瞬态滤波器设计及其在地方地震观测中的应用研究

The Design of a Transient Filter and its Application in Local Seismic Observation

摘要:

目前地震数据采集器使用的最后两级数字滤波器相位有最小相位和线性相位两种。相位特性直接影响着记录的地震波形数据质量，最小相位滤波器的相位特性非线性严重，引起波形失真，线性相位滤波器带来“前缀波”。本文介绍了一种专用于地震波形采集的瞬态滤波器的设计及其观测分析研究。该瞬态滤波器既有最小相位特性，又有较好的线性相位特性，具有良好的瞬态响应波形。地方震实测数据表明，使用瞬态滤波器记录的地震波，没有"前缀"波出现，初动半周期也没有被展宽，波形失真最小。因为通带至过渡带转角平缓和过渡带滚降速率低是所有瞬态特性良好的滤波器的共同特征，故使用瞬态滤波必须付出的代价是提高采样率来确保通带上边沿频率保持不变。

Abstract:

At present，the phase of the last two stages of the digital filter in the earthquake data acquisition system can be divided into two types： minimum phase and linear phase.Phase characteristics directly affect the quality of recorded seismic waveforms. Important nonlinear characteristics of the minimum phase will lead to wave distortion，and the linear phase filter will result in an extra wave in front of the original wave.Based on the analysis of the two types of filter，a new transient filter is designed with the characteristics of both minimum and linear phases.The transient filter belongs to a finite impulse response filter （FIR），which consists of two cascaded stages，FIR1 and FIR2.The FIR1 filter stage has a decimation ratio of four.It compensates for previous filter droop and flattens the magnitude response of the pass band.The FIR2 filter stage has a decimation ratio of two. It creates a low pass brick wall filter to block aliased components of the input signal. In this paper，we introduce the detailed design method using Filter Design Toolbox and Signal Processing Toolbox in MATLAB and the characteristics of the filters，and we compare the testing data are introduced in this paper.The testing results of the seismic wave recorded in a number of local earthquakes show that the extra wave is eliminated and the first half period is not broadened.Thus，wave distortion can be minimized.The smooth interaction angle between the pass band and transition band and the slow roll off rate in the transition band are the common features of all transient filters.As a consequence, the sampling rate of the earthquake data acquisition system using the new transient filter must be increased to maintain the same frequency as that of the pass band edge.