achieve a large detection range and greatly simplify the complexity of the operation thereby saving costs application_number US 201315026032 A description the invention will be further described by description of embodiments with reference to the accompanying drawings according to an embodiment of the present invention fig2 shows a downhole em data transmission system which improves the efficiency of data transmission from downhole to surface or drilling platform by establishing a link between the wells being drilled and the already existing wells in the currently drilling well 1 insulating ring 2 is set on the drill pipe 5 the electrical power source 3 is connected to the pipe 5 on the parts to both ends of the insulating ring 2 well head terminal a of the casing 7 of the currently drilling well 1 is connected with the terminal b of the casing 8 of the already existing well 6 by a wire and a signal receiver 10 is serially connected to the wire between two wells thus the electrical power source 3 the drilling pipe 5 the casing 7 usually a conductor of drilling well 1 the wire between the two wells the casing 8 of existing well 6 and the formation around the two wells forms a large circuit loop of which the formation energy dissipation is reduced it can be used to improve the data transmission efficiency in the fig2 a signal receiver 10 is serially connected into the loop so the data can be received by detecting the current in the wire as an.