS stands for permanent scatterer, and it is the use of these (along with the volume of scenes employed) that distinguishes the met

5、hod from more familiar InSAR techniques. A permanent scatterer is any persistently reflective pre-existing ground feature, such as building roofs, metallic structures, and even large boulders. The use of these features offers the possibility of measurements of ground displacements to a degree of acc

6、uracy, and over periods of time, previously unobtainable from conventional interferometry. Furthermore, it is possible to construct histories of displacements over the full temporal extent of the SAR data archive (started in 1991) for any part of the globe with data coverage. PSInSAR therefore repre

7、sents the equivalent of a newly discovered, superaccurate, extremely dense GPS network that has been in existence for the last twelve years. The high resolution of PSInSAR data, coupled with its being particularly suited to urbanized areas (numerous buildings, therefore many PS points), makes it an

8、excellent tool for studying things such as urban subsidence (Ferretti et al., 2000; Mizuno and Kuzuoka, 2003; Dehls and Nordgulen, 2004). It also has applications in seismology: as a substitute for GPS data where these do not exist, and as an enhancement where they do. In this paper we report on a p

9、ilot project in Japan, the principal aim of which was to calibrate and test the PSInSAR measurements in an area where ground truth is very well established from GPS and leveling data. This work results from a European Space Agency (ESA) Earth Observation Market Development project entitled Developin

10、g markets for EO-derived land motion measurement products, involving NPA (lead), the British Geological Survey (UK), Imperial College (UK), TeleRilevamento Europa (Italy), ImageOne (Japan), the Geographic Survey Institute (Japan), Oyo Corporation (Japan), Fugro (Netherlands), and SARCOM (the ESA dat

11、a distributing entity). TECHNICAL BASIS OF PSInSARConventional satellite radar interferometry involves the phase comparison of synthetic aperture radar (SAR) images gathered at different times (Massonnet et al., 1993; Massonnet et al., 1994; Zebker et al., 1994; Gens and van Genderen, 1996; Massonne

12、t and Feigl, 1998). This technique has the potential to detect millimeter-level target displacements along the line-of-sight (LOS) direction. The aim of the interferometric techniques is to highlight possible range variations of the target by means of a simple phase difference between two images gat

13、hered at different times. If the local reflectivity remains unchanged in time, its phase contribution disappears in the differentiation and possible range variations can then be detected. Since the wavelength of the illuminating radiation is usually a few centimeters (satellite SAR operates in the m

14、icrowave domain), even a millimetric range variation translates to a phase change that can be detected. Due to low signal-to-noise ratio values typically present in SAR phase values (never greater than 12 dB), however, the monitoring of subsidence rates of more than 6-7 cm/year is not feasible. Apar

15、t from cycle ambiguity problems, other limitations are due to temporal and geometrical decorrelation and to atmospheric artifacts. Temporal decorrelation makes interferometric measurements unachievable where the electromagnetic profiles and/or the positions of the scatterers change with time within

16、the resolution cell, so that the reflectivity phase contribution cannot be assumed constant with time. The use of short revisiting times proves to be an unsuitable solution, since very slow terrain motion (e.g., seismic creep) cannot be detected. Reflectivity variations as a function of the incidenc

17、e angle (i.e., geometrical decorrelation) further limit the number of image pairs suitable for interferometric applications, unless the change is confined to a pointwise character of the target (e.g., a corner reflector). In areas affected by either kind of decorrelation, generation of the interfero

18、gram no longer compensates the reflectivity phase contribution, and possible phase variations due to target motion cannot be highlighted. Finally, atmospheric heterogeneity creates an atmospheric phase screen superimposed on each SAR image that can seriously compromise accurate deformation monitorin

19、g. Indeed, even considering areas slightly affected by decorrelation, it may prove extremely difficult to discriminate the signal of interest from the atmospheric signature, at least using individual interferograms. The PSInSAR method, developed by TeleRilevamento Europa of the Politecnico di Milano

20、 in Italy, provides a way to overcome these limitations. Although temporal decorrelation and atmospheric disturbances still strongly affect interferogram quality, reliable deformation measurements can be obtained in a multi-image framework on a small subset of image pixels corresponding to stable ar

21、eas. These points, the permanent scatterers (PS), can be used as a natural GPS network to monitor terrain motion, by analyzing the phase history of each one. Atmospheric artifacts show a strong spatial correlation within every single SAR acquisition (Hanssen, 1998) but are uncorrelated in time. Conv

