垂直磁偶极子2.5维正演模拟

2.5-dimensional forward modeling vertical magnetic dipole

讨论实际生产中常见的垂直磁偶极子2.5维正演问题。从Maxwell方程组出发,沿构造走向做Fourier变换转化到波数域,采用波数来模拟三维垂直磁偶源的特征;然后采用有限差分法计算得到沿走向方向的一系列离散波数场,再进行反Fourier变换得到空间域的场响应,从而实现2.5维正演。通过对比标准模型模拟结果,检验正演模拟的正确性之后,对不同参数下的水平板状体模型进行了试算,结果表明:(1)改变工作频率,磁场强度垂直分量在板顶均出现极小值,两侧有对称的极大值,且频率越高,磁场响应越激烈。(2)改变埋深,埋深越浅,磁场响应越激烈。(3)改变板长,板长越长,两个极大值间隔越大。当板长小于收-发极距时,只有一个极小值;而板长大于收-发极距时,曲线会出现两个极小值。(4)改变装置收-发极距,收-发极距越小,磁场强度垂直分量总体会越强,且当收-发极距比水平板长还小时,响应曲线也会出现两个极小值。分析得知,两个极小值本来是存在的,只是随着收-发极距相对板长不断增大,两个极小值便会重合在一起,造成平时所见的只有一个极小值现象。

The paper discusses 2.5-dimensional forward modeling vertical magnetic dipole which is prevalent in actual production.According to Maxwell equation,it synthesizes the data on three-dimensional source with a series of different wave numbers in geologic structure＇s strike direction based on Fourier transform,and then,calculates a series of discrete wave numbers fields using the finite difference method.Then,it will gain the electromagnetic field response and accomplish 2.5-dimensional forward after inverse Fourier transform.Lastly,this paper models and analyzes some horizontal plates with different conditions after testing the accuracy of forward by comparing the result of standard model.The results are：（1）The minimum of vertical magnetic field shows on the top of the plate,and both sides show a symmetrical maximum.And the higher the frequency is,the stronger the vertical magnetic field is.（2）Changing the depth of the plate,the shallower the plate is,the stronger the vertical magnetic field is.（3）Changing the length of the plate,the longer the plate is,the farther the distance of symmetrical maximum is.When the length of the plate is shorter than the transmitter-receiver distance,the result has only one minimum.When the length of the plate is longer than the transmitter-receiver distance,the result has two minimums.（4）Changing the transmitter-receiver distance,the shorter the transmitter-receiver distance is,the stronger the vertical magnetic field is.And when the transmitter-receiver distance is shorter than the length of the plate,the result has two minimums,too.The result is that two minimums essentially exist,just as the transmitter-receiver distance increases as contrasted to the length of the plate,the two minimums show together.