FINAL REPORT

ESA PROJECT AO-3 201

MONITORING OF NISSYROS VOLCANO (GREECE) USING

ERS REPEAT-PASS INTERFEROMETRY

Dr Athanassios Ganas

ATHENS May 8, 2002

2

Principal Investigator’s Addresses:

Dr Athanassios Ganas,

Institute of Geodynamics, Lofos Nymfon, Thission, PO Box 20048,

Athens 118 10, Greece.

Tel: +30-10-3490186 / 195

Fax: +30-10-3490180

e-mail: aganas@gein.noa.gr

http://www.gein.noa.gr/Ganas/resume.html

Table of Contents

ABSTRACT ................................................................................................................................. 3 A. INTRODUCTION ...................................................................................................................... 4

Principles of Interferometry .................................................................................................... 7 B. DIFFERENTIAL INTERFEROMETRIC SAR METHODOLOGY ................................................... 9 C. DIFFERENTIAL INTERFEROMETRY RESULTS.......................................................................13 D. DIFFERENTIAL SAR: DISCUSSION-CONCLUSIONS ...............................................................15 E. DEM EXTRACTION FROM TANDEM ERS PAIRS ....................................................................18 ACKNOWLEDGEMENTS ............................................................................................................24 REFERENCES............................................................................................................................25 APPENDIX A. .............................................................................................................................28 APPENDIX B. .............................................................................................................................30

ERS1/ERS1........................................................................................................................30 ERS2/ERS2........................................................................................................................30 ERS1/ERS2........................................................................................................................31 ERS2/ERS1........................................................................................................................31

APPENDIX C. .............................................................................................................................33 APPENDIX D. .............................................................................................................................34

3

ABSTRACT

Nissyros is an active volcano in the south Aegean Sea, Greece. Deformation

maps at a resolution of 20 metres were produced using ERS C-band SAR image

pairs acquired in 1995/96, over a time period of 279 days. The interferograms show

one, well-developed fringe trending E-W and a second less developed fringe in the

northwestern part of the island. This is the earlier deformation signal ever mapped

on Nissyros. These results clearly indicate significant off-caldera ground deformation

which is interpreted as indicating a minor inflationary phase of the volcanic centre of

Yali - Nissyros. Surface uplift occurred in Mandraki area (NW Nissyros), and the

affected area coincides with a particular geologic discontinuity, a normal fault that

moved during the August 1997 5.5 Ms earthquake. Derived uplift rates are supported

by the results of geodetic measurements (GPS) during the years 1997-2001.

Successful extraction of interferometric DEMs from 1995 tandem pairs is also

reported.

4

A. INTRODUCTION

The Nissyros volcano is located at the southeastern end of the South Aegean

Volcanic Arc at about 36.5 degrees North and 27.1 degrees East (Georgalas, 1962;

Figure 1). Although the last volcanic activity on Nissyros dates back at least 25000

years, the present geodynamic activity encompasses high seismic unrest,

widespread fumarolic activity, and recent gas and hydrothermal explosions

(Papadopoulos et al., 1998; Dietrich et al., 1998). Earthquakes and steam blasts

accompanied the most recent hydrothermal eruptions in 1871-1873 and 1887.

Mudflows and hydrothermal vapors rich in CO2 and H2S were emitted from fracture

zones that cut the caldera of the volcano and extend towards NNW through the

vicinity of the village of Mandraki. The seismic unrest of the years 1995-1997

prompted us to initiate an interferometric study of the ground deformation using

spaceborne data from the European satellites ERS1 and ERS2. Differential

Interferometry has been an effective tool in mapping ground deformation of active

volcanoes (e.g. Massonnet et al., 1995; Lanari et al., 1998; Lu et al., 2000).

We used ERS data originated in the I-PAF facility (Fucino, Italy) in CEOS-

SLCI (Single Look Complex) format with dimensions 4900 pixels (range) by

approximately 26500 rows (azimuth) resulting in ground resolution 7.9 metres in

range - 3.98 metres in azimuth. The orbital characteristics of the data are shown in

Table 1. The Pulse Repetition Frequency (PRF), a critical factor in data processing

(Gens, 2000) for both satellites is 1679.902 Hz. The radar wavelength is 5.65 cm

(frequency 5.3 GHz) and the antenna depression angle is 67 degrees during data

5

acquisitions. The ERS repeat interval is 35 days except for the tandem missions

(see section E below). The radar data were processed by the software EVInSAR,

(1.2.1 version) made by ATLANTIS (van der Kooij, et al., 1997) on a Pentium III

computer with 392 Mb RAM. EVInSAR1 requires large amounts of RAM to run so 1

Gb of virtual memory had to be reserved. The software can be operated interactively

through a Graphical User Interface (GUI) or on a command line to allow batch

processing. Most data processing was performed through the GUI. Operating

system environments used were Windows NT and Windows 2000.

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Table 1. The ERS SAR scenes used in the project. ID refers to CD production by

the data provider. GS is ground station, PAF is Processing-Archiving Facility, FS is

Fucino, Italy. The tools used to select the orbits below are shown in Appendix A. All

orbits considered for InSAR are shown in Appendix B.

