Sunday Seismometer digest

Just a quick post to announce the appearance of the Sunday seismometer digest, a pdf version of the Sunday Seismometer series.

Sunday Seismometer #12

Rocard (1958)

From last week's prototype, let's move on to a more useful set of instruments, designed specifically to detect nuclear explosions in the context of Comprehensive Test Ban Treaty monitoring.



They are named after Professor Yves Rocard, the physicist who started to develop detection seismology in France in the 1960s, and who founded the division of the French Atomic Energy Commission (CEA) that is in charge of geophysical studies and activities associated with monitoring and the environment (LDG).



The Rocard is a classical electromagnetic seismometer, with a 1s natural period and electronic amplification. Rocard instruments were in operation at the Welschbruch station not far from Strasbourg.

Sunday Seismometer #11

Peterschmitt (1950)

Continuing our mini-series on electromagnetic seismometers (see the Galitzine and Press-Ewing posts), here is a seismometer you are unlikely to see anywhere else. The Peterschmitt was designed and built in Strasbourg in 1950, where it was in use until 1975.



This admittedly ugly looking beast is a prototype classical electromagnetic seismometer (you can see its coils on the near side, very similar to those on the Galitzine instrument) combined with a galvanometer. It has a natural period of 1s, and its amplification is provided by a resistance bridge. The most interesting feature of this instrument is its original inbuilt calibration system.

The design of this instrument is attributed to Elie Peterschmitt, who was recruited by Strasbourg in 1937, took charge of the Strasbourg historical seismological station as well as the stations of Besançon and Bagnères de Bigorre, and later helped develop the European-Mediterranean Seismological Center (EMSC).

Sunday Seismometer #10

Press-Ewing (1953)

Some 40 years after the Galitzine electromagnetic innovation, the same principles of operation are put to work in the Ewing-Press seismograph, built at the Lamont Geological Observatory of Columbia (now the Lamont-Doherty Earth Observatory) by Maurice Ewing and Frank Press.

In the photo below you can see the vertical Press-Ewing instrument on display at the Strasbourg Seismological Museum. It was in use in Strasbourg from 1963 to 1975.



It is an electromagnetic seismograph, coupled with a galvanometer, and has a natural period that can be selected and fixed up to 30s. Recording was optical, on photographic paper. The glass ball you can see on the near side of the instrument reduces the effect of variations in atmospheric pressure on the seismograph recordings, using the Archimedes principle.

This seismograph and its horizontal counterparts are very well adapted for the recording of surface waves. In 1957-58, Press-Ewing instruments were deployed in 125 locations around the globe to establish the World-Wide Standardized Seismograph Network, the first global earthquake monitoring system.

Sunday seismometer #9

Galitzine (1910)

All the seismographs we have discussed up to now (Reuber-Paschwitz, Reuber-Ehlert, Wiechert horizontal and vertical, Mainka, Vicentini, 19-Ton, Mintrop) have been mechanical, with either mechanical or optical recording. Today's instruments, built by Galitzine in St Petersburg (Russia) in 1910, are the first examples of electromagnetic seismometers.


In the above photograph of the vertical Galitzine (mass 10 kg, period 24 s) you can see the new element of this seismometer: the coil placed at the end of the pendulum's rod. This coil oscillates in a magnetic field, and creates an electric induction current which can be measured using a galvanometer.

A copper plate, fixed on the same rod as the coil, oscillates in the field of a second magnet and provides damping via a Foucault current.


The horizontal instrument (above, mass 7 kg, period 12 s) works using the same principle. The object placed in front of the seismometer is a galvanometer that is equipped with a mobile frame and a mirror for optical recording.



The Galitzine instruments amplify Earth motion in two successive stages: an electromagnetic amplification (the galvanometer mirror rotates more than the pendulum oscillates) followed by the optical amplification caused by the distance between the galvanometer mirror and the recording medium.

Sunday seismometer #8

Mintrop (built sometime after 1910)

From the very large (last week's 19-ton seismograph) to the relatively small : the Mintrop portable horizontal seismograph.



The Mintrop is an odd instrument, that measures horizontal motion using a damped inverted pendulum with a horizontal rotation axis. Its relatively small mass is coupled with a vertically oscillating mirror and an optical recording system.



Given the delicate nature of the recording system, the Mintrop must have been rather difficult to install. It is considered to be one of the first portable field instruments, and was used for early prospection studies by German oil companies.

