A new far-field low-frequency electromagnetic method for measurement of temporal variations in subsurface electrical conductivity averaged over a transect
A new far-field low-frequency electromagnetic method for measurement of temporal variations in subsurface electrical conductivity averaged over a transect
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DOI：
10.1002/nsg.12009
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发表时间：
2018-08
影响因子：
1.6
通讯作者：
O. Kiseleva;A. Brandelik;C. Hübner;N. Kalthoff;M. Kohler;F. Königer;C. Kottmeier
中科院分区：
文献类型：
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作者：
O. Kiseleva;A. Brandelik;C. Hübner;N. Kalthoff;M. Kohler;F. Königer;C. Kottmeier
新的远场低频电磁方法使用来自远距离无线电发射机的低频电磁地波来估计影响地下电导率的因素的时间变化，这些因素沿着发射器和接收器之间或与发射器一致的两个接收器之间的传播路径平均。两个接收器之间的相位和相位差取决于影响电导率变化的三个因素：土壤湿度、地下水位深度和土壤温度。通过模拟研究了这种依赖性，并通过实验进行了评价。采用表面阻抗法和Sommerfeld地波衰减函数模拟地波在层状地下的传播。地下采用三层模型，包括土层、非饱和渗流带和饱和地下水带。在77.5 kHz频率下的模拟结果表明，上述三种因素的自然变化对地波相位的影响很大；77.5 kHz是位于德国Mainflingen的德国正常时间服务（DCF77）的载波频率，它被选为实验中使用的低频无线电波的来源。在两年的测量期间，地波的振幅和相位由两个接收器记录，一个距离发射器70公里，另一个距离发射器110公里。此外，还计算了两个接收机之间的相位差。土壤湿度、地下水位深度和土壤温度的原位观测被用来估计相和相位差依赖关系。对实测的相位差进行多元回归分析，发现其对地下水位深度和土壤温度的依赖性较强，而土壤湿度对相位差的影响很小。相反，如果有给定频率的相位和土壤温度信息，所得到的关系可以用来估计地下水位的深度变化。
The new far-field low-frequency electromagnetic method used the electromagnetic ground wave from distant radio transmitters at low frequencies to estimate temporal variations of factors affecting subsurface electrical conductivity averaged along the propagation path between either a transmitter and a receiver or two receivers that are in line with a transmitter. Phase and phase difference between two receivers depend on three factors that influence the changes in electrical conductivity: soil moisture, depth to the groundwater table and soil temperature. This dependence was investigated by simulations and evaluated by an experiment. The measurement layout was based on simulating ground wave propagation over a layered subsurface using the surface impedance method and the Sommerfeld ground wave attenuation function. A three-layer model for the subsurface was used, which includes a soil layer, an unsaturated vadose zone and a saturated groundwater zone. The results of simulations at a frequency of 77.5 kHz showed that the phase of the ground wave is strongly influenced by natural variation of the above-mentioned three factors; 77.5 kHz is the carrier frequency of the Normal Time Service Germany (DCF77) in Mainflingen/Germany, that was chosen as a source of the low-frequency radio waves used in the experiment. Over a 2-year measurement period, the amplitude and phase of the ground wave were recorded with two receivers, one 70 km and the other 110 km away from the transmitter. Additionally, phase difference between the two receivers was calculated. In situ observations of soil moisture, depth to the groundwater table, and soil temperature along the transects under investigation were used to estimate phase and phase difference dependencies. Multiple regression analysis of the measured phase and phase difference revealed a strong dependence on the depth to the groundwater table and on soil temperature, whereas the impact of soil moisture on the phase and phase difference was found to be very low. Conversely, the relations obtained can be used to estimate the variation of the depth to the groundwater table, if the phase at a given frequency and the soil temperature information are available.