磁性源瞬变电磁法零偏移距全期视电阻率计算及其校正方法研究

Study of all-time apparent resistivity calculation at zero-offset and its correction method using magnetic source transient electromagnetic method

计算视电阻率是瞬变电磁数据处理与解释工作的主要内容之一。通常由均匀或层状模型计算得到的视电阻率不足以详细表征地下介质分布,具有较强的体积效应和多解性且计算量大。基于此,针对全期视电阻率的优化问题,研究了半球壳层校正方法,实现了快速计算弱多解性的视电阻率,校正结果显示电性分界面更加明显。为实现数据快速处理并得到视电阻率唯一解,首先分析了感应电动势和电阻率之间的单调关系,对比地质情况从而选取感应电动势的单调区间,然后重新构造核函数以适用于二分法,最后计算连续且完整采样时间的视电阻率。为减弱视电阻率的体积效应,建立由均匀半球壳层组成的视电阻率校正模型且介质层数与时道数量一致,然后基于扩散深度表达式推导出视电阻率校正公式,用以校正全期视电阻率最优化算法计算的视电阻率。理论模型和实测数据的处理结果均表明,全期视电阻率最优化算法相对于晚期视电阻率计算方法,能够将有效时道拓展至全时间段,从而可以更加有效地识别浅层介质的垂向分布规律,而且半球壳层校正方法能够有效约束低阻介质体积效应对深度的影响,提高视电阻率的垂向分辨率。

Calculating apparent resistivity is important for transient electromagnetic data processing and interpretation.The apparent resistivity usually calculated by homogeneous or layered model is not enough to describe the distribution of underground media in detail,with strong volume effect,multiple solutions,and large calculation amount.Therefore,this study aims to solve the optimization problem of the all-time apparent resistivity using the hemispheric shell correction method to quickly find the solution for the weak multi-solution apparent resistivity.The result shows that the apparent resistivity interface is more prominent.The monotone of induced electromotive force and resistivity was analyzed first to achieve fast data processing and obtain the unique solution of apparent resistivity.Then,the monotone interval of induced electromotive force can be selected by comparing geological conditions.Furthermore,the transient electromagnetic kernel function was reconstructed for adapting the dichotomy method,which obtains all-time apparent resistivity within a continuous and complete sampling time period.For reducing the volume effect of apparent resistivity,a hemispherical shell correction model was proposed,in which the underground medium was regarded as a uniform hemispheric shell assemblage whose number of shells was the same as the time channels.The apparent resistivity correction formula used to correct the apparent resistivity,which was calculated using the optimum calculation of the all-time apparent resistivity,was derived from the diffusion depth formula.The calculation results of the theoretical model and field-measured data processing show that compared with the calculation method of late apparent resistivity,the optimization method of all time apparent resistivity can extend the effective time channel to the all-time period,and effectively identify the vertical distribution of the shallow medium.The hemispherical shell correction method effectively limits the volume effect of the low resistivity medium to the depth and improves the vertical resolution of apparent resistivity.