GMES
ESRIN/Contract No. 4000101274/10/I-AM
(formerly 19366/05/I-EC)
TERRAFIRMA PRODUCT
Interpretation Report
V1.0
November 2012
Analysis of ground motion in areas of underground storage of oil and gas
and natural gas production in the North German Lowland
Author:
Corinna Wolf - Federal Institute for Geosciences and Natural Resources (BGR), Germany
Acknowledgements:
This paper received editing input from Dr. Uwe Schäffer, Dr. Friedrich Kühn and Dr. Thomas Lege
(BGR, Germany).
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Table of Content
1. SUMMARY ........................................................................................................... 2
2. AIM OF THE STUDY ........................................................................................... 3
2.1.
Description of the product ................................................................................................. 5
3. INTRODUCTION TO THE AREA OF INTEREST ................................................ 6
3.1.
Geological Background ...................................................................................................... 6
3.2.
Oil and gas in the North German Lowland ....................................................................... 8
4. THE PS DATA USED IN THE STUDY............................................................... 13
4.1.
Available satellite datasets ............................................................................................... 13
4.2.
Choice of the reference point ........................................................................................... 14
4.3.
Geo-referencing ................................................................................................................. 14
5. VALUE ADDED PRODUCT RESULTS ............................................................. 15
5.1.
Comparison & integration methodology ........................................................................ 15
5.2.
Case studies on specific areas ........................................................................................ 16
6. OBSERVATIONS............................................................................................... 30
6.1.
Assessment of impact and benefits ................................................................................ 30
6.2.
Critical analysis of utility for end-user organisation ..................................................... 30
6.3.
Comparison with alternative services and information sources ................................. 30
6.4.
Recommendations for product improvements .............................................................. 31
6.5.
Record of complaints, problems, resolutions ................................................................ 31
7. CONCLUSIONS ................................................................................................. 32
8. REFERENCES ................................................................................................... 33
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1. Summary
This study is a detailed analysis and interpretation of ground motion correlated to underground
storages of gas and oil and natural gas production sites in the North German Lowland. The
investigation is based on Persistent Scatterer Interferometry (PSI) data of the Wide Area Product
(WAP), developed by the German Aerospace Center (DLR) within the framework of the ESA-GMES
project Terrafirma. This dataset was originally analysed and interpreted within the Terrafirma Flood
Theme Service to assess flood risk in coastal lowland areas (see Flood Plain Subsidence Mapping
Product North German Floodplain). However, from this Wide Area Product, not only terrain motion in
the North German Lowland along the North Sea Coast and the rivers Elbe and Weser, also
subsidence in areas of underground storage of gas and oil and natural gas production sites could be
identified. The present study further analyses this ground motion and provides background information
about oil and gas deposits, underground storages and natural gas production sites in North Germany.
In a case study, the PSI data are validated with levelling data, provided by the Mining Authority of
Lower Saxony, for the oil cavern storage in Wilhelmshaven-Rüstringen. The comparison reveals a
strong correlation between both datasets and demonstrates the reliability of the ground motion
detected by PSI data, therefore the subsidence above the oil cavern site in Wilhelmshaven is further
analysed and interpreted. The subsidence, which is caused by convergence, is spread uniformly over
a large area and therefore no critical value of inclination, which can cause building damages, is
reached. As the PSI measurements support the levelling data and extend the dataset into the build-up
area, they are suitable for monitoring purposes. Currently, an ad hoc working group of the German
Mine Surveyor Association (Deutscher Markscheider-Verein, DMV) is working on “guidelines for the
use of radar interferometry for monitoring purposes in mining”.
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2. Aim of the study
In the area of interest, located in the North German Lowland, several land subsidence phenomena
could be identified from PSI-derived ground motion data. In Stage 3 of the ESA-GMES project
Terrafirma, an analysis and interpretation of vertical ground motion data was done in the North
German Lowland for the Flood Plain Subsidence Mapping Product (FSM), which aims at providing
information about terrain motion in flood-prone areas. The analysis, which was based on PSI data
from a Wide Area Product (WAP), developed by the German Aerospace Center (DLR), revealed
several land subsidence in the marshlands along the North Sea Coast and the rivers Elbe and Weser.
In general, the land subsidence in the marshland is characterised by motion rates of -1 to -7 mm/year
and is supposed to be due to compaction of lowland sediments and deposits. Areas of higher rates of
subsidence were identified and further investigated on-site. For example, ground motion rates up to
-2 cm/year occur in the harbour and industrial areas of Bremerhaven due to heavy load on the low
load-carrying ground and insufficient foundation of buildings.
Also a strong correlation between subsidence areas and marshlands in the area of Hamburg could be
revealed from PSI data between 1993 and 2005 during Terrafirma Stage 2. In addition to that,
subsidence also correlates to the outline of a near-surface salt diapir (Terrafirma Atlas). This long-term
subsidence is caused by salt creep of several salt diapirs, located in the deeper underground of the
metropolitan region of Hamburg. In this area also subrosion processes occur, which caused
approximately 40 sinkholes in the city zone of Hamburg during the last two centuries and led to visible
building damages at active sinkholes. Sudden collapses of subrosion-induced cavities constitute a
Fig. 1: PSI-derived ground motion data (coherence ≥ 0.65) in the North German Lowland of the Wide Area Product
ascending dataset between August 1993 and December 2000, displayed on Landsat 7 pan mosaic (4 scenes from May
2000 and 2001, derived from USGS Earth Explorer [1], georeference: WGS84 UTM32N). Subsidence occurs in the
marshlands along the rivers Elbe and Weser and in areas of oil and gas storage and natural gas production, for
example: 1: Oil cavern storage in Wilhelmshaven-Rüstringen, 2: Gas field in Hengstlage, 3: Cavern storage of oil and
gas in Ohrensen and Harsefeld.
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Cuxhaven
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Fig. 2: PSI-derived ground motion data (coherence ≥ 0.65) in the North German Lowland of the Wide Area Product
descending dataset between May 1992 and January 2001, displayed on Landsat 7 pan mosaic (4 scenes from May 2000
and 2001, derived from USGS Earth Explorer [1], georeference: WGS84 UTM32N). Subsidence occurs in the
marshlands along the rivers Elbe and Weser and in areas of oil and gas storage and natural gas production, for
example: 1: Gas field near Rotenburg-Taaken and Söhlingen, 2: Cavern storage of oil and gas in Ohrensen and
Harsefeld, 3: Porous storage of gas in Reitbrook near Hamburg.
moderate to strong georisk in the Hamburg city area (Reuther et al. 2006). A similar ground motion
phenomenon occurs in the city of Lüneburg, which is located in the south-east of Hamburg. There,
subsidence with a ground motion rate of approximately 20 cm/year (between 2002 and 2004) takes
place in the western part of the city due to man-made triggered dissolution of salt and natural
dissolving by ground water (Reuther et al. 2006).
