Sizing up Seismometers
I drove down to North Devon to Hartland Point with a friend Charlotte, to see a seismometer in action.
I wanted to find out what I was hearing with the microphones I had put in the ground in Bristol? (Piezo and AKG pick-ups and a geophone).
What was I hearing from them? How could I find out where the sound was coming from? The understanding of this long distance low frequency microphone would help me find out.
Hartland Magnetic Observatory
The seismometer in it’s case in the ground 6 feet down on bedded on solid rock
I was right about seismometers picking up sound waves traveling long distances from other continents, which was mind blowing, but surprisingly even thelocal dog walkers had an effect on the seismometer. This was strange, but of course they were making their own mini tremors with their foot -falls.
Here’s the data read from the seismometer – showing a blob on the left
I stamped my foot and the seismometer reacted instantly even though it was out side the building 60 METERS AWAY!!
This clever instrument could pick up the quieter range of sounds in the ground. It would be capable of monitoring 0.01Hz-100Hz, and the (inaudible to human ear –which hears from approx 20Hz) natural Hum of the earth (0.02Hz- 7 Hz).
And this being lower than the cultural man made sounds from cities and trains (in particular) that start at approximately 1Hz.
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While I was getting my head round the image of a sound wave making it’s way to effect this instrument, in a box 6 foot under the ground, from several thousands of miles away, I felt the mighty physicality of the sound wave traveling over such a distance was certainly a planetary scale event.
I start to think of earth as a lump of humming, quaking, vibrating buzzing rock.
The work by artist Floriain Dombois, puts this global phenomena into context with pictures of measuring devices in sheds and compounds and seismic stations from different 60 countries. (Seismic Stations Global seismographic Network (GSN) 59 of them.
Up in the Edinburgh the British Geological Society monitor the signals that get picked up from round the UK, those from the UK and those that have traveled from other continents. With data from different station’s the origin of the tremor can be plotted. Primary, Secondary and surface waves, and their different trajectories through and round the planet are compiled and compared with data from other countries. We hear the effects of these forces more often on the news, from places that suffer human tragedy from the destruction caused by the planet.
Here’s pictures of sound through the earth’s mantle: http://en.wikipedia.org/wiki/File:Earthquake_wave_paths.svg
Here’s some big tremors: http://www.iris.edu/seismon/
Here are some recent tremors: http://www.iris.edu/seismon/last30.html
Here are seismograms at different stations: http://www.iris.edu/cgi-bin/wilberII/wilberII_EnO_page4.pl?evname=20110122_172924.4.spyder
That afternoon we learnt that forces effect our planet -not only vibration working from the middle of the planet: from the movement of tectonic plates (making quakes and tremors), but also the geomagnetic forces from round the outside of the ionosphere working in.
It was described to me recently that we, on the surface of our planet, are like innocent children or kittens, to whom things happen. In this case vibrations and forces from outside our local view.
We noticed the amazing display of Heath Robinson looking gadgets/instruments all over the station.
Although we came to see the seismometer, the observation station mainly monitors geomagnetic forces, measuring slight changes in the earth’s magnetic field, and has done for over 100 years (I found a bound record book dated1865). They moved from near Kew Gardens / London in 1957 when the train line was built and started to interfere with the instruments.
Records were meticulously kept for posterity, hand drawn in dusty volumes reminding me of nautical maps of uncharted seas.
On the head land with the on shore breeze, Steve, the manager who kindly showed us around the Observatory, describes how the latest magnetometer no longer needs a whole building, being small, digital and self heating.
I felt inspired to use the geophone we had brought, and was advised by Steve, that a good place to listen to natural sound, was through rock, and the ancient granite rock of the cliffs and harbour wall down the road at Hartland Quay.
Our geophone is an SM24 that picks up signals from 10Hz-240Hz.Like the seismometer has a capability of 2D and and 3D surveys, picking up sounds from all directions, and has a different mechanism to the seismometer. Have a look at it
It turned out we would have heard the sound of the sea, pounding on the ocean floor off Hartland Point, as it added to the natural din of sound in the ground. I never had thought of the sound of the sea emanating INTO the ground before, and even, I fancy, mixing with the earth’s natural low hum, and maybe the sound of more localised quarries and train lines, and low frequency signals from much further a-field, but unfortunately we found we had run out of time to make some good recordings and had to return home, so leaving our recording for another day.
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