Wireless downhole to surface communication has led to the development of super low frequency (SLF) real time electromagnetic telemetry technique. It has such a significant engineering values such as downhole multi-parameter detection and downhole apparatus control, etc.A downhole to surface communication channel consists of a long vertical cylinder (the stem) and an isolated downhole cylinder (the electrode) embedded in a homogeneous earth, the downhole signal detector on the ground. The downhole signal can be detected at the surface as a very small voltage drop, the surface receiver senses the potential between the cylinder and a ground point in order to receive the information sent from the apparatus downhole. However, since the oil wells are cased in steel, the steel casing largely masks EM signals induced into the earth formation, the electromagnetic signal undergo severe attenuation as they diffuse across the casing. After comparing the effect of the electric antenna and magnetic antenna, a method is introduced, that is coupling the voltage into the cylinder directly by providing an insulating break between the stem and the electrode, then there will be a signal voltage drop on the downhole steel casing. The downhole electric antenna which emits the electromagnetic signal into the formation, the ground receiver senses the voltage drop in present of ambient electromagnetic noise in the oil field. Based on the electromagnetic theory, the work is aimed at predicting the receivability on the ground surface and enhancing the acquisition ability of the receiver on the ground.In this paper, it is presented that large amount of the theoretic models concerning the features of the super low frequency electromagnetic signal transmitting through the earth formation, using finite-element method to compute the magnetic and electric field of the downhole SLF signal in a cylindrically symmetric model and the result shows the effect of the magnetic antenna is weaker than the electric antenna. According to the electric field result simulations based on the finite element analytic software of ANSYS, the structure and the space of the downhole electrode, the depth of the downhole apparatus, the power level of the downhole SLF signal, the electrode's depth of the receiver equipment on the ground are crucial to predict such a weak SLF signal going through the earth formations as well as the electric field distribution on the ground surface in oil field. It demonstrated an effective scheme which can largely increases the strength of the detectable signal on the surface of the earth. It is also carried out the 3D simulation, a realistic model for assessing the performance of the communication channel.At the end, the thought of the OFDM communication method and the technique of designing the high temperature circuit are introduced in developing a testing carrier wave transmitting equipment, and also the principle of designing the downhole carrier wave transmitting apparatus is given.
