Research Of Short Dead Time For MRS Based On Q-toggle

Magnetic Resonance Sounding (MRS) is a kind of geophysical method designedfor directly exploring groundwater. The hydro geological conditions of certain regioncan be effectively estimated by the water content and permeability.Because of the limitations of SNMR method and structure of the instruments, theinstruments can’t switch from their receiving state directly to the transmitting state.The time delay between the end of the applied excitation field and the start of theusable recorded data is referred as the―dead time‖. The original designs ofSNMR instrument (e.g., Hydroscope and NUMIS) exhibited dead times in the rangeof30-40ms, while our JLMRS operated with a dead time of38ms.It is known that the amplitude of the free induction decay (FID) signal is largestat the earliest part of the response; therefore, losing the early part of signal will lead toa relative low signal-noise-ratio (SNR) and more errors in estimation of water content.In the tested area where environmental noise is more obvious, and non-saturatedwater layer of smaller porosity, MRS instrument will not give accurate information ofgroundwater, thus limiting the applications of the nuclear magnetic resonance method.Simultaneously, because of the longer dead time, when the signal received is beingdeduced, the earliest part of the response would exhibit a relatively large error ofhydro geological parameters. Conversely, reducing the dead time will improve theSNR and the accuracy of inversion result, particularly the error generated inparameter estimation process can be reduced when the undergraduate magnetic fieldis not uniform, and access to more accurate information on the underground moisture.Besides the signal-noise-ratio of received signal is related to the quality factors ofcircuit, so the quality factors of each level of circuit are need to be taken into account.Firstly, this study gives the general principles underlying the dead time andquality factor, then starts from the basic definition of dead time, and analyzes thetransient EM response、oscillation caused by switching movement of relay and delayof amplifier, which are composition of dead time. Finally, we summarize the causes ofeach time period and the factors influencing the length of each time period. After that, based on the analysis of quality factors of MRS instrument front-endreceiver and post-amplifier, this paper gets the factors of the instrument influencingquality factor, and gets conditions in which the instrument can work in the best qualityfactor. On this basis, we propose the program to shorten the dead time and the timingto control the short dead time circuit, in the process, we analyzes the principle ofselecting the time. After several tests，we finally identified the program short deadtime MRS circuit based on Q-toggle.Finally, many tests have been done in short dead time MRS circuit indoors andoutdoors. Indoor tests have given feasibility results, tests on field obtained MRSresults of short dead time. By inverting and processing the experimental data, as wellas comparing the drilling data, our research demonstrated that Q-toggle circuit cansignificantly shorten the dead time and consequently improve the quality of signal,which will provide a theoretical basis to carry out the design for NMR detection andinfiltration basins and clay layers in the water sector Fu detection applications.