Theoretical Study On Ground TEM System Calibration

Transient Electromagnetic Method (TEM) is a kind of artificial sourcetime-domain geophysical detection methods which is based on electromagneticinduction, the basic principle of this method is to pass a current in transmitter loop,and the current need to turn off in a very short time. the tremendous changes of thecurrent produce tremendous changes of magnetic field, when it is propagated to agood conductor, it will encourage good eddy current generated inside the conductor,the eddy current transients decay with time and generating a secondary magnetic fieldto the surface of the transmission, the transmitter gap current is coupled to thereceiver coil and turn it into a voltage signal, then the signal pre-processing by thereceiving system, and finally get underground and shape of the target volumeresistivity distribution and other related information by inversion.Transient electromagnetic system signal to decay with time signals, signal bandis from low to high frequency omni-band signal, typically a high-frequency signalcorresponding to an early signal and reflects mainly shallow information, advancedsignal corresponding to low-frequency signal and reflects mainly the deep message.Since the transient electromagnetic signals are attenuated, the attenuation from themaximum value to the signal (several hundred to a few thoμsandths) time on theorder of ms, so the signal, in particular an early reflected signal information shallowprone to distortion. It is also a key issue for shallow probing of TEM need to solveone. Establish a way to accurately describe the system performance and early signaldistortion theory and methods to achieve the quantitative evaluation of TEM system,the mechanism for early detection of signal distortion and thμs targeted to beresolved, and ultimately improve the system detection capability is critical shallow.To solve this problem, we established a calibration target body ground loop TEMsystem calibration model, calculate and analyze the impact of the calibration loop response (TLR) accurately obtain the seven non-expected response. The results showthat a field response (TR) and earth response (TER, TRER) the greatest impact can beremoved by removing the background field method. Higher mutual response TRLRreceiving coil and calibration loop between the mutual inductance between the earthand the calibration ring TELR amplitude response calibration ring TLR responsemore than an order of magnitude, serioμsly affecting the calibration to obtainaccurate loop response. By adjμsting the diameter of the ring of the calibration, thecalibration between the ring and the height of the receiving coil, the calibration looptime constant, can effectively suppress the influence TRLR; reasonableconfiguration and calibration loop time constant calibration selective high resistanceground can effectively suppress the influence of TELR. This study on the accuracy ofthe calibration, evaluation and optimization of ground TEM receiving system isinstructive.