Seismic wave measuring devices

Details Seismic wave measuring devices Seismic wave measuring devices

2002-08-15

2004-11-30

Lobo, Ian J. (Department: 3662)

Acoustics

Geophysical or subsurface exploration

Seismic wave detection

C367S014000, C367S178000

Reexamination Certificate

active

06823963

ABSTRACT:

The invention provides an improved device for the measurement of seismic waves.
A large number of victims and enormous economic losses due to the destruction of buildings and industrial plants are caused by natural catastrophic events like earthquakes, landslides provoked by geological instabilities and rock impact caused by mining and large excavation works.
At present seismology does not reliably predict earthquakes. The instrumentation like seismometers and accelerometers, presently used for monitoring such events have been mainly developed in view of measuring only one engineering parameter which is the acceleration. This is used by engineers to calculate the load acting on the earthquake resistant structures using the estimated mass of the structure. Furthermore the present seismometers fail to record with good accuracy the peak values of acceleration, displacement, speed, rise time caused by strong motions in the critical near field of strong earthquakes (P. C. Jenkins, “Engineering Seismology” from “Earthquakes: Observation, Theory and Interpretation”, Editors: H. Kanamori and E. Boschi, North Holland, 1983).
The lack of precise measurements of seismic waves parameters (including peak values of acceleration, displacement and speed) impedes the consistent design of buildings and industrial plants resistant to strong earthquakes in the near epicentre area; this fact has been demonstrated by the collapse of important constructions in the recent strong earthquakes. For a more consistent design of buildings, it would be of an advantage to be able to measure directly the seismic waves acting on the buildings. This is because the acting load can be calculated with more accuracy knowing the extent of the foundations.
It is well known that a reliable scientific prediction capability of earthquakes does not exist. This is tragically well demonstrated by the lack of preparation by authorities and populations during the last big earthquakes (Friuli, Mexico City 1985, Aegion 1995, Northridge 1994, Kobe 1995, Umbria 1997, Turkey 1999) which is one of the reasons for the large number of victims.
Some empirical attempts to predict earthquakes were done in Greece based on the change of electric current in the soil some days before the earthquake, and in Japan based on the change of chloride and sulphur content in mineral water but these were largely unsuccessful.
Earthquakes are generally caused by the energy release from the fracture/slip of large geological material masses; the seismic energy release takes place through seismic wave propagation. It is known that large fractures are preceded by small fractures which give rise to lower levels of energy release and to low amplitude seismic waves. They are the precursors of large earthquakes and to predict earthquakes, they must be resolved from the usual continuous microtremors of the earth.
The present seismometers have a too low signal to noise ratio due to environmental conditions (for example wind, change of ambient temperature, pressure and humidity) and earth tremors of other origin to give reliable records of precursors. Therefore the correlation of the seismic wave characteristics with the different phases of the fracture process of geological masses based on the records of present seismometers is too inaccurate to be able to predict earthquakes.
Damaging effects of strong earthquakes on civil engineering constructions are limited to several tens of kilometers (P. C. Jennings “Engineering Seismology”) from causative faults. Outside of this range, the motion is still perceptible, but is typically associated only with non-structural damage. One consequence of this situation is that details of records from seismographic instruments are not normally of much significance in engineering seismology.
One reason is because seismographs go off scale in the near field; they are on scale only when the motion is too weak, and usually too dispersed to be greatly informative about the strong shaking. A second reason is that the natural frequencies of the transducers of most seismographs are so low that the measurement of the high frequency ground acceleration in the near field of strong earthquakes from the records is a difficult and inaccurate process. Therefore, there is a lack of knowledge of the amplitude, duration, frequency content and extent of strong shaking.
Furthermore, there is a shortage of records in the very near field (e.g. &Dgr;<20 Km), where for a period less than one second the ground speed can reach values of 10 m/s, imposing very large displacement on the building structures.
The high frequency content of seismographic records is important for progress in earthquake engineering and in strong earthquake prediction because strong motions depend crucially on the detail of the fault rupture mechanics. The measurement of the short rise time from high frequency accelerograms is very inaccurate because of the noise and high variability.
Also to monitor seismic waves in the ground caused by landslides, volcanism, mining rock impact and instability of large civil engineering constructions (e.g. dams), a device is needed which is able to record the wave parameters like rise time, pressure, displacement, ground speed both in the case of low amplitude waves and in the case of peak amplitude waves. It is expected that in these cases the effectiveness of disaster prediction will increase faster than for earthquakes because the correlation of the measured wave parameters with the state of the wave source can be established and verified in a much more reliable way.
It is an object of the invention to provide a new monitoring instrument allowing the accurate measurement of all the seismic wave parameters which can be better correlated with the source of the catastrophic event (fault fracture process) to enable progress in the prediction of such catastrophic events.
According to the invention there is provided an instrument suitable for measuring seismic waves in an environment which comprises a wave guide through which a seismic wave may propagate substantially without dispersion wherein the wave guide is provided with means for measuring its deformation.
As a result of the seismic wave propagating through the wave guide without dispersion, there is no modification in the shape and amplitude of the wave which allow its properties to be measured along the length of the wave guide.
According to the invention there is further provided use of an instrument according to the invention to measure a seismic wave.
The wave guide is preferably a bar mounted in a cavity in the environment. The mechanical impedance of the wave guide is preferably substantially the same as the environment in which the instrument according to the invention is placed.
The substantial matching of the mechanical impedance of the wave guide and of the environment is preferably achieved by choosing the bar material and the geometry of the cavity and of the bar such that:
&rgr;
1
C
1
A
1
=&rgr;
2
C
2
A
2
where:
&rgr;
1
and &rgr;
2
are the density of the environment and of the bar, respectively;
A
1
and A
2
are the cross-sectional area of the cavity and of the bar, respectively
C
1
and C
2
are the elastic wave velocity in the environment and in the bar, respectively.
As a result of the substantial matching of the mechanical impedance of the wave guide and of the environment, an improvement in the undisturbed entrance and propagation of the seismic wave in the wave guide is produced.
The bar used in the wave guide is preferably metallic (e.g. aluminium). The diameter and length of the bar and the cavity generally depend upon the physical properties of the environment in which the wave guide is placed. More preferably the dimensions of the bar are of from 100 to 150 mm in diameter and from 500 to 750 mm in length.
Noise sources coming from known directions are preferably suppressed by carefully aligning the axis of the bar so as to minimise their effect.
The bar is optionally hollow, e.g. in the form of the tube, in order to obtain improved impedance matching.
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