Deformation characteristics of the 2016 Mw5.9 Menyuan (China) earthquake by modelling Sentinel-1A and auxiliary data

ABSTRACT

The 2016 M_w5.9 Menyuan earthquake occurred on the Qilian-Haiyuan fault system on the northeastern margin of the Tibetan Plateau. Since the seismogenic source is a blind fault, it is difficult to obtain reliable geometric parameters of the source only from the satellite data. In this paper, we used two aftershock relocation data sets to constrain the seismogenic fault, and obtained two southwest-dipping listric fault models with a strike of about 126° and different dip angles. According to the geological structures, we assumed that the dip is uniformly reduced along the width direction, and modelled the Interferometric Synthetic Aperture Radar (InSAR) data of the Menyuan event using a simulated annealing algorithm to search for the optimal fault dips. The optimal top and bottom dips are about 59° and 1°, respectively. Using the optimal fault model, we obtained the slip distribution and three-dimensional displacement fields for the 2016 M_w5.9 Menyuan earthquake. Sliding occurs mainly on the four sub-faults in the middle of the model, indicating that the rupture zone is relatively small. The displacement fields indicate that the seismogenic source is a thrust fault, and the epicentre experienced a strong northeastward compression deformation.     

The Al Hoceima (Morocco) earthquake of 24 February 2004, analysis and interpretation of data from ENVISAT ASAR and SPOT5 validated by ground-based observations

The magnitude M_w = 6.3 earthquake in Al Hoceima, Morocco of 24 February, 2004 occurred in the active plate boundary accommodating the oblique convergence between Africa and Eurasia. Three different sets of estimates of its source parameters have already been published. We try to resolve the discrepancies between them by using additional data including two remote sensing satellite systems (ENVISAT and SPOT5). Using a model with a dislocation in an elastic half-space, we constrain the source parameters. The hypothesis of two subevents on distinct faults as inferred from seismological inversions is confirmed here by adopting a cross-fault mechanism. The rupture began on a left-lateral strike-slip fault striking at N10° azimuth with 90 cm of horizontal slip and then transferred to a right-lateral strike-slip fault striking at N312° azimuth with 85 cm of horizontal slip. The first fault is at 500 m depth from the free surface and the second fault is at 3 km depth. This model is consistent with ground-based observations, including GPS, seismology, and mapped surface fissures. The pair of faults activated in 2004 appears to constitute part of a complex seismogenic structure striking NNE-SSW that separates the Rif tectonic blocks.     

The 26 December 2003, Bam,Iran earthquake: surface displacement from Envisat ASAR interferometry

On 26 December 2003 a 6.5?Mw earthquake took place near the city of Bam, in south‐eastern Iran. In order to detect the surface displacement field due to the Bam earthquake, we used satellite sensor images from the new European C‐band Envisat Advanced Synthetic Aperture Radar (ASAR). The displacement pattern from InSAR (SAR Interferometry) is well defined. It evidences a north–south high fringe gradient stream located 4?km west of the Bam fault. This result represents an important contribution to knowledge of the rupture process of this event and can be useful for improving and revising the interpretations of geological, geodetic and seismological data.     

获取张北地震同震形变场的差分干涉测量技术

基于合成孔径雷达差分干涉测量的基本原理,利用地震前后的ＥＲＳ－１／２的三景数据,获取了１９９８年１月１０日张北地震（Ｍｓ＝６．２级）的同震干涉纹图。这种差分干涉纹图能表现地震断层和形变场的同震形变特征,而且该方法无需任何的辅助信息,适用于大范围的地震监测与研究,研究表明,通过合成孔径雷达差分干涉测量技术能够获取高精的强震同震位移场数据,这是传统的地面观测方法所无法做到的。     

Seismic unloading and loading in northern central Chile as observed by differential Synthetic Aperture Radar Interferometry (D-INSAR) and GPS

