How Seabed Geophysics Serves Every Offshore Sector
Understanding the seabed and the shallow sub-seabed geology is fundamental to every offshore energy project. From renewables, oil & gas field developments and cable routes, to port expansions and environmental assessments, success depends on accurately understanding the geology of a site. The science of seabed and sub-seabed geophysical analysis has been well established for decades, and while the technologies and applications have evolved, the fundamentals remain the same.
Geophysical surveys and data analyses are used to detect and interpret contrasts in the physical properties of the ground. Changes in material density, magnetic field and sound velocity are recorded and those measurements turned into actionable knowledge.
What has changed is how broadly these fundamentals are now applied. The techniques once developed primarily to identify shallow hazards and reservoir characterisation for offshore oil and gas projects continue to innovate and be applied to optimising the locations of offshore wind turbines, designing subsea foundations or ensuring the safe and efficient installation of pipelines and cables. They also play a growing role in environmental and regulatory studies, where a clear understanding of the seabed and sub-seabed reduces uncertainty and mitigates risk.
This article will explore how the core principles of seabed geophysics remain constant, while their value extends across a wide spectrum of offshore industries. It will also highlight the importance of integrating geophysical data with geotechnical and environmental information to build reliable ground models that support better design, safer operations, and more cost-effective project delivery.
First and foremost: Geological Background
Geological desktop studies are a key first step before facing the financial outlay of deploying vessels and acquiring geophysical data. They set the foundation for survey design, hazard anticipation, and later geological interpretation.
Compiling existing geological knowledge is essential for understanding expected seabed and sub-seabed conditions. This process involves studying academic papers, geological maps, previous survey reports, and legacy seismic and borehole data. Together, these sources help identify lithology, structural trends, and depositional environments, and they also allow early identification of potential geohazards such as shallow gas provinces, submarine landslide areas, fault zones, and karst features. Key efficiencies in routing and/or location selection can be found following a good desktop study and in planning and designing site surveys. Further information can be found here.
Geophysical surveys can be planned and optimised based on the findings of the geological desktop study, whether it is defining survey extents, line spacing or orientation based on geological variability or ensuring appropriate geophysical techniques are used.
This early desktop study is useful in creating an interpretation framework and conceptual geological model prior to the geophysical survey data reaching the geophysicists’ workstation, reducing the ambiguity by linking observed features, objects, and morphologies with known regional and local geological features.
Analysis of the background data can provide an early indication of the likely soil and rock types, providing optimised positioning of boreholes, Cone Penetration Tests (CPT) or sampling sites. Prioritising target zones for hazard testing (e.g. slope instability, soft sediments).
Well-designed geological desktop studies can provide environmental baseline understanding (sediment types, habitats), highlight archaeological or cultural heritage potential (wreck sites, submerged landscapes) and provide baseline information required for permits and stakeholder engagement.
Once the geotechnical and environmental surveys and data analyses are complete, the results can be integrated to ground-truth the geophysical interpretation, providing a comprehensive overview.
Geophysical Survey
Geophysical surveys are the core of seabed and sub-seabed characterisation. By combining multiple geophysical sensors, they provide both the broad coverage and the detail needed to reduce uncertainty and guide engineering decisions at significantly lower cost than high density geotechnical sampling alone. The following methods are routinely employed to deliver a comprehensive understanding of site conditions across a range of offshore project types:
- Multibeam Echosounder (MBES)
- Purpose: High-resolution bathymetry and backscatter mapping.
- Applications:
- Charting of water depth and seabed morphology for safe engineering positioning, cable and pipeline routing
- Identifying bedforms (e.g. sand waves, ripples, dunes, ridges, scours)
- Mapping changes in seabed sediment
- Monitoring morphological changes in seabed conditions over time through seabed mobility and seabed stability studies
- Correlation and positional validation of features observed on other sensors.
- Side-Scan Sonar (SSS)
- Purpose: Acoustic imagery of seabed texture and features.
- Applications:
- Detection of debris, boulders, wrecks, and anthropogenic objects that pose installation hazards
- Characterising sediment types for route planning and environmental baseline studies
- Supporting UXO surveys by identifying anthropogenic objects on the seabed.
- Magnetometer / Gradiometer
- Purpose: Detection of ferrous objects and magnetic anomalies.
