NSG2023 1st Conference on Sub-surface Characterisation for Offshore Wind

The development of offshore windfarms requires an in-depth characterization of large areas that typically have experienced a complex geological history affected by glacial-interglacial cycles as well as sea-level fluctuations. Advanced geo-data integration allows the definition of the Quaternary stratigraphic soil units and their associated depositional environments, which is required to create a more robust and potentially predictive ground model that can complement or replace some datatypes under certain conditions. In this paper, we conduct a qualitative integration to characterize the sedimentary environments for two windfarm sites in the Southern Baltic Sea. We discuss the extent of subglacial tills across the development site and the origin of different vintages of channel systems, and investigate the similarity between lacustrine deposits occurring at different stratigraphic levels, and the homogeneity of outwash sedimentation.

Before constructing wind turbines in the south of Groix island, overseas measurements of the total magnetic field were conducted to locate the Unexploded Ordnances (UXO) buried since the Second World War. A set of two magnetometers pulled by a boat was used to measure the magnetic field in go-and-return trajectories. The measurements along north and southward trajectories were correlated. However, the magnetometers systematically recorded a relatively higher magnetic field during the northward course. Using both north and southward measurements resulted in a heterogenous dataset, which created azimuthal artifacts on the produced maps. A Gaussian transformation was used to adapt the statistical distribution of the southward measurements to the northward ones and make the dataset homogeneous. Therefore, the azimuthal artifacts were considerably reduced, and potential UXO locations were detected more easily. Exploratory data analysis and mapping steps were done entirely in the Isatis.neo™ geostatistical software.

The increasing demand for offshore wind farms and the need for detailed sub-bottom geotechnical information has led to an increase in shallow-tow high-resolution or ultra-high-resolution seismic acquisition. However, the resulting data presents several processing challenges, particularly in receiver deghosting.

Shallow-tow streamers and sources are more susceptible to wave interaction, leading to source variability and receiver datum issues. Additionally, the commonly used flat surface model with a constant reflection coefficient may be inadequate for approximating acoustic waves reflecting from an undulating water surface. While trace-by-trace deghosting is effective for deeper-towed streamers, it may present challenges for shallow-tow data with a single visible ghost notch.

To address these challenges, we propose a multi-layered approach to deghosting shallow-tow high-resolution seismic data. The first layer involves streamer-by-streamer sparse estimation to calculate ghost-free waveforms and ghost delays in short windows around the first arrival times for each trace. The results are used as initial values for the non-linear second layer, where multiple shots are analysed together to yield time- and offset-dependent delay times, reflection coefficients and signal spectra. The final layer utilises this information to deghost the original seismic data, providing improved results compared to traditional receiver deghosting techniques.

A 3D Ground Model was built in Leapfrog™ for the Hesselø site, a pre-bid area announced by the Danish government. First a geomodel was constructed from seismic data and surfaces and then numerical models of cone penetration tests (CPT) and calculated geotechnical parameters were generated. Finally synthetic CPT logs were blind tested and extracted for selected WTG test locations. The under-burden at the Hesselø site is characterized by a fine-grained ‘post-glacial’ sequence overlaying a high strength till. Models of the calculated shear strength (Su) show very low strength for the fine-grained sequence indicating the dominance of soft unconsolidated clay which is furthermore supported by lab tests.

With a thickness ranging from 25–100 meters the fine-grained sequence constitutes very challenging soil conditions for a foundation design. In fact, it has resulted in a suspension of the site by the authorities. Similar conditions might be present in other proposed wind farm sites and might represent an underestimated risk or cost factor.

Efficient workflows and tools that integrate and process geology and under-burden data as well as early involvement of these studies in the planning phase can help to manage geological risks and lead to cost savings.

This study presents evidence of shallow gas and pockmarks interpreted on various types of high-resolution geophysical datasets, within the Morro Bay Wind Energy Area, offshore California. The Morro Bay WEA lies within the Big Sur Pockmark Field, consisting of more than 5000 individual pockmarks. The pockmarks are presently inactive, however, due to lack of data, the triggering mechanism leading to the formation of the pockmarks has not been determined. The paper describes potential pockmark formation mechanisms, including that linked to gas hydrate dissociation. Finally this paper briefly describes the impacts of shallow gas, fluid seepage, gas hydrate dissociation on the geotechnical properties of sediments which can in-turn impact the design of offshore wind farm foundations.

