钻孔瞬变电磁法拟地震全波形反演

Full waveform inversion for the borehole transient electromagnetic pseudo seismic data

钻孔瞬变电磁法是将发射与接收装置布置于钻孔中,因距离目标体近,探测精度得到提升,有利于煤矿钻孔周围含水体精细探测.常规的钻孔瞬变电磁法通过多分量响应近似判断异常体方向,难以对异常体的边界位置进行刻画,而且因瞬变电磁场的扩散特征,常规的反演方法对地质异常体的边界位置分辨率偏低.针对以上问题,结合钻孔瞬变电磁法扫描探测定位方法,通过波场转换将钻孔瞬变电磁法数据转换成虚拟波场数据,然后进行全波形反演与成像.针对虚拟波场直达波能量较强且属于干扰波,采用剪切波变换去除虚拟波场直达波,避免了虚假界面;其次由于波场转换过程中核函数病态性强的特点,引入虚拟电导率,降低了虚拟波场速度,从而降低了波场转换核函数病态性;为了对虚拟波场进行高精度反演与成像,分别采用阻尼最小二乘法和伴随状态法实现虚拟波场全波形反演.结合钻孔瞬变电磁法探测的数值实验得出阻尼最小二乘全波形反演方法的反演结果更加准确,可以实现对钻孔孔壁外围地层信息的精细刻画,提高了钻孔瞬变电磁法扫描探测的效果.

The borehole transient electromagnetic method is arranged in the drill hole with transmitting and receiving devices,which are closer to the target body and suitable for finely detecting the water-bearing body around the coal mine borehole.The conventional borehole transient electromagnetic method approximates the direction of anomaly by multi-component response,resulting in difficulty characterizing the boundary position of anomalous bodies.Moreover,due to the diffusion characteristics of the transient electromagnetic field,the conventional inversion method has a low resolution of the boundary location of the geological anomaly.To address the problems above,this paper,based on the borehole transient electromagnetic scanning detection method attempts to convert the borehole transient electromagnetic field into the virtual wave field by using the wave field conversion formula.Then,a full waveform method can be applied to acquire a resistivity profile.Technically,to address the strong energy of the virtual wave field direct wave and the interference wave,the shear wave transform is adopted to remove the direct wave of the virtual wave and then avoid the false interface;furthermore,to address the strong ill-posed problem of kernel function in the wave field conversion process,the virtual conductivity is introduced to reduce the virtual wave field velocity and thus improve the ill-posed condition;moreover,the damped least squares and adjoint-state full waveform inversion method are performed respectively.In this way,the numerical simulation verifies that the inversion results obtained by the damped least squares full waveform inversion method are more accurate,which enables to achieve the fine portrayal of the layer information outside of the borehole wall,and improve the inversion and imaging results of the scanning detection of the borehole transient electromagnetic method.