Determining the rheology of active lava flows from photogrammetric image sequence processing

We describe a photogrammetric approach used to determine the rheological properties of active lava flows based on stereo image sequences. Bulk rheological properties can be estimated from measurements of flow slope, velocity and dimensions and so, at flow-fronts, they can be calculated from sequential digital elevation models (DEMs) acquired as the flow advances over new ground. For useful flow parameters to be extracted, DEMs may need to be obtained at approximately minute intervals, over durations of up to multiple hours. To deliver such data, we use oblique stereo pair sequences captured by digital SLR cameras and a semi-automated DEM-generation pipeline. Although similar data could be acquired with a terrestrial laser scanner, with deployments in remote and hazardous regions the photogrammetric approach offers significant logistical advantages in terms of reduced equipment cost, bulk, weight and power requirements. We describe the application of the technique to an active lava flow on Mount Etna, Sicily, in 2006. Image sequences were acquired from two tripod-mounted cameras over a period of ~3 hours, as the flow-front advanced ~15 m. Photogrammetric control was provided by 11 targets placed in the scene, with their coordinates determined by dGPS. The cameras were synchronised by a shutter release cable and triggered by an external timer (intervalometer). Image pairs were obtained every minute with DEMs extraction carried out on every fourth epoch; 57 DEMs, with a 0.25-m resolution, were generated. We describe the challenges associated with data collection in this remote environment and the techniques required to automate the photogrammetric analysis and sequence-DEM generation

Aerodynamic characteristics of a large number of airfoils tested in the variable-density wind tunnel

The aerodynamic characteristics of a large number of miscellaneous airfoils tested in the variable-density tunnel have been reduced to a comparable form and are published in this report for convenient reference. Plots of the standard characteristics are given in tabular form. Included is a tabulation of important characteristics for the related airfoils reported in NACA report 460. This report, in conjunction with NACA report 610, makes available in comparable and convenient form the aerodynamic data for airfoils tested in the variable-density tunnel since January 1, 1931

Tests of related forward-camber airfoils in the variable-density wind tunnel

A recent investigation of numerous related airfoils indicated that positions of camber forward of the usual location resulted in an increase of the maximum lift. As an extension of this investigation, a series of forward-camber airfoils has been developed, the members of which show airfoil characteristics superior to those of the airfoils previously investigated. The primary object of this report is to present fully corrected results for airfoils in the useful range of shapes. With the data thus made available, an airplane designer may intelligently choose the best possible airfoil-section shape for a given application and may predict to a reasonable degree the aerodynamic characteristics to be expected in flight from the section shape chosen

Surface temperature measurements of active lava flows on Kilauea volcano, Hawai'i.

Systematic hand-held radiometer measurements of lava surface temperatures in active flows and tubes on Kilauea volcano, Hawai′i reveal complexities that cannot be resolved in remotely sensed data from aircraft or satellites. Using portable infrared Minolta/Land Cyclops radiometers, we measured surface temperatures of flows at various distances from their sources and investigated cooling rates and the development of crust. Our measurements suggest that the upper surface of these lava flows can be split into a minimum of four thermal components: core (>1050°C), visco-elastic skin (750–900°C), rigid solid crust (<750°C), and flow margins (<175°C). For the ′a′a flows investigated, a cool rigid crust characteristically developed in the central part of channels within 30 m of the source vent and incandescent lava was exposed in the marginal shear zones of channels. This affects the heat loss and morphology of lava in active channels. Our investigations of temperature distributions on pahoehoe flow fields reveal temperature anomalies of up to 150°C above active tubes and tumuli

The development of compound lava flow fields:insights from the 2008-9 eruption of Mt. Etna, Sicily

While the early development of basaltic lava flows is relatively well understood, long-lived effusive activity (>~3 weeks) results in complex flow field architectures with multiple ephemeral vents feeding short-lived flows, that may become tubed. The processes controlling flow structures and lifetimes are not fully understood; some may be governed by effusion rate or topography, others by rheological changes at the flow front. Detailed analyses of flow field development will improve our understanding of the above processes, and others including accidental breach formation. During the final month of the 13 May 2008 - 6 July 2009 eruption of Mt Etna, we installed four Canon EOS 450D cameras (3 visible and one modified to collect infrared images) at critical locations to record images (at 5 or 15 minute intervals) of the active flows. The image sequences show that multiple individual 'a'ā channel flows were often simultaneously active for hours to days, with lengths of tens to hundreds of metres. Mean flow lengths decreased over time, indicating diminishing effusion rates. Variations in the number and lengths of active flows, and the occurrence of pulses within channels are suggestive of shorter-term effusion rate fluctuations. Several flows roofed over on timescales of hours to a few days. Measured flow velocities and dimensions can be used to estimate rheological properties. Processes observed in these small flows have also been observed in larger flows, so we discuss implications for interpreting flow behaviour on larger spatial scales

