D A N M A R K S O G G R Ø N L A N D S G E O L O G I S K E U N D E R S Ø G E L S E R A P P O R T 2 0 1 6 / 29 Vindmøller nord for Als i Lillebælt Bathymetrisk og geologisk kortlægning af område til opstilling af vindmøller i Lillebælt imellem Als og Fyn Jørgen O. Leth & Steen Lomholt DE N A T I O N A L E G E O L O G I S K E U N D E R S Ø G E L S E R FOR D A N M A R K O G G R Ø N L A N D, E N E R G I -, F O R S Y N I N G S - OG K L I M A M I N I S T E R I ET
DANMARKS OG GRØNLANDS GEOLOGISKE UNDERSØGELSE RAPPORT 201 6 / 29 Kortlægning af området Lillebælt Syd Bathymetrisk og geologisk kortlægning af et område til opstilling af vindmøller Udført for Sønderborg Forsyning Jørgen O. Leth & Steen Lomholt Fortrolig rapport Kopi nr. Kan ikke frigives DE NATIONALE GEOLOGISKE UNDERSØGELSER FOR DANMARK OG GRØNLAND, ENERGI-, FORSYNINGS- OG KLIMAMINISTERIET
1. Executive Summary 3 2. Introduction 4 2.1 Scope of work... 5 2.2 Geology... 5 3. Data processing, Interpretation and mapping 7 3.1 Positioning... 7 3.2 Bathymetry... 7 3.2.1 Data source and processing... 7 3.2.2 Bathymetric mapping... 7 3.3 Seismic... 8 3.3.1 Data source and processing... 8 3.3.2 Interpretation and results... 9 3.4 Comment on possibly geotechnical hazard.... 15 4. References 16 5. Enclosures and Charts 17 G E U S 2
1. Executive Summary GEUS has performed a bathymetric and seismic mapping of the Lillebælt Syd area north of Als in relation to the construction of wind turbines. The interpretation is based on existing data provided by Sønderborg Forsyning A/S. The bathymetric data have been processed, cleaned and adjusted to existing bathymetry data with reference to DVR90. The data have been gridded and are presented as grid maps with 1 m contour intervals. The seismic data have been interpreted in relation to the distribution of the units regarded as muddy sediment. The data have been gridded and the resulting distribution and thickness is presented as gridded maps with 1 m contour intervals. The comparison between the water depth and the distribution of the muddy sediments indicates a relation between the distribution of the muddy and gassy sediments and the presence of a deep basin in the central part, the deep channel cutting the area to the southeast and at the edges of the study area. Another type of muddy/soft sediment is related to the shallower part and more local valleylike features of the study area including the possibility of lake deposits. G E U S 3
2. Introduction GEUS has been asked by NIRAS on behalf of Sønderborg Forsyning A/S to perform a bathymetric and seismic mapping of the Lillebælt Syd area north of Als, that should be used for the construction of wind turbines. The area is located in the southern part of Lillebælt, between Als and Fyn, near the Helnæs peninsula. The mapping is based on existing data provided by Sønderborg Forsyning A/S (Figure 1). Furthermore data from GEUS data archive have been used for depth correction of the bathymetric data. Figure 1 Extend of the study area in the southern part of Lillebælt. Side scan sonar, sparker seismic and bathymetric data are acquired in a grid of 1 x 1 km, covering the area with a total length of 286 km. Data were originally acquired for raw material exploration in 2009 by Rohde Nielsen A/S. Furthermore, 20 km of bathymetric data acquired by GEUS 2014 for habitat mapping are used for calibration of depth measurement (Figure 2). Six vibrocores are present in the northern part of the investigation area. They have been acquired as part of a pre-investigation survey for a cable route from Jylland to Fyn in 2000. The cores are shallow and cover only the uppermost part of the subsoil down to between 1.9 and 4.3 m (Figure 2). G E U S 4
Figure 2 Seismic data coverage for surveyed area. The seismic and bathymetric data from 2009 are illustrated with black lines and bathymetric from 2014 is data illustrated with blue lines. 3 m vibrocores are marked by red dots. 2.1 Scope of work The purpose with the work is to prepare basis material that could be used to identify potential areas for a 120 MW wind farm in the Lillebælt Syd area north of Als. GEUS has been asked to prepare a bathymetric map of the area, based on an existing dataset acquired in a grid of 1 x 1 km. Data was processed for heave and roll and de-spiked and depth corrected expressed as elevations relative to DVR90. Furthermore, GEUS has been asked to prepare a thickness map of mud based on seismic data in the 1 x 1 km seismic grid. Geological evidence on conditions that could cause geotechnical problems for wind turbines will be described and illustrated on seismic profiles. This could have an impact on the selection of areas suitable for the location of wind turbines. 2.2 Geology The surficial geology of the southern Lillebælt reflects the glacial deformation episodes during the end of the last glaciation. The most important impact of the southern part of Lillebælt is from the so-called Bælthavet Readvance (Houmark-Nielsen 1987), where a glacier intruded the area from the southeast. Meltwater deposits in the deepest part of Lillebælt are believed to originate from this event where water from the melting ice cap flowed G E U S 5
