美国地质工程assignment作业：mapping of the extent of quick clay deposits by 2D electrical resistivity tomography (ERT) was demonstrated

本文通过二维电阻率层析成像（ERT）对快速粘土沉积程度的精确填图进行了论证。作为高度敏感的海相粘土，快速粘土需要被详细地绘制出来，因为它被认为是地质灾害的指示器。然而，事实证明，用常规的ERT方法无法解决近大型异常的小电阻率对比问题。因此，限制参与ERT数据实现快速程度划分后的粘土。实验结果表明，这种约束反演模型表现出清晰的电阻率对比增强的原位测量结果的一致性。
In this article, the accurate mapping of the extent of quick clay deposits by 2D electrical resistivity tomography (ERT) was demonstrated. As highly sensitive marine clay, quick clay needs to be mapped in detail since it is regarded as an indicator for geo-hazards. Nevertheless, it has been proved that small resistivity contrast near large anomalies cannot be resolved by conventional ERT methods. Therefore, constraints were involved in theinversion of ERT data to achieve delineation of quick clay extent. The experimental results revealed that this constrained inversion models exhibit sharper resistivity contrasts and enhanced consistence with the results ofin situ measurements.

1. Synthetic method合成方法

作为对ERT适用的说明,介绍一种类似于典型的挪威快速粘土网站综合模型。该模型用于具有不同电阻率的几个地质单元。在地表附近，三导电体ofmarineclay（ρ= 3Ω•m）被放置在一个ρ= 50的Ω•M.片麻岩电阻的背景（ρ= 1000Ω•m），在35–45米深处的bedrocklies。此外，海洋粘土深体（ρ= 3Ω•m）的基岩之上是位于模型的中心部分。As an illustration of theapplication of ERT investigationsin quick clay mapping, a synthetic model resembling typical Norwegian quick clay sites is introduced.This model consists of several geological units with differentresistivities.In the vicinity of the surface, three conductive bodies ofmarineclay (ρ = 3 Ω•m) are placed within a resistive background with ρ = 50 Ω•m. Made of gneiss (ρ = 1000 Ω•m),the bedrocklies at 35–45 m depths. In addition, a deep body of marine clay (ρ = 3 Ω•m) lies on top of the bedrock locatedin the center of the model section. Byperturbing the theoretical solution of the forwardproblem with 5% Gaussian noise using the software mentioned above, we can obtain a syntheticdataset. The noise added is calculated based on the potentials with a multiple-gradientarray consisting of 81 electrodes at 4 m spacing. Themultiple-gradient array has been designed for use in multi-channel systemsand is optimal for the instrument weused. Afterwards, the synthetic dataset obtained is invertedwith RES2DINVusing standard parameters.

2. Case study案例学习

In this part, the proposed method was applied in two real-life cases.

The first case suggested that interpolation between soundings quick clay investigations usingdiscrete rotary pressure soundings can be significantly enhanced byusing ERT profiles to interpolate. In this case, the resistivities inverted from theERT data were perfectlyaligned with the data provided byin situmeasurements,indicating excellent resolutionof the ERT method.
The second case showed thatthe threshold resistivityfor the quick clay zoning should be adjustedaccording to theRCPT soundings of each case/site. In this case, however, ERT imaging of layers of homogeneous materialrequiredfor the calculation of slope stability was ineffective.
In conclusion, priori constraints play a key in the reduction of the uncertainty inherent in the inversion process ofERT data for quick clay mapping. Firstly, standard processing is employed to provide a basic idea of the subsurface. Then, thebedrock depth is used to reprocess the ERT data with aconstrained inversion. The combined investigation provides a continuous andrelatively detailed picture of the subsurface in costeffective manner.Finally, two cases in Norway were used to verify the proposed method.