Metadata: Neart Na Gaoithe - Proposed Offshore Wind Farm and Cable Routes Geophysical Survey

J-1-02-1447

Emu Limited was commissioned by Neart Na Gaoithe Offshore Wind Limited to undertake a geophysical survey of the Neart Na Gaoithe proposed offshore wind farm site and buffer, and two possible cable routes: Torness (including Thorntonloch and Skateraw connections) and Cockenzie. The proposed wind farm is located offshore the east coast of Scotland in the central North Sea, 61 km northeast of Edinburgh at 52º16’ N, 2º16’ W. It is situated east of the Firths of Forth and Tay and 13-30 km east of Fife Ness. The following objectives were outlined: • To determine accurate depths, giving full coverage across the site and collected to International Hydrographic Organisation (IHO) Order 1 standards; • To determine the nature of the seabed sediments; • To identify any seabed surface features, such as sandwaves, and potential mobility; • To identify any wrecks or surface obstructions; • To identify subsurface strata and relate these to existing known geology. The survey area over the proposed wind farm totalled approximately 181 km2. The Torness and Cockenzie routes have approximate lengths of 32 and 54 km respectively. Data collection required the acquisition of swath bathymetry, sidescan sonar, sub-bottom profiling, acoustic ground discrimination system (AGDS), and magnetometer data.