Using data of two GPS campaigns as well as two ERS-1/2 Single Look Complex (SLC) datasets, we studied the distribution of co-seismic and post-earthquake surface deformation of the major (moment magnitude Mw=8.1) Antofagasta (Chile) event of 30 July 1995. Earthquake-related fault dimensions and inter-seismic surface deformation patterns were achieved by comparing results from the GPS and interferometric Synthetic Aperture Radar (SAR) investigations and by applying interpretative forward dislocation modelling. SAR data post-dating the major earthquake suggest a change in deformation directions after the earthquake within the first 50–80?km normal to the Chilean coast and show opposite signs when comparing results of the western part of the study area with those of the eastern part. We propose that this change in direction might be indicative of a superposition of relatively rapid post-seismic slip along a deeper section of the fault zone and/or distributed relaxation of the lower crust/upper mantle and seismic loading along the coastal part of the fore-arc. Assuming that the Antofagasta earthquake ruptured the entire seismogenic interface, we used the derived depth distribution of the interplate fault for the estimation of seismic moment rates. Taking into account the rate and size distribution of teleseismic events from the USGS and recently derived plate convergence rates, we constrained the size of the maximum earthquake and approximated the apparent recurrence intervals of events similar to the Antofagasta event in the area.     

Detecting changes in surface soil moisture content using differential SAR interferometry

The feasibility of measuring changes in surface soil moisture content with differential interferometric synthetic aperture radar (DInSAR) has received little attention in comparison with other active microwave techniques. In this study, multi-polarization C- and L-band DInSAR is explored as a potential tool for the measurement of changes in surface soil moisture in agricultural areas. Using 10 ascending phased array L-band SAR (PALSAR) scenes acquired by the Japanese Advanced Land Observing Satellite (ALOS) and 12 descending advanced SAR (ASAR) scenes acquired by the European ENVISAT satellite between July 2007 and November 2009, a series of 27 differential interferograms covering a common study area over southern Ireland were generated to investigate whether small-scale changes in phase are linked to measured soil moisture changes. Comparisons of observed mean surface displacement and in situ mean soil moisture change show that C-band cross-polarization pairs displayed the highest correlation coefficients over both the barley (correlation coefficient, r = 0.51, p = 0.04)- and potato crop (r = 0.81, p = 0.003)-covered fields. Current results support the hypothesis that a soil moisture phase contribution exists within differential interferograms covering agricultural areas.     

昆仑山口西8.1级大地震震后库赛湖地区多时相InSAR地表形变遥感探测

利用2003年~2007年间14景欧空局ENVISAT ASAR卫星数据,结合SRTM DEM数据,采用多时相SAR协同配准方法进行二路干涉处理,对2001年11月昆仑山口西8.1级大地震宏观震中—库赛湖地区的地表形变场进行了多时相探测。结果表明:①多时相InSAR技术能够对位于高海拔山区的东昆仑断裂带库赛湖段的地表形变实施连续监测;②震后发震断裂带的调整活动经历了一个动态变化的过程,2003年~2004年、2005年9月至2006年2月相对活跃,调整幅度较大,2004年11月至2005年9月间调整较小;③2006年~2007年发震断裂带地区活动逐渐减弱。     

Integration of radar and Landsat-derived foliage projected cover for woody regrowth mapping, Queensland, Australia

In Queensland, Australia, forest areas are discriminated from non-forest by applying a threshold (∼ 12%) to Landsat-derived Foliage Projected Cover (FPC) layers (equating to ∼ 20% canopy cover), which are produced routinely for the State. However, separation of woody regrowth following agricultural clearing cannot be undertaken with confidence, and is therefore not mapped routinely by State Agencies. Using fully polarimetric C-, L- and P-band NASA AIRSAR and Landsat FPC data for forests and agricultural land near Injune, central Queensland, we corroborate that woody regrowth dominated by Brigalow (Acacia harpophylla) cannot be discriminated using either FPC or indeed C-band data alone, because the rapid attainment of a canopy cover leads to similarities in both reflectance and backscatter with remnant forest. We also show that regrowth cannot be discriminated from non-forest areas using either L-band or P-band data alone. However, mapping can be achieved by thresholding and intersecting these layers, as regrowth is unique in supporting both a high FPC (> ∼ 12%) and C-band SAR backscatter (> ~ − 18 dB at HV polarisation) and low L-band and P-band SAR backscatter (e.g. < =∼ 14 dB at L-band HH polarisation). To provide a theoretical explanation, a wave scattering model based on that of Durden et al. [Durden, S.L., Van Zyl, J.J. & Zebker, H.A. (1989). Modelling and observation of radar polarization signature of forested areas. IEEE Trans. Geoscience and Remote Sensing, 27, 290-301.] was used to demonstrate that volume scattering from leaves and small branches in the upper canopy leads to increases in C-band backscattering (particularly HV polarisations) from regrowth, which increases proportionally with FPC. By contrast, low L-band and P-band backscatter occurs because of the lack of double bounce interactions at co-polarisations (particularly HH) and volume scattering at HV polarisation from the stems and branches, respectively, when their dimensions are smaller than the wavelength. Regrowth maps generated by applying simple thresholds to both FPC and AIRSAR L-band data showed a very close correspondence with those mapped using same-date 2.5 m Hymap data and an average 73.7% overlap with those mapped through time-series comparison of Landsat-derived land cover classifications. Regrowth mapped using Landsat-derived FPC from 1995 and JER-1 SAR data from 1994-1995 also corresponded with areas identified within the time-series classification and true colour stereo photographs for the same period. The integration of Landsat FPC and L-band SAR data is therefore expected to facilitate regrowth mapping across Queensland and other regions of Australia, particularly as Japan's Advanced Land Observing System (ALOS) Phase Arrayed L-band SAR (PALSAR), to be launched in 2006, will observe at both L-band HH and HV polarisations.     