Sunday seismometer #7

Great Pendulum or "19-Tons"

In the last Sunday seismometer post on the Vicentini seismograph, we mentioned that in order for a seismograph to overcome the friction caused by a purely mechanical recording system, it needs a large mass.

The Vicentini instruments actually have the smallest masses (100 kg for the horizontal and 50 kg for the vertical) of the mechanically recorded seismographs we have described so far. The Mainka instrument has a 450 kg mass, the Wiechert horizontal instrument has a 1-ton (1000 kg) mass, and the Wiechert vertical instrument has a mass of 1.2 tons.

The largest mass of all the seismometers in the Strasbourg museum is that of the Great Pendulum: an impressive 19 tons (that is 19 000 kg)!



Its construction was started before the First World War (1910), when the Strasbourg Observatory was part of Germany. The idea was to build an instrument that would be similar to one installed in Göttingen, a 17-Ton seismograph. After the war, Strasbourg became French, and it was the French director of the Observatory, Edmond Rothé, who completed the construction of the Great Pendulum in 1925.



The mass itself is essentially made up of scrap metal from the War, including 12 tons of axles from military trucks and 2 tons of weapon parts.

The 19-Ton has a natural period of 2 seconds, and records both the horizontal directions of motion, like the Wiechert horizontal instrument. Also like the Wiechert, its motion is damped by air pistons.



The smoked paper recording system was abandoned in 1970 in favor of galvanometric recording. In 1987 the recording system was changed once again to digital recording using displacement detectors.

The 19-Ton instrument is still in working order today, and is a great favorite with visitors to the Strasbourg Seismology Museum.

Sunday seismometer #6

Vicentini (1895 and 1899)

Let's go back to the early days, and take a look at a contemporary of the Reuber-Paschwitz and Reuber-Ehlert seismographs. The Vicentini instruments are simple, un-damped pendulums, one to record the two horizontal components of motion, and another to record the vertical component.


The clock at the back of the image above is a precision chronometer that was used to produce the time base for the seismic recordings at the Strasbourg Seismic Observatory.

Built in 1895 and 1899 in Padova, Italy, the Vicentini seismographs had low sensitivity and were designed to record strong motion from local earthquakes. Their natural period is 1-2 seconds.



Where the Reuber instruments were lightweight (200 g masses) and used optical recording to reduce friction, the Vicentini instruments had heavy masses (100 kg for the horizontal and 50 kg for the vertical) and used a mechanical recording system. In order for a seismograph to overcome the friction caused by a purely mechanical recording system, it needs a large mass.

We no longer have the Vicentini recording systems. The horizontal system was installed under the pendulum mass and recorded the two components of motion and a time mark on a single sheet of smoked paper.

The two Vicentini seismographs operated in Strasbourg from 1895 to 1907.

Sunday seismometer #5

Mainka (1910)

The Mainka seismograph is a large, single component horizontal pendulum (two instruments installed at right angles to each other are required to fully describe the horizontal ground motion).



Its 450kg mass is suspended in such a way that it oscillates around a near-vertical axis, with a natural period of 8-10 seconds. Damping is provided by a dash-pot system (a plate moving through a viscous fluid). Seismograms are recorded on smoked paper via a mechanical stylus, as was the case for the Wiechert seismographs.



Various models of the Mainka instrument with differing masses were built. The model shown in the photo above and visible in the Strasbourg Seismology Museum was manufactured by the Society for Optics and precision Mechanics of Paris (SOM). It is an improved model, of average sensitivity, robust and easy to tune, and was chosen in 1925 by the French Central Seismological Bureau (BCSF) for deployment at seismic stations throughout France and in the French colonies.

A Mainka seismograph operated at the Strasbourg Seismic Observatory from 1910 to 1960.

Sunday seismometer #4

Wiechert vertical seismograph (1909)

The Wiechert vertical seismograph was built in Göttingen, Germany, in 1909, five years after the horizontal seismograph described in last week's post. The two instruments together formed a complete recording system, capable of determining the 3 components of ground motion.



The vertical seismograph has a mass of 1200 kg and a natural period of 5 seconds. Its design is more immediately recognizable by today's high-school students, as it is essentially a damped mass on a spring.