The PSI data of the Wide Area Product, used in Terrafirma Stage 3 for the Flood Plain Subsidence
Mapping Product, not only revealed ground motion along the marshlands. Subsidence also seems to
be correlated to areas of underground storage of oil and gas and natural gas production sites (see
figures 1 and 2). As land monitoring of underground storage sites is required by law, levelling data are
available from the Mining Authority. These data can be used to validate the ground motion derived
from the PSI Wide Area Product and to further investigate the causes of the identified subsidence. So
the aim of this study is to validate the PSI-derived ground motion data with levelling data of
underground storage sites and to further analyse and interpret the ground motion phenomena.
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2.1. Description of the product
This product is an analysis of vertical ground motion data in areas of oil and gas storage and gas
extraction in the North German Lowland. It includes the analysis and validation of PSI data with
levelling data provided by the Mining Authority in a Geographic Information System. The derived
results were also checked on-site in Wilhelmshaven-Rüstringen. From these results, this Interpretation
Report and an Interpretation PowerPoint Presentation are compiled with a description of the
investigation area, background information about oil and gas deposits and underground storages and
natural gas production sites in the North German Lowland, information about the used PSI dataset
and levelling data. In a case study, a detailed validation and interpretation of the PSI data is conducted
at the oil cavern storage in Wilhelmshaven-Rüstringen.
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3. Introduction to the area of interest
The area of interest is located in the North German Lowland and covers an area of approximately
21,000 km². It includes the North Sea Coast from Wilhelmshaven to Bremerhaven and Cuxhaven and
covers the Weser Estuary to the city of Bremen and the Elbe Estuary beyond Hamburg (see figure 3).
The North German Lowland is characterised by a flat relief and low areas, which are endangered by
floods.
Fig. 3: Area of interest in the North German Lowland (red: Ascending data stack, blue: Descending data stack).
3.1. Geological Background
In the deeper underground, the first layer with more or less consistent distribution in the North German
Floodplain consists of Permian deposits. After volcanic activity and the deposition of volcanic rocks
with thicknesses up to 2,000 m in the Rotliegend, a sedimentation basin was formed. In the centre of
this North German depression, rock salt was deposited and covered by an approximately 2,000 m
thick layer of red sediments and a layer of Zechstein with a similar thickness. Permian salts rose due
to movements of north-northeast and south-southwest extending faults and formed approximately 200
salt structures, mainly in the area of west Holstein, Hamburg and in the north of Lower Saxony (see
figure 4). During the Jurassic, a stronger salt movement occurred, which lead to a separation in
several troughs. These structures, which extend from south-west to north-east, were filled with
Jurassic sediments. The salt pillows located between the troughs were hardly covered by Jurassic
sediments, because their deposition was incomplete or the sediments were eroded. In the
Cretaceous, approximately 2,000 m thick clayey-sandy and chalky sediments were deposited,
followed by Tertiary layers of clay and sand with a thickness up to 5,000 m in basins at the border of
salt structures. The salt movement continued also during the Cretaceous and Tertiary and nowadays
some of them are still rising. If the salt reaches the ground water, gypsum will be formed. The gypsum
can be pressed out of the land surface, but normally there is subsidence instead (Henningsen and
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Katzung 2006). For example, subsidence due to a near-surface salt dome could be detected in the
urban area of Hamburg by the use of PS data during Terrafirma Stage 2.
The flat relief of the North German Lowland is caused by a 500 m to only several meters thick layer of
quaternary loose sediments, which cover the deeper underground (Henningsen and Katzung 2006,
see figure 5). During the Quaternary, several changes between glacial and interglacial periods
occurred and the original landscape and deposits were reshaped by the force of ice and melt water.
During the Saale glacial period (approximately 330,000 years ago), melt water deposited sand layers
in front of the glacier, which mostly were preserved and now form the geest (Feldmann et al. 2002).
Apart from the moraine material and melt water deposits of the Saale glacial period, the surface is
made of lowland sediments and deposits in the marsh, which were formed in the Holocene. The
marsh area extends along the North Sea from Schleswig Holstein into the Elbe valley beyond
Hamburg and is only separated by geest ridges in some small areas. Along the North Sea coast, bogs
were formed next to the geest border. The deposits between the coast and the geest ridges consist of
marine sands, clayey mudflat deposits, sediments of brackish water and lagoons, deposits of fresh
water and peat. All these Holocene deposits have different size and are mainly interconnected. The
most common deposits are clayey, which are called “Klei”. Some dyked marsh areas are now below
sea level, because of subsidence, early embankments and gradual subsidence of the North Sea area
in the west of the Elbe estuary (Henningsen and Katzung 2006).
Fig. 4: Salt structures in North Germany (Reinhold et al. 2008).
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Fig. 5: Geology in the investigation area (Lahner and Toloczyki 2004).
3.2. Oil and gas in the North German Lowland
The following section will give an overview about oil and gas deposits and underground storages and
natural gas production sites in the North German Lowland.
The underground of North Germany is important due to its oil and gas reservoirs, of which several
were formed along flanks or crests of salt structures (Henningsen and Katzung 2006). Oil reservoir
rocks are of Jurassic and Cretaceous age and occur in depths between 1,000 and 2,500 meters
(WEG 2008). Gas with more than 90% methane was formed due to temperature increase during
carbonization of hard coal and accumulated in sandstones of the younger Carbon, Rotliegend
(Permian) and Buntstandstein (Triassic) and in carbonate rock of Zechstein (Permian). Most of the gas
reservoirs exist at the southern border of the North German Lowland extending in an east-west
direction from Emsland to the region Altmark (close to Salzwedel). Additional reservoirs occur at the
estuary of the river Ems (Henningsen and Katzung 2006, see figure 6).
Oil production
In 2011, the 49 oil production fields in Germany provided approximately 2.7 million tons of oil and
condensate (1%) and covered 2.5% of the German oil consumption. The most important oil fields of
Germany are located in Schleswig-Holstein and Lower Saxony with approximately 91% of the entire
German oil production. Lower Saxony alone produced 36.1% (see table 1) (LBEG 2012).
Table 1: Oil, condensate, oil gas and gas production (crude gas) in Germany in 2011 (LBEG 2012).
Federated states of
Oil
Gas
Oil gas
Germany
(incl. condensate) [t]
%
[m3 (Vn)]
%
[m3 (Vn)]
Bavaria
32,406
1.2
5,969,250
0.0
1,462,516
Brandenburg
16,014
0.6
5,258,120
Hamburg
18,651
0.7
371,603
Mecklenburg4,071
0.2
666,455
Vorpommern
Lower Saxony
966,220
36.1
12,077,925,792
93.8
49,529,245
Rhineland-Palatinate
170,019
6.4
2,069,961
Saxony-Anhalt
487,086,502
3.8
Schleswig-Holstein
1,469,757
54.9
275,138,057
2.1
20,313,236
Thuringia
26,413,248
0.2
Sum
2,677,136
100
12,872,532,849
100
79,671,136
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%
1.8
6.6
0.5
0.8
62.2
2.6
25.5
100
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Fig. 6: Gas production fields in North Germany with the strongest production fields: 1: Rotenburg-Taaken, 2:
Goldenstedt/Visbek, 3: Völkersen, 4: Varnhorn/Quadmoor/Wöstendöllen…, 5: Hemmelte/Kneheim/Vahren, 6: Söhlingen,
7: Bahrenbostel/Burgmoor/Uchte, 8: Siedenburg-West/Hesterberg, 9: Salzwedel (LBEG 2012, modified).
