A gravity survey was conducted at a salt mine in Nemocón, Cundinamarca, Colombia, located in the axial part of the Eastern Cordillera of the northern Andes. This part of the Eastern Cordillera is characterized by successions of synclines and anticlines sometimes containing salt bodies at their core, and the mine is located at the top of one of these anticlines called the Nemocón anticline. As is typical for salt and other mines in Colombia, very little data on the internal structure and geology are publicly available. The purpose of our study is to show how gravity data and modelling can be used to infer near‐surface properties above the mine in the absence of other sources of information. Relative gravity measurements were made over an irregular grid of 107 stations and tied to the Instituto Geográfico Augustin Codazi national absolute gravity network. The total and residual Bouguer anomalies as well as the analytic signal were calculated over the area of the salt body. Using grid data, we first determine the main inversion parameters (e.g., depth weighting) and model resolution using a checkerboard model with the addition of random noise. Then, applying 3D inversion modelling to the residual Bouguer anomaly, we find that the positive and negative anomalies correspond to few zones of positive density contrasts and two main zones of negative density contrasts. Some of the positive anomalies are located above the fault trace putting in contact the Arenisca Dura formation with the Conejo formation containing the salt body. The negative anomalies are not well correlated with the underground position of the salt mine, but some influence of the low‐density ‘rute’ material, consisting of black clays with shaly claystone parts covering the salt body might be important. This study showcases how 3D inverse modelling can provide important information on the near‐surface structure of a salt mine.

中文翻译：

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哥伦比亚Nemocón盐矿的重力调查和建模

在位于安第斯山脉北部东山脉的轴向地带的哥伦比亚昆迪纳马卡内莫松的一个盐矿中进行了重力测量。东部山脉的这一部分的特征是连续的向斜和背斜有时在其核心处含有盐体，而该矿位于这些背斜之一的顶部，即Nemocón背斜。正如哥伦比亚盐矿和其他矿山的典型情况那样，很少公开内部结构和地质数据。我们研究的目的是说明在没有其他信息来源的情况下，如何使用重力数据和建模来推断矿山上方的近地表特性。相对重力测量是在107个站点的不规则网格上进行的，并与InstitutoGeográficoAugustin Codazi国家绝对重力网络联系在一起。计算了整个盐体区域的布格异常总数和残差以及分析信号。使用网格数据，我们首先使用带有随机噪声的棋盘模型确定主要反演参数（例如，深度权重）和模型分辨率。然后，将3D反演模型应用于残余的布格异常，我们发现正异常和负异常分别对应于几个正密度对比区域和两个负密度对比主要区域。一些正异常位于断层迹线上，使阿里尼斯卡杜拉地层与含盐体的Conejo地层接触。由黑色粘土和覆盖盐体的泥质粘土部分组成的“ rute ”材料可能很重要。这项研究展示了3D逆向建模如何提供有关盐矿近地表结构的重要信息。