Coseismic lateral offsets of surface rupture zone produced by the 2001 Mw 7.8 Kunlun earthquake,Tibet from the IKONOS and QuickBird imagery

Analyses and interpretations of 1m‐resolution IKONOS, and 61cm‐resolution QuickBird images reveal that the distribution of strike‐slip offsets and ground deformation characteristics of the coseismic surface rupture produced by the 2001 M _w 7.8 Kunlun earthquake occurred in the western segment of the strike‐slip Kunlun fault, northern Tibet. The 2001 coseismic strike‐slip offsets measured from IKONOS images range from 2 m up to 16.7 m, generally 3–8 m, which are generally consistent with those measured immediately in the field after the earthquake. The coseismic surface ruptures along which offsets were observed are mainly composed of a numerous of en echelon shear faults and cracks which are concentrated on a rupture zone ranging from a few meters up to ～500 m in width. The offsets measured along individual shear faults or crack are typically 2–7 m, but up to >10 m in several locations. Our results show that high resolution remote sensing imagery provides a powerful tool for measuring coseismic strike‐slip offsets and detecting the ground deformation produced by a large earthquake in the remote and high mountain Tibet region.     

Sensitivity of SAR data to post-fire forest regrowth in Mediterranean and boreal forests

Disturbed forests may need decades to reach a mature stage and optically-based vegetation indices are usually poorly suited for monitoring purposes due to the rapid saturation of the signal with increasing canopy cover. Spaceborne synthetic aperture radar (SAR) data provide an alternate monitoring approach since the backscattered microwave energy is sensitive to the vegetation structure. Images from two regions in Spain and Alaska were used to analyze SAR metrics (cross-polarized backscatter and co-polarized interferometric coherence) from regrowing forests previously affected by fire. TerraSAR-X X-band backscatter showed the lowest sensitivity to forest regrowth, with the average backscatter increasing by 1-2 dB between the most recent fire scar and the unburned forest. Increased sensitivity (around 3-4 dB) was observed for C-band Envisat Advanced Synthetic Aperture (ASAR) backscatter. The Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) L-band backscatter presented the highest dynamic range from unburned to recently burned forests (approximately 8 dB). The interferometric coherence showed low sensitivity to forest regrowth at all SAR frequencies. For Mediterranean forests, five phases of forest regrowth were discerned whereas for boreal forest, up to four different regrowth phases could be discerned with L-band SAR data. In comparison, the Normalized Difference Vegetation Index (NDVI) provided reliable differentiation only for the most recent development stages. The results obtained were consistent in both environments.     

Spatial distribution of the surface rupture zone associated with the 2001 Ms 8.1 Central Kunlun earthquake,northern Tibet,revealed by satellite remote sensing data

The Ms 8.1 Central Kunlun earthquake occurred on 14 November 2001 in northern Tibet, China. Landsat Enhanced Thematic Mapper (ETM) and Thematic Mapper (TM), Système Probatoire de l'Observation de la Terre (SPOT) High Resolution Visible (HRV) panchromatic, and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) visible and near-infrared (VNIR) images before and after the earthquake are compared to detect the spatial distribution of the surface rupture zone. A surface rupture zone at least 400 km long produced by this event can be accurately identified from satellite sensor images. It is the longest surface rupture zone among the coseismic rupture zones ever reported worldwide. In spatial terms, the surface rupture zone consists of two segments. The eastern segment extends about 350 km striking N75°-85°W (between 91°07' E and 94°55' E), and the western segment bends south-west extending more than 70 km to west of Sun Lake with a strike of N50°E-N70°W (west of 91°07' E). Analyses of satellite sensor images are well consistent with ground data obtained from the field survey. High spatial resolution satellite remote sensing techniques, therefore, provide a rapid and powerful tool to detect the co-seismic surface rupture in the remote and high mountainous Tibet area.     

