LIDAR (LIght Detection And Ranging) uses a laser beam to scan objects, in our case rock outcrops, and returns series of points that are accurately located in space (within a couple of centimetres) and also have a value for the intensity of the returned laser pulse. This produces what is known as a 'point cloud' dataset. We use a calibrated digital camera to give colour to the points. A differential GPS with sub-metre accuracy allows us to accurately georeference the data. The system that Radiohead used collects up to 1.8 million points a second. The system we use manages 12,500 points a second, which is still fast enough for us to scan an outcrop on a scale of the order of a few hundred metres by a hundred metres in 10 to 15 minutes, with a point spacing of 5 to 10 cm. One of our datasets captures an entire half-graben in the Suez rift, and comprises something on the order of 5 billion points.
Point cloud dataset, coloured using digital images, with geological interpretation added.
Digital Elevation Model derived from one of our Gulf of Suez datsets, and coloured with a 60 cm resolution satellite image. Red dots show the positions of the LIDAR scan stations. The whole dataset comprises more than 5 billion points and covers an area of approximately 9 km2, with a point spacing of 5 to 10 cm.So far this perhaps sounds like an unnecessarily elaborate system for doing geological mapping. After all, you don't actually need much more than a map, compass and a bunch of colouring pencils to create a perfectly acceptable geological map. So what actual advantages does it give us? It allows us to do accurate geological mapping of surfaces that are exposed in vertical cliff faces, which is difficult to do in other ways because the faces are not easily accessible, and because it is difficult to map vertical faces accurately on horizontal maps or aerial photos. The data is digital, and that allows us to export our geological interpretations to modelling software of the type typically used in the oil industry. This allows us to use outcrop data in a similar way to how sub-surface data (mainly seismic) is used in the oil industry, but with much higher resolution (centimetres compared to tens of metres). In the Suez rift, we're using LIDAR data to improve our understanding of how fault systems evolve through time, and how that structural evolution influences the stratigraphy and sedimentology of the strata that are deposited in the rift. To do this, we need to understand how the thickness and rock types of geological strata change across the area, with respect to the structural geology. Those variations are subtle, and we need high resolution data to map them accurately. Finally, it allows us to easily communicate our data and results to the oil companies who sponsor us, and gives us nice, clear images we can use to illustrate scientific papers and conference presentations.
Three-dimensional representation of a geological surface, derived from LIDAR-based geological mapping
This being a fairly new technology for geological mapping, it has not always been straightforward to use. For example, software for geological interpretation of LIDAR data didn't exist. So we had to create our own: my colleague Dave Hodgetts has been working on this, and the resulting software is now being spun out. And we're still trying to figure out how we can use all the data to its best advantage. But we're getting there, and I hope the first publications from this work will be coming out soon. Watch this space...