In this study, we explored the resolving potential of UHR seismic data through the integration of broadband processing with a modern diffraction imaging workflow to improve temporal as well as lateral spatial resolution. The processed data were subjected to deghosting and wave field separation as a prerequisite for diffraction imaging. The conducted work demonstrates the adaptation of broadband processing to be advantageous for diffraction processing and imaging. The results of UHR broadband processing combined with diffraction processing exhibited enhanced resolution of diffracted wave fields, and increased resolution in focused events when compared to the non-broadband approach. This supports an improved identification and imaging of small-scale structural features. The broadband diffraction spectrum displayed a considerably wider bandwidth than the conventional diffracted wave field, highlighting the efficacy of the proposed approach for diffraction imaging. The study indicates the utilization of UHR broadband diffraction data to obtain additional information on small-scale heterogeneities in the subsurface. That has notable benefits for investigating complex geological settings in offshore site characterization for windfarm development.

Poor knowledge of source and receiver positions in ultra-high-resolution marine seismic data is the cause of severe damage which requires novel processing techniques to mitigate. Current positioning technologies are limited partly by their accuracy but also the fact that they are only placed on head and tail buoys of the towed arrays. This leaves receiver locations on the length of the streamer cable to be interpolated. Rather than developing additional processing methods, we propose to improve the quality of the data by introducing a complimentary receiver positioning system to reconstruct the shape of the streamer cable in 3D using Fiber Optic Shape Sensing (FOSS) technology. In this abstract, we present the methods and hardware of this method as well as our progress in the development of experiments designed to establish the viability of FOSS in the field.

The in-depth integration of sparse 1D geotechnical data with 2D UHR seismic reflection in a consistent geological framework forms the back-bone of the data-driven ground model approach. In order to predict CPT or geotechnical parameters (and their uncertainties) across the entire development area, one typically relies on geostatistical methods, like 3D kriging, Considering that the 2D line spacing is often larger than key geological phenomena, this interpolation will lead to uncertainty. In this paper, we investigate the effect of line spacing and geological complexity on the model prediction, using the TNW site (offshore the Netherlands) as a case study. We focus on an area with ultra-high-resolution 3D data, and decimate the volumes of 4 sub-sets with distinct geological features and complexity in order to assess the uncertainty on the interpolation, using both geostatistical and machine learning methods.

2D, and particularly 3D Ultra High Resolution (UHR) seismic surveying has become an invaluable tool for assessing the geological subsurface for offshore wind developments. In the case of The Ten noorden van de Waddeneilanden Wind Farm Zone (TNWWFZ), a number of geohazards have been detected using these techniques including tunnel valleys, deformed sediments, shallow gas, etc. These features are variable in size and type of sediment infill, with some exhibiting deformation characteristic of glacial tectonic activity and others of buried dead ice landscapes. Both the deformation and complexity of sedimentation raises concerns for engineers as it puts into question the load bearing capacity of the subsurface. Glacial tectonics form during fully glacial conditions whilst kettle holes form during glacial regression when sediments bury dead ice within the landscape. Once buried this dead ice is subjected to higher pressure and temperature causing it to melt forming a small valley and depositing any previously frozen sediment. The resulting depressions can accommodate large volumes of sediments and create a 3D distribution of properties that is impossible to model without 3D seismic data or very dense 2D grids.

In recent years, marine geophysical techniques such as marine seismic survey, sea-floor controlled-source electromagnetic (CSEM), marine magnetotelluric (MMT), and marine gravity survey have developed rapidly, resulting in various underwater devices have been developed for detecting different target fields. Meanwhile, since joint observation and inversion of multiple ocean physical fields has become a popular research topic, it is necessary to develop the corresponding integrated underwater observation devices. Considering that the single-function seabed acquisition instruments cannot effectively meet the requirements of marine multi geophysical joint exploration, we developed the OBMIX marine seismic-electromagnetic joint acquisition station, which integrates the functions of seabed observation equipment such as OBS and OBEM, to synchronously collect underwater seismic and electromagnetic signals. By utilizing the joint exploration and inversion of seismic, electromagnetic, and geomagnetic data, we can improve exploration efficiency and inversion accuracy. Two prototypes of the instrument have been installed in 2022. By now, we have completed indoor and offshore testing of the two prototypes, and the test results meet R&D expectations. According to the research plan, the deep-sea testing is also on schedule.