The main fields of oil extraction in Germany are located in the western part of the North German Basin
in the “Bentheimer sandstone” of Early Cretaceous in the Lower Saxony Basin, sandstones of
Rhaetian (Late Triassic), Early and Middle Jurassic in the Greifhorn Basin, sandstones of Middle
Jurassic and Late Cretaceous in the Hamburg Basin and sandstones of Middle Jurassic in the
Ostholstein Basin and the Jade-Westholstein Basin (Henningsen and Katzung 2006).
Gas production
3
In Germany, 81 gas fields produced 11.8 billion m (Vn) clean gas (normalized heating value H o = 9.77
3
kWh/m (Vn)) in 2011, which covered approximately 13% of the German gas consumption. With
93.8%, most of the crude gas was extracted in Lower Saxony. Additionally, approximately 80 million
3
m (Vn) oil gas was gained during oil production, mainly in Lower Saxony (62.2%) and SchleswigHolstein (25.5%) (see table 1). The main production fields are located in the areas between the rivers
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Elbe and Weser, Weser and Ems and in the region in the west of Ems. With 93.8% (12.1 billion m
(Vn)), the majority of the crude gas production was extracted in Lower Saxony, mainly in the regions
between Weser and Ems and Elbe and Weser. The main reservoir rocks are the dolomites of the
deeper Zechstein and sandstones of the upper Rotliegend. Two-thirds of the entire annual gas
production was extracted in the ten strongest gas production fields in Germany (see figure 6 and table
3
2). The field complex Rotenburg/Taaken produced the highest amount with 1.7 billion m (Vn) crude
gas from the reservoir rock Rotliegend (LBEG 2012).
The gas production fields Rotenburg/Taaken, Söhlingen and Hengstlage are located in the area of
interest (see figure 7). In the production fields Rotenburg/Taaken and Söhlingen, the gas is extracted
from the reservoir rock Rotliegend in depths between 3,170 and 3,500 meters (see figure 8). In the
field Hengstlage, the gas is produced in depths between 1,500 and 2,800 meters from Buntsandstein
(Triassic) (see figure 9).
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Table 2: Strongest gas production fields in Germany in 2011 (crude gas without oil gas) (LBEG 2012).
Reservoir
Year of
Gas production
Percentage of inland
Number of
founding
[1000 m3 (Vn)]
production [%]
producing wells
Rotenburg-Taaken
1982
1,657,618
12.9
30
Goldenstedt/Visbek
1959
1,246,348
9.7
19
Völkersen
1992
1,241,257
9.6
14
Varnhorn/Quadmoor/
1968
793,704
6.2
15
Wöstendöllen…
Hemmelte/Kneheim/
1980
731,258
5.7
11
Vahren
Söhlingen
1980
710,506
5.5
20
Bahrenbostel/
1962
571,440
4.4
9
Burgmoor/Uchte
Siedenburg1964
562,976
4.4
10
West/Hesterberg
Salzwedel
1968
487,087
3.8
143
Figure 7: Relevant gas production fields in the area of interest (Lahner and Wellmer 2004, modified).
Tertiary
Quaternary -Tertiary
Cretaceous
Upper Cretaceous
Triassic
Lower Cretaceous
Zechstein
Jurassic
Rotliegend
Buntsandstein
Upper
Carboniferous
Ca2
Zechstein
Productive horizon
Fig. 8: Gas structure in the eastern part of the area of
interest (in the north of river Weser): Gas extraction from
Rotliegend in depths between 3,170 and 3,500 meters, for
example in Rotenburg-Taken and Söhlingen (LBEG 2012,
modified).
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Rotliegend
Upper Carboniferous
Fig. 9: Gas structure in the area south of river Weser,
for example: Gas extraction from Triassic-Buntsandstein in depths between 1,500 and 2,800 meters in
Hengstlage (LBEG 2012, modified).
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Underground oil and gas storage
Porous storage for gas with max.
working gas capacity [mio. m3 (Vn)]
Cavern storage for gas
Cavern storage for crude oil, mineral
oil products and liquefied gas with
number of single storages
25 km
Figure 10: Underground oil and gas storages in the area of interest (LBEG 2012, modified).
In Germany, oil and gas are stored in porous storages and in salt caverns (LBEG 2012).
Porous storages are used to cover the seasonal base load and especially to serve daily peak loads,
as they have efficient injection and withdrawal capacities due to natural flow paths in the capillary pore
volume of the reservoir rock. The storages are installed in sandstone formations, former oil or gas
reservoirs and subordinated also in aquifers in the sediment basin of North, East and South Germany
(Sedlacek 2009).
Cavern storages are installed by creating large cavities in relatively low depths in salt domes
(Henningsen and Katzung 2006, WEG 2008). In this process, water is pumped into the salt dome
through the drilled hole. The salt is dissolved and the brine is pumped to the surface (WEG 2008).
More than 100 cavern storages exist in the North German Plain, mainly in the northern part of Lower
Saxony, where the highly concentrated brine can be drained into the North Sea (Henningsen and
Katzung 2006) or can be used commercially (Sedlacek 2009). In Germany, approximately 97% of the
crude oil is imported. Apart from above ground tanks, gasoline, middle distillate, fuel oil and crude oil
is stored in salt caverns for times of crisis and as compensation for production fluctuations in the
producing industry (LBEG 2012, Sedlacek 2009). The German Oil Storage Association
(Erdölbevorratungsverband, EBV), which consists of all companies producing or importing crude oil or
its products to Germany, is obligated by law to store approximately 20 million tons of crude oil and
mineral oil products for the year 2011. Currently, they store approximately 21 million tons for times of
crisis (LBEG 2012).
Five of the twelve storage facilities for crude oil, mineral oil products and liquefied gas in Germany are
located in the area of interest (see figure 10 and table 3).
Table 3: Cavern storages for crude oil, mineral oil products and liquefied gas in the area of interest (LBEG 2012).
Location
Type of storage
Depth [m]
Number of single storages
Charge
Blexen
Salt dome cavern
640 - 1,430
4
Crude oil
3
Petrol
1
Fuel oil
Bremen-Lesum
Salt dome cavern
600 - 900
5
Light fuel oil
Ohrensen
Salt dome cavern
800 - 1,100
1
Ethylene
1
Propylene
Sottorf
Salt dome cavern
600 - 1,200
9
Crude oil, mineral oil products
WilhelmshavenSalt dome cavern
1,200 - 2,000
36
Crude oil, mineral oil products
Rüstringen
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Table 4: Underground gas storages in the area of interest (LBEG 2012).
