4. Pre-Construction Geophysical and Geotechnical Surveys

4.1. Introduction

  1. Pre-construction geophysical and geotechnical work is anticipated within the Array. Archaeological input into survey specifications will be obtained to secure specialist advice on methods to investigate areas of archaeological potential and avoid impacts on known heritage assets.

4.1.1. Health and Safety

  1. Health and safety considerations are of paramount importance in conducting all archaeological work in relation to the mitigation outlined in this outline WSI and PAD. Safe working practices will always override archaeological considerations. All work must be carried out in accordance with the Health and Safety at Work etc. Act 1974, the Management of Health and Safety at Work Regulations 1999 the Federation of Archaeological Managers and Employers health and safety manual (Federation of Archaeological Managers and Employers 2010) and all other relevant and contemporary health and safety legislation, regulations and codes of practice that apply at the time the works take place.
  2. The RA will ensure that any MSs prepared to meet the requirements of the WSI are compliant with the requirements of the Applicant‘s Health and Safety Plans.

4.2. Planning

  1. When planning geophysical and geotechnical surveys the Applicant will advise the RA (or RPS supported by MSDS Marine if pre-submission) well in advance of the survey planning to secure specialist input into the scope of works.
  2. The RA will advise on measures to optimise archaeological results from the geotechnical and geophysical surveys. Areas to be considered will include:
  • the available details on previously identified sites and/or anomalies and areas of heightened archaeological potential;
  • the archaeological potential of areas where no existing sites and/or anomalies are yet known;
  • the equipment, equipment settings, survey methodologies and data collection points that will optimise the recovery of archaeological information; and
  • the requirements for data analysis, interpretation and archiving.
  1. The advice from the RA may also include:
  • altering vibrocore/borehole positions to maximise the potential for the collection of archaeological data; and/or
  • altering grab sample positions to maximise the potential for the collection of archaeological data.
  1. The results of the surveys will be used to inform the final positioning of wind turbines and other infrastructure. Where a direct impact on a heritage asset is likely to occur, any anomalies identified will be subject to further investigation to determine whether they represent archaeological material. In situations where such investigation suggests an archaeological origin, the anomalies in question will be subject to formal AEZs. The Applicant will consult the RA in advance of the finalisation of the wind turbine and infrastructure positions to ensure that known archaeological constraints identified by these surveys are avoided.
  2. A detailed MS will be produced by the RA in advance of each geophysical and geotechnical survey. Approval by the Archaeological Contractor will be assumed if no response is received within 30 days of submission of each individual MS.
  3. The MSs produced will be in accordance with the clauses set out in the Archaeological Written Schemes of Investigation for Offshore Wind Farm Projects (The Crown Estate, 2021).

4.3. Geophysical Survey

  1. A site-specific survey was carried out between 15 April 2022 and 20 July 2022 wihin the marine archaeology survey area shown in Figure 2.2   Open ▸ . The survey resulted in the mobilisation of a Multi-beam 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. This survey is described in further detail within volume 3, appendix 19.1.
  2. New geophysical data obtained for areas where direct impacts to archaeological receptors are anticipated will undergo analysis by a suitably qualified Archaeological Geophysical Contractor.
  3. Additional survey data may be collected to inform potential modifications to an AEZ that has been implemented to protect identified sites and/or anomalies of interest. In this instance the collection of such data will be supervised by a suitably qualified Archaeological Geophysical specialist who may be present on the survey vessel to ensure the quality of these data.