from the south to the north. In many areas, the glacial deposits are overlain by late glacial clayey and fine sandy deposits, which again can be overlain by early Holocene freshwater deposits often interlayered with organic deposits and peat (Bennike & Jensen, 2011). Marine Holocene sediments gradually cover these freshwater deposits. Also in the study area, there are indications of the presence of such late glacial deposits in incised channels. During the Holocene transgression, muddy sediments were deposited in the area, mostly connected to the deep basins, the fjords and the troughs in the deepest part. Between these basins, the seabed is partly influenced by areas of Holocene sand and gravel deposits with shallow areas of glacial deposits of moraine ridges and boulder reefs outcropping in between. Examples of such ridges within the study area are Lillegrund / Nordlige Lillegrund, Torø Banke and Hesteskoen. Figure 3. Locationmap and seismic section from an area to the northwest of the study area showing the general stratigraphy of the southern Lillebælt region. From Bennike & Jensen, 2011. G E U S 6
3. Data processing, Interpretation and mapping 3.1 Positioning For the data acquisition, two navigation systems were used: Simrad Seatex 20 GPS and Thales Sagitta GPS. Despite no differential correction have been used the positioning was very stable during the whole survey with an accuracy between 3-10 m. All positions in this report is in UTM zone 32N, ellipsoid WGS84. 3.2 Bathymetry A gridded depth map for the investigation area, have been prepared from depth measurement acquired during the seismic profiling in a 1 x 1 m grid (Appendix 1). 3.2.1 Data source and processing The bathymetric data has been acquired in 2009 with a Reson Navisound 215 dual frequency echosounder with a standard GPS system. The original survey was conducted as a regional exploration survey for sand and gravel, where sounding data only were used to determine depths to resources at seabed. X and Y offsets from antenna to Navisound position on the ship can be read in the header of the original raw data in NaviPac files (xxg.npd), and positions are corrected for offset in the navigation program. No. z offsets (the depth of transducer below sea surface) for the echosounder were recorded. An offset of the depth data is therefore introduced using corrected depth data from another survey that covers part of the investigation area. These data are collected by GEUS in 2014 during a habitat survey conducted by the Danish Nature Agency. These data are acquired with RTK positioning and therefore adjusted to DVR90. The overlap between the two surveys shows that the 2009 data z level need to be adjusted -0.6 m to match the new RTK corrected dataset. All data are expressed as elevations relative to DVR90 with negative depth values. Ungridded soundings is stored as (X,Y,Z) values in ASCII data format. 3.2.2 Bathymetric mapping Based on the depth corrected dataset a grid map was prepared with 1 m resolution and a 1 m contour interval map (Figure 4). The uncertainty on depth measurement is approximately 30 cm with a maximum of approximately 50 cm. The map is presented in Appendix 2. The depth variations are between 5 and - 40 m and the - 20 m depth contour line is highlighted to get a better understanding of the seabed morphology. G E U S 7
Figure 4 Bathymetric map of the investigation area, Lillebælt Syd. 3.3 Seismic 3.3.1 Data source and processing The seismic data used was acquired in 2009. The seismic source is a Geo-Resources sparker model Geo-Spark 200. The energy level of the sparker has been set to 300 J and 400 J during the survey. The frequency of the transmitted pulse is within the interval of 500-2000 Hz. The reflected seismic signal was recorded via a 3 m hydrophone array with 8 elements connected in a series. The sparker data were recorded using the Chesapeake SonarWizMap5 system and stored in SEG-Y format. The Geographix Interpretation software is used for interpretation. The sparker data are used for mapping the distribution of the muddy sediments in the survey area. The thickness of the muddy sediments is determined by subtracting the depth to the mud surface from the depth to the seabed. The resulting thicknesses were exported to Excel for converting the TWT-values in milliseconds values to depths in metres using acoustic velocity of 1500 m/s. The gridding of the mud thicknessis done with the gridding software MapInfo Vertical Mapper. The interpolation method used is Nearest Neighbor with a cell size of 150 m and an aggregation distance of 10 m. G E U S 8
3.3.2 Interpretation and results The seismic sparker data, in general, provide information of the upper 10-30 m of the seabed with a vertical resolution of about 0.5 to 1 m. The interpretation performed has focused on the distribution of units regarded as muddy sediment. Muddy sediments are known to appear with a transparent character in the seismic record, with no or indistinct internal acoustic layering due to the relatively high water content in the sediment. Gas plumes often appear in the seismic unit due to de-gassing of organic rich sediment in the underlying layers. No sediment cores have verified the interpretations of the seismic units in the study area. Thus, at this stage the seismic interpretation and the delineation of expected muddy sediments also include semi-transparent seismic units, which by experience is known to represent soft sediments of different origin. The resulting distribution and thickness map of the muddy and soft sediments is shown in figure 5. Figure 5. Isopach map of muddy sediments in the survey area. The maximum thickness of muddy sediment has been mapped in the central part of the study area. Here a northeast-southwest striking continuous basin is infilled with at least 12 m of soft and muddy deposits. The thickness of the unit has been estimated due the pres- G E U S 9
