Looking Beneath the Seabed: How Sub-bottom Profilers and Marine Magnetometers Reduce Offshore Project Risk

When planning an offshore wind farm, submarine cable route, port expansion, bridge crossing, or other marine infrastructure project, accurately mapping the seabed is only the beginning.

Just as important is understanding what lies beneath the seabed.

A bathymetric survey can accurately map water depths and seabed topography, but it cannot reveal whether the sediments beneath are soft clay, dense sand, buried rock, or whether hidden metallic objects lie below the surface.

For engineers and developers, these unknowns can introduce significant construction risks, design changes, project delays, and unexpected costs.This is why many offshore developments go beyond traditional bathymetric surveys by incorporating additional geophysical survey technologies such as Sub-bottom Profilers and Marine Magnetometers.

Together, these technologies help answer two important questions:

• Can the seabed safely support what we're planning to build?

• Are there hidden hazards beneath the seabed that could affect construction?

Rather than simply collecting more data, these investigations help developers make more informed engineering decisions before construction begins.

Why Looking Beneath the Seabed Matters

Imagine you're planning to install dozens of offshore wind turbines. Your bathymetric survey confirms that the water depth is suitable, and the seabed appears relatively flat.

At first glance, everything looks ready for construction. However, beneath the seabed may lie thick layers of soft sediments incapable of supporting turbine foundations, buried rock that could complicate pile installation, or even metallic objects left behind from previous maritime activities.

None of these conditions are visible in a bathymetric survey alone. Similarly, when installing a submarine power cable, the proposed route may appear clear on the seabed, yet buried obstacles or unsuitable sediments beneath the surface could significantly affect cable installation and long-term reliability.

At AB Surveying, we often explain to clients that bathymetry tells us what the seabed looks like, while geophysical investigations tell us what lies beneath it. Both are equally important for many offshore developments.

Sub-bottom Profilers: Understanding the Ground Beneath the Seabed

A Sub-bottom Profiler (SBP) is designed to investigate what lies beneath the seabed. Unlike a Multibeam Echo Sounder, which maps the surface of the seabed, a Sub-bottom Profiler emits low-frequency acoustic pulses capable of penetrating the sediments below.

As these sound waves travel through different geological layers, part of the signal is reflected back to the receiver. Because different materials reflect sound differently, the resulting profile reveals the thickness, layering, and characteristics of sediments beneath the seabed.

Rather than simply knowing how deep the water is, engineers gain valuable insight into the conditions they may encounter during construction.

When Would You Need a Sub-bottom Profiler (SBP)?

Offshore Wind Farms

One of the first questions offshore wind developers ask is whether the seabed can safely support turbine foundations. Although bathymetric surveys confirm water depth and seabed topography, they cannot determine whether the underlying sediments are suitable for foundation installation.

Offshore wind turbines transfer enormous loads into the seabed through their foundations. Before selecting a monopile, jacket foundation, or another foundation type, engineers need to understand whether the underlying sediments can safely support these structures.

A Sub-bottom Profiler helps engineers identify soft sediments, dense sand layers, buried rock, or historical river channels beneath the seabed. This information allows foundation designers to select the appropriate foundation type, anticipate difficult ground conditions, and reduce costly design changes during construction.

Why it matters: Better knowledge of subsurface conditions reduces design uncertainty, minimizes costly foundation redesigns, and helps determine where further geotechnical investigations should be focused. Understanding subsurface conditions early helps reduce construction risk and improves confidence in foundation design.

Submarine Power and Communication Cables

Submarine cables are typically buried beneath the seabed to protect them from anchors, fishing activities, vessel traffic, and natural erosion. However, successful cable burial depends on the sediments beneath the seabed.

Sub-bottom profiling allows engineers to evaluate whether the proposed cable corridor contains materials suitable for trenching and burial, while also identifying geological conditions that could complicate installation.

Why it matters: Selecting the most suitable cable route before construction begins helps reduce installation challenges, construction delays, and long-term maintenance risks.

Port Developments and Marine Infrastructure

Whether constructing a new quay wall, jetty, or marine terminal, understanding the strength and thickness of seabed sediments is essential.

Sub-bottom profiling helps engineers understand the thickness and characteristics of seabed sediments beneath proposed marine structures. This additional information provides a better understanding of subsurface conditions before detailed design and construction begin.

Why it matters: Better understanding of subsurface conditions helps reduce uncertainty during planning and supports more informed engineering decisions.

Dredging Projects

Not every dredging project requires the removal of every layer beneath the seabed.

In many cases, engineers only need to remove accumulated soft sediments while leaving harder underlying materials undisturbed. Bathymetric surveys can identify where sediment has accumulated, but they cannot always determine what lies beneath those sediments.

Sub-bottom profiling allows engineers to distinguish between recently deposited soft sediments and harder underlying geological layers.

Why it matters: More accurate dredging estimates translate to better project planning, improved budgeting, and reduced construction costs.

Land Reclamation and Coastal Development

Large reclamation projects require a thorough understanding of the seabed before fill materials are placed.

