Electromagnetic Compatibility Study Of Acoustoelectric Logging Detector

Permeability logging has been clearly identified as an important development direction in the field of logging technology during China’s "14th Five-Year Plan" period.The acoustoelectric logging detector is a formation permeability-sensitive logging tool that the acoustic logging team at China University of Petroleum(Beijing)has been continuously working on for the past decade.Its main EMC problem is that the vertical interface converted waves,which reflect the most favorable acoustoelectric properties of the formation,cannot be distinguished from the interfering signals in time and spectrum,which greatly affects the analysis of the subsequent data.The interference signals from the auxiliary electrodes originate from stray electromagnetic fields inside the detector as well as from interference current fields in the complex well bore medium.Analyze the intricate electromagnetic coupling effects,with theoretical studies as the main focus,supplemented by experiments.The stray electromagnetic field inside the detector is in the near-field region,and circuit analysis is used.The interference current field of the well bore medium is in the far-field region,and the radiation theory of electromagnetic waves is used.In the near-field region,electrical coupling is the main interference factor.With the help of parallel two-conductor transmission lines and Gauss’ s law for electric fields,the relationship between the interfering signal and the source of interference and the measurement environment is predicted and experimented: the characteristics of the interfering signal such as morphology,phase,and frequency originate from the source of interference,while the resistance and dielectric properties of the measurement environment affect its magnitude.The results of the electrical coupling experiments were in agreement with the predictions,confirming that the electrical coupling of the highvoltage pulsed source was the main interference contributor.The coupling path of the detector is more complex than the electrically coupled experiments,and an electrically coupled interference simulation prediction model of the detector is constructed for predicting the effects of metal skeleton grounding and co-grounding factors on the interference signal.The experimental results are consistent with the predictions: the detector’s current use of the skeleton connection to GND is the one with less interference impact;the transmitter and receiver circuits share a common ground,making the interference larger.The simulations and experiments highly reproduce the interference magnitude and characteristics of the detector,therefore,the suppression measures provided by the model for the electrically coupled experiments are equally applicable to the detector.In the far-field region,a theoretically derived model of the interfering electromagnetic field of a well bore is used to analyze the general laws of the interfering source in the well bore model.The geometric features in the detector structure are further abstracted to optimize the model,which is used to deeply analyze the pattern of the interference signal with respect to the interference source and the measurement environment,and to guide the layout of the detector’s transmitting stubs as well as to improve the optimization of the interference.With the help of simulation,experimental and logging data,the effect of resistivity parameters on the interference signal is evaluated:the interference amplitude is positively correlated with the resistivity of the well bore medium.An effective method of suppressing capacitive coupling is shielding.Analyzing electromagnetic shielding based on transmission line modeling is used to guide the design of shielding structures for transmitting acoustic system.An electrically coupled interference model for shielding experiments is constructed for guiding interference suppression.The simulation results explain the experimental results better.An EMC solution was preferred: well-designed shielding of the transmitting acoustic system,correct grounding,and an outer insulating layer to ensure both common ground and μVlevel measurement sensitivity of the auxiliary electrodes.