David G. Macfarlane

Lava dome growth and mass wasting measured by a time series of ground-based radar and seismicity observations

range and intensity measurements of the change in summit lava (� 1.5 � 10 6 m 3 , 22%), (2) AVTIS range measurements to measure the talus growth (� 3.9 � 10 6 m 3 , 57%), and (3) rockfall seismicity to measure the pyroclastic flow deposit volumes (� 1.4 � 10 6 m 3 , 21%), which gives an overall dense rock equivalent extrusion rate of about 7 m 3 � s � 1 . These figures demonstrate how efficient nonexplosive lava dome growth can be in generating large volumes of primary clastic deposits, a process that, by reducing the proportion of erupted lava stored in the summit region, will reduce the likelihood of large hazardous pyroclastic flows.

340-GHz 3D radar imaging test bed with 10-Hz frame rate

We present a 340 GHz 3D radar imaging test bed with 10 Hz frame rate which enables the investigation of strategies for the detection of concealed threats in high risk public areas. The radar uses a wideband heterodyne scheme and fast-scanning optics to achieve moderate resolution volumetric data sets, over a limited field of view, of targets at moderate stand-off ranges. The high frame rate is achieved through the use of DDS chirp generation, fast galvanometer scanners and efficient processing which combines CPU multi-threading and GPU-based techniques, and is sufficiently fast to follow smoothly the natural motion of people.

Use of a portable topographic mapping millimetre wave radar at an active lava flow

A ground-based millimetre wave radar, AVTIS (All-weather Volcano Topography Imaging Sensor), has been developed for topographic monitoring. The instrument is portable and capable of measurements over ranges up to ∼7 km through cloud and at night. In April and May 2005, AVTIS was deployed at Arenal Volcano, Costa Rica, in order to determine topographic changes associated with the advance of a lava flow. This is the first reported application of mm-wave radar technology to the measurement of lava flux rates. Three topographic data sets of the flow were acquired from observation distances of ∼3 km over an eight day period, during which the flow front was detected to have advanced ∼200 m. Topographic differences between the data sets indicated a flow thickness of ∼10 m, and a dense rock equivalent lava flux of ∼0.20 ± 0.08 m3s−1.

AVTIS - a dual-mode imaging millimetre wave radar/radiometer for volcanological surveying

We present the design and preliminary results for an advanced mm-wave remote sensing instrument. AVTIS (AH Weather Volcano Topography Imaging Sensor) is a dual-mode passive imager and active radar operating at 94 GHz, designed to yield topographic and thermal maps of volcanic lava domes. The mechanically scanned imager uses a 0.5m Cassegrain antenna mounted on a pan & tilt gimbal. The radiometer is of the total power heterodyne type, and the radar operates in an FMCW mode. When images are collected from multiple viewpoints, it will be possible to construct a thermo-spatial 3D map of the lava dome. The project volcanologists will use this data to enhance the modeling of the growth and collapse mechanisms in lava domes, which may ultimately lead to improved hazard warning.

Topographic and Thermal Mapping of Volcanic Terrain Using the AVTIS Ground-Based 94-GHz Dual-Mode Radar/Radiometric Imager

The All-Weather Volcano Topography Imaging Sensor remote sensing instrument is a custom-built millimeter-wave (MMW) sensor that has been developed as a practical field tool for remote sensing of volcanic terrain at active lava domes. The portable instrument combines active and passive MMW measurements to record topographic and thermal data in almost all weather conditions from ground-based survey points. We describe how the instrument is deployed in the field, the quality of the primary ranging and radiometric measurements, and the postprocessing techniques used to derive the geophysical products of the target terrain, surface temperature, and reflectivity. By comparison of changing topography, we estimate the volume change and the lava extrusion rate. Validation of the MMW radiometry is also presented by quantitative comparison with coincident infrared thermal imagery.

Chapter 13 AVTIS observations of lava dome growth at Soufrière Hills Volcano, Montserrat: 2004 to 2011

Abstract To solve the problem of lava dome growth at Soufrière Hills Volcano (SHV) being invisible and unmeasured owing to cloud, we have designed, built and deployed a ground-based millimetre-wave radar/radiometer: the All-weather Volcano Topography Imaging Sensor (AVTIS). In this chapter, after an outline technical sketch of the instruments, we describe the campaigns between 2004 and 2011 used to test their capabilities. We then present results from the campaigns to illustrate how signals of volcanological interest can be retrieved. The primary measurements of AVTIS are range (to within, at best, about 1 m), and, from that, topography, topographical change and effusion rates, and surface temperature (to within a few degrees Celsius). Changes in radar reflectivity can indicate surface processes (e.g. mass wasting). Surface motion within the instantaneous field of view produces a Doppler signal that allows detection of rockfall. Attenuation of the signal by rain along the path can, when stacked temporally, give an image of rain cloud structure and, by calibration, a rate of rainfall. We regard a strategy of two radars – one permanantly mounted (at Windy Hill) autonomous instrument, and the other used as a rover – as being best for capturing dome growth.

