Calculation Of All-time Apparent Resistivity For The SOTEM

As the increase of the mineral development, the shallow mineral resources has been sharply decreased. It has become an emergent and a long-term task to develop the second depth space(500m~1000m) to explore the mineral resources in elaboration. The subject of mineral exploration is developing toward the deep mine and buried ore. Geophysical exploration has been the increasingly significant method for the survey of buried ore and developing large depth and high-accuracy geophysical survey research has been the mainly current scientific problems in geophysical prospecting.Recently, Transient Electromagnetic Method(TEM) has been extensively used in mineral exploration. Loop Transient Electromagnetic Method is just only sensitive to low resistivity anomaly and weak in high resistivity anomaly, and its exploration depth is limited. Though LOTEM could observe the vertical component of the magnetic field and the horizontal component of the electric field and has a good response for low resistivity and high resistivity anomaly, it does not do well for highly effective and accurate prospecting for its large distance transceiver and low signal ratio. According to the basic theory of transient electromagnetic field, grounding wire source could achieve near-source observations when selecting proper transmit waveform to excite electromagnetic field. On this basis, the proposed electrical source of Short-offset Transient Electromagnetic Method(SOTEM) is a method which is large depth prospecting, high accuracy and high effective. Apparent resistivity as a main, even the sole parameter in the electromagnetic interpretation plays a decisive role in SOTEM system. As SOTEM could observe the whole field signals, thus, the precise calculating method of all-time resistivity is fundamental for fully developing the prospecting ability of SOTEM system.This thesis calculates the electrical source transient electromagnetic response on the uniform earth surface and the stratified earth surface based on the Maxwell equations with auxiliary bit function. Separating the long grounding wire source response into line source term and grounding term and calculating these two terms respectively could reduce the calculating time. Secondly, we could study every component of the electrical generated secondary electromagnetic field by planar propagation distribution and single-point decay curve of diffusion field,providing the theory support for selecting appropriate observation component and scope for practical work.We calculate all-time apparent resistivity of vertical component of magnetic field and its derivative by piecewise series approximation, dichotomy and translational algorithm respectively, and analysis its pros and cons of these three methods for calculating all-time apparent resistivity and the resolving power of the all-time apparent resistivity based on magnetic field intensity and induction electromotive force. Compared with early stage and late period of apparent resistivity, all-time apparent resistivity curve figure could reflect the electrical target and information of the geo-electric section more precisely. In addition, all-time apparent resistivity is only in connection with electrical structure of stratum, and has nothing to do with the transceiver distance and azimuth, which facilitates the near-source measure of ground source TEM. At last, applying the all-time apparent electrical resistivity method this thesis propose to certain metal ore exploration obtain a good geophysical results.It is proved that theelectrical source all-time electrical resistivity of is effective by theroy analyse and practical application. All-time electrical resistivity eleminates the false high value in the shallow, being superior to the traditional late period electrical resistivity and reletively truely reflecting the rules of the electrical change of the stratum.