How to bridge the spatial scale gap in Earth's surface observation: the drone solution
The success and popularity of drones have enabled significant technological developments and decreased their market cost. Veerle Vandeginste investigates the opportunities and challenges of using small drones in field geological studies.
Observations of the Earth's surface happen at a range of scales, and vary from on the ground human observation to up in space satellite monitoring. Generally, the smaller the area covered, and thus the smaller the scale of the observation, the better the resolution of the observation. The resolution of on the ground observations is in the order of tens of micrometers to a few millimeters, whereas for satellite remote sensing the resolution is in the order of tens of centimeters to a few meters. The revolution in drone technological development offers the ultimate solution to bridge the spatial scale gap between on the ground and satellite imaging techniques.
Small drones, such as the DJI Phantom quadcopter, enable imaging at the millimeter to centimeter resolution. In addition to filling the gap in effective spatial range and resolution, these drones offer the flexibility of documenting horizontal, inclined and vertical surfaces. Hence, drones are the key to overcome the challenge of inaccessible rock exposures, such as vertical cliff faces. Some world class sedimentary formations occur in cliffs, and multi-rotor systems (e.g. quadcopters) can be used to document these structures at an unprecedented high resolution.
Drones can also be deployed to monitor sites that pose a potential hazard, such as imaging of volcano craters or volcanic lakes. Beyond conventional photogrammetry, other sensors can be attached to drone platforms, such as multispectral and hyperspectral sensors, monitoring devices for CO2 and other gases, etc. Further developments are on-going to miniaturise laser scanning technologies, and other types of sensors that are of interest.
Drone-captured images are used to reconstruct detailed maps of three-dimensional models from surveys of geological study sites, and form the fundament for further quantitative analysis. The advance of extraordinarily fast, cost-effective and high geometric resolution aerial photogrammetry using small drones is revolutionary compared to the traditional methods.
Besides the vast range of applications in which drones can play an important and innovative role, the small drones are low-cost and easy to use. These multi-rotor drones need only a very small area for take off and landing, which makes them very flexible in the field, as they can take off from very irregular rocky surfaces. They have accurate positioning and a good level of automation. The small drones (such as the DJI Phantom quadcopter) are lightweight and can easily be carried by one person in the field, which makes it ideal in terms of mobility in mountainous regions. They can also be used at short notice.
The challenges of using small drones in geological studies are minor. Weather conditions need to be good, since the lightweight drones cannot fly in strong winds. Also their flight time is generally limited to no more than 20 minutes due to battery capacity. However, this last challenge can be decreased, and flight time per day on site increased by having several batteries available.
Legislation needs to be investigated carefully, as it is evolving, and local rules may differ by country or district. The current Civil Aviation Authority UK Dronecode involves the following: 1) always keep your drone in sight, 2) keep your drone below 120m height, 3) follow the manufacturer's instructions, 4) keep 50 m distance from people and properties and 150 m distance from crowds and built up areas, 5) legal responsibility lies with the drone operator, and 6) stay well away from aircrafts, airports and airfields.
Application of diagenetic dolomite geobody mapping
The use of drones to reconstruct three-dimensional surface models is an innovative way to map diagenetic geobodies. Diagenesis is a physicochemical process that affects sedimentary rocks and changes their properties, such as how well fluids can pass through them. Rocks that have zones where this property is significantly different are said to have heterogeneities in the form of geobodies. Such heterogeneity can be caused by diagenesis, but also by sedimentary or structural features. Mapping or predicting the occurrence and distribution of heterogeneities is important to understand the movement of fluids through rocks.
We mapped four km2 in a one-week field survey in the mountainous region of the Picos de Europa, northern Spain. The reconstructed models have a resolution of ≥ 5 cm. Despite the challenges of vegetation or snow cover at the surface and the fact that only the surface and not the subsurface is imaged, the models can provide quantitative information on the surface dimensions of brown dolomite (calcium magnesium carbonate) geobodies occurring among the grey limestone (calcium carbonate) rock.
The distribution and the quantitative dimension data of the dolomite bodies have revealed the predominant control of strike-slip faults on the formation of dolomite bodies. Moreover, the data suggest that the strain magnitude of nearby strike-slip fault zones influence the size of the dolomite bodies formed.
Three-dimensional models of critical rock exposures can be created at low cost and high resolution by the use of small drones. The method enables the generation of large quantitative datasets and is particularly powerful to collect data from sites that are inaccessible or dangerous. Importantly, drone photogrammetry fills the gap in effective spatial scale and resolution, and has the flexibility to document horizontal, inclined and vertical surfaces. Further developments in drone platforms, sensors and software promise to enable further revolutionary opportunities in a wide range of disciplines.
Note: This scientific paper was accepted on 02/02/2018 and can be found online in Marine and Petroleum Geology.
About the Author
Veerle Vandeginste is Assistant Professor in the GeoEnergy Research Centre and the School of Chemistry, University of Nottingham.