Effect of offset on the depth and effectiveness of controlled-source audio-frequency magnetotellurics exploration

IF 2.1 3区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
Dajun Li , Yaoming Wang , Yabin Li , Aihua Weng , Xuanlong Shan , Chuncheng Li
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引用次数: 0

Abstract

An appropriate offset is crucial for the successful application of controlled-source audio-frequency magnetotellurics (CSAMT) exploration. To investigate the effects of offset on the depth of investigation and application effectiveness of CSAMT exploration, inversion with 3D nonlinear conjugate gradient (NLCG) optimization is applied to synthetic and field datasets with various offsets. In the inversion, forward modeling using the staggered-grid finite difference method to discretize the secondary electric field equation is solved via the quasiminimal residual (QMR) method. The results demonstrate that as the offset increases, datasets show a progressive transition from the near-zone field to the middle- and far-zone fields, enabling more low-frequency data to acquire frequency sounding capabilities. The data from the far-zone field are significantly enriched in deep resistivity measurements. The optimal offset represents a trade-off between the maximal signal strength (effectiveness) and the objective depth of exploration. In light of these results, we synthesize and propose a structured five-step guideline for optimal offset selection, derived from established field-zone division principles and the skin-depth formula: (1) simple subsurface electrical structure construction, (2) target depth estimation, (3) frequency determination, (4) wavelength calculation, and (5) offset optimization. Field data from the Lushi Basin, China confirm that the offset affects not only the signal strength but also the reliability of the actual measured data. This study provides a reference for survey design in offset selection for CSAMT practitioners.
偏移量对可控源音频大地电磁勘探深度和有效性的影响
控制源音频大地电磁勘探能否成功应用,关键在于确定合适的偏移量。为了研究偏移量对CSAMT勘探深度和应用效果的影响,采用三维非线性共轭梯度(NLCG)优化方法对不同偏移量的合成数据集和野外数据集进行反演。在反演过程中,利用交错网格有限差分法对二次电场方程进行正演模拟,并利用拟最小残差法求解二次电场方程。结果表明,随着偏移量的增加,数据集呈现出从近区场到中、远区场的渐进过渡,使更多的低频数据能够获得频率探测能力。深电阻率测量的远区数据明显丰富。最佳偏移量代表了最大信号强度(有效性)和目标勘探深度之间的权衡。根据这些结果,我们综合并提出了一个结构化的五步优化偏移选择准则,该准则基于已建立的场区划分原则和skdepth公式:(1)简单的地下电结构构建,(2)目标深度估计,(3)频率确定,(4)波长计算,(5)偏移优化。中国鲁氏盆地的实测资料证实,偏移不仅影响信号强度,而且影响实际测量数据的可靠性。本研究为CSAMT从业者在偏移量选择上的调查设计提供了参考。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Journal of Applied Geophysics
Journal of Applied Geophysics 地学-地球科学综合
CiteScore
3.60
自引率
10.00%
发文量
274
审稿时长
4 months
期刊介绍: The Journal of Applied Geophysics with its key objective of responding to pertinent and timely needs, places particular emphasis on methodological developments and innovative applications of geophysical techniques for addressing environmental, engineering, and hydrological problems. Related topical research in exploration geophysics and in soil and rock physics is also covered by the Journal of Applied Geophysics.
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