Why this matters

Offshore and nearshore projects depend on a clear ground model. Before a cable route is finalised, a berth is dredged, a foundation is designed, or a subsea asset is installed, the project needs to understand what is on the seabed, what is below it, and how the ground may behave during construction.

Marine geophysical surveys and marine geotechnical investigations support the same overall goal, but they do not produce the same information. Geophysics maps the seabed and shallow subsurface across an area or route. Geotechnical investigation directly tests or samples the ground at selected locations.

What a marine geophysical survey gives you

A marine geophysical survey is normally non-invasive. It uses acoustic, magnetic, seismic and positioning sensors to build a continuous picture of seabed and shallow ground conditions.

Typical methods include multibeam echosounder for bathymetry, backscatter for seabed texture, side scan sonar for seabed features and debris, sub-bottom profiler or UHRS for shallow geology, and magnetometer or gradiometer for ferrous targets, possible UXO, cables, pipelines or buried metallic objects.

The strength of geophysics is coverage. It can identify rock outcrop, sediment changes, channels, sand waves, scour, boulders, debris, buried objects and geological boundaries between sample points. The limitation is that the data is indirect. A reflector, sonar contact or magnetic anomaly still needs interpretation and, where the decision matters, ground-truthing.

What a marine geotechnical investigation gives you

A marine geotechnical investigation directly tests or samples the seabed and sub-seabed. It provides the soil and rock information needed for design, installation and construction planning.

Common methods include CPT/CPTu, vibrocore, gravity core, piston core, box core, boreholes and laboratory testing. These methods confirm soil type, strength, density, stiffness, carbonate content, consolidation behaviour, rock quality, dredgeability and other engineering parameters.

The strength of geotechnical data is that it provides direct evidence at the tested location. The limitation is that it is point-based. A CPT or core does not automatically explain what happens between locations. This is why geotechnical locations should be selected using the geophysical interpretation wherever possible.

How they work together

The best offshore investigation usually starts with geophysics. The geophysical survey builds the seabed and shallow geology model first. That model is then used to select smarter CPT, core or borehole locations.

This avoids blind sampling and targets the areas that matter: representative ground, transition zones, rock risk, burial risk, anomaly areas, crossing points, dredging footprints, proposed foundation locations and high-consequence sections.

The result is not more data for the sake of data. The result is a better ground model that supports engineering and construction decisions.

When geophysics may be enough

Geophysics may be enough for early route option screening, seabed feature mapping, reconnaissance bathymetry, preliminary hazard mapping, hydrographic survey, early dredging assessment and selecting target locations for later intrusive investigation.

When geotechnical data is required

Geotechnical data is required when design depends on soil or rock properties. Examples include foundation design, cable burial assessment, trenchability, ploughability, dredging behaviour, anchor holding, jack-up assessment, settlement, slope stability and engineering approvals.

When both are needed

Most offshore engineering projects need both. This includes submarine cables, ports, dredging campaigns, pipelines, offshore renewables, subsea infrastructure, UXO clearance and mooring or foundation scopes. Geophysics gives the continuous spatial picture. Geotechnical testing confirms what the material is and how it behaves.

QOffshore approach

QOffshore treats geophysical and geotechnical work as one integrated ground investigation, not two disconnected deliverables. Our focus is practical: define the seabed conditions, confirm the ground-truth where it matters, and turn the results into decisions for routing, burial, dredging, construction methodology and project risk.

We use geophysics to plan geotechnical testing, and geotechnical results to calibrate the geophysical interpretation. This gives clients a clearer and more defensible ground model before major engineering or construction commitments are made.