ADVANCED IMAGING OF HIDDEN CONDUITS WITH HELICOPTER ELECTROMAGNETICS IN THE COASTAL KARST PLAIN OF TULUM, MEXICO

The study presented is part of a series of international research co-operations between European and Mexican research institutions and NGO’s, started in the year 2007, and still ongoing. The study area is located at the east coast of the Yucatan Peninsula, Mexico, and comprises the northern most part of the Sian Ka’an biosphere reserve as well as the town of Tulum, Quintana Roo, and part of the second largest barrier reef spreading some 300 metres to one kilometre off shore. Two aerogeophysical surveys conducted in 2007 and 2008 covered an area of 200 square kilometres, including cave systems already mapped by exploration divers. In order to get additional ground truth data and input for a hydrological model, extended ground geophysical campaigns have been conducted annually including geoelectrics, GPR, ground-EM, high precision GPS, piezometry and water analysis. First processing of airborne electromagnetic (AEM) data revealed a clear signature from known caves as well as the picture of a vast unexplored hidden conduit network. However, lateral and depth resolution was limited due to measurement system drift and noise as well as due to specific behaviour of the applied inversion technique. Consequently, careful drift and noise analysis have been conducted on a commercial airborne FDEM-system. Based upon the findings new concepts for drift estimation and noise filtering could be developed and applied which enhance the data quality significantly and enable the imaging of well-defined structures in the underground. While data inversion is conducted by means of a software package developed at the University of British Columbia, the inversion output is treated with special filtering for enhancing structural information in single line sections. Stacking of parallel sections yields a 3d-model of the underground structures or conduits down to depths of 40 to 50 metres. This method best preserves and visualizes the information contained in AEM-data. Including these advancements, the AEM-method proves its capability to quickly deliver crucial structural information of karst water regimes over a difficult accessible area as in the case of Tulum, providing unique lateral and depth information compared with previous surveys.