Geophysical Surveillance Methods of Active Volcanoes

Michael Wack
8.6.2004

Contents

Some general remarks about monitoring of volcanoes

Geophysical Parameters

What are the reasons?

How can it be measured?

Why is the result useful?

References

1. Some General Remarks about Monitoring Volcanoes

Why?

To predict eruptions and to protect people in this way

To understand how volcanoes work

It's important to survey a volcano for a longer time in order to be able to detect changes of parameters. An absolute value says nothing, but its change in time a lot about processes inside a volcano. That's why it's generally called monitoring and not measuring.

To have a good chance to predict anything at all it's reasonable to observe as many parameters as possible, since it is not sure that every parameter reflects a forthcoming eruption.

The time we have to observe a volcano is very short in comparison to its lifetime. This makes it complicated to understand long term activities.

For predicting eruptions it's profitable to know the “history” of a volcano and its characteristics.

2. Geophysical Parameters

Seismic activity

Deformation of the surface

Thermal variations

Electrical variations

Magnetic variations

Gravitational variations

Seismic Activity

There are different types of seismic events with different causes

“normal” earthquakes, not directly related to the volcano

earthquakes originating from breaking rocks due to the pressure of ascending magma (picture on the right)

surface events, such as tephra events, rockfalls associated with dome growth, and snow and rock avalanches from the crater walls

harmonic tremor, which is a long-lasting, very rhythmic signal whose origin is not completely understood but probably comes from the flow of magma through cracks in solid rocks (duration minutes to days)

Seismic Activity

Measuring instruments

Seismometers

Acoustic flow monitors (AFM)

Benefit

Volcanic tremors are one of the important indicators for an eruption in the near future

With multiple seismometers it's possible to locate the source position of the signals and probably to track the way over time

Since no S-Waves propagate through magma, you can locate magma chambers

Deformation of the Surface

Deformations have reasons similar to seismic events. The ascending magma expands the volcano. This leads to an increase in the tilt of the slopes, which is used to predict eruptions.

After an eruption a volcano normally deflates

Measuring methods

Tilt Meters

EDM

GPS

InSAR

Mount St. Helens 1982: line is about 1m long, deformation in 2 days

Tilt Meters

An often used construction works like a spirit-level with a conductive fluid and electrodes to determine the tilt.
Basically it's like a voltage divider.

How sensitive are tilt meters?
Good tilt meters measure the amount of tilt in microradians, which is the angle turned by raising one end of a beam one kilometer long about 1 mm (equivalent to 0.00006 degree!).

Satellite Radar Interferometry

Satellite images of the area recorded before and after the deformation can be combined to generate a colorful pattern of fringes representing the phase shift. One fringe corresponds to the the half of the used wavelength (~3cm).

Advantage over other techniques is that you get a continuous picture and not only data from some selected points and that you don't have to install any equipment on the ground.

With multiple pictures like this it's possible to calculate an animation of the ground deformation over time.

Satellite Radar Interferometry

Movement of Mt. Etna

Visualized with data from the ERS (Earth Remote Sensing) satellites of the ESA (European Space Agency)

Electronic Distance Measurement (EDM)

Works with infra-red laser beam near the visible spectrum

Reflectors are positioned on the volcano

From the round-trip travel time of the light the distance can be calculated

Measuring Distance: ~ 1-50km

Accuracy: a few millimeters

EDM at Kilauea Volcano

Global Positioning System (GPS)

The absolute positions given by the GPS-System aren't enough adequate

By comparing the signals (especially their phase shift) from receivers in different locations it's possible to determine the relative positions very accurately (~ 3 mm)

In contrast to EDM you don't need a direct line of sight between the stations.

Thermal Variations

Sources

Magma near the surface

Hot gases

Measuring

Directly with sensors at the ground

Indirectly through the temperature of water sources or changes in the coverage of snow

Infra-red pictures taken from satellites, aircrafts or from the ground

Benefit

An increase in temperature normally is a hint of magma approaching the surface. So it is common that a eruption will occur at a “hot point”.

A hot pan and Stromboli in infra-red

Electrical, Magnetic and Gravitational Variations

Sources

Movement of magma generates magnetic anomalies

Filling or deflating of holes in the ground influences gravity

Changes of the conductivity affects the results of geoelectrical measurements

Measuring

With magnetometers, gravimeters and voltmeters

Benefit

With numerical models (finite elements) one can try to reconstruct the internal structure and processes of the volcano

Merapi: numerical model, measuring data and reality

3. References

Hans-Ulrich Schmincke: “Vulkanismus”, Wissenschaftliche Buchgesellschaft, Darmstadt 2000

Jacques-Marie Bardintzeff: “Vulkanologie”, Ferdinand Enke Verlag, Stuttgart 1999

Ollier Cliff: “Volcanoes—An Introduction to Systematic Geomorphology”, MIT Press, Cambridge MA (USA) and London (UK) 1975

http://vulcan.wr.usgs.gov/Monitoring

http://www.educeth.ch/stromboli/beso/pdf/monitoraggio-stromboli-en.pdf

http://volcanoes.usgs.gov/About/What/Monitor

http://www.esa.int

http://www.geo.uni-leipzig.de/~geosf/merapi/

http://flir.images.alaska.edu/

Gas temperature: ca. 150-800 °C, sometimes >1000 °C

Geyser: a lot of water => it does not evaporate completely in the underground

Christoph Moder

2004-05-13T11:55:31

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