井下视电阻率观测影响系数分析

Sensitivity coefficients analysis of deep­well apparent resistivity measurement

摘要:

采用水平层状均匀介质中点电流源位于任意深度时电位解析表达式，分析了井下对称四极视电阻率观测时影响系数随深度和极距的变化。结果表明对于固定的观测极距，影响系数与电极埋深之间关系复杂；对于某些电性结构和在一定深度范围内，井下观测对表层干扰具有放大作用。对于固定的电极埋深，小极距观测主要体现观测装置所在处的介质层信息，深部介质的影响系数随着极距的加大而增加，浅层影响系数一般先上升后下降；观测极距足够大时，井下观测影响系数逐渐接近于地表观测的影响系数，井下观测的优势得不到体现。本文以天水台为例讨论了实施井下观测时影响系数在选择供电极距和电极埋深过程中的应用。分析结果对在不同电性结构中实施井下地电观测时具有一定的参考意义。

Abstract:

Analytical expression of electrical potential caused by a point current source in horizontally layered model was applied to analyze the distributions of sensitivity coefficients with the spacing and depth of electrodes when Schlumberger arrays were used in deep-well apparent resistivity measurement. The results showed that for a fixed Schlumberger array sensitivity coefficients displayed complex variations with the increase of electrode depth. Disturbances from surface might be amplified when deep-well measurement was made within some depth ranges for some electrical structures. For the fixed electrode depth，measurement using narrow electrode spacing mainly reflected resistivity information of the stratum where the arrays were. Information from deep stratum increased with the expansion of electrode spacing. Sensitivity coefficients of shallow layers generally increased at first and decreased afterward. When electrode spacing was large enough，difference of sensitivity coefficients between deep-well and surface measurement decreased. As an example，sensitivity coefficients of deep-well measurement at Tianshui seismic station were also analyzed. The calculated results could be used as a reference for choosing buried depth and spacing of the electrodes when deep-well configuration was used in different electric structures.