Marine magnetics

In marine geophysics magnetics plays an important role, since practically the entire seafloor of the oceans consists of strongly magnetized basaltic rocks overlain by a cover of sediments. The basalts are formed continuously at mid-ocean ridge systems from cooling magmas and drift apart with the plate motion. From still not fully understood causes the Earth's magnetic polarity reverses at irregular intervals. Along with the ability of magnetic minerals to freeze the direction and strength of the Earth's magnetic field during the cooling of hot rocks, this is opening up significant opportunities for marine magnetics. In virtually all ocean floors a striped magnetization pattern was formed which can be magnetically measured and which is characteristic of the age of the oceanic crust. This way marine magnetics helped the theory of plate tectonics to achieve a breakthrough in the early 1960s.

Measuring principle gradiometer Source: BGR

Magnetic measurements at sea are generally carried out with proton magnetometers that are towed on a cable behind the ship in order to keep a sufficient distance from the strong disturbing iron mass of the respective research vessel. Proton magnetometers measure the total intensity of the magnetic field from which the magnetic anomalies are computed by subtracting a reference field. At BGR typically a gradient magnetometer is used on the basis of a refined principle of the proton magnetometer (Overhauser magnetometer) which enables to suppress the disturbing temporal variations of the geomagnetic field. Furthermore, a magnetometer probe for measuring the vector components of the magnetic field was developed, which opens new possibilities for data interpretation in equatorial regions for example.

Vektor-Gradientenmagnetometer Source: BGR

At BGR, marine magnetics is routinely being used, analyzed, and interpreted in combination with the other geophysical methods (seismics, gravimetry, and bathymetry). The new BGR methodological developments include the use of magnetometer probes on different tools which are deployed in deep water close to the seabed. These are mainly used for the small-scale study of mineral deposits on the seabed. Examples are devices for sonar measurements at the highest resolution, but also camera systems and sampling devices. A newly developed tool is a non-magnetic device carrier which is specially designed for the use of magnetic coil systems and electromagnetic measuring methods on the sea floor.