3. Methodology

3.1. Study Area

  1. The marine archaeology study area is defined as the Array with an additional 2 km buffer. An additional recorded losses study area, defined as the Array with an additional 5 km buffer, was implemented through consultation at the request of HES after the receipt of their Scoping Representation (see volume 2, chapter 19). The recorded losses study area was used for the desktop study of recorded losses in the vicinity of the Array only, which is described in sections 6.1.7 and 6.1.9. These areas are shown in Figure 2.1   Open ▸ . The 5 km buffer for recorded losses is to account for the imprecise and often arbitrary spatial information associated with these records. These study areas were used as the search area for obtaining records from relevant archive databases. The marine archaeology study area allows for a greater understanding of the archaeological baseline environment with the dual purpose of enabling recognition of any archaeological trends in the region and allowing any archaeological sites to be represented in a broader archaeological context.
  2. The site-specific survey data collection area overlaps the Array, and in places exceeds it. The area of seabed geophysical data collection is referred to as the marine archaeology survey area and is shown separately in Figure 3.1   Open ▸ .

3.2. Desktop Study

  1. Marine archaeology is considered within the following categories:
  • Submerged prehistoric archaeology: this includes palaeochannels and other inundated terrestrial landforms that may preserve sequences of sediment of palaeoenvironmental interest, Palaeolithic and Mesolithic sites and artefacts.
  • Maritime archaeology: relates generally to craft or vessels and any of their associated structures and/or cargo.
  • Aviation archaeology: this comprises all military and civilian aircraft crash sites and related wreckage.

3.2.1. Data Sources

  1. Several sources were consulted in order to inform the desktop study of the technical report. These are provided in Table 3.1   Open ▸ .
  2. Data of wrecks and obstructions held by the United Kingdom Hydrographic Office (UKHO) was also consulted, but there were no records within the marine archaeology study area.

 

Table 3.1:
Summary of Key Data

Table 3.1: Summary of Key Data

Figure 3.1:
Marine Archaeological Survey Area

Figure 3.1: Marine Archaeological Survey Area


3.2.2. Data Structure

  1. In order to present a baseline for the purposes of this technical report, the sources in section 3.2.1 were compiled into gazetteers (see volume 3, appendix 19.1, annex A, B and C).
  2. The desktop data have been classified between records where material is known to be on the seabed and ‘recorded losses’. Recorded losses are records of vessels that have been lost in the area but have no verifiable positional data. They provide additional information on the historical maritime traffic of the area, but associated material is not known to survive on the seabed. A recorded losses study area was implemented to account for the imprecise locational information associated with these records.

4. Site-Specific Surveys

  1. A site-specific geophysical survey was carried out between 15 April 2022 and 20 July 2022. The survey resulted in the mobilisation of a Multibeam Echo Sounder (MBES), a Sidescan Sonar (SSS), a magnetometer, a parametric Sub-bottom Profiler (SBP), and a Two Dimensional (2D) Ultra High-Resolution Seismic (UHRS) ‘Sparker’ and hydrophone array.
  2. Survey operations were undertaken within a pre-defined boundary of approximately 859 km², which overlaps and, in places, exceeds the Array ( Figure 2.1   Open ▸ ).

4.1. Technical Specifications

  1. All geophysical data was collected to a specification that fulfils the requirements of Section 3 of Archaeological WSI for Offshore Wind Farm Projects (The Crown Estate, 2021).
  2. The data were collected to a specification appropriate to achieve the following interpretation requirements:
  • Sidescan Sonar: ensonification of anomalies > 0.5 m.
  • Multibeam Echosounder: ensonification of anomalies > 1.0 m.
  • Magnetometer (Transverse Gradiometer (TVG)): 5 nT threshold for anomaly picking.
  • Sub-Bottom Profiler: Penetration of 3 m was achieved.
  • Sparker 2D UHRS: Penetration of over 200 m was achieved.

4.2. Data Quality and Limitations

  1. The data collected across the extents of the marine archaeology survey area are of good quality overall. The survey line spacing was sufficient to achieve 100% coverage for the bathymetry data. SSS data achieved 80% seabed coverage. SBP data were collected to a pre-determined line plan, largely providing suitable coverage and penetration for the interpretation of the palaeoenvironment. The magnetometer data were collected to a pre-determined line plan suitable for the identification of ferrous material with a peak-to-peak amplitude of 5 nT, with the minimum detection size increasing with distance from the tracklines. The data are considered of an appropriate specification, coverage and quality to undertake a robust archaeological assessment to inform the EIA process, noting that additional data collection and interpretation will be required prior to construction.

4.3. Processing

  1. The archaeological assessment of data was undertaken by a qualified and experienced maritime archaeologist at MSDS Marine with a background in geophysical and hydrographic data acquisition, processing, and interpretation.
  2. MSDS Marine were provided with the survey deliverables by RPS, including both raw and processed data, alongside interpretations and operations reports. The primary deliverables are shown in Table 4.1   Open ▸ . The assessment considers the full extents of the survey data, which was collected within an area that covers and overlaps the Array.

 

Table 4.1:
Data Deliverables to MSDS Marine

Table 4.1: Data Deliverables to MSDS Marine

 

  1. The data were analysed in conjunction with the desktop datasets listed in Table 3.1   Open ▸ to determine any potentially significant archaeological material that may be present within the marine archaeology study area. The criteria for the assessment of archaeological material is presented in Table 4.2   Open ▸ .

 

Table 4.2:
Criteria for the Assessment of Archaeological Potential

Table 4.2: Criteria for the Assessment of Archaeological Potential

 

4.4. Combined Assessment

  1. Following the assessment of all datasets the results were imported into Environmental Systems Research Institute (ESRI) ArcGIS Pro 3.1.1, a GIS software package, and reviewed alongside each other, along with GeoTIFF files of the SSS, MBES, and magnetometer data. The concurrent review allows the amalgamation of duplicate anomalies, the assessment of the wider context, and an understanding of the extents of a feature that may be partially buried or span across two or more lines of data.
  2. Data from the UKHO, including the positions of wrecks and obstructions, and the relevant Canmore records, as well as all other relevant data such as third-party assets were assessed to ensure that any additional information is drawn upon, but also that anomalies are not unnecessarily identified as having archaeological potential. The resultant remaining anomalies assessed as having archaeological potential were compiled into a gazetteer (volume 3, appendix 19,1, annex C) and a GIS shapefile.
  3. The interpretation of geophysical and hydrographic data is, by its very nature, subjective. However, with experience and by analysing the form, size, and characteristics of an anomaly, a reasonable degree of certainty as to the origin of an anomaly can be achieved.
  4. Measurements can be taken in most data processing software, and whilst largely accurate, discrepancies can be noted due to a number of factors. Where there is uncertainty as to the potential of an anomaly, or its origin, a precautionary approach is always taken to ensure the most appropriate mitigation for the historic environment.
  5. It should be noted that there may be instances where an anomaly may exist on the seabed but not be visible in the geophysical data. This may be due to being covered by sediment or being obscured from the line of sight of the SSS. The use of both SSS and MBES data mitigates this by visualising anomalies from multiple angles, including from above. Anomalies were named following the standard MSDS Marine convention, [PROJECTYEAR_ID], e.g. OS23_XXX.