In order to reduce the temperature variations inside the instrument, which change the mechanical properties of the springs, the vertical seismometer is entirely enclosed by a metal casing, which makes it much less aesthetically pleasing than its horizontal companion. The small spring at the center of the instrument helps to correct for residual thermal variations.

The air-piston damping and the stylus and smoked-paper recording system are identical to those on the Wiechert horizontal seismograph.

The two instruments ran side by side in the Strasbourg Seismic Observatory until 1968. You can see working examples of both at the Wiechert Earthquake Station in Göttingen

Sunday seismometer #3

Wiechert horizontal seismograph (1904)

The Wiechert horizontal seismograph (built in Göttingen, Germany, in 1904) has an unusual and striking design: it is essentially an inverse pendulum weighing 1 ton, in unstable equilibrium about a universal pivot at its base. Its natural period is 8 seconds.


The horizontal motion of the mass with respect to the casing is decomposed into its two perpendicular components, North-South and East-West, as we are used to seeing in modern-day instruments. The particularity of the Wiechert is its use of the 2D motion of a single mass to measure the two horizontal components of ground motion.

The motion of the mass is damped by air pistons (see schematic drawing). Damping is used in all modern seismometers to permit recording and interpretation of seismic energy after the first arrival. In undamped instruments, the later arrivals are drowned out by the oscillations caused by the first arrival.


The recording system is mechanical: two fine points scratch out the seismograms for each component on a roll of smoke blackened paper that rotates and translates in order for a full day of recording to be contained on a single sheet. This system is not dissimilar to the drum recordings used by the World-wide Standard Seismograph Network (WSSN) in the 1970s and 1980s.

The Wiechert seismograms contain minute marks made by lifting the two recording styluses in response to an electrical impulse that could be given by a contact switch on a precision pendulum, or by any other time-keeping device.

The Wiechert horizontal seismograph was kept working in Strasbourg Seismic Observatry from 1904 to 1968. It is now visible in the Seismology Museum, which is housed in the original observatory building.

Should you wish to see a working Wiechert seismograph, you should visit the Wiechert Earthquake Station in Göttingen.

Sunday seismometer #2

Reuber-Ehlert (1895)

In 1895 Reinhold Ehlert - continuing the work of Reuber-Paschwitz whose instrument provided the first recording of a distant earthquake (see Sunday siesmometer #1) - modified the original Reuber-Paschwitz design to come up with a new instrument.



The Reuber-Ehlert seismometer has not one but three undamped horizontal pendulums weighing 200g each, and has a natural period of 12 seconds.



A mirror is attached to each pendulum, and reflects a light beam back out through the windows at the front of the instrument. The reflected light can then be recorded on a roll of photographic paper. The amplification depends directly on the distance between the mirror and the recording apparatus.



The use of three horizontal pendulums to record two orthogonal directions of motion may seem curious to us today, as does the lack of damping in both this and the original Reuber-Paschwitz seismometer.

The Reuber-Ehlert seismometer was installed at the Astronomical Observatory in Strasbourg in 1895, then moved to the newly built Seismological Observatory in 1900, where it was kept running until 1906.

Sunday seismometer #1

When I first started this blog nearly a year ago, I ran a short miniseries on my favorite historical seismometers from those on display at the Strasbourg Seismology Museum. The original series petered out after on three posts on the Wiechert, Galitzin and Ewing-Press instruments.

I am starting up the series again as a weekly feature (the Sunday seismometer) that will run over the summer months. I hope you enjoy it!

Reuber-Paschwitz (1889)

The seismogram above represents the first recording of a distant earthquake. It was made on April 17th 1889, in Potsdam, Germany, of an earthquake that occurred in Japan.

The instrument that made this first historical recording was built by Ernst von Reuber-Pashwitz.

Three years later, an identical instrument installed in the Astronomical Observatory in Strasbourg recorded another distant earthquake that occurred in Baloutchistan (a region that is shared by modern day Iran, Afganistan and Pakistan).

These two recordings mark the beginning of modern seismology.

The Reuber-Paschwitz was small (about 40cm in diameter) with a single horizontal pendulum. It seems that none of the original instruments have survived. We do, however, have a schematic drawing that you can see below.

Many of the physical characteristics of this seismometer are recognizable to modern-day seismologists: the horizontal pendulum, the three adjustable feet for leveling, the glass dome for protection from atmospheric perturbations.



The Baloutchistan seismogram recorded at Strasbourg: