A hydrogeologic conceptualization is critical to understand, manage, protect, and sustain groundwater resources, particularly in regions where data are sparse and accessibility is difficult. We used airborne electromagnetics (AEM), shallow seismic reflection and refraction, and downhole nuclear magnetic resonance (NMR) logs to improve our understanding of an arid groundwater system influenced by palaeovalleys. We found that there is a limited connection between the palaeovalley and fractured bedrock aquifers because they are separated by a spatially variable layer of saprolite, which is the layer of chemically altered rock on top of the fractured bedrock. The AEM data provided an estimate of the top of the saprolite but failed to effectively image the bottom. In contrast, the seismic data provided an estimate of the bottom of the saprolite but failed to image the top. This geophysical combination of electrical and seismic data allowed us to map saprolite thickness in detail along a 1.7 km long transect that runs perpendicular the main trunk of a well-defined palaeovalley. These data indicate that the palaeovalley is lined with a heterogeneous layer of saprolite (approximately 3–120 m thick) that is thickest near its edges. Despite the observed variability, only a small percentage of the fractured bedrock aquifer (8%–17%) appears to be in contact with the palaeovalley aquifer. Furthermore, the lack of an elastic boundary at the top of saprolite suggests that the porosity of the saprolite is similar to the palaeovalley sediments — an observation that is supported by the downhole NMR-derived water contents. The electrical change at the top of saprolite is caused by a combination of a decrease in total dissolved solids of the groundwater in the saprolite and a change in pore structure associated weathering in situ versus transported weathered materials. The presence of saprolite, which commonly behaves as an aquitard, may limit the groundwater exchange between the palaeovalley and bedrock aquifers, with implications for the regional groundwater resource potential.

中文翻译：

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利用机载电磁学、地震折射和反射以及井下核磁共振改进偏远半干旱古河谷地下水系统的水文地质概念

水文地质概念化对于理解、管理、保护和维持地下水资源至关重要，尤其是在数据稀少且难以获取的地区。我们使用机载电磁学 (AEM)、浅层地震反射和折射以及井下核磁共振 (NMR) 测井来加深我们对受古河谷影响的干旱地下水系统的了解。我们发现古河谷和裂隙基岩含水层之间的联系有限，因为它们被空间可变的腐泥土层隔开，腐泥土是裂隙基岩顶部的化学蚀变岩层。AEM 数据提供了腐泥土顶部的估计值，但未能有效地对底部成像。相比之下，地震数据提供了腐泥土底部的估计值，但未能对顶部成像。这种电气和地震数据的地球物理组合使我们能够沿着一条 1.7 公里长的横断面详细绘制腐泥土厚度，该横断面与一个明确定义的古河谷的主干垂直。这些数据表明，古河谷衬有一层异质腐泥土（约 3-120 m 厚），在其边缘附近最厚。尽管观察到了变化，但只有一小部分破裂的基岩含水层（8%–17%）似乎与古河谷含水层接触。此外，腐泥土顶部缺乏弹性边界表明腐泥土的孔隙度与古河谷沉积物相似——这一观察结果得到了井下 NMR 衍生的水含量的支持。腐泥土顶部的电变化是由腐泥土中地下水总溶解固体的减少和与原位风化相关的孔隙结构变化与运输风化材料的组合引起的。通常表现为透水层的腐泥土的存在可能会限制古河谷和基岩含水层之间的地下水交换，从而对区域地下水资源潜力产生影响。