All posts by COMET2014

Coupling volcanic gas emission

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Earthquake resilience in the East African Rift: the challenge of making baseline measurements

By COMET Scientist Juliet Biggs

Malawi is a small country at the tip of the East African Rift. Small, that is, until you spend several weeks driving around in loops over challenging roads.

Ake Fagereng (Cardiff), Hassan Mdala (Geological Survey) and I covered nearly 5000 km in less than three weeks. Why would we do this to ourselves? To establish a baseline geodetic network at the southern tip of the East African Rift.

We want to know how strain is distributed, and which are the active structures. This latest field season is just another piece in the tectonic jigsaw that we hope will ultimately lead to improving earthquake resilience in developing countries.

Our partner for the last few years has been the Geological Survey Department of Malawi – in particular, Hassan, who operates a small mobile office in the back seat of the car. From here, he fixes all our problems, often before we even realize they exist; making use of a boundless network of friends and family, all of whom seem to be just waiting to help with our latest peculiar request.

Each GPS station is composed of an antenna, a receiver, a car battery, a regulator, a solar panel and a metal pin that we drill into the rock. Most of the kit only stays for 4 days, but we’ll need to re-measure the position of the pin for many years to come. In our search for suitable sites, we met many enthusiastic head teachers, police chiefs and district commissioners and one rather terrifying headmistress, all of whom helped with security arrangements in schools or telecommunications masts.

At Mua Primary school, with the Head Teacher, Deputy Head and our GPS antenna
At Mua Primary school, with the Head Teacher, Deputy Head and our GPS antenna

Everywhere we went, there were spectacular landscapes and interesting geology, most of which we were forced to drive straight past. But so much of my research is done from satellites orbiting at 600 km that I can get a bit over-excited about getting my nose to the rock again, and we took advantage of what opportunity we had.

Malawi is largely composed of various types of gneiss, a high-grade metamorphic rock, with lots of migmatites, blobs that were once partially molten rock. Perhaps the most spectacular landscapes are the ‘inselbergs’, large granite intrusions, which now stick several thousand metres above the surrounding landscape. On our only day off, squeezed somehow into a hectic schedule, we hiked up the largest of these, Mulanje, for some truly spectacular views. We also found some fascinating faults, but you’ll have to wait for the paper to hear about those!

Hiking on Mulanje Massif. According to the Official Malawi Tourism site, the third largest mountain in Africa – a rather dubious statistic, but still steep enough to trouble my knees
Hiking on Mulanje Massif. According to the Official Malawi Tourism site, the third largest mountain in Africa – a rather dubious statistic, but still steep enough to trouble my knees

Many people ask about the personal challenges to fieldwork in Africa, and there are indeed plenty of those: insect bites, the side-effects of malaria medication, power cuts, cold showers, endless meetings getting permission, long days driving over roads covered with pot-holes, goats, dogs, chickens and bicycles, and on this occasion a car break-in (I’m wondering if there’s such a thing as a frequent flyer card for local police stations…?).

But none of that matters much; I love every minute (well, almost, I could skip the police stations). A great trip requires great company: Hassan, for whom every minor disaster is ‘a learning opportunity’, and Ake whose response to the next challenge is usually ‘well, that’ll be fun’.

Ake and Hassan contemplating a job well done. ‘Til next time.
Ake and Hassan contemplating a job well done. ‘Til next time

All I can say, is I learned a lot and had fun doing it, so I guess they are both right.  I simply feel privileged to spend time in a country with such interesting scientific questions, beautiful landscapes and warm, friendly people. I suspect we’ll find a way to come back before too long!

Our fieldwork was funded by Global Challenges Research Fund through an EPSRC Institutional Sponsorship Award to the University of Bristol; a BGS-COMET fieldwork award and a Researcher Mobility Grant from the World Universities Network.

