Probing the interior of the Earth with seismometers

Angelo Pisconti – Installing seismometers in Bavaria (Germany)  AGU on Tumblr

Postcard from the field, EOS Earth & Space Science News, Vol.97, No2, 15-Jan-2016 (get the pdf and go to the second last page!).

What is a seismometer? Why are they so important?

Seismometers have a fundamental role in Geophysics because they allow us to study the Earth’s structure and dynamics just by using the energy released by the same dynamic Earth. Earthquakes themselves are the consequence and expression of an active planet; convergent, divergent and transform plate boundaries “periodically” release elastic energy.

Seismometers are devices built for detecting and recording the seismic waves released during earthquakes. These elastic waves, containing information about the propagation media, reveal both structure and composition of the Earth. Seismometers has been also deployed on the Moon’s surface and there are efforts to deploy new seismic stations on Mars. These instruments are also useful for improving our understanding on faulting processing and brittle rheology, earthquake source mechanism, and the related seismic risk evaluation. Last, but not least, they allow us to find, probe and control, mines, oil and gas fields extraction and waste deposits. Historically, they were also used for detecting and recognizing nuclear explosions.

Since seismology is the most powerful tool for probing the inner of the inaccessible Earth, installing more and more instruments and carrying out seismic-seismological experiment is a very basic forward step for an increasingly knowledge of our planet.

In December 2015, I participated to the installation of some of the seismic stations deployed in southern Germany (figure 1), in the frame of the AlpArray project (http://www.alparray.ethz.ch/home/). The aim of these experiment, which involves several countries, is to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system.

Untitled

Figure 1. Site locations of the installed seismic stations.

IMG-20151209-WA0004

Figure 2. Postcard from the field. Angelo Pisconti and Januka Attanayake both from Institut fur Geophysik (Munster, Germany) installing a seismic station in a solar park.

Installing a seismometer require a certain setup of the equipment. At first, it is important to choose, during the planning, installing sites where the noise is as low as possible. For example, railways, big urban centers and farms, close to the seismic stations, should be avoided.

Some of the seismic stations were deployed in particular sites like a 900 years old cellar in a monastery and a solar-park (figure 2), which also provides the energy necessary to maintain the seismic station active.

In order to have a reference system, in which the seismic waves are recorded, the three component sensor has to be leveled with the Z component in the vertical direction and the horizontal components oriented in the N-S and E-W directions (figure 3 on the left side). It is also important to ensure a rigid coupling between the sensor and the ground, as pictured in figure 3 (right side).

Then, the seismometer has to be synchronized in time and also linked to a GPS sensor, in order to have the exact position of the different instruments. The instruments are also equipped with a memory card (usually a USB drive) which stores the data, that is the continuous movement (displacement or velocity or acceleration) of the ground as function of the time. The waveform can be then analyzed and processed by seismologists.

Untitled3

Figure 3. Deployment of the seismic stations. On the right is shown the coupling between the sensor and the ground achieved using a block of concrete under the sensor.

Installing seismometers, whose relevance and significance has been briefly mentioned in this report, has been an interesting and new experience which is also part of the basic knowledge of a seismologist.

Written by Angelo Pisconti

Comments are closed.