Abstract:

Under the quasi-static conditions, the measurement sector, features and advantages of electric field y-direction (E_y) of controlled source audio-frequency magnetotelluric are discussed according to  the formula of electromagnetic field data from horizontal electric dipole. The amplitude range of electric field y-direction is different from the common scalar CSAMT measurement.  E_y component data can be gathered and uesed to expand the CSAMT field observation sector and reduce the acquisition costs. According to the basic principal for definition of apparent resistivity in electrical prospecting, a new apparent resistivity defined by E_y is put forward and the sounding significance by frequency of which is explained. Through numerical calculation of several typical layered models, such as homogeneous half-space, D, G, H, K, KH and HK typical models, E_y apparent resistivity is compared with wave-area apparent resistivities defined by E_x、H_y、Z_xy、Re(Z_xy) and full-wave apparent resistivities defined by E_x、Z_xy. The results show that the derivation of  E_y apparent resistivity only needs  the coordinate of the sounding station and one direction E-field intensity and the iterative procedures are unnecessary. The new apparent resistivity easily calculated can satisfactorily eliminate the distortion effect of the near and transient area,because it can converge to a stable value at low frequency parts.

Key words: electromagnetic prospecting, controlled source audio-frequency magnetotelluric, E_y apparent resistivity, far-field apparent resistivity, full-wave apparent resistivity