New modeling technique offers geologic insights
Alaska’s Division of Geological and Geophysical Surveys is using a new technique called structure-through-motion photogrammetry to gain better insights into the detailed geology of areas of the state. DGGS geologist Trystan Herriott explained to Petroleum News that, essentially, the technique involves shooting a large number of overlapping photographs around an area with rock outcrops. The overlapping images allow the derivation of stereoscopic images. Data from these images can be fed into a computer application that can construct a highly accurate three-dimensional model of the rocks, with the images tied back to ground observations and GPS survey markers. Actual rock observations on the ground are linked back to how these rocks appear in the photographic images. Depending on the scale of the project, hundreds or thousands of photographs may be required, Herriott said.
Digital photographsIn some applications, people use drones to collect the required photographs. However, the scale and remote nature of Alaska rock outcrops tend to make this approach impractical - the DGGS scientists instead use digital SLR cameras, shooting the photographs from an aircraft that flies a planned pattern around a survey area. Modern digital photographic technology enables resolutions of about 2.5 centimeters per image pixel.
By comparison, a radar-based technology that the federal government is using to develop new statewide relief maps of Alaska has a resolution of about 5 meters, sufficient for relief mapping but inadequate for detailed geologic analysis. A structure-through-motion survey actually results in two products: a digital surface model, and what is referred to as an ortho model, a three-dimensional model generated by the computer processing of the images and with a uniform scale throughout. The result is a model of the geology that enables the mapping out of rock units, the measurement of features such as deposition channel sizes and so on, Herriott said.
Value in AlaskaIn Alaska’s rugged landscape, the technique is particularly helpful in developing detailed geologic maps at locations where ground access is very limited. For example, the technique is being used to map inaccessible outcrops on the west side of Cook Inlet. On the North Slope the technique makes possible the mapping and three-dimensional modeling of rock outcrops, such as the Nanushuk/Torok outcrop on the east face of Slope Mountain, near the Haul Road.“We see this work as having a lot of different potential benefits and we archive the data on the DGGS elevation dataset, so they’re available for anyone to see,” Herriott said. - ALAN BAILEY