The measurement in seismology is fundamental that it is for the study of the wave propagation that for the study of the seisms. Indeed, the study of a seism passes by the study of the processes in action on the fault before and during the seism. But a direct observation of this object as a whole is not possible. The only possibility for the moment is drilling but it is a very expensive solution and it allows only one specific observation of the fault plane. It is thus necessary to resort to indirect observations, the first being the waves generated by the seisms. These last can be indeed recorded even with the other end of the ground in the event of important magnitudes. These waves with their passage make move the ground. It is this movement which is recorded thanks to sensors called seismometers.
A little story
Modern measurement
The seismometer
The problem of seismological measurement comes owing to the fact that the Capteur, called génériquement seismometer, is fixed at the object moving (ground). The basic principle is a mass magnetized associated with a pendulum or a Ressort. The movement of the mass is deadened in order to reduce the duration of oscillations (system introduced by Emil Wiechert in 1898). The seismometer more diffused has a Bobine which surrounds the mass. The movement of the latter thus creates a Electric current whose tension is proportional to the Speed ground. This type of seismometer is called electromagnetic and was proposed for the first time by Galitzin in 1914.The seismometers recording the speed of the ground are called velocimeters. Another type of sensor is as for him sensitive to the Accélération of the ground and is called accelerometer.
A seismometer must have a linear answer stable in time. But in the event of strong movements of the ground, the seismometer quickly shows problems of nonlinearity. This is why the majority of the modern seismometers are brought under control. The principle is to maintain the mass always motionless by injecting a current in a Bobine. This type of instrument is linear and has a higher dynamics (better sensitivity and Band-width broader). With such a seismometer, the mass does not move practically any more. The recorded data are not thus any more the movements of the mass but the current being used to compensate to see cancelling the movements.
The dynamics of the seismometer is a very important argument because it must be sensitive to a very varied range of signals as well in frequency as in amplitude. The normal modes of the Ground arrive until periods of 53 min with displacements of about .02 Nm for the very large seisms. Weak a Téléséisme generates waves of surface about the µm for frequencies of about .05 Hz. But the waves of surfaces associated with a earthquake with magnitude higher than 9 have amplitudes about the cm for observers on other side of the Ground. The telesismic waves P appreciably have same dynamics but for frequencies a little higher (between .1 and 1 Hz). When the seismometer is close to the source, the problems of measurement multiply. Displacement can be metric, associated with deformations (rejection of the Faille) permanent (null Fréquence) and the Accélération with the ground to exceed the terrestrial Gravité for frequencies around 10 Hz (if the Accélération is vertical and of sign opposed to gravity, the objects take off of the ground).
Seismic station
The Capteur only is not sufficient to record the seismic waves. The measurement of the movement of the ground is not a specific measurement in time but continuous. The first instruments were associated with mechanical systems which generally provided recordings known as Analogique S on paper. These instruments were called Sismographe S. the signals showing the temporal variation of the movement of the ground on paper or any other visual aids as for them are called Sismogramme S.Today the electrical signal delivered by the sensor passes by a analog-to-digital Converter which samples the signal according to a step in constant time. The current converters used in seismology serve for the majority of techniques of oversampling (2000 samples a second which under is then sampled) each sample being coded on 24 bits. Thus a whole of signals coming for example from a seismometer says short period (optimal answer around 1 Hz) made up of 3 components (two horizontal, a vertical), sampled with 125 samples a second, coded on 24 bits generate almost 100 Mo data per day.
But only one type of sensor is not able to be sensitive to all the types of waves. The modern seismological stations are thus equipped in general with two, to see three types of different sensors in order to be able to cover all the dynamics of the seismic waves.
In addition to the general problems associated with physical measurements on the ground like the Power supply (the retroactive sensors or the system of recording need to be fed) or the transfer of the data (the satellite transmission by is more and more employed but costs very expensive), two specific problems are related to this type of measurement: the Synchronism and the Insulation. In order to locate the epicentre of a Earthquake, it is necessary to have the reading of the time of arrival of the waves at least to three different seismic stations. It is thus imperative that the temporal reference is the same one on each station. In a still recent past, the Horloge intern of the seismic station was synchronized thanks to signals minute emitted by radio (for example the signal DCF77 for Western Europe). The modern seismic stations are synchronized by using the signal GPS.
The ground moves permanently. The Wind which make vibrate the Végétation or the structures, the Mer or the anthropic activity inter alia things generate movements of the ground permanently called seismic background noise. To have a seismic station of quality, it is important to have a weak seismic background noise. The best means of limiting this noise is to be held to move away from their potential sources and also to bury the sensor, to see installing it in a gallery. This last type of installation has also the advantage of reducing the variations of pressure and of Température which can involve drifts on the answer of the sensors.
Seismic network
The use of several stations makes it possible to locate the Hypocentre seism.
The seismic wave arrives at one moment tA at the station has , and at the moment tB at the station B . The propagation velocity of the signal in the ground being known, this makes it possible to determine the difference between the distance from the hypocentre H at the station has and hypocentre at the station B outdistances it:
- is known.
See the article Triangulation.
References
Internal bonds
External bonds
- IRIS - World network inter-institutions
- MedNet - Mediterranean Network
- CSEM - Euro-Mediterranean Seismological Center (federation of European networks)
- Géoscope - French World network
- RéNaSS - seismic Domestic network of monitoring French
- DASE - seismic inspection Network of the Commissariat à l'Energie Atomique
- SDN - Swiss seismological Network
- ROB - Belgian Network
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