ID Satellite Sensor Prod GS PAF Orbit Frame Date 305066 ERS1 SAR SLCI FS IP 20135 2871 22051995 305067 ERS1 SAR SLCI FS IP 20636 2871 26061995 305068 ERS1 SAR SLCI FS IP 21137 2871 31071995 305069 ERS1 SAR SLCI FS IP 21638 2871 04091995 305070 ERS1 SAR SLCI FS IP 23642 2871 22011996 305071 ERS1 SAR SLCI FS IP 24644 2871 01041996 305072 ERS2 SAR SLCI FS IP 963 2871 27061995 305073 ERS2 SAR SLCI FS IP 1464 2871 01081995 305074 ERS2 SAR SLCI FS IP 1965 2871 05091995 305075 ERS2 SAR SLCI FS IP 2967 2871 14111995 305076 ERS2 SAR SLCI FS IP 5973 2871 11061996 305077 ERS2 SAR SLCI FS IP 6975 2871 20081996

1 The software is described through a series of on-line publications in: http://www.atlsci.com/eoserv.html

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Figure 1. Location map of the Nissyros volcano, Aegean Sea. Inset in the left

shows the study area. The image to the right is a black-and-white space photograph

of the Space Shuttle. The image shows in 2D the circular geometry of Nissyros and

the caldera in the middle. Nissyros is imaged by the ERS satellites in the

descending mode, i.e. the satellites travel approximately from north to south and

look to the west, inclined 23° down from vertical.

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Principles of Interferometry

The SAR imaging system uses a long, straight antenna mounted on a satellite

platform with its long-axis parallel to the orbit path. The antenna emits pulses of

electromagnetic energy downward to the surface of the planet, at right angles to the

orbital path. The radar backscatter carries amplitude and a phase. Phase refers to

the starting point of the wave relative to some time zero. Thus, the phases of images

with a difference of time (i.e. one SAR acquires images at two separate orbits) can

be compared after careful image registration. The resulting phase difference is a

new image called interferogram. Negative phase values indicate movement towards

the sensor, positive phase values away from the sensor. One phase cycle equals 2π

radians or 28 mm of displacement (C band) and defines a fringe. The overall fringe

pattern contains all the information on relative geometry between the two

overpasses, provided a number of “noisy” contributions to the phase are accounted

for. In particular, to map ground movements along the line of sight between the SAR

and the earth’s surface (Figure 2) we need to eliminate the topographic, ionospheric

and orbital contributions to the phase signal.

In addition, the organized fringe pattern can vanish if phase coherence

between the two SAR acquisitions is lost. This is most commonly observed on

“dynamic” surfaces, such as the sea or agricultural fields. Phase coherence is a

measure of similarity between phase information acquired at different times.

Coherence depends on sensor parameters (wavelength, noise), image geometry

(baseline and local incidence angle) and target characteristics. For instance, in

tandem acquisitions forests show low coherence because the target scatterers

8

(leafs, etc) change their moisture level or facing directions, whereas bare soil is

stable (see Figure 6 below). The surface with the greatest variability is water. In

general, the longer the time interval between phase acquisitions the less coherent

the phase returns are. This is called temporal decorrelation.

___________________________________________________________________

Figure 2. The imaging geometry for SAR interferometry. A1 and A2 represent two

antennas viewing the same surface simultaneously, or a single antenna viewing the

same surface on two separate passes. From trigonometry we can derive z (y) = h -

?cos? where h is platform height and ? is the look (incidence) angle of the radar. B

is perpendicular baseline between A1 and A2, ρ is range (distance) to ground.

9

B. DIFFERENTIAL INTERFEROMETRIC SAR METHODOLOGY

The ODISSEO web Server of ESA2 (was searched for the selection of

suitable pairs for InSAR. The search was completed using the DESCW software

available from the Eurimage Web Site (Appendix A). Ten (10) images were selected

from the baseline search (Appendix B) and were delivered as SLCI products (Table

1). Two optimal scenes were selected for further analysis, namely orbits 5973 (11

June 1996) and 1965 (5 September 1995) of the ERS 2 satellite (Table 2). These

scenes had temporal separation of 279 days which span the time period of

increased seismicity rates while preserving adequate phase coherence. Locally

there is a high level of correlation between the two ERS scenes; however, over most

of the island the coherence remains at relatively low levels. The orbital baselines

(B)3 were: perpendicular B = -79.9 m, parallel B = -27.7 m. The orbital parameters of

the two scenes were refined by more accurate estimations of the state vector

positions as calculated by the DEOS4 (Delft Orbits; Scharroo et al., 1998).

We used the 2-pass method (Massonnet et al., 1993) by eliminating

topographic phase using an external DEM with a resolution of 2 metres. First we

produced a 20-m DEM (see Appendix C) that represents overall topography

satisfactorily, however, it was found to locally introduce many artifacts in spaces

between contour intervals. The finer DEM was produced by digitizing of the 1:5000

topographic maps of Nissyros provided by HMGS (Hellenic Military Geographical

Service). The DEM was edited and corrected with field measurements by the

2 http://jupiter.esrin.esa.it/ 3 Baseline is the spatial displacement of the antenna between the two overpasses. 4 http://www.deos.tudelft.nl/ers/precorbs/

10

Institute of Cartography, ETHZ at a final resolution of two (2) metres (Vassilopoulou

and Hurni, 2001). This good quality had two implications: a) it provided ground

control for the co-registration between master SAR scene - slant range projected

DEM between 1-4 m and b) brought down the magnitude of the topographic artifact

to 4 m / 117 m (altitude of ambiguity5 ha) or 0.034 of a cycle or 1mm using Pair 1. All

datasets were reprojected to the WGS84 datum, UTM zone 35.