NDGO0001, Hydrography, Bathymetry and Elevation

-2.3675°W, -2.1356°E, 56.3481°N, 56.1953°S

The following objectives were outlined: • To determine accurate depths, giving full coverage across the site and collected to International Hydrographic Organisation (IHO) Order 1 standards; • To determine the nature of the seabed sediments; • To identify any seabed surface features, such as sandwaves, and potential mobility; • To identify any wrecks or surface obstructions; • To identify subsurface strata and relate these to existing known geology. The following systems were used to achieve this objectives. 1. Horizontal Positioning: To provide navigation information to on board sensors with an accuracy of better than 0.2 m (Fugro Starfix GPS) or 2 m (DGPS), a Fugro Starfix GPS system receiving Starfix.HP corrections was used as the primary positioning system. This enabled the positioning of seabed information within 0.1 m. An additional Starfix GPS receiver, set to receive Starfix.XP corrections, was used as a secondary horizontal positioning system. A Hemisphere Crescent R120 DGPS system provided RTCM differential corrections for a GPS-derived heading and attitude sensor. Corrections were received from the EGNOS differential network via satellite. 2. Swath Bathymetry: Equipment used: Reson Seabat 8101 multibeam Reson Seabat 7125 multibeam Coda Octopus F180 precision motion and attitude system Valeport Mini-SVP QINSy 8 acquisition, navigation and processing software Leica 1200 GPS system – Post-processed kinematic (PPK) tides Reson 7125 and Reson 8101 multibeam echosounders were used, chosen based on their suitability to the differing depth ranges found on the main site and cable routes. F180 attitude data was fed directly into QINSy for real time bathymetric correction. Both multibeam systems were calibrated for heading, pitch and roll prior to survey with a patch test. The Reson 7125 was used to attain IHO Order 1 swath coverage and density on the main site and the deeper sections of the cable routes. The Reson 7125 was chosen for the density of data it was capable of providing in the deeper water depths present on the main site. The Reson Seabat 8101 was used to attain IHO Order 1 swath coverage and density on the shallower sections of the cable routes. The Reson 8101 was chosen for the wider swath of coverage it was capable of providing in the shallower sections of the cable routes. Sound velocity profiles through the water column were taken at the start and at 8 hourly intervals throughout survey operations. The sound velocity profiles were applied online during data acquisition. Data was processed using QINSy. Relevant corrections and filters were applied including the patch test calibration results, removal of outliers with automatic and manual filtering and post-processing of tidal data. The cleaned data were gridded using a 2 m bin size and exported into a GIS-compatible format. 3. Sidescan Sonar Survey: High (400 kHz) and low (100 kHz) frequency sidescan sonar data was acquired to provide 100% coverage. Equipment used: Edgetech 4200 dual frequency sidescan sonar towfish and transceiver Discover acquisition software SonarWiz processing software Sonardyne Scout USBL positioning system High and low frequency channels were recorded as XTF and JSF. The towfish was positioned by Ultra Short Baseline (USBL), calibrated prior to survey. Sonar data was processed using SonarWiz. Time-varied gain and slant-range corrections were applied to provide a sidescan sonar mosaic. Sediment boundaries were identified and classified using grab sampling and drop-down video. 124 grab samples were analysed using Particle Size Analysis (PSA), and the results used to classify sediment type and feature interpretation for the wind farm site and cable routes. 4. Seismic Survey: Boomer and Pinger data was acquired to provide high and ultra-high resolution seismic data respectively. Equipment used: Applied Acoustics boomer catamaran with 200 J boomer plate Applied Acoustics CSP 1500 J seismic energy source C-Products 8 element hydrophone streamer Sonar Equipment Services Digital Transmitter (Pinger) Coda Octopus DA2000 digital acquisition and processing system Octopus 120 thermal printer Prior to survey, the boomer and pinger systems were tested near the survey area to establish optimum, site-specific settings for each system. For the boomer system 100 J power and a trigger interval of 250 ms were selected with a recorded voltage range of ±1.25 V. The boomer catamaran and hydrophone were maintained 30 m astern throughout the survey. This layback was applied during post-processing. For the pole-mounted pinger, a power output of 40 – 100 % was chosen depending on the water depth (lower power values were preferable in shallow water). A frequency of 5 kHz was initially selected although this was reduced to 3.5 kHz in areas where less penetration was achieved at 5 kHz. The recorded digital seismic data sweep time was 200 ms but a display range of 80 ms was maintained for online data QA. Fixes were generated at 50 m intervals and recorded within digital records. All settings and offsets were recorded in online logs. All data were logged digitally (.cod format) in the Coda Octopus DA2000 acquisition and processing system. Post-processing and interpretation were carried out using Coda Octopus GeoSurvey 4.3.0. A combination of band-pass filters were used to remove noise and aid the interpretation. Horizons were interpreted and exported to a database. These were imported into ESRI ArcGIS where the data were contoured and quality checked. The rockhead values were reduced to Chart Datum (CD) by combining bathymetry values with the depth of below seabed. 5. Magnetometer Survey: Magnetometer data was acquired to identify major items of debris, wreck or changes in near surface bedrock. Equipment used: Geometrics G-882 caesium magnetometer MagLog acquisition software Geosoft Oasis Montaj GIS and processing software Sonardyne Scout USBL positioning system Magnetometer data was acquired using a Geometrics G-882 caesium vapour marine magnetometer. The towfish unit housed a total magnetic field sensor and provided absolute readings of total magnetic field in nano-Teslas (nT). The magnetometer was towed approximately 170 m behind the vessel. Cable length was reduced to 100 m within the inshore sections of cable routes. The altitude was maintained between 15 to 30 m above the seabed. The towfish was positioned by Ultra Short Baseline (USBL). Following the survey, post-processing was carried out using Geosoft Oasis Montaj software. The residual magnetic field intensity was calculated by applying a combination of low-pass filters to produce background magnetic intensity for each line. This curve was subtracted from the raw data to give the residual magnetic field intensity. An analytical signal grid was calculated for this residual which represented the square root of the sum of the derivatives in x, y and z directions (magnetic gradient). This was used to identify the edges of magnetic source bodies. Magnetic targets, or anomalies, were calculated from this grid. 6. Acoustic Ground Discrimination System (AGDS) Survey Equipment used: RoxAnn AGDS Knudsen 320M single beam echosounder RoxMap AGDS acquisition software A RoxAnn Acoustic Ground Discrimination System (AGDS) combined with a Knudsen 320M single beam echosounder was used to collect quantitative data to classify the seabed. RoxAnn analysed the primary and secondary (multiple) returns from the transducer to give results for the roughness (E1) and hardness (E2) of the seabed. Data was post-processed using Geosoft Oasis Montaj. Depth data were first processed to remove anomalous results. Seabed classifications were calculated according E1 vs. E2 values. Boundaries between seabed types were compared with the sidescan sonar interpretation. Ground truthing was completed by grab sampling and drop-down video. Particle Size Analysis (PSA) was used to classify the sediment types for the AGDS interpretation.

English

Data is repeatedly and frequently updated

Tuesday 21 February 2012

MEDIN Discovery metadata standard

2.3.4

English