小极距井下地电阻率观测对地表环境影响的抑制分析——以北京地区台站为例

地震地电阻率是观测中强地震孕震过程中微变形或微破裂变化的方法之一。近年来发展起来的小极距井下地电阻率观测,不仅能够抑制地表环境干扰,还能够缓解地表地电阻率占地面积大的困扰。基于北京地区延庆台、平谷台和通州台小极距井下地电阻率观测,通过与同场地地表地电阻率对比观测和数值模拟计算,分析了地表铁质干扰、季节变化、降水和潜水等因素对地电阻率观测的影响。结果显示,与同场地地表地电阻率相比,小极距井下地电阻率对地表自然环境因素的影响有较好的抑制能力,增强了观测效能。     

开展小极距井下地电阻率观测的可行性分析

目前,我国用于地震监测的地电阻率观测面临着两个难题:①测区范围较大导致台网稀疏且分布不均匀;②易受环境干扰。本文结合台站实际的地下电性结构,采用地电阻率解析表达式和有限元数值分析方法,对开展小极距井下地电阻率观测的可行性进行了讨论。结果表明:小极距井下观测方式能有效抑制地表电性异常体类干扰和年变化的影响,也能记录到地表大极距观测和井下大极距观测所能记录到的震前异常变化。小极距井下观测能大幅减小布极区范围,有助于地电阻率的足密度组网成场观测,可为解决目前地电阻率观测面临的难题提供一种可选方案。     

通州台井下地电阻率测深初步分析

2020年7月12日唐山古冶 M_S5．1地震发生前,井下小极距地电阻率出现快速下降-折返的变化,变化形态、异常时长符合地电阻率孕震机理变化,但下降幅度远远小于地表大极距地电阻率孕震过程的变化幅度.因此,文章基于通州台地下电性结构和装置系统,采用数值分析方法,分析地表和井下小极距地电阻率的探测深度.结果表明,当底层电性变化区域介质电阻率发生某种减小时,通州台地表和井下小极距地电阻率装置系统地电阻率观测值会下降,下降的幅度随着深部介质电阻率变化区域上界面向上的扩展而增大.相比地表观测,井下小极距电阻率观测能更显著地接收到深部电阻率变化信号,对孕震有更强的反映能力.     

平凉崆峒台井下地电阻率观测数据分析研究

简要介绍甘肃平凉崆峒台井下地电阻率观测的地质构造背景、场地布设、观测方式等基本情况,通过对井下水平与垂直观测数据曲线的动态变化、观测精度等对比分析得出,井下地电阻率观测年变化与地表有较大的差别,井下观测大大削弱了年变幅度;垂直观测较水平观测数据变化平稳、年变幅度较低;井下观测系统能减少干扰,提高信噪比,缓解地电观测与经济建设的矛盾,可作为地电观测方式的新探索。     

平凉台井下地电阻率观测影响系数分析

根据地电阻率影响系数理论,以平凉台4层电性结构为例,分析了井下对称四极地电阻率观测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的关系复杂;通过计算各层影响系数的大小,认为平凉台井下观测对地表及浅层干扰有较好的抑制作用,其分析结果可为在类似台址电性结构中实施井下地电阻率观测时选择电极埋深和供电极距提供参考。     

河北大柏舍地电台地电阻率观测数据分析

对大柏舍地电台地电阻率的观测数据进行综合分析表明,台站台址条件、观测系统均符合地电阻率台站建设规范要求;观测数据连续、可靠,年变化主要受地下水位的影响;背景场观测系统外线路采用地埋敷设方式,减小了地表干扰对地电阻率观测数据的影响,观测精度有所提高。该台地电阻率观测对本区及附近中等地震有较好的响应。     

