“It is the black box look”, as said the lead researcher Blase LaSala. You know what you see coming in and out, but you can hardly measure what is going on in there.
To the vast majority of visitors, the water system of the Grand Canyon is just a bottle of water filled at a spigot on the rim, or at a hundred yards down a boiling trail. At the back of that little convenience, is but one single cave fed spring, Roaring Springs, on the North Rim, which furnishes water to over 6 million visitors to the park each year. The engineering issue is not merely to convey that water over a rugged landscape, but to maintain it clean when that origin of its supply is channeled through a subterranean landscape which cannot be examined as a pipe system.
That terrain is within the Redwall and Muav limestones, a karst structure commonly referred to as “Swiss Cheese” rock: fractures, openings, sinkholes, and passages that might swiftly and erratically convey water. Dye tracing has already shown how quickly recharge may move – a distance of 20 km between sinkholes on the Kaibab Plateau and canyon springs in a few days. In a typical aquifer, lengthy residence periods, as well as fines furnish filtration; in karst, swift paths diminish that safeguarding buffering. In case the runoff of any ground burned or bacteria is poured into the connected sinkhole, the pumping activities within the park can be halted until the source point is identified and controlled.
The most useful internal view of these caves was until recently a hand-drawn, two-dimensional map – useful in navigation, but of limited utility in diagnosing a three dimensional flow system. It was altered by a team at Northern Arizona University when they took a handheld mobile lidar scanner in remote passages to create the first high-resolution 3D models of three cave systems connected to perennial springs. During the course of the experiment, more than 10.2 km of passages had been recorded, resulting in point clouds that were dense enough to determine features smaller than 10 cm in diameter with registration errors of less than 10 mm, and drift had been less than 0.1 percent over a kilometer, on average. These figures are important in that they convert “rough sketches” into quantifiable geometry surfaces, planes and alignments that can be compared across caves that have been separated by distances of long distances. The least hospitable places gave some of the hardest-paid information.
Being constantly on the move, teams dragged the heavy packs during days, crawled, rappell, and in some cases traversed partially flooded areas to ensure scans were continuous. What is emerged is not simply a more beautiful cave model; it is a map of the structure by which groundwater is likely to follow its paths. In all three of the cave systems, which are spaced out on the North Rim, the lidar imagery revealed a repeat motif whereby water-cut passages would align with sets of sub-vertical joints and the slope of the bedding planes. That uniformity leads upwards, to surface geology, and still further downwards to ancient faulting and uplift that created crack networks as permanent tracks to contemporary flow.
Another practical question that was tested in the study was: does lidar concur with conventional cave surveying? A more experienced survey crew mapped an area in one cave with the existing methodologies, and the products had 98% spatial repeatability when laid over each other. In places of variation, they frequently reduced to the inevitable subjectivity of the process of reducing the complexity of ceilings and walls to a 2D drawing-the very uncertainty that makes the interpretation of water-paths problematic.
In the case of park operations, the payoff that is associated with engineering is traceability. Having a single supplier of a large facility allows one to determine which fractures and layers are probably to bond a surface sink to a discharge point to reduce the ability to reduce contamination searches, set up monitoring, and priorities zones of protection. The subsequent stage of the research continues that rationale to the surface, with airborne lidar and satellite data over decades to map sink features and patterns of snow accumulation on the Kaibab Plateau.
Karst terrain is typical of Arizona and the Colorado Plateau, where collapses and sinkhole development may form direct and rapid interrelationships between the surface and profound limestone routes. The reliance of a single spring and the depth of its subterranean plumbing, which extends well below ground, to depths of over 2,300 feet, in some spots, of the Grand Canyon case, is what is particularly stark. It is where no usual inspection program can trace the flow. Lidar does not render the subsurface transparent, but it renders a black box a mapped inside with quantifiable structure, which brings a bit more engineering confidence to a system that has always operated on geologic time and through stone.