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Table 2. Best Interferometric Pair Validation results. The first stage of SAR

image processing after data input. Spectral overlap between two SAR scenes

indicates amount of noise introduced to the phase difference of the two

interferograms because of their bandwidths centered at different frequencies.

SAR Image Sensor-Orbit Date Spatial

Overlap

Range

Spectral

Azimuth

Spectral

Pair 1 – Master ERS 2 – 5973 11-6-1996

Pair 1 – Slave ERS 2 - 1965 5-9-1995

96.91 % 92.71% 98.99%

5 This is the height sensitivity (in metres) of the pair or ∆h/∆φ = {2 * B * 1 / λ * ρ * sinθ)-1 See Figure 2 for definitions.

11

Figure 3. The interferogram of Nissyros volcano spanning the period between orbits

5973 (11 June 1996) and 1965 (5 September 1995) of the ERS 2 satellite. The

sensitivity to topography of this pair (ha) is 114 m. Note the E-W strike of the first

fringe (black arrow - blue to red colour 28 mm change), that is across topography. A

second fringe can be seen at about 3 km to the North near the Mandraki – Loutra

coastal area. The interferogram depicts line of sight displacements between the two

SAR acquisitions and is geocoded.

SAR data processing starts with image co-registration analysis. This includes:

a) Resampling of DEM to master SAR image

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b) Processing to distort the re-sampled DEM into the master SAR space

creating foreshortening and layover distortions

c) Fine coregistration of DEM to master SAR image using ground control

points at well-defined bright pixels (spots) and

d) Calculation and subtraction of topographic phase from the interferogram

prior to interferogram enhancement.

e) Calculate coherence image

f) Unwrap the phase

g) Convert unwrapped phase to height

h) Correct for terrain distortions and geocode

In step c) an advanced SAR image backscatter simulator is used to generate a

simulated SAR image for use in fine coregistration of DEM to master. For example,

the fine coregistration for the Pair 1 was done using 23 ground control points with

root mean square errors along x=0.54 and along y=0.66 pixels. The “flat earth”

phase is removed during interferogram generation. The phase difference (Figure 3)

is unwrapped using an iterative disk masking algorithm with controlled error

propagation and error correction. The algorithm uses multiple tiling and seeding

techniques, providing automatic masking of low coherence areas and automatic

connection of residues. An unwrapping, interactive editor was also used to correct

remaining errors. Data resampling to 20 m (azimuth) × 20m (range) is applied to

reduce the amount of data to be phase unwrapped and give square dimensioned

pixels in ground range.

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C. DIFFERENTIAL INTERFEROMETRY RESULTS

The interferometric phase is shown in Figure 3. Because of the cyclical nature

of phase, colour is used to represent it. A value of zero is represented by red and as

the phase increases it is represented by smooth transition from red to green to blue

and then back to red as it reaches 2π (i.e. one cycle). Independent phase effects,

which may be present in such an interferogram, are due to topography, groundwater

content, surface change and atmospheric anomalies. Because the baseline of Pair 1

is relatively small (80 m) for ERS the phase is not highly sensitive to the topography

(one cycle of phase corresponds to about 117 m in height variation). Despite that the

maximum height variation in the region is about 700 m the contribution to the phase

due to topography was considered negligible relative to other effects, such as

deformation, in the interferogram. This is because topography follows a conic pattern

(Figure 1 right; Appendix C) whereas the main fringe trends approximately east-

west. Other possible causes such as atmospheric perturbation or groundwater table

fluctuations are also excluded because of the geometry of the main fringe.

Therefore, there is a significant indication of surface deformation (uplift) at the study

area. A nearly circular phase pattern can be seen in the top near the middle of the

scene. Considering the baseline for this InSAR pair this pattern cannot be accounted

for by the topography known to be present.

By unwrapping the phase and applying a straightforward conversion a height

change map is produced. Note that this conversion assumes that the phase effect

measured is due to height change only (no lateral movement). It is important to

recognize that the SAR measures only the component of surface motion along the

14

viewing direction of the antenna. The actual surface motion must be inferred from

additional knowledge about the area of interest. In the case of the Nissyros volcano,

given the relatively conic topography (Figure 4) and the nature of the hydrothermal

activities, it is assumed that the surface motion is height change.

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Figure 4. Field photograph of the Nissyros caldera showing the Stefanos crater

(circular feature at the bottom) and the steep slopes of the Profitis Elias where

layover occurs in ERS SAR images during daytime, descending orbits. Lithology is

mainly rhyodacites. The highest peak is 698 m high. The Stefanos crater is at 120 m

above mean sea level. View to the west.

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D. DIFFERENTIAL SAR: DISCUSSION-CONCLUSIONS

The nearly circular feature that can be clearly seen in the Interferogram

(Figure 3) is most likely due to uplift. The uplift is due to volcanic activity and not to

tectonic uplift by many earthquakes (Figure 5) occurred around Nissyros, as none of

them had enough magnitude to generate surface deformation (see Appendix D).

Note that no height change measurements were recovered for most of the bottom of

the caldera. This was because the phase unwrapping failed due to a) prohibitively

low coherence as the central-northern areas are primarily agricultural fields and b)

geometrical effects (layover of the N-S slopes, i.e. perpendicular to the radar beam

during descending orbits). The above results were compared to the pattern of

deformation measured by local GPS surveys from 1997 onwards. During the last

four years, a series of GPS measurements were made of the heights of 14 fixed

points distributed throughout Nissyros (Lagios et al., 1998; Lagios, 2000). Analysis of

the results provided a preliminary estimate of surface uplift with millimeter accuracy.

