Topographic effects on long offset transient electromagnetic response

The forward modeling procedure used in this article is formulated with the volume integral equation based on the tensor Green's function. The electromagnetic components responses are first calculated in the frequency domain and then transformed to the time domain by digital filtering. The valley and hill topography with a layered earth is stimulated by a horizontal electric dipole (HED) transmitter, which is common in field surveys, and the TEM responses are calculated at the transmitter and receivers. The topography effects on the long offset electromagnetic transient (LOTEM) responses are discussed in detail. The results show that both valley and hill topography has significant effect on the LOTEM measurement. If the HED is located in the bottom of a valley, the distortion of the observed anomalous field at distance is severe. A valley at the receiver locations show a strong effect but are localized in space and time. In general, hill-shaped topography shows smaller effects no matter where its located. When the topography is located between source and receivers, the influence is negligible. We conclude that the location of the source is much more important than the receivers and it is critical to put the transmitter in an open flat area in the field survey.