The MT Response In Superficial Layer Heterogeneous Body And The Research Of Constraint Inversion

The paper paid attention to the problems which suffered from magnetotelluric (MT) application in large and long tunnels of railway and were studied. Static effects were discussed by two-dimension finite element forward. And some laws in practical application from forward research to the model whose application in data processing can enhance credibility in data interpretation. At the same time, posterior information such as borehole and geology etc was transformed to electricity data for inversion with electrical parameters restraint, and increased MT resolution.In the paper, Emphasis on static effects and their calibration research, analysis of their generation and their typical characters were summarized such as shallow small-scale electricity inhomogeneous body in MT sounding producing static effects; static effects having effects only on electric field data observation but not on magnetic data observation; in log-log coordinate apparent resistivity curve along apparent resistivity axis up and down offset without changing the pattern and with no effects on phase curve; easy confusion in static effects and anomaly. Curve translation and numerical analysis were considered for static calibration. By forward to models, five-point (seven times) filter, two-dimension median filtering and adaptive Hanning filtering and so on were used for static calibration. And advantages and shortcomings of various methods of calibration were discussed and effect of calibration was analyzed by practical examples.Then constructing various two-dimension geo-electric models was used to discuss terrain influence and vertical and horizontal resolution in MT application. To the terrain influence, the following laws were summarized. Including there no good reflection on the bottom interface of low-resistance anomaly; in the terrain of ridge they well reflected center position of anomaly and the terrain had little effects on the top interface of anomaly; in the terrain of valley, center position of anomaly can up and down offset, the greater of radian, the better reflection on low-resistance anomaly on the whole. Research of vertical and horizontal resolution found that MT method can very well determine top interface of the low-resistance anomaly in most upward side, but can not determine top interface of each low-resistance, that is, there is bad vertical resolution in the method. For horizontal resolution, in traverse, when there is some separation in low-resistance anomaly distance, the method can reflect horizontal variation of electricity structure very well. And MT application was discussed further by complex structured model.Geological and geophysical former data was transfromed to resistance distribution priori geo-electric model which is needed by MT data processing. Interaction item of priori geo-electric model was added into objective function in OCCAM inversion algorithm—during the iteration known electricity distribution continually closed to priori geo-electric model which came into being the inversion algorithm with the restraint of electricity parameters, iterative formula of objective function was deduced. And the final inversion module with the restraint of electricity parameters was introduced. Models comparison showed the availability of restraint inversion.Finally the methods which have been discussed above the paper were applied into practical processing in Zili tunnel of Nanpei line and Fandoushan tunnel of Yuli line. For Zili tunnel, according to the work area located in structural zone and its complex subsurface picture and so on, removing the"flying spot"of raw data, spatial filtering and Hanning filtering etc were used for static calibration. And obvious improved pre-processing results were obtained. Also final results interpretation of Zili tunnel corresponded to the results of borehole very well. For Fangdoushan tunnel, the information of borehole and logging etc used mainly was transformed to the electricity restraint file which was required by inversion with restraint of electricity parameters for restraint inversion comparison. And detailed comparison showed the inversion with restraint of electricity parameters can improve the resolution of MT exploration in large and long tunnel of railway in some degree.