Research Of The Transient Electromagnetic Receiver System Based On LabVIEW

At present, Transient electromagnetic method is extremely extensive in theapplications of geological exploration. It is widely used in groundwater resourceexploration, mineral resources, underground cave and unexploded demolitionmaterials exploration etc. The principle of transient electromagnetic method is that thesystem uses no-ground loop to emit a primary field to the underground by emissioncurrent. When it spreads to the conductor, it drives geological body generate eddycurrent. Secondary field produced by eddy current transmits to the surface and decayswith time. In the gap of the emission current, it is received by receiving coil andconverted into voltage signal. Then, it is transformed to the signal pretreatment in thereceiving system. Finally, the subsurface target resistivity and shape distribution etc.are calculated by means of data inversion.This paper uses LabVIEW and NI USB-6351to build a data acquisition andprocessing system. The function realization of the system is explained in details atfirst, including data acquisition program design, background elimination programdesign, program design of signal power spectrum analysis, data playback programdesign, curve fitting program design and sampling channel program design; Secondly,the theoretical study and the actual preliminary measurement of the system calibrationis carried out. Whether or not the shielding of signal transmission line is connected tothe ground, noise reduction effect is studied. The influence of system preheating onsignals is measured. Make an analysis of receiver "error synchronization" problem.The "error synchronization" phenomenon can be reduced by setting a highersimulation synchronization trigger threshold. Thirdly, the system noise problem isstudied with the system. Because the system noise is consist of environmental noiseand sensor background noise. The sensor background noise distribution with frequency and its working mechanism are discussed with the sensor noise model.With this analysis method, background noise level of the sensor tested in the field isderived theoretically. Its level is545nV. In the following, transient superposition andtrace superposition of noise reduction are studied. Noise reduction level which isproportional to transient superposition times and amount of data in a trace isconfirmed by theoretical analysis and experiments. After1024times of transientsuperposition and trace superposition, sensor background noise reduces to0.42nV.But, the ambient noise is μV magnitude. Therefore, environmental noise is the mainfactor of the system noise. Then, the change regularity of the channel noise is studiedand that the channel noise is reduced as t^-1/2. Then this paper defines the systemsensitivity, which is used to measure the system performance. The sensitivity of themeasured system is up to6.8nv/m². Finally, two kinds of the trace algorithm torecord the signal attenuation are studied and the optimal sampling rate of the system isfixed.