Abstract The Maâdna structure (Talemzane, Algeria) is located approximately 400 km south of Algiers and is formed within Upper-Cretaceous to Eocene limestones from the Saharan Platform. It is a bowl-shaped crater roughly 1.7 km in diameter. The present-day crater rim is 60 m higher than the bottom of the depression, and is raised slightly compared to the surrounding terrain. So far, several aspects of Maâdna structure cratering process remain poorly known. Recent study, geological and geophysical investigation, do not support an impact origin for this structure but suggest an alternative dissolved diapir with inverted relief. At present, geophysical studies revealed many interesting features of known terrestrial impact craters. In particular, observed signatures yielded useful information on the impact-induced physical changes in targeted rocks. These observations motivated our investigation of Maâdna crater, which uses several different geophysical surveys with particular emphasis on the GPR technique. This later is used as a valuable tool for near surface explorations. Although GPR technique has a limited use in case of impact structures, it is used in this study to discriminate between electromagnetic signatures of supposed ejectas, which were widely used to describe Maâdna structure origin. Typical GPR antennas were used to test a survey protocol in several situations during the GPR field deployment stage. In order to characterize the target thin and shallower layer, most of the common profiles were acquired using a 500 MHz shielded antenna. Otherwise, to ensure maximum waves penetration depths, “NS & EW” profiles were also acquired using a lowest frequency 30 MHz unshielded rough terrain antenna (RTA) system with a flexible “snake-like” design. Moreover, the ensued GPR characterization is then used to constrain occurred cratering process. Our GPR survey results helped us to improve our knowledge of the structure subsurface features expression. A suitable post-acquisition processing and analyses of the collected data lead us to reject the existence of any electromagnetic signature that may be attributed to the presence of any atypical materials, such as melted iron-rich material or those with distinctive physico-chemical properties, which induces electromagnetic signal modification or dissipation. Besides, our different scans could help truthful geological interpretation linked to Maâdna structure. For example, scans operated on the prevailing structural unit surrounding the Maâdna crater show clearly the most documented structural features at this crater, such as bedrocks fracturation, locally collapses, overturned and/or vertical movement of strata, etc. Specifically, the topographically corrected scans are presently interpretative against the alternative scenario of the cratering process at Maâdna, since our various analyses do not support any meteoritic evidence. As a result, valuable insights into the hypothetic diapiric piercement have been then gained from this analysis. This leads us to conclude that GPR data can be used to build another strong argument in favor of the diapiric origin at Maâdna crater.

中文翻译：

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使用地球雷达技术探索 maâdna 结构（阿尔及利亚 Talemzane）周围的假设撞击喷射层

摘要 Maâdna 构造（阿尔及利亚 Talemzane）位于阿尔及尔以南约 400 公里处，形成于撒哈拉地台上白垩纪至始新世石灰岩内。它是一个直径约 1.7 公里的碗状火山口。现在的火山口边缘比凹陷底部高 60 m，与周围地形相比略有升高。到目前为止，Maâdna 结构陨石坑过程的几个方面仍然知之甚少。最近的研究，地质和地球物理调查，不支持这种结构的撞击起源，但建议另一种具有倒浮雕的溶解底辟。目前，地球物理研究揭示了已知陆地撞击坑的许多有趣特征。特别是，观察到的特征产生了有关目标岩石中撞击引起的物理变化的有用信息。这些观察结果激发了我们对 Maâdna 陨石坑的调查，该陨石坑使用了几种不同的地球物理调查，特别强调 GPR 技术。这后来被用作近地表勘探的宝贵工具。尽管 GPR 技术在撞击结构的情况下用途有限，但在本研究中用于区分假定喷射物的电磁特征，这些特征被广泛用于描述 Maâdna 结构起源。在 GPR 现场部署阶段，典型的 GPR 天线用于在多种情况下测试测量协议。为了表征目标较薄和较浅的层，使用 500 MHz 屏蔽天线获取大多数常见轮廓。否则，为了确保最大的波浪穿透深度，“NS & 还使用具有灵活“蛇形”设计的最低频率 30 MHz 非屏蔽粗糙地形天线 (RTA) 系统获取 EW 剖面。此外，随后的 GPR 表征然后用于约束发生的弹坑过程。我们的 GPR 调查结果帮助我们提高了对结构地下特征表达的了解。对收集到的数据进行适当的采集后处理和分析，使我们拒绝任何电磁特征的存在，这些特征可能归因于任何非典型材料的存在，例如熔化的富铁材料或具有独特物理化学特性的材料，这会引起电磁信号的修改或耗散。此外，我们的不同扫描可以帮助与 Maâdna 结构相关的真实地质解释。例如，在 Maâdna 陨石坑周围的主要结构单元上进行的扫描清楚地显示了该陨石坑中记录最多的结构特征，例如基岩破裂、局部坍塌、地层翻转和/或垂直移动等。 具体而言，地形校正扫描目前具有解释性反对 Maâdna 陨石坑过程的替代方案，因为我们的各种分析不支持任何陨石证据。因此，从该分析中获得了对假设的底皮穿刺的有价值的见解。这使我们得出结论，GPR 数据可用于建立另一个强有力的论据，以支持 Maâdna 陨石坑的底辟起源。地层的局部坍塌、倾覆和/或垂直移动等。具体而言，地形校正扫描目前可以解释 Maâdna 陨石坑过程的替代方案，因为我们的各种分析不支持任何陨石证据。因此，从该分析中获得了对假设的底皮穿刺的有价值的见解。这使我们得出结论，GPR 数据可用于建立另一个强有力的论据，以支持 Maâdna 陨石坑的底辟起源。地层的局部坍塌、倾覆和/或垂直移动等。具体而言，地形校正扫描目前可以解释 Maâdna 陨石坑过程的替代方案，因为我们的各种分析不支持任何陨石证据。因此，我们从该分析中获得了对假设的尿嘧啶穿孔的宝贵见解。这使我们得出结论，GPR 数据可用于建立另一个强有力的论据，以支持 Maâdna 陨石坑的底辟起源。然后从该分析中获得了对假设的尿嘧啶穿孔的宝贵见解。这使我们得出结论，GPR 数据可用于建立另一个强有力的论据，以支持 Maâdna 陨石坑的底辟起源。然后从该分析中获得了对假设的尿嘧啶穿孔的宝贵见解。这使我们得出结论，GPR 数据可用于建立另一个强有力的论据，以支持 Maâdna 陨石坑的底辟起源。