江宁台多孔垂向地电阻率观测试验

为了提高地电阻率观测抑制地铁干扰的能力, 江宁台新建了多孔垂向地电阻率观测。 在观测中探讨完善了装置系数计算方法, 解决了缺数问题, 并通过比较同测区井下和垂向地电阻率观测数据, 得出如下结论: ① 垂向观测由于布极方式较为特殊, 其年变化幅度大于井下地电阻率观测同等极距; ② 与井下地电阻率观测相比, 垂向地电阻率观测信噪比更高, 具备更好的抗地铁干扰能力, 以期为地电观测抗干扰的方法和技术应用提供基础; ③ 垂向地电阻率观测建设中, 可能需要重视电极的位置固定。     

冬奥会保障项目井下地电阻率观测专用电缆设计与测试

根据冬奥会保障晋冀蒙监测能力提升项目的设计方案,在晋冀蒙交界及附近区域的宝昌、集宁、阳原、大同、代县、临汾、通州和平谷等8个地电阻率台站原观测场地建设井下小极距全空间地电阻率观测装置系统。参照该项目对井下地电阻率观测的技术要求,设计了井下地电阻率专用电缆,在项目实施过程中对专用电缆的导体直流电阻、线间绝缘电阻等主要指标进行了技术测试。测试结果表明,该专用电缆适用于本项目井下地电阻率观测,可保障井下地电阻率观测装置的长期稳定性。     

晋冀蒙交界及附近地区小极距井下地电阻率观测装置设计

为提升2022年北京冬奥会期间地电阻率测项的震情保障能力,将对晋冀蒙交界及附近区域的宝昌、集宁、阳原、大同、代县、临汾、通州和平谷8个台站在原有观测基础上增加具有全空间性质的小极距井下地电阻率观测。本文根据台站钻孔岩芯剖面和电测深数据反演测区水平层状电性结构,利用介质对地电阻率观测的影响系数方法,计算影响系数随极距和埋深的变化,选择合适的极距和埋深,使浅层介质对地电阻率观测的影响尽可能小,使目标层位对观测的贡献尽可能大,减少浅层介质对观测的影响。由此得出了各台站观测极距及埋深等装置系数,在此基础上完成了8个台站观测装置的方案设计。     

深井地电阻率观测的几个技术问题

<正>近年由于城市化进程的加快和地表人为干扰的增多,部分大极距的地面地电观测受到了严重的影响,数据质量下降,中国地震局在部分有干扰的地电台站开展井下地电观测。主要解决:1井下多深可以避开地表的干扰和潜水的变化,观测数值主要反映地是探测目标层地电阻率变化;2当测量目标层介质的电性发生变化时,设计的观测系统能否记录下地电阻率变化。井下地电观测系统的建设主要思路:非均匀各向     

小极距井下地电阻率观测数据质量浅析

为加强2022年北京冬奥会举办地区的震情保障工作,2019—2020年,依托原有地电台网,建设平谷、通州、阳原、大同、代县、临汾、宝昌、和林格尔等8个小极距井下地电阻率观测站（点）,目前均运行良好,与同台地表大极距地电观测相比,观测数据精度提高。选取2021年9—11月井下地电阻率观测站数据记录,就相对均方差、日精度、月精度、月离散度、变化幅度等指标,分析小极距井下地电阻率观测数据质量。结果表明,8个井下地电阻率观测站（点）运行稳定,观测数据精度较高,具备一定映震效能,可为冬奥会举办区域及周边地区的震情跟踪有效服务。     

RESISTIVITY TYPE CURVES OVER OUTCROPPING VERTICAL DYKE—I *

The theoretical horizontal resistivity profiles over an outcropping vertical dyke for various parameters-electrode spacing, vein-width and resistivity contrast—with inline alpha, beta and gamma-Wenner electrode systems are described. The resistivity profiles present a most bewildering variety of shapes as compared to those in resistivity soundings. The analysis of type curves suggests suitable electrode configuration for detection of wide, moderately wide, and thin veins. The negative apparent resistivity values on the gamma anomalies for resistive vein over certain vein-widths of higher positive values of resistivity reflection factor contradict the concept of apparent resistivity hitherto held.     

