Ground-Based Thermal Imaging of Coastal and Riverine Sediments Ground-Based Thermal Imaging of Coastal and Riverine Sediments Sliwinski, T. M., Graduate Researcher, Center for Applied Coastal Research, University of Delaware, Newark, USA, McKenna, T. E., Hydrogeologist, Delaware Geological Survey, University of Delaware, Newark, USA, Puleo, J. A., Associate Professor, Center for Applied Coastal Research, University of Delaware, Newark, USA, Meehan, C. L., Assistant Professor, Department of Civil and Environmental Engineering, University of Delaware, Newark, USA, ____________________________________________________________________ I. Motivation and Objective Ground-based remote sensing can provide information on spatio-temporal distributions of sediment and geotechnical properties in dynamic coastal and riverine environments where it can be difficult to collect representative in-situ data. The spatio-temporal variability of grain size, moisture content and biological activity in these environments presents a major challenge in the development of robust remote sensing applications. For ground-based thermal imaging, the radiation received by the imager is a function of the temperature and emissivity of the sediment, observation geometry, atmospheric transmittance (distance and humidity), and reflected background radiation. This study examines the effects of observation geometry on emissivity and the apparent temperature of sediments. A bench-scale multi-spectral imaging system was developed to assess the effects of viewing angle, heating/cooling and moisture variation on thermal imager response. Preliminary results show the expected decrease in emissivity with increasing view angle. Our goal is to parameterize this effect so that imagery from uncontrolled field conditions can be corrected. Combined with corrections for atmospheric transmittance and reflected radiation and an emissivity separation routine, this will allow for a more accurate evaluation of spatio-temporal variations in surface temperature and enable evaluation of the heat-transfer processes driving temporal variations. Of primary interest is estimating the thermal and hydraulic properties of the sediment which serve as proxies for grain size, porosity and moisture content (fundamental parameters for geotechnical applications). II. Long Term Goals A) Develop methodologies for thermal imaging in environmental studies related to tidal marsh inundation, wetland loss, point and non-point source pollution, hydroecology, and groundwater discharge. B) Develop methodologies enabling remote determination of bearing capacity and trafficability in intertidal and riverine environments. III. Lab Setup Thermal infrared (FLIR) imager attached to a short rotating arm and aimed at warm water in a flume. VII. Future Work In the future, data from time-sequenced thermal imagery will be used to drive inverse heat- transfer models to determine thermal properties and sediment properties such as moisture content, lithology and porosity. V. Lab Results Figure 1. Apparent surface temperature variations as a function of camera viewing angle and distance on an exposed tidal flat. The tidal channels in the background are not frozen, yet the imager indicates temperatures well below freezing. Recording images in the field. Reference Emitter Figure 3. Left - Thermal images at oblique angle (85° from nadir) (top) and nadir (bottom). Right – Corresponding thermal images of Mylar. Temperature scale is in °C and changes with image. Abstract Number: OS51B-1278 Using basic geometry it was possible to determine the intersection angle of the ray projected from the center of each pixel. This angle only changed in the vertical direction of the images shown in Figure 3. Experimental results were averaged across 5 images and then across each set of pixels in the horizontal direction within the region of interest (insulation, tank edges, and sensors were removed). Figure 2. Time series of thermal images showing increasing temperature (yellow, orange, and red) as warm tidal water flows over a saltmarsh in Delaware, USA during a summer evening (June 2009). In situ data were collected every minute using a Campbell Scientific data logger, 16 temperature sensors, air temperature sensor, and a relative humidity sensor. Water and Mylar surface temperatures were recorded at nadir using a Fluke 561 IR thermometer. These measurements provide a temperature baseline when the imager is sampling from oblique angles. A custom-fabricated, constant temperature reference emitter acts as an external near-blackbody source to further validate the IR thermometer and FLIR imagery. The emitter is made of solid copper with a milled internal channel. The emitter has a constant temperature maintained by a precision water bath that circulates water through the channel. A material with a known emissivity is placed on the face of the emitter. IV. Experiments Images of water surface at multiple angles. Procedure 1.Fill flume with warm water and mix 2.Adjust arm to 85° from nadir 3.Take 5 images of water surface 4.Take radiant temperature of water surface using IR thermometer 5.Place Mylar reflector boards and take 5 more images 6.Take radiant temperature of Mylar surface using IR thermometer 7.Remove reflectors and repeat steps 3-6 at 80°, 75°, 70°, 65°, 60°, 55°, 50°, 45°, 40°, 30°, 20°, 10°, and 0° from nadir Center for Applied Coastal Research Figure 4. Experimental radiant temperature as seen by FLIR with respect to the intersection angle (from nadir) for rays projected from the center of each pixel. The black dots represent the averaged data for a particular nominal angle, the red line is the theoretical predicted temperature for each nominal angle, and the blue line is the temperature measured at nadir with an IR thermometer at the times corresponding to the FLIR images. Theoretical predictions were made using standard Fresnel relationships from electromagnetic theory. FLIR 7.5 to 13 μm Flume covered in insulation to retain heat throughout experiment. Flaps of insulation allowed access. Area surrounding the imager was covered during the experiment. Visual images show FLIR view at an oblique angle. Left) water surface. Right) Mylar reflector. Mylar images used to remove reflections from water images. During the experiment the laboratory lights and heaters were turned off. Figure 5. Left - Thermal image taken at 80° from nadir, Right - Corrected thermal image. Temperature scale is in °C and changes with image. VI. Field Results Figure 6. Left – RGB image of field site with Regions of Interest (ROI), Right – Thermal image of field site. The cables seen are Campbell kinetic temperature sensors. Figure 7. Raw data for the field experiment. FLIR temperature data are averaged within the ROI. Standard deviation within ROI is small and not shown. Variations between regions is consistent for FLIR and Campbell. However, the Campbell data differs from the FLIR data. The amplitude offset is due to blackbody and the fact that sensors are 3 cm below the surface. The phase offset is due to the time for heat transfer between the surface to the buried sensor. Field experiments took place on a beach of the Wolf River in southern Mississippi. Thermal and visual imagery and in situ measurements were taken through time. Acknowledgements: We are grateful for the financial support provided NASA EPSCoR, NSF EPSCoR, the Office of Naval Research, the Naval Research Laboratory and The University of Delaware. Field assistance was provided by Sam Vaughan, Christine Sutkowski, Melissa Stewart, Autumn Kidwell, Thijs Lanckriet and Lauren Munoz ROI North ROI South ROI Sand