Abstract
Abstract. Mexico’s Yucatan Peninsula is predominantly formed as a karstic terrain on limestone substrates where hundreds of kilometres of now submerged caves constitute extended networks of galleries partially accessible through cenotes, or karstic windows. These cenotes and underwater caves contain numerous objects of great biological, historical, cultural and/or paleontological significance: human and animal remains, ancient Maya artifacts, ancient mines and traces of early human activities. Photogrammetric studies by technical divers allow researchers to access a vast array of data pertaining to objects or features which would otherwise remain inaccessible. However, the harshness of the study environment poses technical acquisition challenges which must be specifically addressed. This paper introduces these environmental challenges, including the geological properties of local caves, bottom composition, ceiling percolation and topography, as well as the intrinsic difficulties linked to the absence of light and the limited time available in an underwater work setting. Technical acquisition solutions, setups and standard operating procedures are then proposed to overcome these challenges. A further consideration is added pertaining to the overall goal of creating a global database for the study objects, thus requiring a unified format for all data: unique identifier, geolocation, minimum resolution, and the like. The authors, focusing on the fieldwork and the data-collection aspect of the photogrammetric study, rather than the data processing and metrological steps, define a new image acquisition methodology built around a compact and replicable setup which will allow the documenting of objects previously out of reach of traditional setups.
Highlights
1.1 EnvironmentThe Yucatan Peninsula is predominantly formed as a karstic terrain on limestone substrate
The geological nature of the Yucatan Peninsula’s underwater caves, with their main conduit mainly formed by the dissolution of limestone substrates, induces the presence of dissolution products in the form of various sediments
In cave areas where the water level, at current or past times, is or was lower than the ceiling height, formation of calcite rafts at the water surface and the subsequent collapse of this thin layer under its own weight or external disturbance has generated an accretion of calcite flakes on the cave floor. These various sediments create a new type of challenge for photogrammetric studies in underwater caves, which requires exact assessment depending on the nature and location of the sediments, as well as the definition of a technical solution tackling this challenge
Summary
The Yucatan Peninsula is predominantly formed as a karstic terrain on limestone substrate. Acidity generated at the surface in later periods featuring lower sea levels, notably rain percolating through the more superficial soil layers, lead to the formation of dry underground caves (Smart et al, 2006) sporting numerous speleothems Besides this chemical formation process, the underground landscape has been shaped through the emergence of mechanical fracture zones (Bauer-Gottwein, 2011), among them the one resulting from the shock wave around the Chicxulub impact crater, the landing site of the meteorite determined to be the cause of the great Cretaceous-Tertiary extinction (Alvarez, 1980). At locations where the ceilings collapsed due to external factors, karstic windows, locally called cenotes, have formed and give access to the result of all these combined forces: one of the most extended cave systems in the world (see figure 1), mainly flooded since the sea level rises following the end of the Pleistocene (Smart et al, 2006)
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