磁法勘探专业英语.ppt

3 Magnetic methods Department of Geophysics Oil Resources H.W. Zhong, 2007.5 History of the magnetic method History of the magnetic method Now, The magnetic methods are among the cheapest geophysical methods and from the operational point of view, also among the easiest and fastest. 1. Basic concepts Magnetic Force, Flux, and Field Magnetic field said to exist around bar magnet or wire, and exerts a magnetic force that aligns the iron filings. Magnetic flux, corresponds to closeness of field lines, and converges at magnetic poles near ends of magnet. Induced Magnetization and Magnetic Susceptibility For low magnetic fields, magnetization J is proportional to the magnetizing field H: J = k H where k is called the magnetic susceptibility. Total Magnetic Field B The Total Magnetic Field B represents the sum of the magnetizing field strength and the magnetization of the medium: B = m0(H + J) = m0(H + k H) = mrm0 H = m H where m0 is magnetic permeability of free space. mr is relative magnetic permeability. m is absolute magnetic permeability. B is also called the magnetic flux density or magnetic induction. Induced and Remnant Magnetization Induced Magnetization Ji is produced within a rock in response to an applied external magnetic field. Magnetic field may exist within rock even in absence of external field due to permanently magnetic particles. This is remnant or permanent magnetization. Classifications of magnetic materials Some paramagnetic are ferromagnetic minerals which have alignments of magnetic moments in small areas called magnetic domains. These materials are not naturally occuring on earth, so again they are not of interest in exploration. Classifications of magnetic materials minerals, on the other hand, are common and naturally occuring. These minerals (e.g. magnetite(磁铁矿), pyrrhotite(磁黄铁矿)) have a net magnetic moment (磁矩) and thus relatively high susceptibilities. In general, igneous and metamorphic rocks have higher susceptibilities than sedi