Gravity survey and modelling of the Nemocón salt mine, Colombia

Abstract

ABSTRACTA 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.