Estimation of Arctic glacier motion with satellite L-band SAR data

Offset fields between pairs of JERS-1 satellite SAR data acquired in winter with 44 days time interval were employed for the estimation of Arctic glacier motion over Svalbard, Novaya Zemlya and Franz-Josef Land. The displacement maps show that the ice caps are divided into a number of clearly defined fast-flowing units with displacement larger than about 6 m in 44 days (i.e. 50 m/year). The estimated error of the JERS-1 offset tracking derived displacement is on the order of 20 m/year. Occasionally, azimuth streaks related to auroral zone ionospheric disturbances were detected and dedicated processing steps were applied to minimize their influence on the estimated motion pattern. Our analysis demonstrated that offset tracking of L-band SAR images is a robust and direct estimation technique of glacier motion. The method is particularly useful when differential SAR interferometry is limited by loss of coherence, i.e. for rapid and incoherent flow and large acquisition time intervals between the two SAR images. The JERS-1 results, obtained using SAR data acquired by a satellite operated until 1998, raise expectations of L-band SAR data from the ALOS satellite launched in early 2006.     

The 7 September 1999 Athens 5.9 Ms earthquake: Remote sensing and digital elevation model inputs towards identifying the seismic fault

The meisoseismal area of the (Ms = 5.9) earthquake of 7 September 1999 in Athens, Greece was localized in the western suburbs of the city (38.1° N, 23.7°E) where no active faulting had been mapped before. Here we show that remote sensing can provide conclusive evidence for the identification of seismogenic structure. Methods applied were: interpretation of Landsat TM images, digital overlays of field observations and aftershock distribution patterns, construction of a 20-m DEM and application of shading techniques, and comparison with fault plane solutions and dominant slip direction in striation populations. Our results imply that the earthquake source is located within the NW-SE trending valley in the Fili region across the south foothills of Mt Parnitha. The earthquake occurred along a normal fault with 110°N-130°N strike, which exhibits typical morphotectonic features of an active fault.     

Regional assessment of geohazard recovery eight years after the Mw7.9 Wenchuan earthquake: a remote-sensing investigation of the Beichuan region

The earthquake of 12 May 2008 in Wenchuan County, Sichuan Province, China, devastated the entire Beichuan region. Sitting at the intersection of the Yingxiu-Beichuan and Pengguan faults, the region experienced seismic intensities of VIII–XI on the Liedu scale. High seismic intensity combined with inherent geomorphological and climatic susceptibility to slope failure resulted in widespread co-seismic geohazards (slope failures of various types), which decimated the region. The seismic characteristics of the Wenchuan earthquake and the co-seismic geohazard distribution in relation to various conditioning factors have previously been examined in depth. However, there has been a lack of regional assessment of temporal and spatial recovery from co-seismic geohazards. Triggered by the authors’ field observation of rapid recovery, this study presents a temporal series of geohazard maps, produced by manual interpretation of satellite imagery, to present an initial assessment of changes in geohazard occurrence in the Beichuan region since the Wenchuan earthquake. In particular, landscape recovery at the co-seismic geohazard sites, as indicated by re-vegetation, is analysed based on temporal/spatial characteristics of geohazard distribution, in relation to co-seismic deformation, distance from the rupture zone and slope angle. Eight years after the Wenchuan earthquake, the overall recovery stands at 65.48%, with approximately uniform annual rates of recovery at 13.45% a year between 2009 and 2011 and 10.56% a year between 2012 and 2016. Whilst co-seismic geohazards are concentrated on the hanging wall of the seismic fault, landscape recovery is more significant in the very highly deformed zone than in other areas. Recovery has been the greatest on slopes of <50^° and peaks on 40^°–50^° slopes, where the area occupied by co-seismic geohazards was the largest. The block-slides and rock topples, which characterize high angle slopes, show much slower recovery, possibly due to greater instability and the lack of soil to support re-vegetation.     

