Metal Detection by Multi-Component TEM Method

During exploration using electromagnetic methods, secondary electromagnetic (EM) fields due to induced currents from conductors, together with the primary EM field, are recorded with a suitable receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the information regarding underground 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-domain 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 undertaken. A forward modeling approach was used to compute the voltage response of half-space containing one or more conductive bodies excited by a bi-polar square waveform. Although this method utilizes conductor scattering, it is particularly useful as a practical use for unexploded ordnance (UXO) detection. Unlike 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 target location, and target size determination, orientation, and characteristics, especially for the targets in the horizontal plan. A three-component TEM field experiment at a well-documented well site (NCU campus) was consistent with the effects predicted by our theoretical modeling. As a result, the three-component TEM survey is an essential element for high-resolution EM engineering survey.