Measurements of water-film thickness on cave walls and speleothems

Abstract

A confocal optical sensor is used to quantitatively measure on-site the thickness of thin water-films on cave walls and speleothems. Results obtained from a sample of 4 prehistoric caves and 1 concretion cave from southern France, indicate that the measurable thickness of water-films is generally in the 25-70 µm range for cave walls and reaches 200-300 µm on the horizontal plane of active speleothems. In most cases, the behavior of cave wall water-film is primarily controlled by surface-tension effects and wall roughness. The high measurement frequency enables the monitoring of water-film dynamics, especially the tracking of thickness changes triggered by dripping and water flow. This study paves the way for elaborating new strategies for the conservation of rock art paintings using water film thickness as an additional monitoring parameter.