单斜地形情况下的磁共振测深方法反演研究

doi:10.11720/wtyht.2021.1238

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Abstract

Magnetic resonance sounding is a geophysical exploration method based on the difference of relaxation characteristics of hydrogen proton in geomagnetic field. It can detect the content and distribution of groundwater without damage. In recent years, with the expansion of the application of magnetic resonance sounding, this method is often faced with the detection when the surface has a large slope. When traditional inversion methods are used, inaccurate inversion results often occur. In this paper, the angle between the geomagnetic field vector and the coil is introduced, which is called the effective geomagnetic inclination, and the inversion method of the magnetic resonance sounding under surface slope is given. According to the prior information provided by geological data or other geophysical methods, the distribution of parallel or horizontal layered kernel functions of the magnetic resonance sounding is obtained, so as to carry out the inversion of the NMR signal under the condition of surface slope. In this paper, 1D and 2D aquifer models are constructed with the sloping surface, the corresponding NMR signals are obtained by forward calculation, and the kernel functions calculated by effective geomagnetic inclination and traditional method are used for inversion. The results show that, in the case of surface slope, the traditional inversion method of magnetic resonance sounding will inevitably produce errors, while the kernel function calculated by effective geomagnetic inclination can suppress the influence of surface slope. The validity and feasibility of the method were verified by the inversion of the filed NMR data in Baishuihe landslide area. Therefore, the inversion method based on the effective geomagnetic inclination proposed in this paper can greatly improve the application scope of magnetic resonance sounding.

Key words： magnetic resonance sounding surface slope inversion effective geomagnetic inclination

Received: 07 May 2020 Published: 27 July 2021

ZTFLH:

P631

Corresponding Authors: LI Zhen-Yu E-mail: lifan@cug.edu.cn;zhenyuli626@126.com

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	Fan LI
	Ming ZHOU
	Kai-Tian LI
	Kai LU
	Zhen-Yu LI

Cite this article:

Fan LI,Ming ZHOU,Kai-Tian LI, et al. Inversion of magnetic resonance sounding under the condition of surface slope[J]. Geophysical and Geochemical Exploration, 2021, 45(3): 712-725.

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https://www.wutanyuhuatan.com/EN/10.11720/wtyht.2021.1238 OR https://www.wutanyuhuatan.com/EN/Y2021/V45/I3/712

Schematic diagram of the angle relationship between the coil and the geomagnetic field
a—horizon;b—inclined surface

Diagram of the coordinate transformation under the condition of horizontal and inclined surface
a—the schematic diagram of angle relationship of horizontal coordinate(x,y,z), auxiliary coordinate(x₁,y₁,z₁) and tilt coordinate (x₂,y₂,z₂);b—the schematic diagram of angle relationship between horizontal coordinate and auxiliary coordinate;c—the schematic diagram of angle relationship between auxiliary coordinate and tilt coordinate

Cross section of the MRS kernel function under different terrain inclination
a—α=0°;b—α=30°;c—α=45°;d—α=90°;each subplot has been placed according to the real dip angle, and the red coordinate axis is the horizontal coordinate system. The parameters used in calculation are:T_x/R_x is a circle with the radius of 10 m, 2 turns. The underground resistivity is 100 Ω·m, the excitation pulse moment is 10 A·s, the geomagnetic inclination is 45°N, the dip direction is south, and the geomagnetic declination is 0°

Diagram of underground aquifers distribution
a—horizontal aquifers in horizontal terrain; b—horizontal aquifer layers in inclined terrain; c—parallel aquifer layers in inclined terrain; d—2D aquifers distributed along the trend in inclined terrain

Schematic diagram of 2D kernel function calculation in case of inclined surface
a—the 2D kernel function distributed along the dip direction; b—the 2D kernel function distributed along the trend direction. The attached subplot on the right is the schematic diagram of spatial geometric position;the calculation parameters are:T_x/R_x is a circle with radius of 10 m, 2 turns. The underground resistivity is 100 Ω·m, the terrain dip angle is 30°, the dip direction is south, the geomagnetic inclination is 45°N, and the geomagnetic declination is 0°. The excitation pulse moment is 10 A·s

Schematic diagram of 1D kernel function calculation in case of inclined surface
a—parallel stratification; b—horizontal stratification

Diagram of 1D aquifer setting and kernel function distribution in condition of surface slope
a—theoretical aquifer model, which represents that the aquifer is located at 5~9 m and 15~21 m underground; b—the relationship between the initial amplitude of NMR signal and the excitation pulse moment of the parallel aquifer, the horizontal aquifer and the aquifer with no topographic inclination; c—kernel function distribution in horizontal terrain; d—kernel function distribution of parallel stratification in inclined terrain; e—kernel function distribution of horizontal stratification in inclined terrain

1D inversion result of NMR signal in case of parallel stratification
a~d—the inversion results ofE_p using parallel kernel function K_p after topographic correction and the uncorrected kernel function K, respectively; e—the fitting results of inversion data

1D inversion result of NMR signal in case of horizontal stratification
a~d—the inversion results ofE_h using horizontal kernel function K_h after topographic correction and the uncorrected kernel function K, respectively; e—the fitting results of inversion data

Diagram of 2D aquifer setting and forward modeling result of NMR signal in case of inclined surface
a—the theoretical aquifer model, the red triangle in the figure represents the position of the center point of theT_x/R_x coil; b—the theoretical NMR signal for each coil

2D inversion result of NMR signal in case of inclined surface
a~d—represents the inversion results using the kernel functionK_s2D after topographic correction and K_2D without topographic correction with or without 5% Gaussian white noise, respectively;e~i—the fitting results of inversion data for each coil

Geological section of Baishuihe landslide
a—photo of the study area;b—geological section of Baishuihe lanslide

Comparison of inversion results of Baishuihe landslide by MRS method
a—inversion results of measuring point 01; b—inversion results of measuring point 02;the red dotted line in the figure indicates the position of the landslide mass calibrated by the borehole

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