海浪引起的地磁场波动的观测与初步分析

Observation and preliminary analysis of magnetic field changes induced by ocean waves

近岸海浪是影响近岸水域环境状态的重要环节.海洋近岸波的波动性质和变化规律的研究对于海岸防护、近岸航运、军事活动等具有重要意义.感应磁场波动可为探测海洋近岸波的非线性过程提供一种有效手段.本文首次对海洋近岸波引起的感应磁场进行了频谱指数分析,获得了能量注入和耗散过程对应的谱段.2016年5月15日—6月30日在昌黎海岸(东经119.3,北纬39.7°)利用KDM-2型磁通门磁力仪对海洋近岸波引起的磁场变化进行了测量,数据分辨率为0.01 nT/(HZ)^(1/2),时间采样率为1 Hz和128 Hz.观测数据显示,在距海岸线2 m远处,清晰地观测到了海洋近岸波引起的磁场波动,在0.001~10.0 Hz频段的波动最大振幅约为1 nT.波动的小波频谱呈现出多次谐波结构,符合海洋近岸波的谐波特征.傅里叶频谱显示不同频段谱的斜率不同,反映了海洋近岸波发展与耗散的非线性物理过程,进一步开展长期观测和统计分析有助于深入了解海洋中的电动力学过程.

Near shore ocean wave has significant impact on the near shore ocean environmental status. The characteristics of near shore ocean wave are substantial to human activities at near shore, such as coast protection work, near shore transportation, and military activity. The measurement of the induced magnetic field changes is an effective tool to detect and study the nonlinear processes of near shore ocean wave. In this paper, we conduct a spectral index analysis on the induced magnetic field generated by near shore ocean waves for the first time. The spectral sections for the processes of energy input and dissipation are finally obtained. A survey of the induced magnetic field at the beach was conducted with the KDM-2 fluxgate magnetometers at Changli city from 15^th May to 30^th June 2016. The fluxgate magnetometer has the resolution of 0.01 nT√Hz, with two sampling rates of 1 Hz and 128 Hz, respectively. Our results clearly show that the magnetic field changes caused by the ocean waves can be detected and stored by our measurements, with the observing distance of 2 meters from the coast. The maximum amplitude of magnetic signals is about 1 nT within the frequency band of 0.001~10.0 Hz. The wavelet spectra show that the configuration of the magnetic energy has multiple harmonic structures, which is consistent with the characteristics of near shore ocean waves. Through the FFT analysis, the spectral line is shown separated to several sections with different slopes, suggesting a development of nonlinear physical process and dissipation of ocean near shore waves. Long term observation and statistical analysis in the future are necessary, which can helpful to understand the electrodynamic process in the ocean.