A novel apparent resistivity for land-based controlled-source electromagnetic method

Apparent resistivity is a significant tool in the controlled-source electromagnetic method (CSEM). The Cagniard apparent resistivity, which is defined by the ratio of the magnetic and electric fields, has been widely used in CSEM interpretation. However, it is only suitable for the far zone, where the signals are weak. Although another apparent resistivity based on horizontal electric field E-x can work in transition zone, its measurable zone is narrow. To better take the advantage of strong signals in non-far zone, we present a novel apparent resistivity by sophisticatedly combining the horizontal electric field and vertical magnetic field. We analyze the asymptotic characteristic of our apparent resistivity in the near and far zones using a theoretical formula. Synthetic data from the layered model show that when in the far zone, our apparent resistivity obtains the same performance as the Cagniard resistivity and E-x resistivity. Besides, our apparent resistivity can still wok well when azimuth approaches 45 degrees in the tranzition zone. Field data collected in Tongling, China further demonstrate the practicability and advantages of our apparent resistivity.