Abstract: A magnetometer sensor is described for a borehole survey. The magnetometer features a bobbin made from a metal alloy such as copper-nickel, which has a very low magnetic permeability, on the order of 1.0001, which can be joined to a magnetic metal alloy, such as Permalloy, by welding, which can be treated by annealing at a temperature of 1500 degrees Fahrenheit and which has a thermal coefficient of expansion generally on the order of the magnetic alloy. In one particular embodiment, the magnetometer sensor comprises a strip of magnetic metal alloy wrapped around the aforesaid bobbin to define a magnetic core, one excitation winding wrapped around the core, a plastic frame for carrying the magnetic core and excitation winding and at least one sense winding wrapped around the frame, the core and the excitation winding.