Metal Detection by Multi-Component TEM Method


During exploration using electromagnetic methods, secondary electro magnetic (EM) fields due to induced currents from conductors, together with the primary EM field, are re corded with a suit able receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the in for mation regarding under ground conductors, may be several orders of magnitude smaller than the primary field. Under these conditions the separation of the measured total EM field into its primary and secondary parts is difficult. For the purpose of measuring the secondary fields in the absence of the primary field, time-do main electromagnetic measurements, often referred to as transient electromagnetic (TEM) techniques are employed. Comparative numerical modeling using a single component (measuring the vertical component of the field, Hz) TEM receiver and a three-component (measuring Hx, Hy, and Hz) TEM receiver was under taken. A for ward modeling approach was used to compute the voltage response of half-space containing one or more conductive bodies ex cited by a bi-polar square wave form. Al though this method utilizes conductor scattering, it is particularly useful as a practical use for unexploded dordnance (UXO) detection. Un like single component data, results from the three-component data are unambiguous as to the location and orientation of conductors. Measurements with the addition of horizontal- component data for secondary magnetic fields lead a better indication of tar get location, and tar get size determination, orientation, and characteristics, especially for the tar gets in the horizontal plan. A three-component TEM field experiment at a well-documented well site (NCU cam pus) was con sis tent with the effects predicted by our the oretical modeling. As a result, the three-component TEM survey is an essential element for high-resolution EM engineering survey.

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