Direct current electrical resistivity method has experienced significant breakthroughs during the last decades with the development of advanced instrumentation and sophisticated inversion algorithms. These have substantially benefitted the field of archaeological prospection to extract quantitative information for the buried archaeological material in a complete three-dimensional (3-D) context. The obvious continuation of past human occupation and associated settlements in the ultra-shallow part of coastal zones generated the necessity to compile methodologies for mapping submerged cultural assets. This study investigates the efficiency of dynamic floating and submerged 3-D Electrical Resistivity Tomography (ERT) for mapping archaeological relics buried beneath the sediments in ultra-shallow marine environments. Extensive testing is performed through numerical simulation and 3-D inversion of synthetic apparent resistivity tomographic data. The generic resistivity model including a complex resistive structure embedded in a stratified conductive environmental regime simulated typical scenario of an archaeological feature submerged below the sea water layer. Different array configurations including Dipole–Dipole (DD), Gradient (GRD), Reciprocal Wenner (RecWEN) and Wenner-Schlumberger (WS), which can be appropriately programmed for continuous off-shore measurements with associated multichannel resistivity instruments, are validated in order to determine the most efficient one for such surveys. Densely spaced multiple parallel two-dimensional (2-D) ERT lines along a single direction composed the survey layout to extract the 3-D apparent resistivity data set for the different electrode arrays using a 3-D finite element program. The synthetic tomographic data were corrupted with 3% Gaussian noise and inverted with an iterative smoothness constrained inversion algorithm. The comparative results from the various tested arrays manifest the superiority of the DD in reconstructing the optimum resistivity inversion model for both the floating and submerged survey modes. Additional tests were made concerning the resolving capabilities of ERT with variable seawater thickness and target characteristics. Although accurate ‘a priori’ information regarding the water resistivity and thickness are essential for constraining the 3-D inversion, erroneous estimation of these parameters can result to misleading results, especially for submerged survey modes. The simulation of floating and submerged 3-D ERT surveys through synthetic modelling documented the benefits of such approach in reconstructing structured cultural material in shallow off-shore environments. Finally, the results of a 3-D dynamic floating ERT survey from a submerged archaeological site in Greece verified the theoretical outcomes, proposing at the same time techniques to overcome problems that can occur due to the unique conditions of the ultra-shallow marine environment. Overall, this work enhances the conclusion that 3-D marine ERT is a robust method to reconstruct submerged archaeological structures related to ancient built environment in ultra-shallow marine regions.

中文翻译：

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使用 3-D 电阻率层析成像进行浅埋水下考古勘探的实际考虑

在过去的几十年中，随着先进仪器和复杂反演算法的发展，直流电阻率方法取得了重大突破。这些极大地有益于考古勘探领域，以在完整的三维 (3-D) 环境中提取埋藏考古材料的定量信息。过去人类占领的明显延续和沿海地区超浅水部分的相关定居点产生了编制用于绘制淹没文化资产地图的方法的必要性。本研究调查了动态浮动和水下 3-D 电阻率断层扫描 (ERT) 的效率，用于绘制埋在超浅海环境中沉积物下的考古遗迹。通过数值模拟和合成视电阻率层析数据的 3-D 反演进行了广泛的测试。包括嵌入分层导电环境体系中的复杂电阻结构的通用电阻率模型模拟了淹没在海水层以下的考古特征的典型场景。不同的阵列配置，包括偶极子-偶极子 (DD)、梯度 (GRD)、倒数温纳 (RecWEN) 和温纳-斯伦贝谢 (WS)，可适当编程以使用相关的多通道电阻率仪器进行连续海上测量，按顺序进行验证为此类调查确定最有效的调查。沿单个方向密集分布的多条平行二维 (2-D) ERT 线组成了测量布局，以使用 3-D 有限元程序提取不同电极阵列的 3-D 视电阻率数据集。合成的断层扫描数据被 3% 的高斯噪声破坏，并用迭代平滑度约束反演算法进行反演。各种测试阵列的比较结果表明了DD在重建浮动和水下测量模式的最佳电阻率反演模型方面的优越性。对具有可变海水厚度和目标特征的 ERT 的分辨能力进行了额外的测试。尽管有关水电阻率和厚度的准确“先验”信息对于约束 3-D 反演至关重要，对这些参数的错误估计可能会导致误导性结果，尤其是对于水下调查模式。通过综合建模模拟浮动和水下 3-D ERT 调查记录了这种方法在浅海环境中重建结构化文化材料的好处。最后，来自希腊水下考古遗址的 3-D 动态浮动 ERT 调查结果验证了理论结果，同时提出了克服由于超浅海环境的独特条件而可能出现的问题的技术。总体而言，这项工作增强了以下结论：3-D 海洋 ERT 是重建与超浅海区域古代建筑环境相关的水下考古结构的稳健方法。