A New Approach to Dense Spectrum Analysis of Infrasonic Signals

Spectrum analysis is very important in geological hazards of infrasonic signal observation systems.The spectrum of infrasonic signal is a dense spectrum which can leads to potential erroneous spectrum analysis.Hereby we propose a dense spectrum analysis algorithm combining all phase Fast Fourier Transform(apFFT)and Chirp Z-transform(CZT)to analyse dense low frequency signal.This is called all phase Chirp Z transform(apCZT).The apFFT spectrum analysis can reduce spectrum leakage,but does not enhance resolution while the CZT vice versa.The novel algorithm apCZT can suppress spectral leakage and improve the resolution at the same time.Simulation results demonstrate that the apCZT algorithm can distinguish the frequencies whose intervals are less than the ordinary frequency resolving power of Discrete Fourier Transform(DFT)and apFFT.The apCZT it is not only suitable for infrasonic signals but also in other dense spectrum analysis applications,such as voice,vibration,noise,electrocardiography,radar signals,power system harmonics and other engineering practice.

Experimental Research on the Low Frequency Wave That Radiates into the Air before the Failure of Rock

Experiments on sonic transmission show that a slabstone can directly transmit part of the energy of a wave excited by knocking or by a transducer into the air. The other part of the wave energy can generate the normal mode of vibration on the slabstone and excite measurable acoustic signals in the air. The dominant frequency is related to the size of the slabstone. These results indicate that the acoustic emission (AE) in rock also displays similar behavior if the source is shallow. It is demonstrated that with the nucleation and propagation of cracks, the dominant frequency of the radiated wave will be lower. When the frequency becomes very low, the wave can be transmitted through the rock into the air and be received by a microphone. According to the theory of similarity of size, there will be low-frequency waves before strong earthquakes because of nucleation of cracks, which can be received by special low-frequency transducers or infrasonic detectors. Before earthquakes, the mechanism of precursors could be very complicated. They might be produced by plastic creep or attributed to liquids but not brittle fracture in most cases. So the periods of the produced waves will be longer. This perhaps accounts for the lack of foreshocks before many strong earthquakes.

Low-Frequency(LF) Passive Seismic Imaging

Infrasonic Passive Differential Spectroscopy (IPDS) technology is employed for direct detection of hydrocarbons.Low-frequency seismic gives geophysicists a new seismic imaging technique to look into wealth & health of reservoir.Hydrocarbon reservoirs can be the origin of a continuous source of low-frequency seismic waves.These phenomena are sometimes called"hydrocarbon microtremors".Low-frequ-