Static Problem. Application Of Csamt Method And Its Inversion

Controlled-source Audio-frequency Magnetotelluric(CSAMT) are developingfrom the Magnetotelluric(MT), which is used to correct the weakly MT signal. TheCSAMT is a kind of geophysical methods that are detecting the different resistivitieson different depths by changing the transmitting and receiving frequencies. In recentyears, CSAMT is successfully used in the area of mining, terrestrial heat, groundwater,oil and gas, and so on. Static shift is one of the difficult problems that have to besolved in CSAMT. Since the static shift disturbs the data and leads to wronginterpretation, so we have to do static correction in order to achieve a better qualityCSAMT data. The generation mechanism and characteristics of the static shift arediscussed in this paper. The galvanic distortion due to small-scale surface and shallowelectric inhomogeneities often leads to distortion of observed and inverted resistivitydata. The appearant resistivity curve in the loglog coordinate system has parallel staticshift effect while the phase curve has no effect.To do the static correction, first we need to distinguish the static shift from theanomalies in the collected data, then considering the field geological conditions tochoose a suitable technical method. However, there are many static correction ways inMT data, including: the curve-shifting method which has a friendly man-machineinteractive interface. The filtering methods based on the numeric analysis, and themethod using the phase data analysis or using magnetic data analysis. The ways ofuniting other electromagnetic sounding geophysics which do not have the problem ofstatic shift, and the multidimensional inversions with static shift parameterization, andso on. Since the human source and nature source is the only difference betweenCSAMT and MT, therefore the CSAMT static correction method can refer to the MTstatic correction methods.Based on the principles of the static corrections, this paper establishes theunderground layered static shift model, computes the appearant resistivity and phasedata by using 2.5D forward arithmetic, attempt to distinguish the static shift fromanomalies using the correction coefficient or wavelet analysis. After doing theresearch studies of some kinds of static correction, three kinds of static correction arechosen in this paper: the EMAP filtering method, the curve-shifting method, and 2.5Drapid relax inversion. The advantages and disadvantages have been summarized inthis paper by comparing each method's effect. The key of EMAP filtering method isto choose a filtering parameter, but sometimes the result is not very good for the staticshift caused by small-scale high resistivity geological body. The key of curve-shiftingmethod is to distinguish the static shift from the anomalies and to choose theresistivity curve that reflects the geological background. It is proved that thecurve-shifting method was useful in the static correction except cannot show theimage of the surface small-scale inhomogeneties. The 2.5D rapid relax inversion is aneffective way to do the static correction. It can make image for both the electricstructure underground and the small-scale anomalies. But it has some defects toidentify the stratum boundary clearly, and it is often seen that the scale of inversionanomalies are bigger than the real anomalies. The CSAMT copper exploration whichhave done in XiaoErQuan district XinJiang province, there is a serious static shift inthe collected data. After static correction, a good quality data was achieved. Inconclusion, these static correction methods gained a good applicable effect.