A 94-GHz dual-mode active/passive imager for remote sensing

We present the development and field testing results of a dual-mode radar/radiometric imager operating at 94GHz. This instrument combines an FMCW radar with a total power radiometer in a compact, portable unit which is designed for ground based remote sensing. The radar produces range maps with a range bin resolution of 1m out to a maximum range of approximately 5km and target reflectivity can also be retrieved. The radiometer produces co-aligned thermal images of the scene with a thermal resolution of the order of one kelvin. The instrument, which uses a single 0.45m Cassegrain antenna, is rastered over the scene using a commercial pan and tilt gimbal. Image acquisition times are of the order of tens of minutes. The principal application for which the instrument was designed is to survey volcanic lava domes, irrespective of weather conditions - it has been named AVTIS for All-weather Volcano Topography Imaging Sensor. We will present results obtained with AVTIS from the Soufrière Hills Volcano, Montserrat, as well as more general terrain mapping imagery gathered locally in Scotland. Besides volcano surveying, AVTIS could be deployed for other remote sensing applications.

Imaging a growing lava dome with a portable radar

The tiny Caribbean island of Montserrat has been in a state of crisis since the Soufriere Hills Volcano (SHV) began its current eruption in July 1995. With its main town, Plymouth, destroyed by pyroclastic flows in 1997, the islanders who have remained have had to rebuild their society on the northern half of the island under varying degrees of threat from the volcano to the south. During this time, the Montserrat government continues to receive advice on the volcanic hazards from the Montserrat Volcano Observatory (MVO). The continuing eruption has provided a wealth of research opportunities for many international groups who sought to study the growth and repeated partial destruction of a Peleean andesitic lava dome. There have been three episodes of dome growth: November 1995 through March 1998, November 1999 through July 2003, and August 2005 to present. Pyroclastic flows and explosions have been the main source of hazard.The pyroclastic flows have been generated mainly by gravitational collapse of the lava dome, but also by collapse of explosive ash columns. The dome collapses tend to occur from the area of the dome where new lava is being added. Similarly collapses are more likely when the rate of lava extrusion varies. Also, the propensity of explosive evacuation of the magma in the conduit is partly controlled by the magma supply rate, with high rates favoring explosions.

Complex Permittivity of Volcanic Rock and Ash at Millimeter Wave Frequencies

Millimeter wave radar has shown to be of great use in the field of volcanology in terrain mapping applications. One area less studied to date is its use in ash cloud monitoring applications. In order for this to be realized, a quantitative study of the dielectric properties of volcanic rock and ash is required. In this letter, we present a method for accurately determining the complex dielectric permittivity of volcanic samples using a quasi-optical technique on the pellets of pulverized rock and present the results obtained. When averaged across our sample set, the results show agreement with those obtained by previous authors for both the permittivity of the volcanic material at lower frequencies and the permittivity of other rocks at millimeter wave frequencies. The results also show good levels of consistency between the multiple splits of each sample. In order to relate the measurements of the porous collections of ash to a continuous rock equivalent that is needed for use in the distributed target radar equation, mixing formulas are required. A measurement technique is presented for validating the mixing formulas using pressed pellets and loose ash. Böttchers formula is found to be both accurate and sufficient for our purposes and is used further to relate our measurements to the continuous rock equivalents. These equivalents show that, for andesitic ash, the radar reflectivity factor may be taken to be an average value of K = |(ε - 1)/(ε + 2)|2 = 0.39 ± 0.03 , showing no measurable deviation from that reported previously at frequencies of up to 19 GHz.

A 94 GHz dual-mode imaging radarometer for remote sensing

We report on the continued development of our 94GHz dual-mode radar/radiometric imager, AVTIS. To date we have concentrated on refining the radar mode and can now acquire state-of-the-art long range, high resolution radar images. More recently we have worked to improve and integrate the radiometric mode, to complete the dual-mode functionality. One notable problem of the monostatic architecture is that of radar transmitter leakage via the circulator/antenna assembly, even with the transmitter signal heavily attenuated. A related issue is the high level of AM noise in the local oscillator signal due to the IMPATT multipliers and power amplifiers used. This far-off-the-carrier noise, in combination with the leakage, desensitises the receiver and raises the noise floor to give a receiver noise temperature of approximately 6000K. This yields a thermal resolution of 2K in 4ms that is considered adequate for the intended volcanological application. In comparison, tests with a high power Schottky diode multiplier chain as the LO source has yielded a noise temperature of 750K in a separate radiometer. Thermal calibration is also of concern and we have implemented an IF noise adding circuit in which the radiometer alternates between the scene and hot and cold IF noise references. This accounts for dominant receiver fluctuations in the IF amplifiers but sky tipping curves are used to calibrate the overall response. Whilst the radar is already capable of producing high resolution topographic maps with surface reflectivity overlaid, it is hoped that co-aligned radiometric brightness temperature data will lead to a better understanding of the emissivity and surface roughness of the terrain being surveyed.