InSAR signatures surface expression of natural disasters and human activities

ABSTRACTThe use of Synthetic Aperture Radar Interferometry (InSAR) technique to the detection and measurement of surface effects caused by natural disasters and human activities has become a common issue. It is not far from truth the statement that InSAR is considered an effective tool in most of Earth Sciences. SAR is an active sensor positioned on a moving platform ex-ploiting the flight path to generate a synthetic antenna and to significatively increase the spatial resolution. InSAR is a signal processing technique based on the combination of the phase components from two SAR images relative to the scene and acquired at different time from the same geometry. The result is the interferogram, a per pixel difference of the two SAR images that provide an accurate measurement of surface displacements. Since 1991 when the European Space Agency (ESA) launched the first SAR satellite, ERS-1, scientists developed ad hoc algorithms leading to InSAR processing. In 1995 ESA launched the twin, ERS-2, thus improving its capabilities to investigate the whole Earth surface. In the following years, another C-Band (5.6 cm wavelength) SAR sensor was available, Radarsat-1. Today a number of C-, X-, and L-Band satellites daily provide an overall “microwave” picture of the Earth and allow an effective monitoring of those phenomena, natural or man made, resulting in surface effects. In particular InSAR is applied to the detection of earthquake induced surface movements, to the monitoring of active volcanoes and to investigate landslides. Concerning those deformations due to human activities, well known results concern the subsidence from fluid extraction, oil and hydrocarbon extraction, surface deformation from mining and slow long term sub-sidence related to abandoned coal mines. A key issue is the characterization of each deformation pattern and the correlation with the possible causes. The InSAR signatures may differ one each other, as for earthquakes and volcanoes, or maybe similar. In this latter case, for instance oil extraction and mining, the use of external ancillary data (in-situ data from ground survey, geological maps, seismological data) provide narrow constraints to detect the source. This approach is more effective when the possible deformation source is a subsurface nuclear test. To this aim InSAR has been used in the Nevada Test Site to investigate the short and long term surface effects induced by tests before and after 1989, using SAR images collected from 1992 to 1996. The variety of patterns reflect the different subsidence rates, which are affected by the subsurface geologic conditions for each test region. The monitoring shows fault-controlled deformation from the dissipation of residual ground-water pore-fluid pressure changes in response to past underground nuclear weapons testing.

Salvatore Stramondo Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

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InSAR concepts

Since its first application in seismology in 1992, when the surface displacement field due to the Landers earthquake was clearly detected and measured (Massonnet et al, 1993), InSAR has become a reliable technique for surface deformation studies. It is not far from truth saying InSAR revolutioned a relevant part of Earth Sciences. In fact in the last 15 years InSAR demonstrated to have unique capabilities for mapping the topography and the deformation of the Earth surface. The early studies concerning InSAR used Seasat satellite data to detect vertical motions caused by soil swelling of irrigated fields in the Imperial Valley, California. In Gabriel et al., 1989, Authors affirmed InSAR “can measure accurately extremely small changes in terrain over the large swaths associated with SAR imaging, especially since the sensor can work at night and through clouds or precipitations”. Despite at initial stage InSAR showed its promising capabilities confirmed by a huge number of applications.Two SAR satellite images can be combined to generate an interferogram. Moving along the orbit the SAR sensors can simultaneously acquire the scene whilst at different time if a single antenna is available and the satellite overpasses the investigated area twice. The former configuration is called single-pass, the latter is repeat-pass interferometry. In the repeat-pass configuration the tem-poral baseline is the time difference between acquisitions. This latter configuration is the more reliable for measuring the effects of natural disasters generating surface displacements. The interferogram is the combination of the signals S1 and S2 received in Antenna 1 and 2 as for the geometry of Figure 1a. More precisely the interferogram is the product of S1 and the complex conju-gate of S2. The interferometric phase can be schematically split into five terms, the flat Earth component , the topographic phase, the displacement phase, the atmospheric term and the error phase. Except for this latter each term contains information relevant to specific issues.

