The Magnetometer -This instrument senses objects having magnetic properties. Basically, the method works on the premise that the world has a relatively static magnetic field, and cultural (and some natural) forces interrupt that field. Magnetometer surveys measure the irregularities in the magnetic field in a given area.
The Gradiometer- This instrument has two flux gate magnetic fixed sensors that are passed closely to and over the ground. When it passes over a buried iron or steel object so that the field is significantly different at the two sensors, and locally there is a high magnetic gradient, the instrument emits a high frequency sound. The frequency is a function of the gradient between the two sensors.