Sub-bottom profiling helps engineers identify weak sediment layers, buried channels, or geological features that may influence settlement and long-term ground performance.

Why it matters: Understanding these conditions early supports more reliable reclamation designs and reduces the likelihood of costly ground improvement works later in the project.

Bridge Foundations

When constructing bridges across rivers or coastal waters, the stability of each pier depends not only on the water depth but also on the materials beneath the riverbed.

Sub-bottom profiling helps engineers understand sediment thickness and identify buried geological features that may influence foundation design.

Why it matters: Early identification of subsurface conditions reduces uncertainty during foundation construction and helps determine whether additional geotechnical investigation is required.

Marine Magnetometers: Detecting Hidden Metallic Hazards

While a Sub-bottom Profiler investigates geological conditions, a Marine Magnetometer focuses on detecting buried ferrous objects that could interfere with construction or create safety hazards.

Rather than producing an image of the underwater terrain, the instrument measures subtle variations in the Earth's magnetic field caused by metallic objects.

These anomalies may indicate the presence of:

• Buried pipelines

• Abandoned anchors

• Shipwreck debris

• Metallic infrastructure

• Unexploded ordnance (UXOs)

Many of these hazards remain invisible during standard bathymetric surveys.

When Would You Need a Marine Magnetometer?

Offshore Wind Farms

Offshore wind farms often involve installing dozens or even hundreds of turbine foundations across large offshore areas. Marine Magnetometer surveys help identify magnetic anomalies that may require further investigation before construction begins.

Marine Magnetometer surveys help identify magnetic anomalies that may require further investigation before construction begins.

Why it matters: Identifying potential hazards during the planning stage helps improve construction safety while reducing costly interruptions offshore.

Submarine Cable Installation

Submarine cable routes frequently cross areas containing existing marine infrastructure.

Marine Magnetometers help identify buried pipelines, abandoned cables, anchors, or metallic debris that may interfere with cable installation or require route adjustments.

Why it matters: Detecting these obstacles before installation helps avoid unexpected delays and protects both existing infrastructure and newly installed cables.

Pipeline and Existing Marine Infrastructure Investigations

Many offshore developments are constructed near existing subsea infrastructure.

Marine Magnetometers help identify buried pipelines, abandoned cables, anchors, and other metallic assets that may not be accurately reflected in historical records.

Why it matters: Confirming the location of existing infrastructure helps avoid accidental damage during excavation, dredging, or new installations while supporting safer construction planning.

Areas with Historical Maritime Activity

The Philippines has a long maritime history, including significant naval activity during World War II. While not every project site contains historical hazards, some offshore areas may still contain shipwreck debris or unexploded ordnance (UXOs).

For offshore developments involving pile installation, dredging, or submarine cable installation, Marine Magnetometer surveys provide an additional level of confidence by identifying magnetic anomalies that may require further investigation.

Why it matters: Early identification of potential UXOs or historical debris supports safer construction and helps developers address risks before work begins.

Pre-construction Marine Site Clearance

Before major marine construction begins, developers often perform site clearance investigations to identify hazards that could affect construction equipment or installation activities.

Marine Magnetometer surveys form an important part of these investigations by locating buried metallic objects requiring removal or further inspection.

Why it matters: Completing these investigations before mobilization reduces unexpected construction delays, minimizes project risk, and provides greater confidence that the site is ready for offshore works.

Working Together with Bathymetric Surveys

Sub-bottom Profilers and Marine Magnetometers do not replace bathymetric surveys.

Instead, they answer different engineering questions.

Technology	The Question It Answers
Multibeam Echo Sounder (MBES)	What does the seabed look like?
Sub-bottom Profiler (SBP)	What lies beneath the seabed?
Marine Magnetometer	Are there buried metallic objects or hazards that could affect construction?

Together, these technologies provide engineers with a far more complete understanding of the offshore environment than any single survey method alone.

Choosing the Right Investigation

Not every offshore project requires every geophysical survey. A routine bathymetric survey for navigation or reservoir mapping may only require information about the seabed itself. However, projects involving offshore wind farms, submarine cables, bridge foundations, marine infrastructure, or major dredging operations often require a deeper understanding of both the geological conditions and potential hazards beneath the seabed.

At AB Surveying, we begin by understanding each project's engineering objectives before recommending the appropriate combination of survey technologies. In many cases, integrating bathymetric surveys with Sub-bottom Profilers and Marine Magnetometers provides engineers with the information they need to reduce uncertainty, improve safety, and make more confident construction decisions. Our goal is not simply to map the seabed. We help our clients understand the conditions beneath it, identify potential risks early, and provide the information needed to design and build with greater confidence.

Final Thoughts

Successful offshore developments depend on much more than knowing the depth of the water. Understanding the seabed is important but understanding what lies beneath it can be the difference between a smooth construction process and costly unforeseen challenges.

By integrating bathymetric surveys with Sub-bottom Profilers and Marine Magnetometers where appropriate, developers gain a more complete understanding of the offshore environment before construction begins.