The dominant  main field originates in the Earth's fluid core. The second internal contribution comes from magnetized rocks in the lithosphere. The third contribution, varying rapidly in time, comes from outside the Earth (External field). Amongst the sources which contribute to the geomagnetic field, the oceanic magnetic field is the faintest.

A detailed description can be found here.

The Geomagnetic Core Field

The dominant component is the so-called main field, generated by a hydrodynamic dynamo operating in the Earth's fluid core.

The Magnetic Field of the Lithosphere

The second internal contribution of the Earth's magnetic field comes from the Earth's lithosphere, induced from magnetised rocks.

External Magnetic Fields

The third contribution, often varying rapidly in time, comes from outside the Earth, from current systems in the ionosphere and magnetosphere, driven by the activity of the Sun.

The oceanic magnetic field

Amongst the many sources which contribute to the geomagnetic field the magnetic field generation due to ocean circulation is the most faintest. It has been recognized that ocean flow generates magnetic field through the process of motional induction. Most of them were theoretical predictions and/or observationally supported in a regional scale. Land based magnetic and electrical field measurements and undersea-cable voltage measurements were used to validate the predictions. Recent studies have highlighted the potential of high quality satellite magnetic data to be utilized to study the magnetic signal of ocean tides. Several attempts have been made to study the weaker and time invariant magnetic signals of ocean currents. The signals generated by the steady motion of highly conducting oceans are predicted to be on the order of several nT at the Earth's surface. The ocean circulation signal is well pronounced in the southern hemisphere, where the eastward motion can circulate without hitting continental landmass.

Ocean tidal dynamo identified in CHAMP satellite magnetic data