Andy Hooper receives 2016 AGU Macelwane Medal

Congratulations to COMET scientist Professor Andy Hooper, who has been awarded the American Geophysical Union (AGU) James B. Macelwane Medal in recognition of his contributions to the geophysical sciences.

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Established in 1961, the medal is given to outstanding early career scientists who have shown depth, breadth, impact, creativity and novelty in their research.

Professor Hooper, who is also Co-Director of the Institute of Geophysics and Tectonics at University of Leeds, pioneered the development of new software (StaMPS) to extract ground displacements from time series of synthetic aperture radar (SAR) acquisitions.  StaMPS is now used widely across the Earth Observation community.

He also discovered a new link between ice cap retreat and volcanism via geodetic monitoring from space and subsequent modelling of the 2010 Icelandic volcanic eruptions, and played a significant role in the €6m FUTUREVOLC project, leading the long-term deformation effort to integrate space and ground based observations for improved monitoring and evaluation of volcanic hazards.

Alongside other COMET researchers, he was part of a team contributing to the international scientific response to the earthquake which devastated Nepal in April 2015.

Most recently, working with colleagues from Iceland, he has shed new light on how volcanoes collapse during major eruptions, focusing on the 2014-15 eruption at Bárdarbunga.

Eruption column and lava flow from the air on 22 September 2014. Credit:Thórdís Högnadóttir
Eruption column and lava flow at Bárdarbunga, 22 September 2014. Credit: Thórdís Högnadóttir

Professor Hooper will be presented with the award at the 2016 AGU Fall Meeting, where he will also be giving a talk at the Union Session focusing on the new generation of scientists, where he will also be conferred an AGU fellow.

Congratulations Andy from all your colleagues at COMET.

Read more about the latest AGU awards

Iceland volcano collapse explained,

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Coupling volcanic gas emission measurements to computational models of conduit gas flow

A new paper published in Geophysical Research Letters by Tom Pering and Andrew McGonigle has combined fluid dynamical modelling of gas flow in conduits with high time resolution measurements of volcanic gas discharge for the first time, revealing new insights into the dynamics of Stromboli volcano.

Their work is based on a recently developed approach using ultraviolet cameras which enable measurements of volcanic gas emission rates with unprecedented time resolution – around 1 Hz – such that gas release patterns associated with rapid explosive and non-explosive basaltic processes, can be resolved for the first time.

Data were captured on Stromboli, where an intriguing coda of lifetime on the order of 10s of seconds was identified following each explosion. Computational models were also developed to simulate the upward flow of conduit filling, so called “Taylor bubbles”, which are believed to be responsible for explosions on Stromboli when they burst at the surface.

The numerical models reveal the fissioning of smaller bubbles from the Taylor bubble bases to generate a train of “daughter bubbles”, thought to be responsible for generating the post-explosive coda upon arrival at the surface.

This process could play a primary yet hitherto unconsidered role in driving the dynamics of strombolian volcanism, both on Stromboli and other targets worldwide, with significant implications for the magnitude of resulting eruptions.

Combining models with field observations in this way shows considerable promise for improving our understanding of how gases drive volcanic activity.

The full reference is:  T.D Pering, A. J. S McGonigle, M. R James, G. Tamburello, A. Aiuppa, D. Delle Donne, M. Ripepe Conduit dynamics and post-explosion degassing on Stromboli: a combined UV camera and numerical modelling treatment, Geophysical Research Letters 2016 DOI: 10.1002/2016GL069001

Airborne volcanic ash detection using infrared spectral imaging

A new paper in Scientific Reports, co-authored by COMET’s Tamsin Mather, has demonstrated for the first time that airborne remote detection of volcanic ash is possible.

Airborne volcanic ash is a known hazard to aviation, but there are no current means to detect ash in-flight as the particles are too fine for on-board radar detection and, even in good visibility, ash clouds are difficult or impossible to detect by eye.

The economic cost and societal impact of the Icelandic eruption of Eyjafjallajökull generated renewed interest in finding ways to identify airborne volcanic ash in order to keep airspace open and avoid aircraft groundings.

