云南南部地区深部电性结构特征研究

在云南南部地区布设了一条孟连-罗平的北东向大地电磁测深剖面,以开展该地区的深部电性结构探测和孕震环境探查.沿该剖面进行了114个大地电磁测深点的观测,经过对观测资料的远参考Robust处理、定性分析和二维反演,得到了沿该剖面地壳、上地幔电性结构模型,从模型的电性结构特征进一步探讨了剖面穿过的3个地震区的深部地震孕育环境.研究结果表明：沿剖面的地壳上地幔电性结构反映出与区域地质构造资料基本一致的构造特征;该区的三个强震带地球深部都存在壳内低阻体,地震发生在电阻率梯度带上;断裂带的两侧块体介质的电阻率差异是强震活动带重要的深部背景.     

Relating electrical conduction of alluvial sediments to textural properties and pore‐fluid conductivity

Electrical conductivity of alluvial sediments depends on litho‐textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho‐textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain‐size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two‐component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three‐component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid‐grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume.     

Optimized arrays for 2D cross‐borehole electrical tomography surveys

The use of optimized arrays generated using the ‘Compare R’ method for cross‐borehole resistivity measurements is examined in this paper. We compare the performances of two array optimization algorithms, one that maximizes the model resolution and another that minimizes the point spread value. Although both algorithms give similar results, the model resolution maximization algorithm is several times faster. A study of the point spread function plots for a cross‐borehole survey shows that the model resolution within the central zone surrounded by the borehole electrodes is much higher than near the bottom end of the boreholes. Tests with synthetic and experimental data show that the optimized arrays generated by the ‘Compare R’ method have significantly better resolution than a ‘standard’ measurement sequence used in previous surveys. The resolution of the optimized arrays is less if arrays with both current (or both potential) electrodes in the same borehole are excluded. However, they are still better than the ‘standard’ arrays.     

浅部不均匀体对目标体电阻率异常影响的研究

浅部不均匀体能使电阻率测深曲线产生严重的畸变. 为了从根本上消除浅部不均匀体的干扰,本文采用三维有限元数值模拟的方法,计算出浅部不均匀体异常和混合体（目标体+不均匀体）异常,通过一定的校正方法,把不均匀体影响从混合异常中校正掉,从而实现对浅部不均匀体影响的定量校正.     

新沂地震台地电阻率与地下水位和降水的关系研究

由于受台址条件的限制,地电阻率观测值受到地下水位、降水等气象因素的影响。运用新沂台地电阻率月均值与地下水位、降水进行相关分析,从而对地电阻率进行校正,尽可能揭示地电阻率的真实变化。     

Frequency‐dependent multi‐offset phase analysis of surface waves: an example of high‐resolution characterization of a riparian aquifer

Multi‐offset phase analysis of seismic surface waves is an established technique for the extraction of dispersion curves with high spatial resolution and, consequently, for the investigation of the subsurface in terms of shear wave velocity distribution. However, field applications are rarely documented in the published literature. In this paper, we discuss an implementation of the multi‐offset phase analysis consisting of the estimation of the Rayleigh wave velocity by means of a moving window with a frequency‐dependent length. This allows maximizing the lateral resolution at high frequencies while warranting stability at the lower frequencies. In this way, we can retrieve the shallow lateral variability with high accuracy and, at the same time, obtain a robust surface‐wave velocity measurement at depth. In this paper, we apply this methodology to a dataset collected for hydrogeophysical purposes and compare the inversion results with those obtained by using refraction seismics and electrical resistivity tomography. The surface‐wave results are in good agreement with those provided by the other methods and demonstrate a superior capability in retrieving both lateral and vertical velocity variations, including inversions. Our results are further corroborated by the lithological information from a borehole drilled on the acquisition line. The availability of multi‐offset phase analysis data also allows disentangling a fairly complex interpretation of the other geophysical results.     

Research Note: The mean sensitivity depth of the electrical resistivity method

Until now, a simple formula to estimate the depth of investigation of the electrical resistivity method that takes into account the positions of all of the electrodes for a general four‐electrode array has not been available. While the depth sensitivity function of the method for a homogeneous infinite half‐space is well known, previous attempts to use it to characterize the depth of investigation have involved calculating its peak and median, both of which must be determined numerically for a general four‐electrode array. I will show that the mean of the sensitivity function, which has not been considered previously, does admit a very simple mathematical formula. I compare the mean depth with the median and peak sensitivity depths for some common arrays. The mean is always greater than or equal to the median that is always greater than the peak. All three measures give reasonable estimates to the depths of actual structures for most circumstances. I will further show that, for 1D soundings, the use of the mean sensitivity depth as the pseudo‐depth assigns an apparent resistivity to a given pseudo‐depth that is consistent between different arrays. One consequence of this is that smoother depth soundings are obtained as “clutches,” caused by a change in the depth sensitivity due to moving the potential electrodes, are effectively removed. I expect that the mean depth formula will be a useful “rule of thumb” for estimating the depth of investigation before the resistivity structure of the ground is known.     

垃圾填埋场地下环境污染检测方法技术研究

本文应用电阻率成像观测方法对北京通州某垃圾填埋场开展了面积性测量,应用模糊数学隶属函数的理论给出了分辨地下污染状况的量化指标,并对填埋场地下环境污染状况进行了分区.钻孔资料表明,分区结果真实地反映了填埋场地下环境污染的分布状况.     

宁-芜矿集区及其西缘深部结构初探

为了探查宁(南京)-芜(湖)矿集区及其西缘地区上地壳结构及其与区域成矿作用的关系,在宁芜矿集区及其西缘布设两条大地电磁测深剖面(江宁区-姑山,巢湖-当涂),通过数据处理、分析和反演,获得了宁芜矿集区10 km以内地壳电性结构模型.大地电磁测深及重力多尺度边缘检测结果表明:(1)宁芜矿集区自南向北表现为低-高-低的电性结构特征,南段深部存在低阻体,向上延伸至浅部,中段深部发育巨型高阻体上隆至浅部,倾向南西,推测可能是巨型花岗岩体,北段低阻体发育深度大,向上延伸至浅部时分叉并上升至地表;(2)揭示了滁河断裂和长江断裂带的存在; 3线剖面东段与2线南段的低阻体位置较为一致,推测为岩浆喷发通道,在含山附近发现两条电性梯度带,3线剖面的电性结构特征可能反映了区域经历了先挤压、后拉伸的演变过程.     

山丹多极距与单极距电阻率对比分析及干扰因素研究

利用山丹地震台2年多的多极距观测资料,并结合该台单一极距的观测资料,对比分析了2种观测方法的结果。结果显示,多极距观测在干扰排除和异常落实方面具备一定的优势;同一场地不同极距电阻率测值的相互佐证,可以为识别干扰和判定地震前兆异常的真实性提供帮助。