井下地电阻率观测影响系数分析——以江宁地震台为例

采用水平层状均匀介质中点电源位于任意深度时的电位解析表达式,以江宁台3层电性结构为例,分析了井下对称四极地电阻率观测时各层影响系数随深度、极距的变化,并结合探测深度探讨了实施井下观测时影响系数在选择供电极距和电极埋深时的作用。结果表明,对于"K"型电性结构,江宁台井下观测对地表、浅层干扰有较强的抑制作用,其短极距观测对地表、浅层干扰的抑制能力显著优于长极距观测;长极距观测在电极埋深H小于100m时对地表介质季节性的干扰具有放大作用;浅层影响系数一定时,电极埋深和供电极距需同时增加;江宁台井下观测供电极距AB/2取100～150m、电极埋深H为250m较为合理。     

An application of three-dimensional electrical resistivity imaging for the detection of an underground waste-water system

A 3D electrical resistivity imaging survey is presented in this paper. The objective was to investigate an underground wastewater system at the University of Malaya, Malaysia. Apparent resistivity data were collected along ten parallel lines using a Wenner-Schlumberger configuration; electrode cables were oriented in the x-direction with 3 m spacing. Roll-along measurements using a line spacing of 3 m were carried out covering a grid of 20 × 10 electrodes. All data sets were merged into a single data file in order to perform a 3D inversion. Two different 3D least squares algorithms, based on the robust inversion method and the smoothness-constrained technique, were used for the inversion of the apparent resistivity data. Both the horizontal and vertical extents of the anomalous zones found by inversion are displayed. The results indicate the superiority of the robust inversion method over the smoothness-constrained technique at this site. The results are in sufficient accordance with previously known information about the investigation area. The results show that 3D electrical resistivity imaging surveys, in combination with an appropriate 3D inversion method, can be highly useful for engineering and archaeological investigations as well as for environmental applications.     

AN ELECTRICAL RESISTIVITY PROFILE IN HAWAII WITH NOVEL ELEVATION CORRECTION*

In order to locate relatively optimum sites for drilling exploratory holes for fresh water, an electrical resistivity survey was conducted along the new Mahukona-Kawaihae Road on the west flank of the Kohala Mountain. Two resistivity soundings made at the same stations, using the Schlumberger electrode configuration, determined an a spacing of 275 feet for horizontal profiling with the Wenner array. The correlation coefficient of the elevation to profile data was 0.41. A procedure for removing elevation effect from observed apparent resistivity was developed. Based on the reduced resistivity profile, four relatively optimum sites for additional exploration, such as by drilling, are specified. There is no specific interpretation of the data that can definitely indicate the occurrence of large underground reservoirs of fresh water anywhere along the profile. This is because the interpretation of horizontal profiling data is essentially relative and not absolute.     

A Low‐Cost,In Situ Resistivity and Temperature Monitoring System

We present a low‐cost, reliable method for long‐term in situ autonomous monitoring of subsurface resistivity and temperature in a shallow, moderately heterogeneous subsurface. Probes, to be left in situ, were constructed at relatively low cost with an electrode spacing of 5 cm. Once installed, these were wired to the CR‐1000 Campbell Scientific Inc. datalogger at the surface to electrically image infiltration fronts in the shallow subsurface. This system was constructed and installed in June 2005 to collect apparent resistivity and temperature data from 96 subsurface electrodes set to a pole‐pole resistivity array pattern and 14 thermistors at regular intervals of 30 cm through May of 2008. From these data, a temperature and resistivity relationship was determined within the vadose zone (to a depth of ~1 m) and within the saturated zone (at depths between 1 and 2 m). The high vertical resolution of the data with resistivity measurements on a scale of 5‐cm spacing coupled with surface precipitation measurements taken at 3‐min intervals for a period of roughly 3 years allowed unique observations of infiltration related to seasonal changes. Both the vertical resistivity instrument probes and the data logger system functioned well for the duration of the test period and demonstrated the capability of this low‐cost monitoring system.