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InSAR application fields: natural events

Therefore the use of SAR Interferometry in different fieldworks is nowadays a common issue. Hy-drology, Geology, Geomorphology, Environmental pollution, Natural disasters are thus the main topics where InSAR allowed providing its contribution. The surface signature from atmospheric phase, earthquake displacement, volcanic deformations are characterized by variable patterns (Figure 2). Since Landers earthquake InSAR has been applied to a huge number of further test cases concerning the earthquake cycle, in particular the co-seismic surface deformation: Umbria-Marche 1997 (Stramondo et al., 1999), Izmit-Turkey 1999 (Feigl et al., 2002), Bam-Iran 2003 (Talebian et al., 2004; Stramondo et al., 2005). Concerning volcanic eruptions the inflation and deflation of the Mount Etna edifice has been measured (Massonnet et al., 1995). Already, a number of successful volcano deformation studies provide some unusual and promising results. These include investiga-tions of several active Alaskan volcanoes (Lu et al 1997, Lu & Freymueller 1998), rifting and volca-nism on Iceland (Sigmundsson et al 1997, Vadon & Sigmundsson 1997), localized inflation on Izu Peninsula, Japan (Fujiwara et al 1998a), dike intrusions on Fernandina volcano, Galapagos (Jo´nsson et al 1999) and Piton de la Fournaise volcano on La Reunion island (Sigmundsson et al 1999), and deformation of the active calderas of Yellowstone (Wicks et al 1998) and Long Valley (Thatcher & Massonnet 1997).

InSAR application fields: human induced events

Human induced deformation have been studied also, as for oil extraction (Figure 3), hydrocarbon extraction (Figure 4) and coal excavation (Figure 5), mining, nuclear. Concerning human induced movements, InSAR has been successfully applied to detect and measure a complex subsidence pattern associated with hydrocarbon extraction (Figure 4) where active faults play also a vital role in subsidence. Surface movement caused by mining is a very dynamic process with high spatial and temporal variabil-ity. Subsidence maps of different time intervals clearly indicate the prog-ress in the sub-surface coal excavation. In particular the InSAR data have been used to investigate the ground movements associated with aban-doned coal mines. Miners made large shafts and left behind pillars of coal to support the roof that weakened, leading to slow downward movement of the roof until it collapsed. (Figure 6) InSAR monitoring shows fault-controlled deformation from the dissipa-tion of residual ground-water pore-fluid pressure changes (Figure 7) in response to past underground nuclear weapons testing. Coseismic surface-deformation signals from three underground nuclear tests (white dots in Figure 8) conducted at the Nevada Test Site in 1992 were collected by InSAR over a 14-month time span. The top images show nearby craters (red dots) from other underground tests prior to 1992. The color interferograms derived from the InSAR data (middle row) show sur-face displacement that occurred both during and following the explo-sions. The profile plots (bottom row) show near-vertical displacement (left scale) and surface topography (right scale). The variety of patterns reflect the different subsidence rates, which are affected by the subsurface geologic conditions for each test region. The SAR images for interferogram in Figure 9 were collected from 1992 to 1996. The stars indicate tests conducted after 1989, and the circles indi-cate tests prior to 1989. The triangles indicate the largest or deepest tests conducted from the late 1960s to mid-1970s.

Figure 1a

Figure 1b

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Figure 3 ( from USGS)

Figure 4 ( from USGS)

Figure 5Coal excavation site in the Ruhrgebiet. The colors indicate the deformation along the SAR observation direction. One color cycle corresponds to a displacement of 1 cm in 35 days. The image width is 4 km.

Figure 6 (from Alberta Geological Survey)

Figure 7 (R. Laczniak, U.S. Geological Survey, written commun., 2003)

Figure 8 (Science&Technology Review)Bibliografia

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