Reseaech On Joint Time-Frequency Electromagnefic Method In Deep Mine Resources Prospecting

The results of exhaustion of mineral source, deficiency of substitution source and declination of economy have been produced with the expliotion of several decades. The crisis ore problem brought becomes the economic and social problem. Metallogenic studies and the international exploition experiences ores show that there exists the second exploring space in China, in which large mineral sources deposit below 500～1500 meters depth. This foundation provides basis guarantee for solving the problem of crisis mines. However, the environment in mining area is very unfavorable for the geophysical exploration, so the large improvement of prospecting techniques is needed.CSAMT and LOTEM methods have those advantages such as:artificial source, controlled source direction, large signal strength, and sensitive to low resistivity. Thus both methods are the good choices for deep mine resource prospecting. Therefore, on the basis of the numerical modeling of these two time frequency methods, this article proposed the joint time-frequency electromagnetic method, and payed the main attention to the research of observation scheme and data process method. The conclusions are as follows:(1) The principles of CSAMT and LOTEM for deep mine prospecting have been introduced based on Maxwell's equations, and the principles confirm the feasibility of those methods. The electric dipole transmitting source is determined by the research of transmitting equency and excited field. The relationship between resistivity, offset and electromagnetic field makes the knowledgement of signal characteristics of time domain.(2) In the CSAMT modeling simulation, the precise and fast method for the electromagnetic calculation is determined by analysis and comparision of several methods for the convergence problem of Bessel function under the large offset condition. The different reflections in electromagnetic curve of low and high resistivity provide theoretical knowledge for recognization of geologic characteristics. In addition, the resolution ability for low and high resistivity bodys is drawn from the characteristics of electromagnetic curves. Non-pl ne wave effect, false peak effect and static effect are found in the far-zone and all-zone electromagnetic curves, by the analysis of their characteristics and influences, the conclusions are obtained as follows:the existence of those effects shields the ture geologic information, especially the medi and deep information, adds the interpretation difficulty, lowers the reliability of results, and finially hinders the development of CSAMT in the deep detecting space. This problem lays the foundation for the effects correction by joint LOTEM data.(3) Guptasarma digital linear filter method with stability and high precision solves the elelctromagnetic field calculation in time domain using the frequency domain method. In the modeling simulation of LOTEM method, different reflections of high and low resistivity in induced voltage and magnetic curves are enduced. The sensibility of induced voltage to resistivity and offset makes the curve complicated, and non-sensibility of magnetic field make the curve simple and smooth. There are no false peak effect, and no static effect and trends in LOTEM curves because of magnetic field or induced voltage collection. The large storage by long period observation, complex calculation of time domain, overloaded early-late apparent resistivity calculation separately and immature data interpretation problems lower the utilization ratio of LOTEM method directly. Those advantages and disadavantages of LOTEM method further the combination of CSAMT and LOTEM in deep mine resources detection.(4) The joint time-frequency electromagnetic method for deep mine resources detection is proposed by the similar observation configuration, complementation of two methods. In frequency domain, the frequency lower than the optimal transmitting frequency can be taken as the minimum transmitting frequency, taking this frequency as the base frequency, and transmitting with 3 or 5 pseudo-random frequency will make the electric field increase byΔfmin1.5. In time domain, in order to improve the data quality by bipolar preset stacking methods, transmitting bipolar period wave, acquiring repeated, and enlarging the storage volume are needed. Combining with the convience of metal electrode and high performance of nonpolarized electrode, the "dumbbell" enclosing solid nonpolized electrode forms taper metal electrode integration construction. The electrode is plated by electrolysis, and then is enclosed by solid electrolyte uniformly, and this method can improve the stability of electrode. The taper electrode is like a needle, can penetrate easily with high efficiency. Several electrode needles forming ring array electrodes can suppress noises using digital signal processing method.(5) The stability and precision of electrodes are affected by the volecity of net reaction, circumstance concentration, circumstance temperature, pH, and diffusion potential. The influence to the potential difference of the factors will be lowered if the electrolytic solution is at the saturated or oversaturated situation. At the same time, increasing the length will extend the use life of the electrode. The calculation by resistivity network simulation method has the results as followings:different type, radius, bury and resistivity of soil will have different grounded resistivity. The bigger radius, the deeper bury, the smaller soil resistivity, and the smaller grounded resistivity, and this is good to the collection of weak signal. The potential difference will change by the influence factor, named as trends. The curve characteristic is near linear or nonlinear, and the spectrum focuses on zero or low frequency as the presence of drift. The drift will make the calculated resistivity at low f(?)equency change abruptly, shield the real information at deep depth, and lead to wrong inversion results.(6) In the data procession method research, the all-zone apparent resistivities of CSAMT and LOTEM methods are calculated, and the results can reflect the geologic information objectively, reproduce the deep geologic imformation, lower the work load and complexities, and improve the reliability of results. Man-machine combination (auto-iteration for CSAMT method) inversion can obtain the geo-electric model quickly, and thus improves the competitive power of LOTEM method in deep resources prospecting. Besides, the effects of CSAMT method can be revised by the LOTEM data, and the methods are f=186/t time-to- frequency transform and inversion correction method. The static effect and trends can be revised effectively, and the false peak effect can be recognized. Those data processing methods make the reflection of deep information more reality, and improve the interpretation effiency and reliability.The innovations of this paper are as followings:(1) The influence to CSAMT results by trends of electrode is found, and the correction method is then proposed. The positive trends will make the resistivity in depth increase abruptly, and vice versa. This kind of change can shield the real information in depth. However, the inversion correction with LOTEM data can reveal the true geologic information in depth, and finally improves the reliability of CSAMT and joint time-frequency electromagnetic results.(2) The design method of ring array solid non-polarized electrode is proposed. Those methods adopted overcome the problems of low stability in metal electrode and inconvience in non-polarized electrode such as, electrolysis in electrode manufacture, electrode enclosed by solid electrolyte uniformly in solid non-polarized electrode manufacture, ring array and taper design structure, and iteration suppression method. The realization of electrode with high precision and effiency provides technical support for weak electric signal acquisition in deep mine resource detection.(3) Joint time-frequency electromagnetic method for deep mine resources is proposed. Those methods such as,3 or 5 pseudo-random transmitting with the base frequency lower than the optimal transmitting frequency, bipole preset iteration method, all-zone apparent resistivity calculation, man-machine joint inversion, time-frequency transform and inversion correction method etc., are used to improve the prospecting ability and effiency of deep resources.Those results show that, joint time-frequency electromagnetic method can break the technique bottleneck of deep mine resources prospecting, provide guarantee for the successful detection of deep mine resources, is very important for the solving of mine crisis, and can shorten the gap of deep prospecting level between our country and developed countries.