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Eyjafjallajökull.  Credit: Ingólfur B

The research, led by COMET Board Member Fred Prata, involved designing and building a bi-spectral, fast-sampling, uncooled infrared camera device (AVOID) to examine its ability to detect volcanic ash more than 50 km ahead of aircraft.

Experiments conducted over the Atlantic Ocean, off the coast of France involved an artificial ash cloud being created from a second aircraft, using ash from the Eyjafjallajökull eruption itself.

The measurements made by AVOID,  along with additional in situ sampling, confirmed the ability of the device to detect and quantify ash in an artificial ash cloud.  This is the first example of airborne remote detection of volcanic ash from a long-range flight test aircraft.

The full reference is Prata, A. J. et al. Artificial cloud test confirms volcanic ash detection using infrared spectral imaging. Sci. Rep. 6; doi: 10.1038/srep25620 (2016).

Vertical distribution of volcanic SO2 plumes measured by IASI

Read Elisa Carboni et al.’s new paper on volcanic SO2 plumes and IASI, available now in Atmospheric Chemistry and Physics. viagra generic sildenafil generico guatemala

The paper describes how the Infrared Atmospheric Sounding Interferometer (IASI) on the METOP satellite can be used to study volcanic SO2 emissions, in terms of both amount and altitude. can take half pill viagra

It sets out measurements of volcanic SO2 for 14 explosive eruptions that took place between 2008 and 2012, including those at Eyjafjallajökull (2010) and Grimsvötn (2011) in Iceland, comparing them with alternative methods of measurement. cheap calis order cialis

Grimsvotn 2011 eruption viagra generico em curitiba

Grimsvotn 2011 eruption

The results show that IASI SO2 measurements are not affected by underlying cloud and are consistent with the other measurements. generic viagra cialis or levitra online where to buy legit viagra online

Also, they show that, of the eruptions studied, the biggest emitter of volcanic SO2 was Nabro (Eritrea), followed by Kasatochi (Aleutian Islands) and Grímsvötn, and that the volcanic SO2 reached the tropopause during many of the moderately explosive eruptions studied.

Detecting geohazards with GPS

COMET’s Marek Ziebart and Chris Atkins, both at UCL, have been using sidereal filtering to detect geohazards in near-real time.  This article describes how they have applied this to earthquakes in the Aleutian Islands, Alaska.

Geodetic quality GPS (Global Positioning System) data – fundamentally high precision range measurements made between orbiting spacecraft and tracking stations, often rigidly attached to bedrock – is one of the key observables available to us to analyse plate tectonics and the earthquake cycle. Such tracking stations now often make measurements at 1 Hz or higher. This enables us to calculate high rate position time series for the tracking stations, revealing various kinds of motion related to seismic waves and other forms of deformation. viagra online sweden

 Figure 1: Deep braced GNSS tracking antenna at the Aleutian Islands, Cape Sarichef, Alaska (AV24-WestdahlNWAK2008). Photo: UNAVCO.org

Figure 1: Deep braced GNSS tracking antenna at the Aleutian Islands, Cape Sarichef, Alaska (AV24-WestdahlNWAK2008). Credit: UNAVCO.org

One such time series is shown below, calculated by a standard processing method called PPP (precise point positioning).  The plot shows the change in position in the north-south direction. When do you think the earthquake seismic waves begin to arrive?

Marek2

Figure 2: 25 minutes of the 1Hz GPS time series for AV24 – a deep braced GNSS antenna on the Aleutian Islands, Cape Sarichef, Unimak Island, Alaska at the time of the 2011 Tohoku earthquake (WestdahlNWAK2008)

GPS Seismology

How can we use this type of data? The time series in figure 2 shows an estimate of the GPS antenna movement in the North-South direction during the 2011 magnitude 9.0 Tohoku earthquake over a period of 25 minutes. This is a measurement of the ground shaking – the data is used to understand the nature of the earthquake and to compare it to other earthquakes in different places around the world. Earthquakes often cause a change in the shape of the ground that is retained after the earthquake is over, and which can be determined from a time series. This kind of information can be used to estimate the size of the earthquake, how much energy was released and tells us about the earthquake’s epicentre location.