Comparison of the 1997-1999 survey results to those of the 1995/96 InSAR

deformation map is favourable, as the GPS data indicate up to 70 mm of uplift.

Therefore, areas affected by uplift are clearly evident from the 1995-1996

interferogram.

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Figure 5. Seismicity map of the Nissyros area (Data from the NOA National

Network). Notice the 2 strong earthquakes of Magnitude 5 near the west coast.

The cause of the 1995-1996 uplift observed by differential InSAR is most

probably upward magma movement at the eastern edge of the south Aegean

volcanic arc. This movement was accompanied by extensive seismic activity (32

events - Figure 5) which includes two (2) intermediate-depth earthquakes of

magnitude 5.5 Ms on 30/11/1995 and 12/4/1996, respectively. This correlation

between volcanic and seismic activity has been recently established worldwide by

Marzocchi (2002). In the Nissyros case, the triggering mechanism seems to be

stress transfer that increased the pressure on the magma chamber which penetrates

17

through the upper crust up to depths of 1-2 km (Nikolova et al., 2002). However,

most of the earthquakes are shallow (see Appendix D) and are concentrated along a

NE-WS to E-W general direction which suggests that they occurred within the Kos

neotectonic graben (Papanikolaou and Nomikou, 2001). It is worth pointing out that

the area of greatest uplift seen in Figure 3 is the northwestern part of the island

(which probably includes a significant offshore part) where the destructive, August

1997 earthquake sequence occurred.

Our master SAR scene (11-6-1996) was also used by Parcharidis and Lagios

(2001) in their study of the surface deformation of Nissyros. The 3-5 cm vertical

deformation mapped by our interferogram (Figure 3) was followed during the years

1996-1999 by mostly horizontal movements to the east and southwest, respectively.

This change in the mode of deformation could be due to the activation of surface

fault zones that cross-cut the volcano.

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E. DEM EXTRACTION FROM TANDEM ERS PAIRS

Interferometric SAR can be also used to extract topography with reasonable

errors (10-30 m; Kenyi and Raggam, 1997). To test this capability we produced an

interferometric DEM from a tandem ERS pair, orbit 21137 of ERS 1 (31 July 1995;

master image) and orbit 1464 of ERS 2 (1 August 1995; slave image). Their scene

parameters are reported in Tables 4 and 5, respectively. The spatial overlap of this

pair was 98.3%, the spectral overlap in range is 96.3% and in azimuth 76.2%. The

baselines were relatively small (perpendicular B = 40 m, parallel B = 26 m) which

implied a rather long height difference per one phase cycle (2π) or 239.2 m.

Practically, one fringe represents about 240 metres in elevation. The master to

slave SAR image registration (along range) was achieved using 37 valid tiepoints

with an RMS error of 0.09 pixels.

On the other hand, the coherence image (Figure 6) shows that a good

correlation exists which was not the case for other tandem pairs. The wrapped

phase image is shown in Figure 7 (bottom) in slant-range view. The interferogram is

quite clear with a well developed pattern of fringes except in the layover region,

inside the caldera. Phase unwrapping was done using the iterative disk masking

method with the coherence threshold set to 0.15. The extracted DEM is shown in

Figure 7 (top) with intensity values increasing towards the highest elevations. The

model has a pixel size of 20 metres. For comparison, a digitized DEM (20-m) of

Nissyros is shown in Appendix C.

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To check the vertical accuracy of the DEM we measured heights from 5 ground

control points against those of the tandem DEM (Table 3). The points are located at

topographic highs on the 1:10,000 map (see Figure 6 for locations). The RMS error

of the height difference is 15.6 metres which is satisfactory given the rough terrain of

Nissyros. Given that the orbital parameters used in processing were those of CEOS

and not any refined ones we estimate that our RMS error is an overestimation.

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Table 3. Quality control of the ERS tandem DEM 21137-1464. Number of

elevation control points: 5. The location of points is shown in Figure 7 (top). All

figures refer to WGS84 datum, UTM projection zone 35.

-----------------------------------------------------------------------------------------------------------------------------------------------

Point Feature GCP GCP GCP GCP DEM GCP Height Error

Row Col Lat Long Height Height (DEM Height - GCP Height)

(degrees) (degrees) (m) (m) (m)

-----------------------------------------------------------------------------------------------------------

1(I) Yes 1014.83 1802.57 36.5781 27.1681 136.0437 120.0000 16.0437

2 Yes 985.96 1880.28 36.5832 27.1858 578.5669 583.0000 -4.4331

3(I) Yes 862.27 1807.42 36.6055 27.1694 436.7401 445.0000 -8.2599

4(I) Yes 888.00 1767.84 36.6009 27.1604 181.5031 161.0000 20.5031

5(I) Yes 940.75 1696.65 36.5914 27.1448 403.5529 425.0000 -21.4471

-----------------------------------------------------------------------------------------------------------

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Figure 6. Phase coherence image of the ERS tandem DEM 21137-1464. This

product is an 8-bit image (see side scale), therefore a perfectly coherent value would

be 255 (bright) while a zero coherence would be 0 (dark). Image is shown in slant-

range and is flipped vertically. Notice that coherence is lost on water and on steep

slopes facing the radar beam.

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Figure 7. Interferometric DEM of Nissyros (top) and wrapped phase (bottom). The

GUI of EVInSAR is also shown. Bottom image is slant-range and flipped vertically.

The colour wheel defines one fringe or approximately 250 m of topography.

