Analysis on response characteristics and detectability of transient electromagnetic transverse magnetic field

Transient electromagnetic method (TEM) is an important method to recover the underground geo-electrical structure. The traditional loop-source and grounded-wire TEM mainly observe the transverse electric field, which is only sensitive to the conductive target and not sensitive to the resistive target. The transverse magnetic field has a high resolution for highresistivity targets, but has not been effectively used. Double-line grounded-wire TEM can enhance the proportion of transverse magnetic field in the observed electric field. However, there is a lack of systematic research on the response characteristics and resolution of this method. Therefore, the response characteristics and detectability of double-line grounded-wire TEM are analyzed in this paper. The response amplitude of the horizontal electric field of the double-line source is smaller than that of the traditional single-line source. On the perpendicular line of the double-line source, the observed horizontal electric field can only be generated by the 3D structure, and the response on surface of the layered earth is zero. The transverse magnetic field and the transverse electric field of the double-line and the traditional single-line sources are extracted respectively. The proportion of the TM field in horizontal electric field of the double-line source is much larger than that of the traditional single-line source. The proportion of the TM field is obviously enhanced and much larger than that of TE field at late time. The horizontal electric field of double-line TEM shows a higher resolution to resistive targets than the traditional single-line source based on root mean square (RMS) misfit and 3D numerical simulation. The resolution of horizontal electric field to resistive target is improved obvious in the equatorial direction.