Enhancing the Positional Accuracy of Merged Regional Acoustic Corer Data Sets: A Case Study from an Offshore Wind Farm in the Baltic Sea

In offshore wind farm (OWF) construction, mitigating the risk of sub-seabed geohazards and investigating the foundation conditions for fixed bottom wind turbine generator (WTG) and offshore substation (OSS) platform locations is of paramount concern. In October 2023, Kraken Robotics carried out a sub-bottom survey using one of our cutting-edge imaging systems, the Acoustic Corer (AC), for an OWF development project in the Baltic Sea. The aim was to assess the conditions of the potential foundation legs for two OSS platforms, each with four legs. This involved interpreting and mapping cobbles and boulders below the seabed. The AC is equipped with low- and high-frequency chirp projectors and a parametric source on each boom that enables data collections spanning from 1.5 kHz to 12.5 kHz (Guigné et al., 2010; Guigné et al., 2012; Guigné & Blondel, 2017). This configuration operates from a stationary landed survey platform. The AC produces a 14 m diameter volumetric dataset, known as an “acoustic core”, down to 60 m below the seabed, depending on soil conditions. For this project, the main objective was to assess the shallow soil conditions for the survey area down to 15 m below the seabed while identifying acoustic responses that are suggestive of the presence of cobbles and boulders 0.2 m in length and width or greater. Kraken Robotics proposed to the client the utilization of the AC triple core application (Abbott et al., 2023). This technique had previously been employed at a proposed wind turbine generator (WTG) site in the Baltic Sea for a similar investigation into the soil conditions of the location, including the interpretation of geohazards, obstructions and stratigraphy. In the triple-core application, three acoustic cores are acquired separately, however they are positioned to partially overlap to provide increased data coverage over the location of interest. Subsequently, all three acoustic cores are merged into a single unified volumetric dataset and corrected laterally and vertically based on a common anomaly detected by each acoustic core, with the support of an ultra-short baseline acoustic positioning system (USBL). This approach integrates all volumetric datasets into a common framework, enabling a more coherent summation of anomalies within that volume. However, shallow water USBL positioning for the OWF survey typically resulted in an uncertainty of ±50 cm due to the shallow water in the survey area. Kraken Robotics devised a new approach to reduce positional uncertainty. A beacon was placed onto the seabed at each triple acoustic core location where all three acoustic cores could capture it. The beacon remained in the same position on the seabed until all three AC data sets were acquired. Subsequently, the beacon response captured by all three acoustic cores is vertically and laterally shifted to the exact location of the beacon during the pre-migration data conditioning. This adjustment, applied to the entire dataset, provided higher confidence towards merging three regional acoustic cores and the lateral and vertical correction of acoustic cores. This was followed by the standard seismic processing steps and three-dimensional (3-D) pre-stack Kirchhoff time migration using Kraken Robotics proprietary signal processing software, ZoomSpace. After processing and interpreting each triple-core, Kraken Robotics successfully reported 1057 anomalies suggestive of cobbles or boulders equal to or greater than 0.2 m in length and width at the two OSS platform legs. Additionally, the AC results were compared and correlated with stratigraphic interpretations obtained from previous geotechnical and geophysical surveys, showcasing that the acoustic cores represented the sediment stratigraphy of the survey area.