22

Table 4. Tandem pair 31 July 1995 (ERS-1) scene parameters report. Read_CEOS_data_from_KV = 0 CEOS_DATA_SET_SUMMARY. Processing_system_identifier = VMP CEOS_DATA_SET_SUMMARY. Processing_facility_identifier = I-PAF CEOS_DATA_SET_SUMMARY. pixel_spacing = 7.904000 CEOS_DATA_SET_SUMMARY. Pulse_repetition_Frequency = 1679.902000 CEOS_DATA_SET_SUMMARY. Line_spacing = 3.981000 CEOS_DATA_SET_SUMMARY. Scene_centre_time = 19950731085317849 CEOS_DATA_SET_SUMMARY. Total_processor_bandwidth_in_range = 15.550000 CEOS_DATA_SET_SUMMARY. Total_processor_bandwidth_in_azimuth = 1378.000000 CEOS_DATA_SET_SUMMARY. Range_sampling_rate = 18.962468 CEOS_DATA_SET_SUMMARY. Nominal_resolution_in_range = 10.000000 CEOS_DATA_SET_SUMMARY. Nominal_resolution_in_azimuth = 5.000000 CEOS_DATA_SET_SUMMARY.Zero_doppler_range_time_of_first_valid_range_pixel = 5.541193 CEOS_DATA_SET_SUMMARY.Zero_doppler_azimuth_time_of_first_azimuth_pixel = 31-JUL-1995 08:53:09.939 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_centroid_const = 435.185000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_centroid_linear = -330058.0000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_centroid_quad = 851000000.0000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_const = -2152.487000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_linear = 386920.738000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_quad = -10000000.000000 CEOS_PLATFORM_POSITION_DATA. Year_of_data_point = 1995 CEOS_PLATFORM_POSITION_DATA. Month_of_data_point = 7 CEOS_PLATFORM_POSITION_DATA. Day_of_data_point = 31 CEOS_PLATFORM_POSITION_DATA. Number_of_data_point = 5 CEOS_PLATFORM_POSITION_DATA. Seconds_of_day_of_data = 31989.401000 CEOS_PLATFORM_POSITION_DATA. Time_interval_between_data_points = 4.167000 CEOS_PLATFORM_POSITION_DATA.Position_vector_X0 = 4971887.630000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y0 = 2939271.170000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z0 = 4230350.890000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X0 = 4652.055850 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y0 = 663.972290 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z0 = -5911.957880 CEOS_PLATFORM_POSITION_DATA.Position_vector_X1 = 4991226.450000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y1 = 2942004.420000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z1 = 4205676.430000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X1 = 4630.038080 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y1 = 647.919350 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z1 = -5931.081180 CEOS_PLATFORM_POSITION_DATA.Position_vector_X2 = 5010473.330000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y2 = 2944670.780000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z2 = 4180922.510000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X2 = 4607.923550 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y2 = 631.867550 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z2 = -5950.092610 CEOS_PLATFORM_POSITION_DATA.Position_vector_X3 = 5029627.860000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y3 = 2947270.260000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z3 = 4156089.610000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X3 = 4585.712660 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y3 = 615.817260 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z3 = -5968.991780 CEOS_PLATFORM_POSITION_DATA.Position_vector_X4 = 5048689.640000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y4 = 2949802.860000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z4 = 4131178.190000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X4 = 4563.405840 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y4 = 599.768840 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z4 = -5987.778350 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_left_border_pixels_per_line = 0 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_right_border_pixels_per_line = 0 CEOS_IMOP_FILE_DESCRIPTOR.Total_number_of_data_groups_per_line_per_SAR_channel = 4900 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_lines_per_data_set = 26578