Another kind of instrument, a seismometer, can be used to determine similar information. The drawback of seismometers is that they can ‘clip’ when the seismic waves become too large to handle. A GPS antenna, on the other hand, has no such limitation – no matter how big the amplitude of the seismic waves, they can still be measured using GPS technology. Because of this, GPS antennas such as the ones shown in figures 1 and 6 below are sometimes called broadband GPS seismometers. In practice we combine information from both GPS antennas and seismometers – they are complimentary. There are now thousands of such GPS antennas installed around the world, connected to the internet and taking measurements 24 hours a day. best time take cialis 20mg

Marek3

Figure 3: an example of a GPS tracking station network – this one is in the Gulf of Alaska and forms part of the US Plate Boundary Observatory (PBO)

So what’s the problem? The problem is Multipath – this is akin to radio interference where the signals transmitted by the orbiting satellites have more than one way of getting to the antenna. The strongest, direct signal – which is the one we want to measure – is distorted by signals bouncing off adjacent reflecting surfaces in the antenna’s vicinity. This causes spurious patterns of motion in the receiver time series. These can vary rapidly over a few seconds in a noise-like way (with amplitudes at the few mm level), or they can appear as a random walk – a slowly varying position error (with changes at the level of a few centimetres). Both these effects limit our ability to use GPS data for seismometry. To get some feel for the effect consider Figure 4. generic cialis in the us

On the day of the earthquake (shown as the black time series) the onset of the arrival of the largest seismic waves is shown clearly at around 6:03:00. A not dissimilar signature is also visible around 5:50:00. However, that same signature (an apparent ground movement of some 25mm over a few minutes, shaking and then reducing in magnitude) is visible at almost the same time on the previous day (in the grey time series). It doesn’t mean the earthquake was repeating – these are multipath distortions. They appear indistinguishable from seismic waves. Our problem, then, is how to remove the spurious, apparent motion from the time series, whilst retaining the actual signature of the earthquake. So what’s the solution?

Marek4

Figure 4: the GPS time series (North-South component) for station AV24 on the day of the earthquake and on the day before. The time series on the vertical axis are offset by 60mm for clarity

Sidereal Filtering

Sidereal filtering is a technique used to reduce errors caused by multipath in the positioning of GPS receivers. It relies upon the receiver remaining static from one day to the next relative to its surrounding environment and takes advantage of the ground track repeat time of the GPS satellites, which is just less than a sidereal day (usually around 23 hours, 55 minutes, 55 seconds). The repeating multipath error can thus be identified and largely removed from the following day by subtraction. red viagra price

A conventional position-domain sidereal filter (PDSF) identifies and removes this repeating pattern from a position time series. However, the ground track repeat time of individual GPS satellites can differ from each other by a few seconds, whereas a PDSF has to assume that all satellites have the same repeat time. This can cause problems for the PDSF, especially when the oscillations caused by multipath interference are particularly high. We have developed an observation-domain sidereal filter (ODSF) that identifies and removes multipath errors from the GPS phase measurements themselves.

Unlike the PDSF, it can account for the fact that the ground track repeat times of the GPS satellites differ by a few seconds from one another. This means that it is more effective at removing the effects of high-frequency multipath error and is less sensitive to satellite outages. For each phase measurement, the ODSF algorithm searches for an appropriate correction based on the azimuth and elevation of the relevant satellite. That correction is derived from the measurement residuals on the previous day that most closely correspond to that azimuth and elevation. The precision of these measures of azimuth and elevation needs to be high – a hundredth of a degree or better. This is readily achievable in our algorithm.

