A Novel Calculation Method of Transient Electromagnetic Resistivity Logging Through Casing

A novel calculation method of transient electromagnetic (TEM) resistivity logging through casing is proposed. The traditional through casing resistivity tool (TCRT) measures voltage difference using adjacent DC electrodes, while the TEM resistivity logging through casing gauges the induced voltage by using low frequency periodic pulses as the excitation current of the transmitting coil. TEM has advantages over TCRT, such as high efficiency and low price. However, it is very difficult to calculate the response of TEM through casing because of the high ration of metallic conductivity to the formation. An efficient method based on numerical mode matching algorithm in frequency domain transforming to time domain is presented. The calculation results of frequency response calculated by using the numerical mode matching method are transformed into time domain response based on inverse Fourier transform. It can be concluded that the frequency domain response of casing cylindrical layered formation is equivalent to band-pass filtering. Then, the simulation is made to reveal the coupling characteristics among metallic casing, different layered formation in time domain, and calculate the resolution of vertical geometric factors of TEM logging. The vertical resolution is inversely proportional to the measured moment. The simulation calculation shows that the vertical resolution of the measured value of the technology to the formation at 1ms is about 1.0 m, and it is 4 m at 10 ms. The future instrument design must consider how to eliminate the impact of metal casing, and make use of early signal inversion as much as possible to improve the formation resolution.