- Applications:
- Identifying unexploded ordnance (UXO) and metallic debris
- Informing the presence of igneous intrusions
- Mapping buried infrastructure such as pipelines and cables
- Supporting archaeological studies of shipwrecks or cultural heritage features.
- Sub-Bottom Profiler (SBP)
- Purpose: Imaging of the shallow sub-seabed (<10m) to reveal stratigraphy and sediment thickness.
- Applications:
- Locating geohazards such as shallow gas, sediment variability, faulting, and buried channels relevant to foundation design
- High resolution mapping of shallow soil units and their thickness for dredging, reclamation, port developments, and cable route engineering
- Providing inputs to ground models.
- Ultra-High Resolution Seismics (UHRS)
- Purpose: Detailed imaging of sub-seabed structures down to tens to hundreds of meters depth (dependant on acquisition set up).
- Applications:
- Mapping of shallow faults, channels and subsurface geohazards
- Supporting design of offshore wind turbine foundations and subsea infrastructure
- Refining stratigraphic models for integrated geophysical and geotechnical interpretation
- Extrapolating geotechnical properties beyond geotechnical sampling sites.
Ground-truthing
While geophysics provides broad coverage and imagery of the seabed and sub-seabed, geotechnical investigations deliver the direct, quantitative measurements required for engineering design. Geotechnical investigations confirm, calibrate, and expand upon geophysical interpretations, ensuring that offshore infrastructure is founded on a reliable understanding of the ground conditions. When combined with Geophysical survey techniques, geotechnical investigations form the backbone of any robust ground model. The following methods are routinely employed to deliver a comprehensive understanding of site conditions across a range of offshore project types:
- In-Situ Testing
- Purpose: Determine soil type and behaviour in situ.
- Types of in-situ test:
- Cone Penetration Test (CPT / CPTu): Soil stratigraphy, density, strength, stiffness, and pore pressure profiles
- Seismic CPT (SCPT): All of the above plus shear wave velocity for dynamic soil properties
- Borehole Logging
- Purpose: Provide continuous ground truth and monitoring capability.
- Applications:
- Geological logging of lithology and structure
- Downhole and cross-hole seismic for shear and compression wave velocities
- Resistivity, density, neutron, sonic/acoustic, calliper logging, Spontaneous potential, gamma ray and borehole imaging
- Pore pressure.
- Sampling and Laboratory Testing
- Purpose: Collect physical samples to determine soil and rock properties.
- Types of testing:
- Classification tests: Grain size distribution, Atterberg limits, bulk and dry density, unit weight, moisture content
- Strength and deformation testing: Triaxial, direct shear, oedometer, permeability, and consolidation behaviour
- Advanced testing: Resonant column for dynamic moduli, and cyclic tests
Conclusion
The techniques and technologies that originated in offshore oil and gas exploration have evolved into indispensable tools for today’s diverse offshore industries. Their heritage ensures proven reliability, while ongoing innovation enables applications that support the global energy transition. From optimising offshore wind turbine foundations to safeguarding subsea infrastructure and meeting environmental obligations.
By integrating geophysical, geotechnical, and environmental data, we reduce uncertainty, mitigate risk, and deliver cost-effective solutions across project lifecycles. Success depends on working with partners who are industry-agnostic yet deeply experienced in applying these methods across energy, infrastructure, and regulatory domains. This holistic approach ensures projects are not only technically sound but also sustainable and ready for the next generation of offshore projects.
Ternan’s Expertise
Ternan Energy provides full life cycle geological, geophysical and geotechnical expertise for a range of offshore projects and industries from Offshore Wind, Oil & Gas, Wave and Tidal energy to cable/pipeline design and route optimisation, port and coastal studies.
Ternan’s team of in-house geophysical and geotechnical experts are experienced in the full suite of geophysical and geotechnical survey methods and operations. They have expert knowledge in data interpretation and integration to support and critically inform clients on the needs of their project from the desktop study through the site investigations to a final ground model, allowing for a bespoke approach to meet the project objectives. Understanding the geology of the seabed and sub-seabed is integral to a project that’s successful both ecologically and economically. We have a full suite of ocean engineering expertise to support you – no matter where you are in the project lifecycle or the geographical location of your project.
For more information on our work in emerging renewable energy sectors, read our article on supporting Portugal’s wave energy future. To explore our specific capabilities in subsea cable projects, download our cables capability statement or contact info@ternan-energy.com