4.3.1. Survey Specifications

  1. Should further site-specific geophysical surveys be required for the purposes of informing approaches to marine archaeology receptors, the following specifications are advised to complement advice from the RA. These specifications are based on the Historic Environment Guidance for the Offshore Renewable Sector (COWRIE, 2007) and Marine Geophysics Data Acquisition, Processing and Interpretation Guidance Notes (Plets et al., 2013):
  2. Surveys will be carried out to a single datum and co-ordinate system. All survey data, including navigation (position, heading and velocity), will be acquired digitally in industry-standard formats. Care will be taken to maintain the orientation and attitude of sensors online. Track-plots will be corrected for layback (including catenary effects) and made available in digital, Geographic Information System (GIS) form.
  3. SSS will be carried out at frequency, range and gain settings capable of resolving all objects that are 0.5 m and above throughout the survey area. Preferably, line spacing will be equal to or less then the effective range and no more than 1.75 times the effective range. Anomalies of apparent archaeological potential will be ‘boxed’ by at least two, preferably four, lines along and across the principal axis of the anomaly. These lines will be offset so that the anomaly does not lie immediately beneath the survey fish and run at optimal frequency and range settings for imaging the anomaly. SSS data will be made available to the interpreting archaeologist in the form of raw, un-mosaic files in a suitable proprietary format.
  4. A sub-bottom survey (seismic) will be carried out using a source capable of resolving internal structures to the full depth of impact from the Array within Quaternary deposits. Line and cross-line spacing, and orientations will be sufficient to resolve the extents and characteristics of the principal Quaternary deposits. A single beam echosounder will be run in conjunction with the sub-bottom survey; the first reflector (seabed) will be levelled with reference to a tide gauge. Sub-bottom data will be made available in a suitable proprietary format.
  5. A magnetometer survey will be carried out using a caesium gas or equivalent system capable of resolving anomalies of 5 nT and above. Lines can be run in conjunction with other sensors (i.e. on the same line spacing and orientation) but provision will be made to run additional lines and cross-lines in areas of apparent archaeological potential, as indicated by the desk-based information or any of the other sensors. Magnetometer data will be made available as cleaned, de-spiked and tidally-corrected text (x, y and z) files for each line, including layback.
  6. Where a MBES is to be carried out solely for archaeological purposes, a system capable of achieving an effective cell/bin size better than 1 m is preferred. Use of a beam-forming system is preferred. The entire survey area will be ensonified. Where an anomaly of apparent archaeological potential is identified, an additional single slow pass will be carried out at the highest possible ping rate. Single beam and multi-beam data will be made available as de-spiked and tidally-corrected text (x, y and z) files for each line, in addition to any gridded/rendered surfaces.
  7. Geophysical surveys may be required for a number of purposes and may be to a lower specification than is generally required to be optimum for archaeological surveying purposes as identified above. If a lower specification is proposed, then it is important that early consultation is undertaken with the RA to allow scope for additional lines of survey in order to fully understand the nature and extent of anomalies identified.
  8. If a UXO survey is conducted the magnetometer dataset will be subject to archaeological analysis in order to clarify the nature and extent of the known sites and anomalies and to identify as yet unknown buried sites.

4.3.2. Interpretation

  1. Once the surveys have been processed to meet their primary objectives, the survey data, together with factual reports will be made available in digital formats to the RA or a suitably qualified Archaeological Contractor for archaeological analysis and interpretation.
  2. Archaeological interpretation will include:
  • examination of SSS, magnetometer, MBES and seismic data for areas within the vicinity of known wreck sites and previously identified archaeological anomalies;
  • examination of SSS, magnetometer, MBES and seismic data within areas that will be subject to direct impacts from the Array to identify any unknown wreck remains;
  • the assessment of seismic data to plot the general trend of the sub-surface sediments with archaeological potential; and
  • further detailed interpretation of seismic data following the initial assessment within those areas that will be subject to impacts from the Array.
  1. The archaeological results of any further geophysical survey will be compiled as a report by the RA or the Archaeological Contractor (if different to the RA) and will include likely requirements (if any) for further archaeological work. The report will be submitted to the Applicant by the RA and then on to the Archaeological Contractor for approval. The scope of any further work will be agreed between the Applicant (through the RA) and the AC. The work may be used to inform the final design.