23

Table 5. Tandem pair 1 August 1995 (ERS-2) scene parameters report. Read_CEOS_data_from_KV = 0 CEOS_DATA_SET_SUMMARY. Processing_system_identifier = VMP CEOS_DATA_SET_SUMMARY. Processing_facility_identifier = I-PAF CEOS_DATA_SET_SUMMARY. pixel_spacing = 7.904000 CEOS_DATA_SET_SUMMARY. Pulse_repetition_Frequency = 1679.902000 CEOS_DATA_SET_SUMMARY. Line_spacing = 3.979000 CEOS_DATA_SET_SUMMARY. Scene_centre_time = 19950801085320490 CEOS_DATA_SET_SUMMARY. Total_processor_bandwidth_in_range = 15.550000 CEOS_DATA_SET_SUMMARY. Total_processor_bandwidth_in_azimuth = 1378.000000 CEOS_DATA_SET_SUMMARY. Range_sampling_rate = 18.962468 CEOS_DATA_SET_SUMMARY. Nominal_resolution_in_range = 10.000000 CEOS_DATA_SET_SUMMARY. Nominal_resolution_in_azimuth = 5.000000 CEOS_DATA_SET_SUMMARY.Zero_doppler_range_time_of_first_valid_range_pixel = 5.582959 CEOS_DATA_SET_SUMMARY.Zero_doppler_azimuth_time_of_first_azimuth_pixel = 01-AUG-1995 08:53:12.637 CEOS_DATA_SET_SUMMARY.Cross_track_Doppler_frequency_centroid_const = 128.440000 CEOS_DATA_SET_SUMMARY.Cross_track_Doppler_frequency_centroid_linear = 6644.000000 CEOS_DATA_SET_SUMMARY.Cross_track_Doppler_frequency_centroid_quad = -46000000.0000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_const = -2135.636000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_linear = 380870.908000 CEOS_DATA_SET_SUMMARY. Cross_track_Doppler_frequency_rate_quad = -10000000.000000 CEOS_PLATFORM_POSITION_DATA. Year_of_data_point = 1995 CEOS_PLATFORM_POSITION_DATA. Month_of_data_point = 8 CEOS_PLATFORM_POSITION_DATA. Day_of_data_point = 1 CEOS_PLATFORM_POSITION_DATA. Number_of_data_point = 5 CEOS_PLATFORM_POSITION_DATA. Seconds_of_day_of_data = 31992.236000 CEOS_PLATFORM_POSITION_DATA. Time_interval_between_data_points = 4.167000 CEOS_PLATFORM_POSITION_DATA.Position_vector_X0 = 4971865.450000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y0 = 2939315.880000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z0 = 4230363.460000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X0 = 4652.031380 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y0 = 664.045550 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z0 = -5911.952140 CEOS_PLATFORM_POSITION_DATA.Position_vector_X1 = 4991204.180000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y1 = 2942049.440000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z1 = 4205689.020000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X1 = 4630.013850 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y1 = 647.992480 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z1 = -5931.075420 CEOS_PLATFORM_POSITION_DATA.Position_vector_X2 = 5010450.950000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y2 = 2944716.100000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z2 = 4180935.120000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X2 = 4607.899560 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y2 = 631.940550 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z2 = -5950.086810 CEOS_PLATFORM_POSITION_DATA.Position_vector_X3 = 5029605.380000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y3 = 2947315.880000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z3 = 4156102.240000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X3 = 4585.688920 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y3 = 615.890130 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z3 = -5968.985960 CEOS_PLATFORM_POSITION_DATA.Position_vector_X4 = 5048667.060000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Y4 = 2949848.790000 CEOS_PLATFORM_POSITION_DATA.Position_vector_Z4 = 4131190.850000 CEOS_PLATFORM_POSITION_DATA.velocity_vector_X4 = 4563.382340 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Y4 = 599.841570 CEOS_PLATFORM_POSITION_DATA.velocity_vector_Z4 = -5987.772510 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_left_border_pixels_per_line = 0 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_right_border_pixels_per_line = 0 CEOS_IMOP_FILE_DESCRIPTOR.Total_number_of_data_groups_per_line_per_SAR_channel = 4900 CEOS_IMOP_FILE_DESCRIPTOR.Number_of_lines_per_data_set = 26386

24

ACKNOWLEDGEMENTS

The ERS data were provided to the author free of charge as part of AO3

series of ESA contracts awarded during 1998 (Proposal AO-3 201). I am grateful to

the ERS Help desk at ESRIN and the ATLANTIS Support team in Ottawa, CA for

valuable help. I am indebted to Prof. Evangelos Lagios (NKUA, Athens), Prof. Volker

(Wumme) Dietrich (ETH Zurich) and Dr George Stavrakakis, Director of NOAGI, for

their continuous support. I also thank Alexis Rigo (IPGP), Alain Arnaud (ESRIN),

Prof Geoff Wadge (ESSC), James Ikkers (ATLANTIS), George Vouyoukalakis

(IGME), Pepi Vassilopoulou (NKUA), Lorenz Hurni (ETH), Isaak Parcharidis (NKUA),

Vassilis Karastathis (NOAGI) and Vassilis Sakkas (NKUA) for many useful

discussions. Mr. Yannis Cobopoulos and IIS SA provided resources and space

during the first stages of this project. The production of the fine DEM was funded by

the GEOWARN project (IST 1999 - 12310). The InSAR Processor is 1996 Atlantis

Scientific.

25

REFERENCES

Dietrich, V. J., Kipfer, R. and Schwandner, F. 1998. Mantle-derived noble gases in the South Aegean volcanic

arc: Indicators for incipient magmatic activity and deep crustal movements. Newsletter of the European Centre

on Prevention and Forecasting of Earthquakes (Council of Europe) 2 (Sept. 1998), 28-32.

Gens, R., 2000. The influence of input parameters on SAR interferometric processing and its implication on the

calibration of SAR interferometric data. Int. J. Remote Sensing, 21 (8), 1767-1771.

Georgalas, G.C., 1962, Catalogue of the active volcanoes of the world including solfatara fields; Part XII Greece:

International Association of Volcanology, Rome, Italy, 40 p.

Kenyi, L., and Raggam, H., 1997. Accuracy assessment of interferometrically derived DTMs. In Proceedings of

the “Fringe 96” Workshop on ERS SAR Interferometry, Zurich, 30 September – 2 October 1996 (ESA SP-406).

Lagios E., 2000. Intense crustal deformation rates on Nissyros Island (Greece), deduced from GPS studies, may

foreshadow a forthcoming volcanic event. Proc. 2nd Intern. Conf. On Earthquake Hazard and Seismic Risk

Reduction, Sept. 15-21,1998, Yerevan, Armenia, S. Balassanian et al. (Eds.), Kluwer Academic Publishers, 249-

259.

Lagios E., Chailas S., Giannopoulos J. & Sotiropoulos P., 1998. Surveillance of Nissyros Volcano: Establishment

and remeasurement of Rn & GPS networks. Proc. 8th Intern. Congress Geol. Soc. Greece, Patras, May 27-29,

Vol. 32/4, 215-227.

Lanari, R., Lundgren, P., and Sansosti, E., 1998. Dynamic deformation of Etna volcano observed by satellite

radar interferometry. Geophysical Research Letters, 25 (10), 1541-1544.

