首都圈数字地震台网对微弱爆破信号的检测能力

Weak explosion signal detection by the Beijing metropolitan digital Seismic Network

利用首都圈数字地震台网接收人工地震信号,进行地下结构研究具有重要意义.但人工震源释放的能量小,激发的地震波以短周期为主,因此本文较全面地研究了地震台网对短周期微弱信号(1～20 Hz)的检测能力:(1)分析了台网的背景噪声,结果表明基岩台址的地震台噪声比沉积盖层台址的地震台噪声低约13 dB,这相当于近1个震级的检测阈值;夜间的噪声比白天低约5 dB;噪声有逐年增高的趋势,2006年比2001年噪声提高约4 dB.(2)分析了在台网内进行的药量为25 kg的陆地井下爆破实验,一次爆破相当于0.69级(M_L)的天然地震,有18个地震台可辨认爆炸产生的 Pg、Pm 或 Pc 波;离爆破点218 km 的基岩台,仍可以接收到振幅只有1.6 nm 的 Pm 波.这个结果可为地震勘探实际工作提供参考.(3)研究了台网外核爆试验的信号特征,2006年发生在朝鲜的地下核试验是一次检验台网检测微弱信号能力的好机会.波形记录经1～5Hz 滤波后,台网中噪声小的18个基岩台可以清晰辨认核爆破产生的 P 波或 Lg 波,P 波平均振幅为16 nm,计算的平均震级为 m_b 4.3,和 NEIC 给出的震级相同;分析还表明背景噪声是影响台站信号检测能力的主要因素之一.

It is meaningful to use the Beijing metropolitan digital Seismic Network （BSN） of China to study deep crustal structure by recording artificial earthquakes. However, the energy released by artificial sources is small and they generally generate short-period seismic signals. In order to study the weak signal detectability of the Seismic Network between 1 and 20 Hz, this paper carries out the following work. （1） The analysis of background noise of BSN shows that the noise level at seismic stations on bedrock is about 13 dB lower than that at stations located on sedimentary layer. The difference of noise level is equivalent to variations of detectable earthquake magnitude 1.0. The noise between night and day time varies by about 5 dB. There is a trend of noise level increasing year-by-year. The noise level in 2006 is 4 dB higher than that in 2001. （2） A borehole explosion experiment with 25 kg chemical charges was conducted with its energy equal to an ML0.69 natural earthquake. The seismic phases Pg, Pm and Pc generated by explosion can be identified at 18 seismic stations. At a permanent seismic station 218 km away, the seismic phase Pm can be detected with the peak-to-peak amplitude being about 1.6 nanometers. （3） An underground nuclear explosion carried out by North Korea in 2006 is a good opportunity to examine the weak signal detectability of the BSN. The records filtered with 1~5 Hz shows that the P or Lg waves generated by the nuclear explosion can clearly be recorded by 18 seismic stations located on bedrock. The average amplitude of P waves is 16 nanometers. The magnitude calculated is rob4.3, the same as that given by NEIC. The result also shows the noise level of station is one of the most important factors for signal detectability.