Soil moisture limitations on monitoring boreal forest regrowth using spaceborne L-band SAR data

A study was carried out to investigate the utility of L-band SAR data for estimating aboveground biomass in sites with low levels of vegetation regrowth. Data to estimate biomass were collected from 59 sites located in fire-disturbed black spruce forests in interior Alaska. PALSAR L-band data (HH and HV polarizations) collected on two dates in the summer/fall of 2007 and one date in the summer of 2009 were used. Significant linear correlations were found between the log of aboveground biomass (range of 0.02 to 22.2 t ha^-1) and σ° (L-HH) and σ° (L-HV) for the data collected on each of the three dates, with the highest correlation found using the L-HV data collected when soil moisture was highest. Soil moisture, however, did change the correlations between L-band σ° and aboveground biomass, and the analyses suggest that the influence of soil moisture is biomass dependent. The results indicate that to use L-band SAR data for mapping aboveground biomass and monitoring forest regrowth will require development of approaches to account for the influence that variations in soil moisture have on L-band microwave backscatter, which can be particularly strong when low levels of aboveground biomass occur.     

A study on measuring surface deformation of the L’Aquila region using the StaMPS technique

This research compares two time-series interferometric synthetic aperture radar (InSAR) methods, namely persistent scatterer SAR interferometry (PS-InSAR) and small baseline subset (SBAS) to retrieve the deformation signal from pixels with different scattering characteristics. These approaches are used to estimate the surface deformation in the L’Aquila region in Central Italy where an earthquake of magnitude Mw 6.3 occurred on 6 April 2009. Fourteen Environmental Satellite (ENVISAT) C-band Advanced Synthetic Aperture Radar (ASAR) images, covering the pre-seismic, co-seismic, and post-seismic period, are used for the study. Both the approaches effectively extract measurement pixels and show a similar deformation pattern in which the north-west and south-east regions with respect to the earthquake epicentre show movement in opposite directions. The analysis has revealed that the PS-InSAR method extracted more number of measurement points (21,103 pixels) as compared to the SBAS method (4886 pixels). A comparison of velocity estimates shows that out of 833 common pixels in both the methods, about 62% (517 pixels) have the mean velocity difference below 3 mm year^?1 and nearly 66% pixels have difference below 5 mm year^?1. It is concluded that StaMPS-based PS-InSAR method performs better in terms of extracting more number of measurement pixels and in the estimation of mean line of sight (LOS) velocity as compared to SBAS method.     

PSGRN/PSCMP—a new code for calculating co- and post-seismic deformation,geoid and gravity changes based on the viscoelastic-gravitational dislocation theory

We present a new numerical code for modeling co- and post-seismic response of the Earth's crust to earthquakes. The code consists of two FORTRAN programs: the first program, PSGRN, calculates the time-dependent Green functions of a given layered viscoelastic-gravitational half-space for four fundamental dislocation sources [the strike–slip double–couple, the dip–slip double–couple, the compensated linear vertical dipole (CLVD) and the point inflation] at different depths. The results provide a data base for the second program, PSCMP, which automatically discretizes the earthquake's extended rupture area into a number of discrete point dislocations and calculates the co- and post-seismic deformation by linear superposition. According to the correspondence principle, the same propagator algorithm used in our previously published elastic modeling software, EDGRN/EDCMP, is adopted to compute the spectral Green functions. The temporal Green functions are then obtained by the fast Fourier transform extended with an anti-aliasing technique, that ensures numerical stability when calculating the post-seismic transients. Moreover, the new software considers the coupling between the deformation and the Earth's gravity field, so that its output includes not only the complete deformation field consisting of 3 displacement components, 6 stress (strain) components and 2 tilt components, but also the geoid and gravity changes. In particular, the gravity effect is treated using a new consistent approach that remedies an incorrect formulation used in many earlier publications. The performance of the software is shown by an example.     

