ABSTRACT The geological setting in the north of Ireland, especially concerning the origin of the Moffat Shale Group, has long been under discussion. Within the Tellus Programme of the Geological Survey Ireland, airborne electromagnetic measurements revealed high‐conductivity anomalies that have been interpreted as the response of a black shale. In order to petrophysically characterize the Moffat Shale, a laboratory study using material from two shallow boreholes was carried out. The study focuses on spectral induced polarization measurements on 23 oriented samples in the frequency range from 10−4 to 105 Hz. The sample material can be categorized into two groups. A mudstone‐like rock type shows weakly frequency‐dependent, porosity‐driven conductivities with a strong anisotropy. On the other hand, black shale samples are characterized by moderately anisotropic but strong polarization effects especially at low frequencies and a strong conductivity increase towards higher frequencies. The polarization in the black shale is controlled by the texture and volume fraction of the polarizable components. The spectral induced polarization data are processed by means of a Debye decomposition approach. The anisotropy of the complex electrical conductivity is determined by utilizing the foliation dip angle and assuming tilted transverse isotropic conditions. The relevance of the laboratory findings for airborne electromagnetic surveys is addressed with a synthetic one‐dimensional modelling study.

中文翻译：

---

来自 Moffat 页岩群（爱尔兰）的黑色页岩和泥岩的各向异性复合电导率

摘要 爱尔兰北部的地质背景，特别是关于莫法特页岩群的起源，长期以来一直在讨论之中。在爱尔兰地质调查局的 Tellus 计划中，空中电磁测量揭示了高电导率异常，这些异常被解释为黑色页岩的响应。为了对 Moffat 页岩进行岩石物理表征，使用来自两个浅钻孔的材料进行了一项实验室研究。该研究的重点是在 10-4 到 105 Hz 的频率范围内对 23 个定向样品进行光谱激发极化测量。样品材料可分为两类。泥岩类岩石类型显示出弱的频率依赖性、孔隙度驱动的电导率和强各向异性。另一方面，黑色页岩样品的特点是中等各向异性但强烈的极化效应，尤其是在低频下，并且电导率向更高的频率增加。黑色页岩中的极化受可极化组分的质地和体积分数控制。光谱激发极化数据通过德拜分解方法进行处理。复电导率的各向异性是通过利用叶理倾角并假设倾斜的横向各向同性条件来确定的。实验室发现与机载电磁调查的相关性通过综合一维建模研究得到解决。黑色页岩中的极化受可极化组分的质地和体积分数控制。光谱激发极化数据通过德拜分解方法进行处理。复电导率的各向异性是通过利用叶理倾角并假设倾斜的横向各向同性条件来确定的。实验室发现与机载电磁调查的相关性通过综合一维建模研究得到解决。黑色页岩中的极化受可极化组分的质地和体积分数控制。光谱激发极化数据通过德拜分解方法进行处理。复电导率的各向异性是通过利用叶理倾角并假设倾斜的横向各向同性条件来确定的。实验室发现与机载电磁调查的相关性通过综合一维建模研究得到解决。