Abstract Quantifying groundwater storage in weathered/fractured basement rock aquifers can be challenging owing to both their high degree of heterogeneity and their overall low storage capacity. Therefore, in these aquifers, the use of direct borehole hydraulic data is usually insufficient. Here we assessed the popular method of electrical resistivity tomography (ERT), combined with borehole data and including associated uncertainties, to resolve the spatial variability of groundwater storage properties at high resolution within a fractured mica schist aquifer in Ireland. Porosity distributions across both the saturated and unsaturated zones were calculated from two-dimensional (2D) ERT resistivities using two standard petrophysical models, Archie and Waxman & Smits (WS), the latter accounting for the influence of clay minerals on resistivity data. Our results demonstrated the importance of the hydrogeological conceptual constraints provided by ERT when parametrizing the 2D petrophysical models from borehole point data. They also confirmed the importance of accounting for clay minerals (the products of bedrock weathering processes) in the WS model, whereas predictions from Archie’s model produced unrealistically high porosity values of over an order of magnitude higher than the WS model. The WS model predicted porosities decreasing exponentially with depth, with values ranging from a few % in the shallowest, most-weathered part of the bedrock (upper 5 m on average) and deep fractured zones (to about 20 m deep), to less than 1% in the underlying fissured aquifer, and possibly down another order of magnitude in the deep massive bedrock. WS-derived porosities were in agreement with independent vertical water content profiles derived from magnetic resonance sounding (MRS), as well as point storativity values estimated from borehole hydraulic testing at the study site, with particularly good matches in the upper weathered/fractured bedrock and deeply weathered/fractured zones associated with regional faults. Detailed comparison suggested that WS provides an upper-bound estimate of groundwater storage in this environment. In the deep massive, un-weathered, and poorly fractured bedrock, however, discrepancies between groundwater storage estimate obtained from the three methods (ERT, MRS, and hydraulic) prevented reliable storage quantification, owing to the methods’ inherent technical limitations in such low porosity rocks. Our results demonstrated the suitability of resistivity tomography to quantify groundwater storage heterogeneity in weathered/fractured basement rock aquifers at high resolution and with reasonable overall uncertainty given the relative high uncertainties in petrophysical parameters at the kilometric scale. The results are promising for better characterization of groundwater storage variations in these hydrogeological systems, which are crucial to predict their response to climate variability and human exploitation.

中文翻译：

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使用电阻率层析成像技术量化风化/破裂基岩含水层中地下水储存的异质性：与岩石物理建模相关的敏感性和不确定性

摘要 风化/破裂的基底岩含水层中的地下水储量由于其高度非均质性和总体低储存容量而具有挑战性。因此，在这些含水层中，使用直接钻孔水力数据通常是不够的。在这里，我们评估了电阻率层析成像 (ERT) 的流行方法，结合钻孔数据并包括相关的不确定性，以高分辨率解决爱尔兰裂隙云母片岩含水层内地下水储存特性的空间变异性。使用两个标准岩石物理模型 Archie 和 Waxman & Smits (WS)，根据二维 (2D) ERT 电阻率计算饱和区和非饱和区的孔隙度分布，后者考虑了粘土矿物对电阻率数据的影响。我们的结果证明了 ERT 提供的水文地质概念约束在从钻孔点数据参数化二维岩石物理模型时的重要性。他们还证实了在 WS 模型中考虑粘土矿物（基岩风化过程的产物）的重要性，而 Archie 模型的预测产生了不切实际的高孔隙度值，比 WS 模型高出一个数量级。WS 模型预测孔隙度随深度呈指数下降，其数值范围从基岩最浅、最风化部分（平均 5 m 以上）和深裂隙带（至约 20 m 深）中的百分之几，到小于1% 在底层裂隙含水层中，并可能在深大块基岩中下降另一个数量级。WS 导出的孔隙度与从磁共振测深 (MRS) 导出的独立垂直含水量剖面以及从研究地点钻孔水力测试估计的点存储值一致，在上部风化/破裂基岩和与区域断层相关的深度风化/断裂带。详细比较表明，WS 提供了该环境中地下水储存量的上限估计。然而，在深层块状、未风化和裂隙较差的基岩中，通过三种方法（ERT、MRS 和水力）获得的地下水储量估计值之间的差异阻碍了可靠的储量量化，由于这些方法在这种低孔隙度岩石中固有的技术限制。我们的结果证明了电阻率层析成像适用于以高分辨率量化风化/破裂基岩含水层中地下水储存的异质性，并且考虑到千米尺度岩石物理参数的相对较高的不确定性，并且具有合理的整体不确定性。结果有望更好地表征这些水文地质系统中的地下水储存变化，这对于预测它们对气候变化和人类开发的响应至关重要。我们的结果证明了电阻率层析成像适用于以高分辨率量化风化/破裂基岩含水层中地下水储存的异质性，并且考虑到千米尺度岩石物理参数的相对较高的不确定性，并且具有合理的整体不确定性。结果有望更好地表征这些水文地质系统中的地下水储存变化，这对于预测它们对气候变化和人类开发的响应至关重要。我们的结果证明了电阻率层析成像适用于以高分辨率量化风化/破裂基岩含水层中地下水储存的异质性，并且考虑到千米尺度岩石物理参数的相对较高的不确定性，并且具有合理的整体不确定性。结果有望更好地表征这些水文地质系统中的地下水储存变化，这对于预测它们对气候变化和人类开发的响应至关重要。