Imaging Magma Under St. Helens

Here are some reports from the field for the active experiment.  This blogger was not part of this work and thus is reporting only what I think went on.  I apologize for any inaccuracies.  I like to make things up sometimes.  A very large crew has been working like mad for over a week but a few members had time to take some photos as things progressed.

 

Lots of meetings for explanations and training were held.  Here all attend a safety briefing.

 

 

The seismic recorders are RT-125 ("Texans") that will run for several days on C-cell batteries.  The recorders must be loaded with these batteries and then programmed and have their clocks properly set to provide accurate timing of the data.  Fifteen recorders are packed into a sturdy case for transport.  One team of two two people may have two cases (30 instruments)  to install in a day.  Often this can be done from a vehicle along a back road, but sometimes it means loading the sensors and recorders in backpacks and hiking trails or even cross-country installing an instrument every few hundred meters.

 

 

 

One may recall that it was quite cool and wet at the beginning of this past week.  Indeed, there was plenty of rain during the field deployment on at least one day.  Here two teams are together in the mist and fog hoping that they are going in the right direction.

 

About 2,500 individual instruments were deployed during a three day stretch.  Here is a map showing the locations of fewer than half of the sites.  The station density is the highest right in the vicinity of the volcano and is stretched out in a NE-SW line along which the shots for this deployment are located.

 

 

 

 

 

Late Wednesday evening Alan Levander, the chief scientist for the active experiment, proudly points out seismograms showing up in near realtime on PNSN telemetered stations, from the first two shots. The crew of staff and students who were in attendence at the instrument center broke into a cheer.  After years of planning and fund raising, months of logistics, weeks of organization and preparation work and days of deployment work, it is quite heartening to see the first data role in.

 

Over the next few days the teams again take to the field to retrieve all of the instruments. The data are down-loaded from them, the batteries replaced and then, next week its back to the field to deploy them yet again in a whole new pattern, this time stretched in a SE-NW line.

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Late addition - some data just downloaded and plotted up for a quick check.

Here are some plots (record sections) from a few of the stations from one line.  On the left are the recordings of a shot and on the right those of an earthquake.  Record sections such as these show the traces from each station on a vertical axis with time increasing from top to bottom.  The traces are ordered by their position in a line.  The first arriving waves are from the P-wave or fastest traveling compressional wave.  Later arrivals are from the slower velocity S-waves (shear waves) .  Because explosive shots don't generate much shear enery there are very weak S-waves on the record section for the shot compared to that for the earthquake.  With sophisticated data processing and many more such records a hazy image of the subsurface geologic structure (possibly including partially molten rock) can be made.