ence of gas in the sediment, which blurs the seismic record and by that eliminates the exact delineation of the unit i.e. the actual thickness can be higher than estimated. In the following a series of examples are presented to illustrate, the different types of seismic records, which have been included in the mapping of units, regarded as muddy or soft sediments. In figure 6 the location of the seismic profiles is presented. Figure 6. Overview map showing the location of the seismic profile described in the text. In the deepest part of the study area in general deeper than 20 m water depth muddy deposits with gas are widespread (see figure 9). The 5 seismic examples in figure 7 show the presence of the muddy unit in different parts of the study area. The basin-like structure is typical for this type of muddy sediments. G E U S 10
G E U S 11
Figure 7. Seismic profiles showing transparent seismic units interpreted as muddy sediments bordering glacial deposits (moraine). Gas plumes appearing in the mud indicates degassing of underlying sediments. Outside the basin like structures, muddy sediments have been designated in closed basins or in infilled valleys. Here gas is not present and the bottom layers of the units, in general, are easy to determine. The seismic display of the unit in these areas often show weak internal seismic layering which indicate a layering in the sediment e.g. changes in the fine grained sediments types from muddy sand to sandy mud with interlayering of fine sands or the presence of peat layers. Examples of this type are given in figure 8. Some of the units in the figures are named soft sediment/mud to indicate the difference between these units and the muddy units with gas in figure 7. G E U S 12
G E U S 13
Figure 8. Seismic profiles showing transparent seismic units interpreted as muddy and/or soft sediments bordering glacial deposits (moraine), in general above 20 m water depths. In figure 9 the relation between the water depth and the distribution of the muddy sediments is shown. The blue line indicates the 20 m depth contour line. A comparison between the seismic profiles in figure 7 indicates the distribution of the muddy and gassy sediments are related to the deep basin in the central part, the deep channel cutting the area to the southeast and at the edges of the study area. The opposite relation to the other type of muddy/soft sediments shown in seismic profiles in figure 8 is related to the shallower part and more local valley-like features of the study area. Figure 9. The relation between the water depth and the distribution of the muddy sediments. The blue line indicates the 20 m depth contour line. G E U S 14
3.4 Comment on possibly geotechnical hazard. It is well known that muddy sediments, containing gas, could cause geotechnical problems, such as instability of seabed and gas leakage, as seen at Sæby, Nearshore Wind Farm project and the Anholt Wind Farm. The present study has illustrated, that the muddy and gassy sediments are related to the deep basin in the central part of the Lillebælt area and the deep channel cutting the area to the southeast and at the edges of the study area, as seen in figure 7. The maximum thickness of muddy sediment varies from 8 to 12 m, based on the seismic interpretation. In other areas muddy sediment could superimpose deeper soft late glacial sediments, which could be the case at Profile EW 5000 figure 8, where gas signature is annotated. Local basins with freshwater lake deposits could be a potential hazard for exploration and establishment of wind turbines. The presence of organic rich peat layers in these basins blurs the seismic data below and potential soft sediments will be invisible on the seismic. This could be the case in the western part of the profiles EW-10000b and NS-2000. It should be emphasized that there is a lack of cores in the survey area and by that the seismic interpretations have not been verified. The interpretations of the seismic might be adjusted if vibrocores are performed at a later stage in the investigation of the area. G E U S 15
4. References Bennike, O. & Jensen, J. B., 2011: Postglacial, relative shore-level changes in Lillebælt, Denmark. Geol. Surv. of Denmark and Greenland Bull. 23, 37-40. Houmark-Nielsen, M., 1987: Pleistocene stratigraphy and glacial history of the central part of Denmark. Bull, geol. Soc. Denmark, vol. 36, pp. 1-189. G E U S 16
5. Enclosures and Charts G E U S 17
Appendix 4 G E U S
CMP Shot 0 0 m 20 20 gas moraine 15 mud 40 40 30 60 60 Profile EW 0000 1000 m 45
m moraine gas 15 mud 30 Profile EW 10000a 1000 m 45
m gas moraine 15 mud mud 30 Profile NS 5000 1000 m 45
m gas 15 moraine mud mud moraine 30 Profile NS 10000 1000 m 45
m gas 15 moraine mud moraine 30 Profile NS 8000 1000 m 45
m soft sediment/mud 15 moraine 30 Profile EW 7000 1000 m 45
m moraine mud ga s moraine soft sediment/mud 15 30 Profile EW 5000 1000 m 45
m soft sediment/mud gas moraine mud 15 30 Profile EW 10000b 1000 m 45
m moraine soft sediment/mud moraine 15 30 Profile NS 2000 1000 m 45