4.4. Geotechnical Surveys

  1. Geotechnical data assessment will be subject to a staged programme of assessment and analysis by a suitably qualified Geoarchaeological Contractor. Early planning and liaison with the Geoarchaeological Contractor to enable the archaeological recording of intact cores will be a key requirement of this dataset.
  2. As summarised in section 6.5 of volume 3, appendix 19.1, the potential for surviving submerged prehistoric archaeology is low. The staged approach to geotechnical data assessment presented below illustrates the potential full realisation of an assessment programme. Each stage, however, is justified based on the satisfactory results and justification from the previous stage. The requirement for geoarchaeological assessment will be established in consultation with a Geoarchaeological Contractor and the AC and with advice from the RA.
  3. The stages are based on the Historic Environment Guidance for the Offshore Renewable Sector (COWRIE, 2007). Each stage of this phased assessment of the cores is subject to the results of the preceding stages:
  • Stage 1: Archaeological Assessment of geotechnical logs by a competent geoarchaeologist upon completion of the geotechnical investigations. This will provide an overview of the sedimentary sequence within the area, including whether any organic material is present and whether there are homogenous sedimentary layers throughout the area.
  • Stage 2: Geoarchaeological recording of geotechnical cores identified in Stage 1 if the Stage 1 assessment identifies horizons of archaeological potential. This will entail the detailed recording of the sediments within selected cores for a range of palaeoenvironmental indicators and dating material. The geotechnical cores need to be retained until the selection for archaeological recording has been made. Ideally one undisturbed half of each core is required for archaeological recording. The assessment programme will comprise the longitudinal splitting of each core section, the cleaning of half of each section and the detailed archaeological recording of each section, noting sediment colour, sediment type, sedimentary architecture and inclusions. A Stage 2 outline report will present the results of the archaeological recording and will indicate whether a Stage 3 laboratory assessment of the cores is warranted. The scope of further work will be agreed by the Applicant (via the RA) and the AC. If no further work is recommended a final (Stage 5) report will be produced by the Geoarchaeological Contractor.
  • Stage 3: Geoarchaeological Assessment of selected cores identified during the Stage 2 recording as warranting further laboratory assessment. Stage 3 will comprise the sampling and laboratory assessment to a level sufficient to enable an assessment of the value of the palaeoenvironmental material (pollen, diatoms, ostracods and foraminifera) surviving within the cores along with sampling for radiocarbon (C-14) and/or optically stimulated luminescence (OSL) dating purposes. The assessment seeks to further establish the preservation, diversity and quantity of palaeoenvironmental material to further refine the interpretation of the sedimentary sequence. A Stage 3 outline report will present the results and will indicate whether further (Stage 4) analysis of samples is required. The scope of further work will be agreed by the Applicant (via the RA) and the AC. If no further work is recommended a final (Stage 5) report will be produced by the Geoarchaeological Contractor.
  • Stage 4: Geoarchaeological analysis of the core material. It typically involves the full counts and analysis of pollen, diatom, ostracod and foraminifera samples and will be supported by C-14 dating of suitable sub-samples. This phase will result in an account of the successive environments within the coring area, a model of environmental change over time and an outline of the archaeological implications of the analysis. It will include the incorporation of the results into a model of the seabed sediments and palaeotopography based on the analysis of the seismic data. If a full seismic analysis has not been undertaken prior to this point it will be required.
  • Stage 5: A final report will be prepared by the Geoarchaeological Contractor at the end of the last stage of works. This will include a synthesis of all aspects of the palaeotopography, geoarchaeology and prehistory of the area affected by the Array. This will be based on the results of the archaeological work carried out in support of the Array Application in addition to the pre-construction surveys. It may include relevant data generated from the Marine Archaeology Technical Report, geophysical surveys and particularly seismic data. The report will incorporate appropriate 2D and Three Dimensional (3D) illustration of the sediment sequences identified in different parts of the development zone. This will include a geoarchaeological deposit model but will also include plans and sections of areas where more detailed analysis has been possible. The RA will submit the report to the Applicant and the AC for approval.