Marek5

Figure 5: The same GPS time series for station AV24, but with the time series resulting from the two types of sidereal filter also shown. The time series on the vertical axis are offset by 50mm for clarity

Figure 5 shows the same 25-minute time series of 1 Hz displacements in northing as shown in Figure 4. Also plotted in red and green are the time series that result after applying the two types of sidereal filter: the PDSF and ODSF respectively. Notice that both the PDSF and the ODSF are largely successful at removing these oscillating errors. They reveal a surface wave arrival time of about 06:00 with the largest Love waves arriving at around 06:03. These events would not have been so easy to distinguish in the original unfiltered time series.

Marek6

Figure 6: Deep braced GNSS tracking antenna on the Aleutian Islands, Cape Sarichef, Unimak Island, Alaska (AV27-WestdahlSWAK2008). Credit: UNAVCO.org

 

Marek7

Figure 7: GPS time series for station AV27 including those resulting from the two types of sidereal filter. The time series on the vertical axis are offset by 50mm for clarity

Figure 7 similarly shows 1 Hz displacements at station AV27 which is only 11 km from AV24. One would therefore expect a very similar displacement signal. However, it is clear that the standard PPP time series, plotted in black and grey, are severely affected by strong short-period (~11 s) multipath error. In this case, the PDSF was unable to remove such a high-frequency error and instead increased the amplitude of these oscillations. The ODSF on the other hand was far more effective because it could take into account the differing repeat times of the satellites.

Conclusions

The precision of GPS technology has improved dramatically since 1995 when the system reached FOC (full operational capability). It is an effective tool for measuring geohazards and other geophysical phenomena in near real-time. However, data processing and interpretation must be handled carefully. Multipath effects on position time series can introduce spurious signatures that resemble seismic events and hamper our ability to exploit the technology. Sidereal filtering offers a very effective way to clean up GPS positioning time series to reveal more clearly geophysical events, effectively in real-time.

 

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Over what distances do volcanoes interact?

In the geological past, large eruptions have often occurred simultaneously at nearby volcanoes. Now, a team of COMET scientists from the University of Bristol uses satellite imagery to investigate the distances over which restless magmatic plumbing systems interact.

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In a study published in the journal Nature Geoscience, the scientists use deformation maps from the Kenyan Rift to monitor pressure changes in a sequence of small magma lenses beneath a single volcano. Importantly, they find that active magma systems were not disturbed beneath neighboring volcanoes less than 15 km away.

The lead author, Dr Juliet Biggs, explained: “Our satellite data shows that unrest in Kenya was restricted to an individual system. Inter-bedded ash layers at these same volcanoes, however, tell us that they have erupted synchronously in the geological past. This was our first hint to compare observations of lateral interactions based on recent geophysical measurements with those from petrological analyses of much older eruptions.

The team, which includes a recently graduated PhD student Elspeth Robertson and Bristol’s Head of Volcanology Prof. Kathy Cashman, took this opportunity to compare observations from around the world with simple scaling laws based on potential interaction mechanisms. They found that stress changes from very large eruptions could influence volcanoes over distances of up to 50 km, but that smaller pressure changes associated with unrest require a different mechanism to explain the interactions.

Prof Cashman explained ‘Volcanology is undergoing a scientific revolution right now – the concept of a large vat of liquid magma beneath a volcano is being replaced by that of a crystalline mush that contains a network of melt or gas lenses. The interactions patterns observed in Kenya support this view, and help to constrain the geometry and location of individual melt and gas lenses.”

The study was funded by two major NERC projects: COMET, a world-leading research centre focusing on tectonic and volcanic processes using Earth observation techniques; and RiftVolc, which is studying the past, present and future behavior of volcanoes in the East African Rift.

The research paper, The lateral extent of volcanic interactions during unrest and eruption, was published online in Nature Geoscience on 15th February 2016.

 

Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics