随钻条件下井旁界面声电效应的辐射电磁波响应特征

孔隙地层中存在着动电耦合现象,弹性波作用在声阻抗或者电化学性质差异地层会产生辐射电磁波.本文探讨了随钻条件下,利用孔隙地层中的动电耦合效应探测井旁地质体的可行性.为此模拟了井外存在声阻抗差异和电化学差异界面时声波诱导的辐射电场响应特征.求出柱坐标下介质中声场和电场函数表达式,由此计算并比较了均匀介质和弹性差异界面情况下井中接收到的电场.发现随钻条件下,井外的弹性差异界面会产生以光速传播的辐射电磁波.该电磁波反映了井外地质信息,并先于井中直达声波的伴随电磁场到达接收器,因而可以直接观测到.此外,还发现界面辐射电磁波可以用来探测油、水等电化学性质差异大而弹性阻抗差异小的界面;声源频率以及界面两侧地层性质差异程度是辐射电磁波幅度的影响因素.

Characteristics of radiating electromagnetic waves induced by the interface outside the borehole during seismoelectric logging while drilling

Porous formations often exhibit seismoelectric effects. When there is an interface in the medium, the discontinuity of the seismoelectric property gives rise to electromagnetic radiation. We study the feasibility of using the electrokinetic effect generated in porous formations to explore the geologic structure outside the borehole. In particular, we study the characteristics of the radiation of electromagnetic waves from an elastic impendence interface or an electrochemical interface outside the borehole with a seismoelectric logging tool. We first derive the solution for elastic and electric fields in the cylindrical coordinates. Then we calculate the electric fields in the borehole for a homogenous formation and for a formation with an elastic impendence interface. The results show that during the seismoelectric logging while drilling, the electromagnetic radiation can be induced at the interface outside the borehole and travels at speed of light. Because of the radiated electromagnetic waves arrive earlier than the seismoelectric signals traveling along the borehole, it can be used to detect the geologic structure outside the borehole. In addition, these radiated electromagnetic waves can be used to detect a low elastic impendence or high electrical impendence interface, such as an oil-water interface, which is usually difficult to recognize using the elastic wave reflection method only. We also show that the dominant frequency and contrast between two formations can influence the amplitude of the radiated electromagnetic waves.