Abstract

Electrical resistivity imaging survey was carried out at the warm spring site in Kasane, northwest Botswana, to investigate the subsurface distribution of geological structures and how they influence the existence of the warm spring. This investigation was carried out along eight 480-m-long parallel profiles with an electrode spacing of about 10 m and 50 m line spacing. We used a Pole–dipole electrode array to acquire the data. The data inversion employed in this study was based on the smoothness constrained nonlinear least-square inversion algorithm. 2D data inversion for all the eight 2D datasets was performed, and the eight 2D datasets were then collated into a single 3D data file before performing a 3D data inversion. The resultant 2D inversion models display the variation of electrical resistivity beneath the respective profiles; whereas, the resultant 3D models show the distribution of electrical resistivity in both the lateral and vertical directions. The 2D inversion results revealed two distinct anomalies with low resistivity values, e.g., < 46.2 Ω m of the topmost layer indicating the presence of silty clays and relatively high resistivity material with resistivity values exceeding 249.3 Ω m, which was attributed to basalts. Also observed in the 2D inversion results are low-intermediate resistivity zones of about 46.2 Ω m extending to greater depths beyond the boundaries of the models, which are attributable to the existence of fluid-filled fractures/faults. The 3D inversion results resolved a laterally extensive high-resistivity basaltic unit, two NE–SW-trending vertical/near-vertical low-resistivity features which are attributable to fluid-filled fractures or faults, and a broad low-resistivity zone occurring beneath a depth of 40 m on the southeastern side of the study area, which is likely to be indicating part of the geothermal reservoir. The low-resistivity vertical/near-vertical features observed in this study are likely to be enabling the flow of geothermal fluids, and the high resistivity basaltic unit acts as a caprock overlying the reservoir.

中文翻译：

---

博茨瓦纳西北部卡萨内温泉区的 3D 电阻率调查和地下成像

摘要

在博茨瓦纳西北部卡萨内的暖泉地点进行了电阻率成像调查，以调查地质构造的地下分布及其对暖泉存在的影响。这项调查是沿着 8 个 480 米长的平行剖面进行的，电极间距约为 10 m，线间距约为 50 m。我们使用极偶极电极阵列来获取数据。本研究采用的数据反演基于平滑约束非线性最小二乘反演算法。对所有八个 2D 数据集进行了 2D 数据反演，然后在执行 3D 数据反演之前将八个 2D 数据集整理成单个 3D 数据文件。由此产生的二维反演模型显示了各个剖面下方电阻率的变化；而生成的 3D 模型显示了横向和垂直方向上的电阻率分布。二维反演结果揭示了两个具有低电阻率值的明显异常，例如，最上层的电阻率<46.2 Ω m，表明存在粉质粘土和电阻率值超过249.3 Ω m 的相对高电阻率材料，这归因于玄武岩。在二维反演结果中还观察到约 46.2 Ω m 的中低电阻率区域延伸到模型边界之外的更大深度，这归因于充液裂缝/断层的存在。 3D 反演结果解析了横向广泛的高电阻率玄武岩单元、两个 NE-SW 走向的垂直/近垂直低电阻率特征（归因于充满流体的裂缝或断层）以及出现在玄武岩单元下方的广泛低电阻率区域。研究区东南侧深度达40 m，可能是地热储层的一部分。本研究中观察到的低电阻率垂直/近垂直特征可能使地热流体能够流动，而高电阻率玄武岩单元则充当储层上方的盖层。