Estimating rheological properties of lava flows using high-resolution time lapse imaging

During effusive eruptions, property and infrastructure can be threatened by lava flow inundation. In order to maximise the effectiveness of the response to such an event, it is necessary to be able to reliably forecast the area that will be affected. One of the major controls on the advance of a lava flow is its rheology, which is spatially and temporally variable, and depends on many underlying factors. Estimating the rheological properties of a lava flow, and the change in these over space and time is therefore of the utmost importance. Here we report estimates of rheological properties made from geometric and velocity measurements on integrated topographic and image data using the method of Ellis et al. (2004) (Ellis B, Wilson L & Pinkerton H (2004) Estimating the rheology of basaltic lava flows. Lunar & Planetary Science XXXV Abst. 1550). These are then compared to the viscosity predicted from composition and temperature by the GRD model (Giordano D, Russell JK, & Dingwell DB (2008) Viscosity of Magmatic Liquids: A Model. Earth & Planetary Science Letters, 271, 123-134). During the 13 May 2008 - 6 July 2009 eruption of Mt Etna, Sicily, lava flows were emplaced into the Valle del Bove, reaching a maximum length of >6 km. Towards the end of the eruption, multiple channelized aa flows were active simultaneously, reaching tens to hundreds of metres in length. Flow lifetimes were of the order hours to days. In the last month of the eruption, we installed a Canon EOS 450D camera at Pizzi Deneri, on the north side of the Valle del Bove, to collect visible images at 15-minute intervals. On one day, topographic data (using a Riegl LPM-321 terrestrial laser scanner) and thermal images (using a FLIR Thermacam S40) were also collected from this location. The fronts of some of the larger flows were tracked through the time lapse image sequence. Using knowledge of the camera imaging geometry, the pixel tracks were reprojected onto the topographic surface to determine flow advance in 3-D geographic coordinates. Integrating the tracking results with the topographic data allows flow lengths and velocities to be extracted. Using these parameters together with estimates of the flow width and thickness, we estimate effective yield strengths, apparent viscosities and Gratz numbers for the tracked flows. We then evaluate the success of this method using predicted viscosities from the GRD model of Giordano et al. (2008)

Monitoring volcanoes:a review

Minimising risk in volcanic regions involves a number of inter-related studies. The collection of historical data on previous eruptions is the first step, followed by detailed mapping around the volcano. The resulting volcanic hazard map can be used during volcanic crises to minimise casualties. The range of tools currently deployed on volcanoes include seismometers, a range of gas monitoring equipment, ground deformation equipment including differential GPS, borehole strainmeters, thermal imaging cameras, micro-gravimeters, magnetometers, laser scanners, radar scanners and magnetotelluric equipment to create sub-volcanic images based on electrical conductivity. Satellites are also becoming more important, and new techniques involving solid state sensors may have a significant role to play in the future. Case studies confirm the usefulness of each of these techniques, though, even in well-monitored volcanoes, some eruptions take place without detectable precursors. To minimise the possibility of unexpected eruptions in the future will require both increased instrumentation and the application of methodologies such as advanced neural networks and expert solicitation

Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna

Detailed mapping during the 1991–1993 eruption of Mount Etna has shown that there is a relationship between tumuli, ephemeral vents, lava tubes, and their parent lava flows. During this eruption, many tubes formed in stationary, inflated ‘a‘a lava flows. Ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows, many of which subsequently developed new tubes. The resulting complex network of tubes, ephemeral vents, and secondary flows was responsible for most of the widening, thickening, and lengthening of the 1991–1993 Etna lava flow field. The supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel-fed lava flow. Our observations suggest that lava tubes play a more important role in the formation of extensive ‘a‘a flow fields on Etna than has previously been recognized