Research On Transient Electromagnetic Method For Resistivity Detection Through Casing And Data Acquisition System

Transient electromagnetic method(TEM)for formation Resistivity Detection through casing is a new type of logging method,which has become one of the hot spots in related fields.The basic detection principle of this method is as follows: When the emissions probe inside the casing excits the transient electronmagnetic field,there will be a response in the well,casing,cement ring,strata and other media,which contains the resistivity information of each media.In the detection process,the response inside the casing will change when the formation resistivity changes,which means the resistivity information can be extracted from the response waves acquired by different receiving probes in casing.Compared with the other resistivity detection methods through casing,this one can realize the high power excitation without contact,and obtain more information from the response.Based on the logging method of TEM,this paper worked on the both aspects of related theory and the circuit system design.In the aspect of theory research,this paper started from the exact solution in cylindrical coordinate which is based on the Maxwell equations,then applied the method of real axis integration to deduct and calculate the response of the transient electromagnetic field excited by coils in the casing well model.In this paper,the transient response of the whole casing model was calculated and descripted for the first time,and the response distribution characteristics was also analyzed.Moreover,after researching on the relationship between the secondary field response and the formation resistivity,this paper concluded that the secondary field response is proportional to the conductivity difference,and the secondary field response at any moment can reveal the formation resistivity,which have a certain guiding sense for the actual detection method and the instrument design.As for the circuit system design,the paper mainly developed the software and hardware of the data acquisition system,along with the corresponding ground receiving system and PC software.The data acquisition system contains two parts,including the multi-channel data acquisition part and the wire-line communication part.The acquisition part realized the synchronous sampling on four response signals and measurement of two DC voltage signals,as well as the control on the excitation sequence of the transmitting board.According to the two different wire-line protocols,the communication part is designed for providing the communication interface for the system.After the verification by experiments,the whole system can meet the design requirements,and the secondary field response can also be detected.