Location
Type of storage
Depth [m]
Reservoir rock
Reitbrook
BremenLesum_EMPG
Bremen-LesumSWB
Harsefeld
Huntorf/
Neuenhuntorf
Porous storage
(former oil field)
Cavern storage
640 - 725
Late Cretaceous
Total volume
[mio. m3 (Vn)]
530
Working gas volume
[mio. m3 (Vn)]
350
1,300 – 1,780
Zechstein
247
160
Cavern storage
1,050 – 1,350
Zechstein
87
73
Cavern storage
Cavern storage
1,150 – 1,450
650 – 1,400
Zechstein
Zechstein
189
435
140
311
Germany is supplied with gas by import (80%) and inland production (Sedlacek 2009). To ensure the
gas supply, gas is stored half in porous storages and half in cavern storages. In 2011, the maximum
3
usable working gas volume was 20.4 billion m (Vn) (LBEG 2012), stored in approximately 40
underground storages in Germany (WEG 2008). Apart from storage for times of crisis and optimisation
of gas supply (temporal injection or withdrawal in winter or summer periods), gas is stored to balance
daily and seasonal peak demand, as changes in the production rates are only possible to a limited
extent and the import volume is fixed by contracts (Sedlacek 2009). Depending on the geological
3
3
structure, petrophysical properties and depth, between 100 million m and several billions m gas can
be stored in porous storages. Approximately half of the gas amount is working gas, whereas the other
half (cushion gas) is serving to buffer the pressure and to keep away the reservoir water from the
3
storage drill hole. In cavern storages, approximately 30 million m gas can be used and 10 to 30
3
million m cushion gas is necessary to maintain adequate pressure (WEG 2008).
In the area of interest, four gas storages are operated in Reitbrook near Hamburg, Bremen-Lesum,
Harsefeld and Huntorf/Neuenhuntorf (see table 4). Additionally, it is planned to install a new gas
3
cavern storage with approximately 730 million m in Ohrensen near Stade. In the first project stage,
3
four caverns with gas volume of approximately 400 million m are planned, which will start to operate
between 2015 and 2018 (Sedlacek 2009).
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4. The PS data used in the study
The investigation is based on vertical ground motion data in the North German Lowland, derived from
a new Persistent Scatterer Interferometry (PSI) standard product, which was designed and processed
by the German Aerospace Center (DLR). This Wide Area Product (WAP) gives an overview of vertical
terrain motions over large areas and was originally used to investigate subsidence in coastal lowland
areas and flood-prone river basins to assess flood risk (see Flood Plain Subsidence Mapping Product
(FSM) North German Floodplain).
4.1. Available satellite datasets
The area of interest in the North of Germany is covered by two ascending and two descending
Synthetic Aperture Radar (SAR) data stacks of the ERS-1 and ERS-2 satellites (figure 11). Each of
the stacks was processed into two independent Wide Area Products.
The ascending stack with the number T129 consists of 32 radar scenes, which cover the time range of
October 1993 to December 2000 and the ascending stack with number T172 includes 25 scenes from
August 1993 to November 2000. The acquisition time frame of the descending stack T22 ranges from
May 1992 to November 2000 and consists of 68 scenes. The 66 scenes of the descending stack with
the number T65 cover the time range from June 1992 to January 2001.
Approximately half a million Persistent Scatterers (PS) of high reliability (coherence ≥ 0.65) were
identified, providing measurements of the average annual terrain velocity for each point across the
study area with an estimation accuracy of +- 1mm/year assuming linear displacement. Negative
values of ground motion (away from the satellite) indicate subsidence, positive values indicate uplift.
T129
T65
T172
T22
Hamburg
Bremerhaven
Fig. 11: Ascending datasets T172 and T129 (red) and descending datasets T65 and T22 (blue) of the Wide Area Product,
displayed on Landsat 7 pan mosaic from May 2000 and 2001 derived from USGS Earth Explorer [1], georeference:
WGS84 UTM32N).
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4.2. Choice of the reference point
The reference point determines the point within the study area against which all ground velocity
measurements are relative to and therefore this point should be stable. During the processing, the
system proposes points of the reference network with high medium amplitude and low amplitude
dispersion as possible candidates. From these, the operator selects a point which is probably located
in a stable area. At the end of the processing, the deformation value of the reference point is verified.
However, should there be a deformation after the processing, another reference point can be selected.
The reference point of scene T129 of the ascending track is located near the town Kayhude (N 5 957
970, E 575 902 m) in the north of the city Hamburg and the one of scene T172 is situated in Bad
Bederkesa (N 5 942 596 m, E 487 656 m) in the northeast of Bremerhaven. In the descending track,
the reference point of scene T22 is set in the area of the Hamburg airport (N 5 942 505 m, E 566 506
m) and the one of scene T65 is located in Holste (N 5 913 155 m, E 488 214 m), which is in the
southeast of Bremerhaven.
Mostly, the reference points are located upon glacial deposits of sand and gravel with middle to good
bearing capacity. From geological point of view, these points are probably stable and therefore
suitable as reference points.
4.3. Geo-referencing
In the automatic process of geo-referencing, the coordinates of the PS in the radar image, the satellite
orbits and geographic data were used to convert the satellite geometry into a reference system. The
PSI data of the Wide Area Product have been delivered by the DLR in the reference system WGS 84
UTM zone 32 N with a georeference accuracy of 25 m.
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5. Value added product results
This section gives an overview about the used data and methodology and presents the case study of
the underground storage site in Wilhelmshaven-Rüstringen with a validation of PSI data with levelling
data, provided by the Mining Authority, and an analysis and interpretation of the ground motion
velocity.
5.1. Comparison & integration methodology
The PSI data of the Wide Area Product were integrated in a Geographic Information System (ArcGIS
10) and geological data, levelling measurements and auxiliary data were added to analyse and
interpret areas of ground motion.
As background image, a satellite mosaic of four Landsat 7 pan scenes from May, 15, 2000 (Path/Row
196/22 and 196/23), May, 8, 2000 (195/22) and May, 11, 2001 (195/23) were compiled with ENVI 4.8.
Additionally, topographic maps (1:25,000) and digital orthophotos with ground resolution of 40 cm
were used.
The PSI-derived ground motion data were compared to geological maps of Lower Saxony, SchleswigHolstein and Hamburg (see table 5). For the analysis of ground motion related to underground
storages of gas and oil and area of gas production, a geohazard map of Lower Saxony, an overview
map of underground storages and a map of hydrocarbon resources in Germany were used. To
validate the PSI measurements, levelling data, provided by the Mining Authority, were analysed for the
cavern sites in Wilhelmshaven-Rüstringen, Sottorf near Hamburg and Bremen-Lesum.
To evaluate the detected ground motion phenomena, investigations on-site were conducted at the oil
cavern storage located in Wilhelmshaven-Rüstringen.
