THE PROTON PRECESSION MAGNETOMETERThe proton precession magnetometer is most commonly used for terrestrial magnetic surveys. This magnetometer measures only the total amplitude (size) of the Earth's magnetic field. These types of measurements are usually called total field measurements. The proton precession magnetometer is so called because it uses the precession of spinning protons or hydrogen atom nuclei in a sample of hydrocarbon fluid to measure the total magnetic intensity. The sensor component is a cylindrical container filled with a liquid rich in hydrogen atoms surrounded by a coil. Kerosene, alcohol and water are the commonly used liquids. Upon closing the switch, a direct current from the battery is directed through the coil, producing a relatively strong magnetic field in the fluid-filled cylinder. The hydrogen nuclei (protons) behave like minute rotating magnetic dipoles, aligning themselves along the direction of the applied field (i.e. along the axis of the cylinder). Opening the switch cuts off power to the coil, as the Earth's magnetic field generates torque on the aligned and rotating hydrogen nuclei, they begin to precede around the direction of the Earth's total field. As the protons precede, the precession produces a time-varying magnetic field that induces a small alternating current in the coil such that the frequency The precession of the nuclei is equal to the frequency of the alternating current. Proton precession measures the frequency of the oscillatory field and since it is the same as the proton precession frequency it can be used to determine the strength of the external field. the constant of proportionality that relates the frequency to the field intensity is a well-known atomic constant, the gy...... in the center of the paper ......xshows the scheme usually common to a small magnet , as shown below in fig 1. The direction of the field lines at the equator is horizontal while at the north and south magnetic poles it is vertical. This geometry is important in the interpretation of magnetic anomalies. The total strength of the Earth's field is not perfectly asymmetric with respect to the geographic north pole, for example the magnetic north pole in northern Canada is more than 1,000 miles from the geographic pole. The Earth cannot be represented exactly by a single bar magnet, but it has numerous higher-order poles and very large-scale anomalous features due to unknown characteristics of the generating mechanism in the Earth's core. As shown in Figure 1, the solar wind, or the constant flow of particles and electric currents from the sun, distorts the field lines. The deviation from the asymmetric field is the anomalous set of characteristics of the field