Written by:
John Rankin, B.Sc.
Geomatics Consultant
Eagle Mapping Ltd.
In the realm of mineral exploration, utilizing LiDAR stands out as the most effective surface visualization method, especially in densely vegetated areas. Through its active light source, LiDAR, short for Light Detection and Ranging, provides precise measurements down to a few centimeters. By calculating the time taken for a light pulse to bounce off an object and return to the sensor along a known vector, LiDAR accurately determines the position of the reflected point in three-dimensional space.
Furthermore, each laser pulse emitted by LiDAR can yield a single return on solid surfaces like beaches or parking lots. Conversely, when encountering obstructive elements such as trees, buildings, or power lines, multiple returns are generated. Today’s advanced LiDAR systems have the capacity to capture millions of points per second, facilitating efficient and precise coverage of extensive areas.
The effectiveness of LiDAR data in mineral exploration hinges on point density and uniform distribution. A minimum of 4 to 6 evenly dispersed points is required to initiate the visualization of a feature. Simpler attributes necessitate fewer points for identification, whereas more intricate features demand a greater number of points. Inadequate point coverage can lead to misinterpretations, potentially mistaking a complex object for a simpler one, like misidentifying a car as a large boulder.
While LiDAR technology offers the ability to collect an unlimited number of points per square meter, the balance between point density and cost is crucial for effective project management. Increasing pulse density may initially seem beneficial, but the cost-effectiveness can fall rapidly due to diminishing returns and the difficulty that comes with managing a large data set.
On hard surfaces or bare earth each additional pulse gains a ground point offering a good benefit to cost at lower point densities. Going from 4 points with a resolution of 0.5m to 8 points increases your point density 100% for an increased ground resolution of 0.353m or a ~ 29% increase in resolution. As seen, a doubling of points is required each time to increase ground resolution by ~ 29%. This rapidly increases the cost of a survey and balloons file size making it more difficult to process, manage, and distribute.
In forested areas, more pulses over the canopy do not necessarily translate to more usable ground points, as most additional points end up in the canopy. The laser pulses tend to penetrate the same canopy holes, registering points on the ground where they already exist. These points usually have poor distribution and contribute little to the ground surface which is of the highest importance in mineral exploration. By increasing flight lines with varying overlaps new angles can be achieved to find more openings in the canopy, but it increases costs quickly often making this collection impractical.
In ideal conditions resolution under canopy almost never exceeds 0.5m or 4 points/m². At a 0.5m resolution objects under canopy, on a forest floor free of debris, would begin to appear once they were larger than roughly 2m. If significant forest debris were present, it makes features equal in size to the debris difficult to identify.
Mineral Explorationists focus on identifying various geological elements such as glacial features, ancient hydrology, outcrops, pegmatites, faulting, folding, and overburden. There is also great interest in discovering historical workings (pits, piles, trenches, tailings, trails, roads) as well as locations for drill collars, routes for accessibility, and active hydrology. These features are relatively large and to achieve their visualization even under canopy 6 to 8 pulses/m is often the sweet spot with 10 pulses/m being on the high end of point collection and sufficient in almost every mineral exploration project.