26

Lu, Z., Mann, D., Freymueller, T., and Meyer, D. J., 2000. Synthetic aperture radar interferometry of Okmok

volcano, Alaska: Radar observations. Journal of Geophysical Research, 10791-10806.

Marzocchi, W., 2002. Remote reismic influence on large explosive eruptions. Journal of Geophysical Research,

107 (B1), 1029.

Massonnet, D., Rossi, M., Carmona, C., Adragna, F., Peltzer, G., Feigl, K., and Rabaute, T., 1993. The

displacement field of the Landers earthquake mapped by radar Interferometry. Nature, 364, 138-142.

Massonnet, D., Briole, P., and Arnaud, A., 1995. Deflation of mount Etna monitored by spaceborne radar

interferometry. Nature, 375, 567-570.

Nikolova, S., Ilinski, D., Makris, J., Chonia, T., and Stavrakakis, G., 2002. 3D velocity tomography of the Kos -

Nissyros volcanic area - east Aegean sea. EGS Abstracts – Nice, April 2002 (EGS02-A-06157).

Papadopoulos, G. A., Sachpazi, M., Panopoulou, G., and Stavrakakis, G., 1998. The volcanoseismic crisis of

1996-97 in Nissyros, SE Aegean Sea, Greece. Terra Nova, 10, 151-154.

Papanikolaou, D., and Nomikou, P., 2001. Tectonic structure and volcanic centers at the eastern edge of the

Aegean volcanic arc around Nissyros Island. Bulletin of the Geological Society of Greece, 34(1), 289-296.

Scharroo, R. and P. N. A. M. Visser, and G. J. Mets, 1998. Precise orbit determination and gravity field

improvement for the ERS satellites, Journal Geophysical Research, 103 (C4), 8113-8127.

Parcharidis, I., and Lagios, E., 2001. Deformation in Nissyros volcano (Greece) using Differential radar

Interferometry. Bulletin of the Geological Society of Greece, 34(4), 1587-1594.

27

Van der Kooij, M.W.A., Armour, B., Ehrismann, J., Farris-Manning, P., and S. Sato, S., 1997. The EarthView

software for Spaceborne Interferometric SAR processing. Proceedings 3 rd ERS Symposium, (ESA) 18-21 March

1997 Florence (Italy).

Vassilopoulou, S., and Hurni, L. 2001. The Use of Digital Elevation Models in Emergency and Socio-Economic

Planning: A Case Study at Kos - Yali - Nisyros - Tilos Islands, Greece. Proc. 20th International Cartographic

Conference, Beijing, China, pp. 3424-3431.

28

APPENDIX A.

1. Working window of the DESCW software used to locate suitable baselines for

differential InSAR in Nissyros (white circle). The list of the available 1997 ERS 2

scenes is shown. Note that frames 711 and 729 indicate ascending (night) and 2871

descending (day) orbits, respectively. Highlighted is orbit 10160 of 30 March, 1997.

29

2. Working window of the ODISSEO software tool used to locate suitable baselines

for differential InSAR in Nissyros. The list for the April 22, 1997 ERS 2 scene is

shown. Suitable pairs form when frame Id=2871 and baseline length is less than

1000 metres.

30

APPENDIX B. The Nissyros Baselines. Yellow highlights indicate suitable pairs.

FROM http://odisseo.esrin.esa.it/ TRACK 107 FRAME 2871

ERS1/ERS1 ORBIT 24644 1 APRIL 1996 BASELINE PARAL BASELINE PERP

23642 22 JAN 1996 55 251 21638 4 SEPT 1995 -29 -145 21137 31 JUL 1995 273 659 20636 26 JUN 1995 -124 -456 20135 22 MAY 1995 78 166

ORBIT 23642 22 JAN 1996 BASELINE PARAL BASELINE PERP

21638 4 SEPT 1995 -84 -396 21137 31 JUL 1995 218 408 20636 26 JUN 1995 -179 -707 20135 22 MAY 1995 23 -85

ORBIT 21638 4 SEPT 1995 BASELINE PARAL BASELINE PERP

21137 31 JUL 1995 302 804 20636 26 JUN 1995 -95 -311 20135 22 MAY 1995 107 311

ORBIT 21137 31 JUL 1995 BASELINE PARAL BASELINE PERP

20636 26 JUN 1995 -397 -1115 20135 22 MAY 1995 -195 -493

ORBIT 20636 26 JUN 1995 BASELINE PARAL BASELINE PERP

20135 22 MAY 1995 202 622 ERS2/ERS2

ORBIT 6975 20 AUG 1996 BASELINE PARAL BASELINE PERP 5973 11 JUN 1996 -64 -221 1965 5 SEPT 1995 -36 -139 1464 1 AUG 1995 276 706 963 27 JUN 1995 -155 -490