22、ersely, target motion is usually strongly correlated in time and can exhibit different degrees of spatial correlation depending on the phenomenon at hand (e.g., subsidence due to water pumping, fault displacements, localized sliding areas, collapsing buildings, etc.). Atmospheric effects can therefo

23、re be estimated and removed by combining data from long-time series of SAR images, such as those available in the ESA ERS archive, which has been gathering data since late 1991. To exploit all the available images, and improve the accuracy of the estimation, only scatterers that are not greatly affe

24、cted by temporal and geometrical decorrelation are selected. Possible stable and point targets, known as permanent scatterers (PS), are detected on the grounds of the stability of their amplitude returns (Ferretti et al., 2001): i.e., how constant their brightness or intensity remains from one SAR i

25、mage to the next. This allows pixel-by-pixel selection with no spatial averaging. Due to the high spatial correlation of the atmospheric contribution, proper sampling of the atmospheric components can be achieved with a sparse grid of measurements, provided that the PS density is high enough (greate

26、r than 4-5 PS/km2; Ferretti et al., 2000, 2001). A sufficient number of images is needed (usually more than 30) to identify PS and separate the different phase contributions. Even though precise satellite position and velocity state vectors are available for ERS satellites, orbit ambiguities and the

27、ir impact on the interferograms cannot be neglected. The estimated atmospheric phase screen is actually the sum of two contributions: atmospheric effects and fringes due to orbital errors. The latter correspond to low-order phase polynomials, however, and do not change the low wave-number character

28、of the signal to be estimated on the sparse PS grid. At the PS point, submeter accuracy elevation and millimetric terrain motion detection (due to the high phase coherence of these scatterers) can be achieved once atmospheric contributions are estimated and removed. Relative target LOS velocity can

29、be estimated with unprecedented accuracy, sometimes even better than 0.1 mm/year, due to the long time span of the data used. The higher the accuracy of the measurements, the more reliable the differentiations between models of the deformation process under study. PILOT PROJECTThe area selected for

30、the pilot project was the Tokai area in Japan, initially around Hamamatsu and then extended to cover the rest of the west side of Suruga Bay and the northern part of the Izu Peninsula (Figure 1). This area was attractive for the project for several reasons. It is one of the most intensively studied

31、areas in Japan, because it was identified as the likely location of the next major earthquake as long ago as the 1970s. It is an area of active tectonics in a complex structural setting. The principal component of the tectonic structure in the Tokai district is the collision of the northward-moving

32、Philippine Sea Plate (PSP) with Japan (Figure 1). This collisional process started about 6-7 million years ago (Niitsuma, 1982) and has been responsible for most of the seismicity of southern Japan as the PSP subducts under the overlying Eurasian Plate. In southern Honshu the situation is relatively

33、 simple and follows the conventional subduction model. The plate boundary geometry around the Tokai district is very much more complex, however (Takahashi, 1994). The subduction trench, which, as the Nankai Trough, is oriented northeast-southwest to the south of Honshu, bends to an almost northsouth

34、 orientation as the Suruga Trough in Suruga Bay. The most northerly point of the PSP is occupied by the Izu Peninsula, which is colliding with Honshu rather than being subducted beneath it. The reason for this is believed to be the relative lightness of the volcanic rocks of the Izu Peninsula, the b

35、uoyancy of which prevents subduction (Takahashi, 1994). The northward movement of the PSP in the Izu area is therefore a process of collision tectonics akin to continental collision rather than normal subduction (Niitsuma and Matsuda, 1985; Koyama, 1991). Prior to the collision of the Izu Peninsula,

36、 the Tanawa Block collided with the Honshu mainland during the Miocene, and the process by which the Tanawa Block accreted to the mainland is now being repeated with the Izu Peninsula (Amano, 1991). The process is described in detail by Takahashi (1994). The possibility, or even the probability, of

37、a large and disastrous earthquake in the Tokai district has been of concern since the area was identified as a danger area and seismic gap by Mogi (1970) and Ishibashi (1976). The subduction front from Shikoku to Hamamatsu has been identified as being partitioned into several principal fault planes

38、that appear to rupture in characteristic earthquakes. These segments were labeled A to D (from west to east) by Ando (1975), and this system was expanded by Sugiyama (1994) to include segment Z (Bungo Channel) in the west and segment E (Suruga Bay) in the east (Figure 1). Any large earthquake may rupture one of these segments entirely, or in the worst case all six at once, as apparently occurred in the 1707 Hoei earthquake (Sugiyama, 1994). For histori