Vector sensor for measuring particle movement in a medium

vector sensor that in a passive mode measures movements in a medium, preferably caused by sound waves, characterised by that it comprises a magnetic body (1, 2) with a density that is substantially buoyancy-neutral in the medium, a magnetometer (3), and a signal processing arrangement, wherein the magnetic body is held at a certain distance from the magnetometer in such a way that the magnetic body can more in time with a passing particle movement, the magnetometer is disposed to detect the oscillations in the magnetic field that the movements in the medium give rise to and the signal processing arrangement calculates the direction of the movement based on the output signals from the magnetometer.