浅水区海浪电磁噪声对大地电磁阻抗估计的影响

在浅水区海洋大地电磁(MT)勘探中, 海浪运动频带范围内(约0.05~0.8 Hz)阻抗估计结果欠佳, 这一直是困扰地球物理工作者的难题.本文基于三层海洋地电模型推导了二维和三维海浪运动感应电磁场(简称海浪感应电磁场)表达式, 分析了海浪感应电磁场的平面波传播特性.借鉴MT场趋肤深度和阻抗的概念, 推导出海浪感应电磁场的趋肤深度和阻抗表达式, 分析了浅水区海浪感应电磁场和MT场趋肤深度和阻抗的差异.在海底介质中, 海浪感应电磁场的衰减速度要比MT场衰减速度大约快2个量级, 因而它无法用于探测海底深部电性结构.海浪感应电磁场阻抗幅值比MT阻抗幅值小2~3个量级.海浪感应电噪声强度约为大地电场强度的10%, 而海浪感应磁噪声比地球变化磁场大1~2个量级, 故海浪感应电噪声对大地电场的影响较小, 但海浪感应磁噪声对地球变化磁场的影响较大.海浪感应电磁阻抗相位在-90°(TE模式)和90°(TM模式)附近, 偏差小于0.06°, 而均匀半空间介质MT阻抗相位为-45°(TE模式)和135°(TM模式).通过引入"复合阻抗"概念, 探讨了海浪感应电磁场对海洋MT阻抗估计结果的影响.

Impacts of electromagnetic noise of shallow water ocean waves on magnetotelluric impedance estimation

In shallow water marine magnetotelluric (MT) exploration, the impedance estimation within the wave motion frequency band (about 0.05~0.8 Hz) is poor. Based on the three-layer ocean geoelectric model, the expressions of ocean waves induced electromagnetic (OWEM) fields are derived. And the propagation characteristics of OWEM fields are analyzed. The expressions of the skin depth and impedance expressions of OWEM fields are derived, and the differences between the skin depth and impedance of OWEM and MT fields in shallow water areas are analyzed. In the seabed medium, the attenuation rate of the OWEM field is about two orders of magnitude faster than that of the MT field, so it cannot be used to detect deep electrical structure. The amplitude of OWEM impedance is 2~3 orders of magnitude smaller than that of MT impedance. The intensity of the wave-induced electric noise is about 10% of that of the telluric field, and the wave-induced magnetic noise is 1~2 orders of magnitude larger than the varying magnetic field of the Earth, so the wave-induced electric noise has little impact on the telluric field, but the wave-induced magnetic noise has a great impact on the Earth's varying magnetic field. The phase of OWEM impedance is around -90° (TE mode) and 90° (TM mode), the deviation is less than 0.06°, while the MT impedance phase in homogeneous half-space is -45° (TE mode) and 135° (TM mode). By introducing the concept of "composite impedance", the influence of OWEM fields on marine MT impedance estimation is discussed.