Table 5: Maps and data used for the investigation.
Map/Data
Scale/Resolution
Landsat 7 pan mosaic
15 m
Topographic maps
1:25,000
Digital orthophotos (DOP40)
0.4 m
Distribution of salt structures in North
1:500,000
Germany
Geological map of Schleswig-Holstein
1:500,000
Geological map of Hamburg
1:25,000
Geological map of Lower Saxony
1:50,000
1:25,000
Distribution of peat and bogs in Lower
1:50,000
Saxony
Geohazard map of Lower Saxony
1:25,000
(areas of sinkholes and subsidence)
Engineering geology map of Lower
1:50,000
Saxony (ground stability)
Hydrocarbon resources in Germany
N/A
Overview map of underground storages
N/A
of gas, oil, mineral oil products and
liquefied gas in Germany
Levelling data of underground storages
N/A
in Wilhelmshaven-Rüstringen, Sottorf
and Bremen-Lesum
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Source
USGS Earth Explorer [1]
Bundesamt für Kartographie und Geodäsie (BKG)
Bundesanstalt für Geowissenschaften und Rohstoffe (BGR)
(Federal Institute for Geosciences and Natural Resources)
Landesamt für Natur und Umwelt Schleswig-Holstein
Geologisches Landesamt Hamburg
Landesamt für Bergbau, Energie und Geologie,
Niedersachsen (LBEG)
(State Authority for Mining, Energy and Geology of Lower
Saxony)
Mining Authority of LBEG
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5.2. Case studies on specific areas
As land monitoring of underground storage sites is required by law, levelling data are available from
the Mining Authority and can be used to validate the ground motion data derived from the PSI Wide
Area Product. The Mining Authority of Lower Saxony provided levelling data of the oil cavern storages
in Wilhelmshaven-Rüstringen, Sottorf and Bremen-Lesum. The analysis revealed that the storage site
in Wilhelmshaven-Rüstringen is best suitable for a detailed validation and analysis, as a subsidence
bowl is visible in the PSI dataset and both datasets show a strong correlation. In contrast to the
storage in Bremen-Lesum, the cavern in Wilhelmshaven is located on the edge of the city and
therefore further influencing factors by the city can be excluded. In Sottorf, PSI data and levelling data
both do not show ground motion above the storage. Due to these facts, the storage in WilhelmshavenRüstringen was selected as case study.
The cavern storage of Wilhelmshaven-Rüstringen, located on the edge of the city Wilhelmshaven, is
the biggest of four caverns storages owned by the German Oil Storage Association
(Erdölbevorratungsverband, EBV) to store oil for times of crisis ([2]). In 1968, it was started to install
35 caverns in the salt dome of Rüstringen in depths between 1,200 and 2,000 meters. In the cavern
3
3
storages, which are up to 420,000 m in size, approximately 7 million m crude oil is stored. With three
pipelines, the cavern sites are connected with an oil pipeline company, located 9 km away at the North
Sea Coast. Via these pipelines, crude oil and other mineral oil products can be delivered to the
caverns. If necessary, oil can be transferred from the cavern storage to the storage tanks of the oil
pipeline company and from there, the oil can be transported to refineries in the Rhein-Ruhr region or
Hamburg or can be shipped on tankers ([3], [4]).
Fig. 12: PSI data of ascending track T172 in the area of Wilhelmshaven (average annual ground motion data with
coherence ≥ 0.65, displayed on Landsat 7 pan mosaic of May 2000 and 2001, derived from USGS Earth Explorer [1],
georeference: WGS84 UTM32N).
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From the Wide Area Product, the ascending track T172 covers the area of Wilhelmshaven. This
dataset is based on ERS-1/2 data between August 1993 and November 2000. In the overview of
figure 12, the PSI-derived ground motion data reveal a subsidence bowl above the oil cavern storage,
located at the edge of the city.
In a first step, the PSI-derived ground motion data and levelling data of the cavern storage in
Wilhelmshaven-Rüstringen were compared. The levelling dataset consists of yearly measurements
between the years 1993 and 2000, whereby inconsistent levelling measurements in the original
dataset were not incorporated in the analysis. The comparison of both datasets demonstrates, that the
PSI ground motion data fit very well to the levelling measurements (see figure 13 and 14). While the
levelling measurements are concentrated on the cavern sites, the PSI data mainly exist in the build-up
area and therefore extend the levelling measurements into the urban area.
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Fig. 13: PSI-derived average annual ground motion rates from 1993 to 2000 in the area of Wilhelmshaven-Rüstringen
(background image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
Fig. 14: Levelling data from 1993 to 2000 of the oil cavern storage Wilhelmshaven-Rüstringen (background image:
digital orthophotos DOP40, georeference: WGS84 UTM32N).
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Validation
For the validation of the PSI measurements from the Wide Area Product, 36 neighbouring points were
selected all over the investigation area (see figure 15). The distance between the PS points and
levelling benchmarks is on average about 23 meters and the difference between the average
displacement rates is between -0.07 mm/year and 2.63 mm/year. From this compiled dataset, the
Root Mean Square Error (RMSE) amounts to 1.07 mm/year. This value is comparable to the ones
calculated in the Terrafirma Validation Case Study in the Netherlands. In Alkmaar, PSI measurements
from 83 ERS-1/2 images were compared to 945 levelling benchmarks and the values of the calculated
RMSE ranged between 1.0 and 1.5 mm/year (see [5]). Figure 16 presents a diagram of the
comparison of the two datasets. The calculated correlation coefficient of 0.93 and the displayed
trendline demonstrate a linear correlation. The selected points for validation were checked on-site in
Wilhelmshaven-Rüstringen. The investigation demonstrated that the selected PS points and levelling
bechmarks were suitable for comparison, as they occur in close spatial proximity.
To analyse the both datasets more detailed, four sites with suitable PSI and levelling measurements
for comparison (several points in close spatial relation) were selected for the analysis of the timeseries. Figure 17 gives an overview about the selected sites. When comparing time-series it has to be
considered, that they do not include redundant information and therefore are very sensitive to phase
noise. For example, in the Terrafirma Validation Case Study in the Netherlands (see [5]), the Root
Mean Square Error for average annual displacement rates ranged between 1.0 and 1.5 mm/year. In
contrast to that was the calculated RMSE for the comparison of time-series between ERS-1/2 data
and levelling benchmarks between 6.2 and 8.7 mm (average RMSE of single deformation
measurements).
Fig. 15: Selected points for validation (light blue), PSI-based annual motion rates (points) of ascending dataset T172
and levelling data (triangles) between 1993 and 2000 at the oil cavern storage Wilhelmshaven-Rüstringen (background
image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
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Fig. 16: Comparison of PSI-derived average annual ground motion rates and levelling bechmarks.
