Magnetic Detection

Diagram of a towed marine magnetometer detecting the magnetic anomaly around a shipwreck

Marine magnetometry supports the search for ferrous objects lying on the seabed or buried in sediments. Towed behind the survey vessel, the magnetometer measures variations in the magnetic field to map anomalies. It complements sonar surveys, particularly for wreck searches and the assessment of potential unexploded ordnance (UXO).

Principle

A marine magnetometer measures variations in the Earth’s magnetic field. These variations may arise from geology, natural changes in the field or ferrous objects lying on or beneath the seabed.

The survey identifies magnetic anomalies for interpretation in their geological and operational context. An anomaly alone does not establish the nature of a target.

Equipment

CERES uses Geometrics G-882 marine magnetometers, either individually or in a gradiometer configuration.

Multiple sensors measure differences in the magnetic field between their positions. This configuration supports anomaly analysis and target positioning.

Operating range: 20,000–100,000 nT

Heading error: less than 1 nT

Depth rating: approximately 2,730 m

Sensitivity: 0.004 nT/√Hz

Maximum sampling rate: 20 Hz

GEOMETRICS G882

Methodology

Acquisition-Processing

Mapping

Survey-line spacing and sensor altitude above the seabed are defined for each mission, according to the target size, required detection objectives and site conditions.

CERES specifies these parameters in the technical proposal, which may include a site plan with proposed survey lines. Detection performance depends on the target and survey conditions.

Data acquisition and processing use HYPACK. The International Geomagnetic Reference Field (IGRF) provides a reference model for mapping magnetic anomalies.

Survey speed is adapted to the mission, generally around 3 knots. Processing and interpretation take account of survey conditions and the positioning of the sensors.

Magnetic anomaly maps can be compared with side-scan sonar mosaics to help locate and characterise targets. Combining these datasets supports the assessment of potential unexploded ordnance (UXO).

Further investigation may be needed to establish the nature of a target. Contact CERES to define the survey methods suited to your project.