Issuu on Google+

Maxwell Scott Geo-Phys Electromagnetic methods are becoming more popular in engineering and physical research because of their high resolution, speed and ease of use. Due to the presence of subsurface materials like fluids, and metals etc, practices in electromagnetism are being used increasingly to map these regions of enhanced conductivity. The instruments used in field tests induce an electromagnetic current in conducting materials within the earth. They collect data rapidly and accurately without the mess of cables and ground penetrating radar methods. Electromagnetic waves include wavelengths over a multiple range of spectra, from radio waves to gamma rays. The waves are produced by the acceleration of electrons or other charged particles in space which propagate in the direction B x E (magnetic field cross electric field). All waves are vector fields and propagate according to this simple vector addition. These electromagnetic waves were first proposed by Scottish physicist James Maxwell who derived equations based on the prior works of Gauss, Ampere and Faraday. His equations revealed that a time varying electric field generates a magnetic field and vice versa. His first equation is Gauss’ law for electric charge contained within a closed surface, which shows how the divergence of an electrical field is affected by charges. Also states Flux in=Flux out. His second equation is a derivation of Gauss’ law for magnetism and states that total flux through a Gaussian surface is zero. Third is a derivation of Faraday’s law which states that an electric field can be produced by a change in magnetic field (or dB change in flux). Fourth is derived from Ampere’s law which states that magnet fields can be generated by an electrical current or by changing electric field. Maxwell made a slight (but very important) correction to Ampere’s law, that linked displacement of current to the time varying electric field , ultimately determined that light and magnetism are of the same nature and propagate through a field according to the same laws that govern all of electromagnetics. Through these equations unified the separate laws of electromagnetism and optics. Some other important physics properties that describe mobility of an electric charge are resistivity and conductivity. Of course, in a conductive material charge can move freely. But when resistance is added to the field, the wave will propagate less efficiently. Because electromagnetic surveys are less sensitive to variations in resistivity, they are often described in terms of conductivity. In general, relative conductivity increases with porosity and conductivity of pore fluids. Understanding dielectric permittivity and magnetic permeability are also important in determining subsurface consistency. Dielectric permittivity measures the ability of a material to store a charge when an E field is applied. Greater conductivity means less dielectric permittivity and thus energy will attenuate at a shallower depth. Magnetic permeability measures the ability of a medium to become magnetized when an electric field is imposed on it. Greater permeability means greater electromagnetic energy during transmission.


As electromagnetic waves propagate, they lose energy through attenuation and absorption. Absorption is the loss of wave intensity due to an exothermic process. Heat is released as the wave propagates and thus slows. This is why food gets heated in the microwave or why your cell phone does not work in tunnels. Attenuation is a result of absorption and scattering due to high frequencies of oscillation. This is generally neglected because much of the transmitting and receiving is done at lower frequencies.


qualitative look electromagnetism and methods