Fig. 17: Selected sites (light blue points) for the comparison of the time-series of PSI data (points) and levelling
benchmarks (triangles) (background image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
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Fig. 18: Location of time-series 1 and 2 (cavern sites) for the validation of PSI data with levelling measurements
(background image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
Fig. 19: Location of time-series 3 (company site) and 4 (housing area) for the validation of PSI data with levelling
measurements (background image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
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Time-series 1
The time-series 1 is located at a cavern site (see figure 18). At this site, the measurements of two PSI
points and two levelling benchmarks are compared (figure 20). The average annual displacement
rates of the two datasets are very similar: between -7.6 and -7.7 mm/year from the PSI data and
-7 mm/year in the levelling dataset. The calculated correlation coefficient between 1993 and 2000 (no
data from 1994) is 0.99, which demonstrates a strong linear correlation between the PSI dataset and
the levelling measurements.
Fig. 20: Time-series 1: Two PSI measurements (blue) compared to two levelling benchmarks (green) between 1993 and
2000, measurements are relative to master scene in March 1998, where ground deformation is set to zero.
Time-series 2
The location of time-series 2 is also at a cavern site (see figure 18) and again two PSI measurements
are compared to two levelling benchmarks (figure 21). While the levelling data show subsidence with
an average annual displacement rate of approximately -6.6 mm/year, the PS data indicate a smaller
motion rate between -5.1 and -5.7 mm/year. But the correlation coefficient of 0.97 and the comparison
of the time-series indicate a linear correlation.
Fig. 21: Time-series 2: Two PSI measurements (blue) compared to two levelling benchmarks (green) between 1993 and
2000, measurements are relative to master scene in March 1998, where ground deformation is set to zero.
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Time-series 3
At the company site of the storage operator, three Persistent Scatterer points were compared to five
levelling benchmarks between 1993 and 2000 (figure 19 and 22). The average annual ground motion
rate is between -6.9 and -8.1 mm/year in the levelling dataset and between -6.5 and -8 mm/year in the
PSI dataset. With a calculated correlation coefficient of 0.97, the time-series indicate a linear
correlation between the both datasets.
Fig. 22: Time-series 3: Three PSI measurements (blue) compared to five levelling benchmarks (green) between 1993
and 2000, measurements are relative to master scene in March 1998, where ground deformation is set to zero.
Time-series 4
Time-series 4 is located in the housing area of Wilhelmshaven. There, three PSI measurements are
compared to two levelling benchmarks between 1993 and 2000 (figure 19 and 23). The average
annual displacement rates show values between -7 and -7.1 mm/year in the levelling dataset and
between -6.5 and -6.7 mm/year in the PSI dataset. The correlation coefficient for the time-series is
with a value of 0.98 very high and demonstrates a strong linear correlation between both datasets.
Fig. 23: Time-series 4: Three PSI measurements (blue) compared to two levelling benchmarks (green) between 1993
and 2000, measurements are relative to master scene in March 1998, where ground deformation is set to zero.
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Interpretation of subsidence
The validation study demonstrated that the RMSE is approximately 1 mm/year for the comparison of
average annual displacement rates of PSI data and levelling benchmarks. In the validation of the timeseries, strong correlation coefficients indicated a strong linear correlation between both datasets.
These investigations confirm that the PSI measurements do have a high quality and therefore a good
significance and reliability. On basis of this validation, the subsidence above the oil cavern storage in
Wilhelmshaven-Rüstringen is further analysed and interpreted.
The ground motion data of PSI and levelling measurements between 1993 and 2000 can be seen in
figure 24 with the outline of the salt dome Rüstringen. The majority of levelling benchmarks have
subsidence rates between -5 and -8 mm/year in the centre of the salt dome with only single scatterer
points with higher values (maximum -11.9 mm/year). From the PSI measurements, which mainly occur
at the border of the salt dome in the build-up area, most values are stable (-1 to 1 mm/year) or range
between -1 and -3.5 mm/year. The maximum displacement value from the PSI dataset is
-8.9 mm/year, located at the edge of the housing area.
To understand the causes of the subsidence, some background information about the salt dome of
Rüstringen and its characteristics is necessary. The salt dome of Rüstringen consists of Zechstein
salt, which rose from a depth of approximately 5,000 meters up to 1,100 meters and is now covered
by rocks of Early and Late Cretaceous, Tertiary and Quaternary age (Esso AG 1976, see figure 25).
The oil storage caverns, constructed by solution mining, were installed in depths between 1,200 and
2,000 meters (LBEG 2012). A main characteristic of salt is its creep behaviour, which is dependent on
temperature, pressure and time (IVG Caverns GmbH 2011). The rate of this deformation is influenced
by the salt properties, cavern depth and overburden characteristics, which control the temperatures
Fig. 24: PSI-derived average annual ground motion data with coherence ≥ 0.65 (points), levelling data (triangles) and
outline of salt dome Rüstringen (blue line), derived from salt structures map 1:500,000 in the area of Wilhelmshaven,
displayed on Landsat 7 pan mosaic of May 2000 and 2001, derived from USGS Earth Explorer [1], georeference: WGS84
UTM32N).
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Fig. 25: Geological model of salt dome Rüstringen with cavern site (Esso AG 1976, translated and modified).
and lithostatic pressure, difference of pressure between the cavern and its surrounding, cavern shape
and configuration of multiple caverns (Neal 1991, Warren 2006). Large differences between the
natural lithostatic pressure and the cavern pressure cause an increase of the creep rate and the salt
creep will continue until differential pressures area equalised (Warren 2006). This reduction in cavern
volume due to salt creep is called convergence. The storage products of the cavern act against the
convergence, but this counter-pressure is small compared to the overall formation pressure, which is
acting on the salt (IVG Caverns GmbH 2011). The creep deformation occurs radially, with the largest
amount at the cavern bottom. In this process, the salt flows from all directions towards the cavern and
therefore the surface is gradually lowered (Neal 1991, figure 26). How this subsidence is expressed at
the surface is depending on the cavern depth, mode of cavern operation, thickness of salt cover and
the character of overburden materials or roof beams (Warren 2006, Neal 1991). Due to the depth of
the cavern, the subsidence is normally spread uniformly over a large area (IVG Caverns GmbH 2011)
and if the subsidence at the land surface is ongoing, a subsidence bowl or depression is formed
(Warren 2006). In the process, the rate of subsidence is depending on the actual convergence and the
resulting reduction of cavern volume (IVG Caverns GmbH 2011). Land subsidence above salt caverns
can be considered as a normal process, especially above storage caverns in shallow depths.
Normally, low deformation rates are spread over a large area and therefore no problems are caused
by the subsidence (Warren 2006).
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Fig. 26: Convergence and subsidence processes associated with oil storage in a salt cavern (Warren 2006, see also
Neal 1991).
Fig. 27: Tension zone and compression zone of a subsidence bowl (P: point of maximum subsidence, h: depth of seam
under surface, : limiting angle) (Nishida and Goto 1969).
Fig. 28: Shear planes underneath a building in a compression zone (a) and tension zone (b) of a subsidence area (Deck
and Anirudh 2010).
