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Charakterystyka częstotliwościowo-czasowa drgań sejsmicznych wywołanych wstrząsami górniczymi

Marcak H., Kłosiński J.

Archives of Mining Sciences

2004

Vol. 49, nr 2

193-204

Wstrząsy sejsmiczne wywołane pracami górniczymi mogą być źródłem uszkodzeń budynków w rejonach górniczych. Prowadzi się ciągłą rejestrację drgań sejsmicznych wywołanych wstrząsami górniczymi. W artykule pokazano, że istotnym parametrem, który wpływa na wielkość wzmocnienia drgań przez konstrukcje budowlane, jest długość trwania wzmocnionej części widma sygnałowego, którą można wyznaczyć z transformacji Stockwella sygnałów sejsmicznych. Taka zależność dotyczy zarówno rejestracji sejsmicznych pochodzących od trzęsień Ziemi jak również od wstrząsów górniczych.

Generally, Poland is aseismic country. Damages of buildings in our country are caused mainly by mining tremors related with sudden upsetting of equilibrium in exploitation area. Seismic oscillations sometimes have energy from […] Joules. Influence of these tremors on building objects is described in nurnerous scientific publications (Dubiriski, Mutke 1998; Maciqg 2000; Lasocki et al. 2000; Cianciara et al. 2000; Dubinski. Mutke 2001; Mutke et al. 2002). There is a similarity between reacting of buildings on seismic forcing caused by earthquake and mining tremors but there are differences resulting from different structures of seismic signals of these two sources. Frequency band, in which the oscillations caused by these different sources propagate, is shifted (seismic oscillations from mining tremors propagate in higher band), seismic signals from these tremors are shorter and less energetic. There are difficulties in assesment of seismic oscillations effects in the mining areas. Especially, it concerns Copper Basin where appear very strong tremors under the urbanized area. There is continuous recording of seismic oscillations in this area. Measure points are 3-component (vertical Z-component, horizontal X- and Y-component) stations installed on the building in Polkowice. Between other places they are located on: [...] floor, in the pillar and in the ground near the building on Hubal Str. These records were used in presented calculations. These records were from 2002, February 19 at 9:33 pm. in the distance 1236 m from tremor focus with seismic energy 1,6 x […] Joules. We will show that assesment of seismic oscillation effects on the base of maximum value can be inappropriate. To investigating the changes of seismic signal spectrum can be used the method proposed by Stockwell (1996). Calculations of local spectrum is conducted in Gaussian window, which width depends from parameter [tau]. This spectrum is assigned to time instant, in the middle of time window. By shifting this time window two-dimensional image of frequency-time structure can be obtained. On the figures 3a, 3b, 3c are showed records in the ground, on pillar and […] floor, respectively on the building on Hubal Str. in Polkowice. These records concern X, Y, Z components. Looking on these distributions, it can noticed that the value of maximum acceleration in the ground and pillar is not connected with value of acceleration on the 12th floor. Ratio between acceleration of Y-component on the […] floor to the pillar is almost equal to 1,7 but for Y-component this ratio exceeds 3. For Z-component this ratio is equal 0,7. Maximum acceleration in pillar is twice lesser than in the ground for Y-component and 3,5 times bigger for Z-component and almost as big as for Y-component. This diversification of maximum accelerations in the ground indicates that quantity can't be the basis to assesment of deformation degree of building under the influence of seismic oscillations. Principal element which differs these results from records in the ground is occuring relatively strong anomaly in Stockwell transform of X-component in the frequency range below 5 Hz. This strong effect goes on relatively long in records from pillar and on the […] floor (from 9 to 12 seconds). The strongest effects in the Y-component transform in the ground occur at frequency 8 Hz (they are longer on records from pillar). Maximum intensity of Stockwell transform anomaly for Z-component is twice bigger than X-component for the ground and […] floor about 30 times) and occurs in the 20-25 Hz frequency range. For X- and Y-components intensity of high-frequency oscillations recorded on the ground is approximately equal to pillar. But for low frequencies of records from pillar are stronger in compa-rision with those from the ground (maximum intensity for Y-component in the pillar in the range of a few Hz arises several times in comparision with the ground). In the case of Z-component we observe amplification in high-frequency range what can be result of elimination of attenuation in the nearsurface ground layer. It must be given attention to the fact that intensive elements of Stockwell transform for Z-component in the pillar are rather short. It appeared that only oscillations length in the low frequency range has the influence on the oscillations magnitude on the 12th floor. X-component, which was the weakest in the pillar, was strongly amplified creating in the records on […] floor the structure, which can be dangerous for construction. Whereas the strongest oscillations in the Z direction are scaterred by building. This example seems to confirm the conviction that Stockwell transform of seismic records allows much fully to estimate the influence of oscillations on the buildings. Showed example points that effects caused by both earthquakes and mining tremors depends considerably on the length of low-frequency seismic signals characteristics and this parameter should be taken into account in the assesment of the risk of desctructing the buildings under the influence of seismic waves.