ORBIT 5973 11 JUN 1996 BASELINE PARAL BASELINE PERP

1965 5 SEPT 1995 28 82 1464 1 AUG 1995 340 927 963 27 JUN 1995 -91 -269

ORBIT 1965 5 SEPT 1995 BASELINE PARAL BASELINE PERP

1464 1 AUG 1995 312 845

31

963 27 JUN 1995 -119 -351

ORBIT 1464 1 AUG 1995 BASELINE PARAL BASELINE PERP 963 27 JUN 1995

ERS1/ERS2 ORBIT 24644 1 APRIL 1996 BASELINE PARAL BASELINE PERP

E2 1965 5 SEPT 1995 -65 -226 E2 1464 1 AUG 1995 247 619 E2 963 27 JUN 1995 -184 -577

ORBIT 23642 22 JAN 1996 BASELINE PARAL BASELINE PERP

E2 1965 5 SEPT 1995 -120 -477 E2 1464 1 AUG 1995 192 368 E2 963 27 JUN 1995 -239 -828

ORBIT 21638 4 SEPT 1995 BASELINE PARAL BASELINE PERP

E2 1965 5 SEPT 1995 -36 -81 E2 1464 1 AUG 1995 276 764 E2 963 27 JUN 1995 -155 -432

ORBIT 21137 31 JUL 1995 BASELINE PARAL BASELINE PERP

E2 1464 1 AUG 1995 -26 -40 E2 963 27 JUN 1995 -457 -1236

ORBIT 20636 26 JUN 1995 BASELINE PARAL BASELINE PERP

E2 963 27 JUN 1995 -60 -121 ERS2/ERS1

ORBIT 6975 20 AUG 1996 BASELINE PARAL BASELINE PERP E1 24644 1 APRIL 1996 29 87 E1 23642 22 JAN 1996 84 338 E1 21638 4 SEPT 1995 0 -58 E1 21137 31 JUL 1995 302 746 E1 20636 26 JUN 1995 -95 -369 E1 20135 22 MAY 1995 107 253

ORBIT 5973 11 JUN 1996 BASELINE PARAL BASELINE PERP

E1 24644 1 APRIL 1996 93 308 E1 23642 22 JAN 1996 148 559 E1 21638 4 SEPT 1995 64 163 E1 21137 31 JUL 1995 366 967 E1 20636 26 JUN 1995 -31 -148 E1 20135 22 MAY 1995 171 474

32

ORBIT 1965 5 SEPT 1995 BASELINE PARAL BASELINE PERP

E1 21638 4 SEPT 1995 36 81 E1 21137 31 JUL 1995 338 885 E1 20636 26 JUN 1995 -59 -230 E1 20135 22 MAY 1995 143 392

ORBIT 1464 1 AUG 1995 BASELINE PARAL BASELINE PERP

E1 21137 31 JUL 1995 26 40 E1 20636 26 JUN 1995 -371 -1075 E1 20135 22 MAY 1995 -169 -453

ORBIT 963 27 JUN 1995 BASELINE PARAL BASELINE PERP E1 20636 26 JUN 1995 60 121 E1 20135 22 MAY 1995 262 743

33

APPENDIX C. (Top) Nadir view of Nissyros topography using a 20-m DEM. Also showing is

the overlay of 20-m contours that were digitized from the 1:10,000 map provided by

IGME (G. Vouyoukalakis, 1999). A linear histogram stretch has been applied using

the EASI PACE software package. North is towards the top. (Bottom). Image fit

showing ground control points used to georeference the 1:10,000 topographic map.

34

APPENDIX D.

Earthquakes recorded by the National Observatory of Athens (NOA) near

Nissyros between 8/3/95 and 31/8/1996 (32 events).

YEAR M D HR MIN SEC LAT LON DEP MAG 1995 MAR 8 08 28 9.3 36.43 27.04 10 3.5 1995 AUG 22 14 26 38.8 36.86 27.41 5 4.0 1995 SEP 11 08 12 59.3 36.60 26.91 32 3.8 1995 NOV 3 20 21 18.9 36.55 27.17 5 3.6 1995 NOV 25 09 04 13.3 36.55 27.26 39 3.7 1995 NOV 25 09 16 39.9 36.61 27.25 37 3.6 1995 NOV 27 13 26 5.2 36.44 27.33 39 4.2 1995 NOV 30 11 49 35.6 36.59 27.12 130 4.8 1995 DEC 14 20 53 24.5 36.50 27.38 5 3.8 1995 DEC 18 07 49 8.4 36.54 26.98 38 3.5 1996 JAN 2 14 54 28.1 36.57 27.02 10 3.4 1996 JAN 2 15 14 36.3 36.56 27.04 10 3.6 1996 JAN 2 20 51 23.8 36.64 27.25 37 3.7 1996 FEB 18 20 03 6.0 36.72 27.22 5 3.7 1996 FEB 29 06 13 21.9 36.76 27.39 37 3.7 1996 APR 8 11 44 59.5 36.65 27.15 33 3.9 1996 APR 12 15 39 12.1 36.63 27.06 153 5.0 1996 APR 25 10 47 39.7 36.85 27.44 1 3.8 1996 APR 25 15 53 3.6 36.89 27.41 1 3.8 1996 APR 25 15 59 58.6 36.89 27.45 10 3.8 1996 APR 27 21 41 51.8 36.68 27.27 10 3.8 1996 APR 30 06 29 7.9 36.73 27.34 10 3.6 1996 APR 30 15 07 23.5 36.66 27.26 10 3.9 1996 MAY 15 06 02 24.4 36.61 27.17 10 3.8 1996 JUN 2 23 52 49.8 36.71 27.39 49 3.6 1996 JUN 26 20 28 37.6 36.57 27.23 5 3.6 1996 AUG 4 18 57 50.6 36.42 27.17 15 3.8 1996 AUG 10 15 23 55.5 36.42 27.18 10 4.0 1996 AUG 19 23 01 35.4 36.62 27.12 5 4.0 1996 AUG 20 15 44 18.4 36.59 27.04 5 4.2 1996 AUG 26 14 01 28.8 36.43 27.11 5 3.6 1996 AUG 31 11 08 37.2 36.59 27.08 5 4.0