Investigation Of Multiple Sources Long Offset Transient Electromagnetic Tensor Measurement

A single source could only generate a single direction current vector under the ground, which is just suitable for1-D layered media, not for3-D structure. As the actual geological condition is complex and changeable, so this paper put forward the multiple sources long offset transient electromagnetic sounding method, short for MLOTEM. MLOTEM is developed on the basis of the electric dipole source transient electromagnetic method. Unlike the electric dipole source transient electromagnetic method, MLOTEM uses multi-source electric dipole as artificial source, by measuring the total electric field, using current density vector as the normalized parameters, introducing the apparent resistivity tensor as parameters, and analyzing the coordinate invariants and the ellipse parameter in order to investigate underground medium structure and geological characteristics.This paper starts from the theory of MLOTEM. At first, introducing the concept of apparent resistivity tensor and parameters such as coordinate invariants and ellipse; then deduced the1-D layered media LOTEM electric expression, and calculate the apparent resistivity tensor response using numerical simulation method, it turn out that:comparing with the early-time and late-time resistivity, the coordinate invariants could reflect the underground structure, which verify the validity of parameter for the method. Secondly, using IE method to modeling the apparent resistivity response in several typical3-D model, the result proves that the coordinate invariants corresponding to different geological variables in different time at different depth, which the result in low low-resistivity is better than the one in high-resistivity. The change of source location will have an influence on the detection of tensor apparent resistivity, but basically can reflect the variation of abnormal situation. The apparent resistivity ellipses at different times can be made into a sequence of images which provide a dramatic way of visualizing the tensor data.The paper show that:the apparent resistivity tensor could be used as MLOTEM interpretation parameters to explain the change of underground medium, Different coordinate variables have different response characteristics, interpretation of tensor data should not be confined to a single parameter in order to achieve the best results.