Abstract
Magnetic viscosity (MV) effect is a disturbance to the transient electromagnetic method (TEM). This effect will cause slow decay at the intermediate-late time of the TEM responses, leading to erroneous interpretation. Although there are extensive researches on the MV distortion to TEM data, they are focused on conventional loop source configuration, while little has been studied for grounded-wire source configuration that has become the essential role for relatively deep-buried targets. Hence, we present the study of MV effect on the grounded-wire source TEM data. We first derived the formulas for calculating the secondary field due to MV effect, based on the Chikazumi model. The dependence of the MV effect on model parameters, such as resistivity, susceptibility, offset and geometry of the superparamagnetic layer, is then examined to illustrate the intrinsic physical mechanism of MV effect. We find that the MV effect on the grounded-wire source TEM data can be suppressed by adjusting the parameters of the survey design. Thereafter, we propose the method to quantify the MV effect when estimates of resistivity and susceptibility are available. Finally, the method for choosing the optimal offset to suppress the MV effect is proposed, which can be used as a guide for the installation of fieldwork of grounded-wire TEM survey.
Highlights
Transient electromagnetic method (TEM) is an important tool to obtain the electric properties in near-surface detection
Macnae [27] compared the magnetic viscosity (MV) effect of the land-based and airborne TEM configuration and summarized their regulations and differences. All these researches are based on conventional loop source TEM configuration, little has been conducted to the grounded-wire source configuration except that Antonov and Kozhevnikov [28] derived the formula for calculating the effect of magnetic relaxation on the horizontal electric field and cooresponding apparet resistivity
We focused on the MV effect on groundedwire source TEM method
Summary
Transient electromagnetic method (TEM) is an important tool to obtain the electric properties in near-surface detection. The TEM method uses the grounded-wire or ungrounded-loop to transmit an electromagnetic signal to the earth and records the induced electromagnetic field after the source is switched off. Macnae [27] compared the MV effect of the land-based and airborne TEM configuration and summarized their regulations and differences All these researches are based on conventional loop source TEM configuration, little has been conducted to the grounded-wire source configuration except that Antonov and Kozhevnikov [28] derived the formula for calculating the effect of magnetic relaxation on the horizontal electric field and cooresponding apparet resistivity. The study of MV effect on grounded-wire source TEM method is important to guarantee the accuracy of the inversion of the data when SPM medium is presented.
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