Ground deformation due to tectonic, hydrothermal, gravity, hydrogeological, and anthropic processes in the Campania Region (Southern Italy) from Permanent Scatterers Synthetic Aperture Radar Interferometry

We apply the Permanent Scatterers Synthetic Aperture Radar Interferometry (PS-InSAR) technique to the Campania Region (Southern Italy), which includes the Southern Apennines chain and Plio-Quaternary structural depressions, with the aim to detect ground displacements at a regional scale. The study area, which extends for 13,600 km², is characterized by intense urbanization, active volcanoes (Phlegraean Fields, Vesuvius and Ischia), seismogenic structures, landslides, hydrogeological instability. PS-InSAR technique allows us to identify a set of radar benchmarks (PS) where accurate displacement measurements can be carried out. About 1.7·10⁶ PS are identified by processing Synthetic Aperture Radar (SAR) images acquired in ascending and descending orbits from 1992 to 2001 by the European Remote Sensing satellites (ERS). The PS-InSAR application at regional scale detected ground deformations ranging from + 28 to − 39 mm/yr. The calculated velocity values are consistent with the available GPS and levelling data from selected areas. We identify volcanic areas in which the deformation is mainly related to the depressurization of the local hydrothermal systems, and recognize deformations along seismogenic and aseismic NNW-SSE and NW-SE faults. The deformations localized along the Southern Apennines chain are mainly related to landslides while those occurring in the plains are due to subsidence processes induced by intensive drainage from wells, i.e. anthropic activity. The review of 9 years of SAR data shows that tectonic, volcanic/hydrothermal, gravity, and anthropic processes are responsible for the ground deformation of Campania. The proposed joint interpretation of deformation fields related to natural and anthopogenic factors provides a comprehensive view of the dynamics of the Earth’s surface.     

Co‐seismic surface ruptures produced by the 2005 Pakistan M w7.6 earthquake in the Muzaffarabad area,revealed by QuickBird imagery data

High‐resolution QuickBird imagery data have been used to analyse and detect the co‐seismic surface ruptures produced by the 2005 Pakistan M _w7.6 earthquake in the Muzaffarabad area. The analytical results and interpretations of the QuickBird images reveal that the co‐seismic surface ruptures are mostly concentrated on the pre‐existing active faults striking northwest–southeast. Most of co‐seismic surface ruptures show a deformation feature of compressional cracks having a right‐stepping echelon geometric pattern. Individual cracks vary from metre order to 1‐km in length, generally 10 to 100 m. In the northern Muzaffarabad city, an east–west striking co‐seismic surface zone of ～1 km length occurred in the jog area between two northwest–southeast striking surface rupture zones. A strong damage zone along which all buildings completely collapsed is concentrated in a deformation zone of ～60 m wide on the uplift side of the east–west striking surface rupture zone. Large‐scale landslides caused by strong ground motion are mostly constricted on the uplift side along the co‐seismic surface rupture zones. The deformation features and spatial distribution patterns of the co‐seismic surface ruptures and the ground motion direction indicate that the co‐seismic fault that triggered the 2005 Pakistan M _w 7.6 earthquake is a thrust fault with a right‐lateral slip component.     

The contribution of Seasat to ice sheet glaciology

Abstract

The suite of sensors flown onboard Seasat during 1978 has provided glaciologists with valuable tools for the study of ice masses, particularly in the polar regions. Of the sensor package, the most useful instruments for glaciology have been the radar altimeter and the synthetic aperture radar. The former has demonstrated the ability to map the surface of ice sheets in considerable detail (possibly to better than 50 mm over ice shelves) and over a very short period of time. Such maps provide the first step towards evaluating the long term mass balance of these ice masses. Such studies are of central importance to global climate modelling, investigation of the ‘greenhouse effect’ and prediction of world sea levels. Radar altimeter mapping has also provided unparalleled detail on surface topography relevant to ice dynamics investigations. The small dataset of Seasat Synthetic Aperture Radar (SAR) imagery gathered over ice masses, principally in Iceland and Greenland (there was no coverage of Antarctica), has begun to reveal useful detail of surface and near-surface phenomena such as flowlines, meltwater percolation, and snow and ice facies invaluable for glaciolog-ical reconnaissance. In particular recent studies have shown the value of a multi-sensor approach with the combination of SAR and multi-spectral imagery. It is likely that X- and C-band SARs will prove better for snow and ice discrimination than the L-band system on Seasat. The Scatterometer and Scanning multi-channel microwave radiometer instruments on Seasat have yielded data over ice masses which are still in the early stages of evaluation. Nevertheless there are strong indications of the value of these data for investigation of surface melt phenomena and temperature-accumulation patterns.