Challenges in characterisation and mapping of solution pipes

Solution pipes are vertical or near-vertical cylindrical tubes occurring within the vadose zone of limestones during the eogenetic stage of their diagenesis, characterised by high permeability and matrix porosity (e.g., Quaternary calcarenites). The pipes vary in size and can be wider than 2 m and deeper than 100 m; depths between 1 m and 4 m and diameters between 20 cm and 80 cm are most common. The radius of a single pipe within a homogeneous rock is usually either constant or tapers slightly downwards. Some of the pipes, particularly the ones in coastal areas in the Mediterranean climate, have well cemented calcrete rims. These rims are usually less porous and more resistant to weathering than the host rock, and may consequently stand out after erosion of the surrounding material.

The unifying process responsible for their formation is a focused vertical flow of water, which depends primarily on sufficient water supply – e.g., rainfall. A detailed understanding of the formation of solution pipes can therefore provide us with a tool to estimate the climatic conditions prevailing at the time of their formation based on the density and shape of the pipes. The first important component here is the distribution of pipes. In addition to manual mapping and measuring, a combined photogrammetry and 3D laser scanning can be used to record their distribution on a larger scale. However, the machine learning algorithm needs to be developed to automatically detect their appearance and radius. This is challenging because pipes can appear in various shapes: as flat circles filled with sediment (with no relief on the surface), as holes, or as elevated cylindrical pinnacles due to erosion of the surrounding bedrock. In addition, their visibility is often limited due to sediment and vegetation cover. Cliff faces offer a glimpse of their interior, but their true spatial distribution is unknown. In contrast the eroded coastal platform shores provide a horizontal cross-section and distribution, but their vertical morphology and their depths are unknown. Similar situation appears in anthropogenic outcrops such as road cuttings and quarries. Promising methods for non-invasive mapping of the pipes are ground penetrating radar (GPR), magnetic gradiometer and electrical resistivity tomography (ERT), but with certain limitations, mainly related to unclear detection of the depths of the pipes, and the reliability of the mineralogy, geochemistry and texture of the fill of the pipes.

The second important component is the morphology of the pipes. In order to properly estimate their shapes, especially their depths, a denudation factor must be considered. This can be partially assessed with numerical modelling of reactive-infiltration instability, which incorporates the lowering of the landscape during the formation of the pipes. However, this remains limited to the accuracy of dating of solution pipe formation, and estimations of post-formation landscape denudation.

ACKNOWLEDGEMENT: We acknowledge the financial support of Slovenian Research Agency (P6-0101; I0-0031; N1-0162; J6-3142).