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Compared to mining areas in Germany, the subsidence with a maximum value of approximately
9 mm/year at the cavern storage in Wilhelmshaven is relatively low. Subsidence due to coal mining in
the German Ruhr District is in the magnitude of several centimetres per months. For example in the
region of Essen, a maximum subsidence rate of 2 cm/month can be assumed from GPS
measurements and Digital Elevation Model between August 1998 and February 1999 (Spreckels
2000). In a not further specified area in the Ruhr District, an analysis of ERS-1/-2 SAR data revealed a
maximum displacement of 4 cm in 35 days in the time range between September 1995 and April 1997
(Strozzi et al. 2001). Levelling measurements in the city of Gelsenkirchen show a maximum
subsidence rate of 3, 6 and 15 cm/month along the levelling lines 6, 3-20-19 and 5 between July 1996
and February 1997 (Spreckels et al. 2001) and a total subsidence of more than 4 meters between
1948 and 2010, which equals a deformation rate of approximately 6.4 cm/year (Harnischmacher
2010). In Recklinghausen, levelling data along the levelling lines 1, 2 and 3 indicate a maximum
subsidence rate of approximately 7 cm/month (approximately 25 cm between September and
December 2000, Spreckels et al. 2001).
Black coal mining between 1914 and 1997 in the town of Wassenberg, located in western Germany at
the border with the Netherlands, led to land subsidence of up to 3 meters between 1953 and 1997.
After the drainage of water stopped with the end of the mining, ground water re-entered the mine
causing uplift. The strongest uplift occurred between 2001 and 2004 with 8 to 9 cm (Caro Cuenca and
Hanssen 2008).
In the Terrafirma case study of Stassfurt, PSI data between 1992 and 2005 revealed subsidence of
approximately 10 mm/year in Stassfurt and 23 mm/year in Bernburg at abandoned potash and rock
salt mining areas. At an active salt mining area in Ilberstedt, a maximum ground motion of
approximately 26 mm/year could be identified. In these areas, convergence and subrosion cause
large-scale land subsidence (see [6]).
Potential damages on structures like buildings and roads are dependent on the induced tensile or
compressive horizontal ground strains (Deck and Anirudh 2010). The area of subsidence can be
divided in a compression zone located in the centre of the subsidence and a tension zone at the edge
of the subsidence bowl (see figure 27). In the two zones, shear planes with different directions are
formed (see figure 28). Apart from these horizontal ground strains, inclination is a factor concerning
building damages. According to Prinz and Strauss (2006), the limit value, below which cracks in loadbearing walls or further damages can occur, is 1:500, which equals an inclination of 2 mm per meter
(see table 6).
To analyse the potential of building damages induced by inclination at the cavern site in
Wilhelmshaven, an approximately 1,500 meters long transect line was laid from the company site to
the housing area of Wilhelmshaven (see figure 29). From the PSI measurements along the transect
line, the total amount of subsidence between 1993 and 2000 and the resulting inclination were
calculated. Figures 30 and 31 show that the subsidence is spread over a large area and therefore only
small inclinations occur. With a maximum inclination of 1:1,300 (approximately 0.8 mm per meter), no
critical limit value for serious building damages is reached (see table 6).
For example, the subsidence area of Lüneburg has an inclination of approximately 1:6 in the distal
area and 1:2 in the center, which equals approximately 180 mm per meter and 630 mm per meter
(Reuther et al. 2006). In Fromme Street, where average subsidence rates are approximately 13 cm
per year, numerous damages on the street and buildings occur. Due to risk of collapse, several
buildings had to be evacuated and pulled down (see [7]).
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Table 6: Limit value for inclinations and associated damages (Prinz and Strauss 2006).
Inclination limit value
damage
1/1000 (1 mm per meter )
no damage
1/500 (2 mm per meter)
limitation for mining damage, safety limit for the absence of cracks
(small cracks are possible)
1/300
cracks in load-bearing walls
1/250
visible tilt of tower-like structures
1/150 (7 mm per meter)
extensive cracks in walls, structural damages
Fig. 29: Transect line between company site (south-west) and housing area of Wilhelmshaven and ground motion data
of the Wide Area Product track 172 with coherence ≥ 0.65, based on ERS-1/-2 data from 1993 to 2000 (background
image: digital orthophotos DOP40, georeference: WGS84 UTM32N).
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Fig. 30: PSI-derived total ground motion between 1993 and 2000 along the transect line.
Fig. 31: Calculated inclinations along the transect line with limit values for damages after Prinz and Strauss 2006.
Another factor has to be considered, when cavern storages area located in lowland areas. With
ongoing subsidence the depth to the water table is lowered and the area may be flooded in future. In
this case, pumps have to be installed to keep the area dry.
To mitigate the effects of subsidence, hydrocarbon storage caverns are usually operated at the
highest practical pressure at all times to minimise salt creep. In consideration of the specific cavern
rock mechanics, economic and regulatory factors and the potential effects of subsidence, the highest
practical cavern pressure and operation schedules can be determined (Rokahr et al. 1998). To
minimise the effects of subsidence from salt creep, monitoring of land subsidence and cavern shape is
necessary. The cavern shape reflects the state of the cavern and if anomalous effects occur,
preventive action can be implemented (Warren 2006).
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6. Observations
In the following section, observations about the presented product are summarised. This evaluation
include an assessment of impact and benefits, critical analysis of utility for end-user organisation,
comparison with alternative services and information sources, recommendations for product
improvements and records of complaints, problems and resolutions.
6.1. Assessment of impact and benefits
The validation study conducted with PSI data of the Wide Area Product and levelling benchmarks at
the cavern storage in Wilhelmshaven revealed a strong correlation between both datasets and
confirmed that the PSI measurements do have a high quality and therefore a good significance. By the
use of PSI data, which provide reliable ground motion information with mm-precision, motion
phenomena can be identified and analysed. From this analysis, geohazards can be assessed, for
example flood-prone areas in coastal lowlands.
Furthermore, the PSI method can be used to monitor subsidence correlated with storage operations.
The study of Kühn et al. (2009) showed that PSI time-series can monitor the initial phases of gas
storage operations as well as the alternating stages of gas injection and withdrawal at the porous
storage of gas in Berlin.
Currently, monitoring of subsidence with PSI method alone cannot be accepted by the responsible
Mining Authority, as mining concepts have to fulfil several accuracy regulations requested by the
German Mining Law. Therefore, implementation of new methods like radar interferometry needs legal
acceptance. An ad hoc working group formed by the German Mine Surveyor Association (Deutscher
Markscheider-Verein, DMV) is currently working on “guidelines for the use of radar interferometry for
monitoring purposes in mining” (see [8] and [9]).
Maybe in future, conventional land surveying methods and radar interferometry methods can be
combined. By the use of Persistent Scatterer Interferometry, density of measurements in build-up
areas can be increased and levelling benchmarks can be reduced in these areas.
