Study on the response and optimal observation area for SOTEM

SOTEM is a kind of time domain electromagnetic method with great detecting depth and high resolution. In order to further understand and promote this method, studies on its distribution of electromagnetic field response and diffusion characteristics was conducted in this paper. Calculation results based on the 1D forwarding theory of SOTEM indicate that grounded wire source can excite both horizontal and vertical induced current under ground. The horizontal current includes upper and lower parts (also called return current). The maximum of horizontal induced current mainly focus on the area close to the source and diffuses downward vertically. The maximum of vertical induced current diffuses along the direction of 45 degrees with the ground surface With a weaker amplitude and faster speed than horizontal induced current. All the six EM components have the ability for geophysical detection. However, in consideration of the diffusion characteristics of each component and observation convenience, vertical magnetic component H-z (partial derivative B/partial derivative t) and horizontal electric component E-x are the mostly used in practical. Magnetic component (H-z) is more sensitive to low resistance body and it mainly focuses on the region close to equator area. Electric component (E-x) shows same sensitivity to both low and high resistance body, while low resistance sensitive area mainly focuses on the region close to equator and high resistance sensitive mainly focuses on axial region, and the distance between sensitive region and source is depended on the buried depth of targets and resistivity of overburden.