6.2. Critical analysis of utility for end-user organisation
The end-user is the Section of “Fundamental Mining Questions, Monitoring and Danger Prevention,
Lost Mining” of the State Authority for Mining, Energy and Geology (LBEG). They are interested in
data to identify, analyse and monitor ground motion in mining areas. Especially in abandoned mining
areas, where continuous monitoring with conventional methods are not carried out any more,
information about terrain motion is desired. Therefore, PSI data with mm-precision and wide area
coverage are suitable to identify and monitor ground motion in these areas of interest.
6.3. Comparison with alternative services and information sources
In contrast to conventional land surveying methods, which are only conducted at sites where
monitoring is required by law or ground motion is suspected and further investigated, the main
advantage of the PSI method is the coverage of large areas. Therefore, ground motion can be
detected at sites not covered yet by monitoring, like natural gas production sites. Also abandoned
mines can be monitored and so far unknown vertical ground motion can be identified. Over such large
areas, the analysis of ground deformation with the Wide Area dataset of the North German Lowland is
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easier, faster and more cost-efficient compared to conventional levelling. At mining sites, where
conventional methods are used for monitoring, the PSI-derived terrain motion data can be used to
extend the surveillance area and therefore support the monitoring.
6.4. Recommendations for product improvements
This study is based on PSI data of the Wide Area Product, which was developed by the German
Aerospace Center (DLR). As these data were processed with a linear deformation model, non-linear
deformations cannot be displayed in the data and therefore seasonal ground motion cannot be
identified from the PSI dataset. A method to overcome this limitation, as well as GPS calibration of the
PSI dataset, are currently being developed.
Further detailed investigations can be made if more data and information of the cavern storage such
as amounts of injection and withdrawal, depth, extend and shape of cavern storage and geological
information can be provided.
If a higher density of PSI data is needed, a subset of the WAP can be processed. As for the
processing of the WAP a large processing network is needed and more error propagation is included
due to topography, atmosphere and distance to the reference point, processing of a smaller dataset
will probably increase the density of detected Persistent Scatterers with high quality. To increase the
density or to get PS at desired locations, corner reflectors can be installed for future investigations or
monitoring.
6.5. Record of complaints, problems, resolutions
This investigation was only possible with the collaboration with the Mining Authority and storage
operators to get and use levelling data for validation. Validation of the PSI data is necessary, as
several factors during acquisition and processing can have impacts on the accuracy of the data. With
permission of the storage operator, levelling data could be used to validate the Wide Area Product PSI
data in the North German Lowland.
Meanwhile it is a common convention to display PSI-derived ground motion with colours ranging from
blue (uplift) over green (stable) to red (subsidence). As subsidence areas are presented with red
colour, which is a signal colour and often associated with hazards, misunderstandings can occur. It
has to be noticed, that in most cases the scale of the colours is relative to the maximum displacement
value or according to visualisation purposes and not relative to hazards, risks or damages. In the
investigation area, only small deformations occur, which are spread uniformly over a large area and
therefore only small inclinations result, which are far away from any critical limit for damages.
Concerning building damages, inclination and tensile and compressive horizontal ground strains are
the most important factors.
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7. Conclusions
The aim of this study was to analyse ground motion identified from PSI data of the North German
Lowland in correlation to underground storages of oil and gas and natural gas production sites. This
investigation was realised in collaboration with storage operators. Thanks to the kind support of the
Mining Authority of Lower Saxony, which provided levelling data, and the permission of the storage
operator, a detailed analysis of PSI-derived terrain motion data at the oil cavern storage in
Wilhelmshaven-Rüstringen was possible. The levelling data were used to validate the PSI data of the
Wide Area Product, a dataset with large coverage, developed by the German Aerospace Center
(DLR) within the ESA-GMES project Terrafirma. The comparison of both datasets revealed a strong
correlation and therefore a high reliability of the PSI-derived ground deformation data. These data
allowed to further analyse the subsidence above the cavern storage in Wilhelmshaven, which is
caused by convergence. The subsidence is spread uniformly over a large area and therefore only low
inclinations results, which do not cause building damages.
This study was conducted within the ESA financed project Terrafirma, which is part of the GMES
service. The aim of GMES is to provide easy accessible information based on earth observation
satellites and in situ sensors to improve the management of the environment and civil security. The
emergency response and downstream service Terrafirma supports civil protection agencies and
disaster management organisms to assess and mitigate geohazards. The ground motion data
described in this study enable local authorities to analyse wide areas and to identify vertical ground
deformations. Concerning the Mining Authority of the State Authority for Mining, Energy and Geology
of Lower Saxony, information about ground motion in abandoned mining areas is appreciated for
evaluation and monitoring. The collaboration with company representatives provided the chance to
inform the industry sector about the methodology, which can be used to support and spatial extend the
conventional levelling or to do monitoring. For this purpose, an ad hoc group of DMV is currently
working on guidelines for the use of radar interferometry for monitoring purposes in mining. This
Terrafirma service makes ground motion and geohazards information available to the public and with
the development of GMES services, European citizen will be increasingly provided with extensive
information in future. This study contributed to validate the new developed Wide Area Product of DLR,
introduce the methodology of PSI for the analysis of ground motion to potential users and to inform
about subsidence in areas of underground storages of oil and gas and natural gas production. For the
future it is to be hoped that the methodology can be established, as satellite based measurements
make an important contribution to the identification, evaluation and monitoring of geohazards.
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Online sources
[1] USGS Earth Explorer: http://earthexplorer.usgs.gov, November 24, 2011
[2] EBV website: http://www.ebv-oil.de/cms/cms2.asp?sid=92&nid=&cof=60, October 10, 2012
[3] NWKG website: http://www.nwkg.de/betriebe/ruestringen/whv.html, July, 17, 2012
[4] Kbbnet website: http://www.kbbnet.de/referenzen/projekte/rustringen, October 10, 2012
[5] Terrafirma Validation Project: http://www.terrafirma.eu. com /Terrafirma_validation.htm, July, 4,
2012
[6] Terrafirma presentation “GMES –Terrafirma II Stassfurt Case Study - Preliminary Results” of
Friedrich Kühn (BGR): http://www.terrafirma.eu.com/Documents/presentations/TF%20WS5/Germany_
Stassfurt _BGR.pdf, November 9, 2012
[7] Lüneburg website: http://meetings.copernicus.org/www.cosis.net/abstracts/EGU06/01868/EGU06J-01868.pdf, November, 8, 2012
[8] Ad hoc working group of DMV: http://www.dmv-ev.de/arbeitskreise/messverfahren.html, October,
29, 2012
[9] Terrafirma presentation “Inactive/Abandoned Mines – Ruhr” of Norbert Benecke (DMT GmbH &
Co. KG) from November, 30, 2010: http://www.terrafirma.eu.com/Documents/Workshops/WS%206/
Inactive%20and%20Abadoned%20Mines%20in%20Southern%20Ruhr,%20Germany.pdf, November
8, 2012
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