The Experiments Of Using Small Artificial Seismic Sources To Explore Regional Deep Structure Of The Earth

The 4D map of regional structure of deep earth provides important information for earthquake prediction and understanding the continental geodynamics. Suitable seismic source and advanced recording and processing systems are necessary to produce the 4D map. Although natural earthquakes can release huge amounts of energy, the epicenters of natural earthquakes are not accurate enough to be used for producing the 4D map. Traditionally, explosives are used as the seismic sources for exploring depth structure. However, explosives are expensive and have a large negative effect on the environment, so it is difficult to use them. Alternative seismic sources are necessary. If we can use a regional seismic network or seismic array to detect the seismic signal generated from small artificial seismic sources far away, it will provide a practical way to explore the regional deep structure of the Earth.From 2004 to 2007, China Earthquake Administration with some other organizations held several artificial seismic experiments. In this paper, I will discuss some topics of using small artificial seismic sources to explore regional structure of the deep earth, including features of the artificial seismic sources, signal detection, data processing technics and application. The data used in this paper is based on the seismic datasets recorded by Capital Area Seismograph Network(CASN) and portable seismic stations.Explosion is the most common source for seismic exploration in the land. However, is a small explosive a good source to investigate the deep structure? A borehole explosion experiment with 10-25kg chemical charges was conducted to answer this question. The energy of the explosive equals to a about M_L0.69 natural earthquake. The seismic wave generated by 25kg explosive can be detected by a permanent seismic station with offset up to 218km, and the seismic phases Pg, Pm and Pc can be identified. Pm waves can be detected with the peak-to-peak amplitude being about 1.6 nanometers and the depth of propagation is about 40km. The comparability of seismic waves generated by several 10-25kg explosions shows that the explosion is a fairly repeatable source. The relationship between amplitude of seismic wave and charge of the explosives was calculated. The result shows that the explosive can be used as the seismic source for deep structure exploration if the source and receiver conditions are good. However, it is not easy to control the conditions and the explosive has a limited application.The airgun is the most important seismic source in marine exploration. We conducted two experiments using airguns in a reservoir. The airgun-array (4 airguns) had a volume of 6000-8000 cubic inches and the energy released by one shot is equivalent to a 1.4-1.6 kg TNT explosion. Because more energy is transformed into seismic waves in the water than in the land, the energy of an airgun-array is equivalent to a M_L0.5-0.6 earthquake. In order to enhance the signal, we stack the recordings of about 272 shots for each receiver. The phases of Pg, Pc, PmP, Pn and Sg, SmS, Sn can be picked easily in the stacked seismic recordings. We found that once stacked, the signal can be detected by receivers with offset up to 185km and by a permanent station with offset upto 400km. The crosscorrelation coefficient (CC) of the signal is higher than 0.96. The results shows that airgun is also a green, cheap source with high controllability and repeatability, so it is the suitable source for exploring regional deep structure of the Earth.The regional seismic network plays an important role for studying deep crustal structure and monitoring the changes of rock properties via artificial seismic source. The artificial sources generally generate short-period seismic signals between l-20Hz. In order to study the ability of the seismic network to detect short -period weak signals, I analyse noise datasets of the CASN. The results show that the background noise level at seismic stations on bedrock is about 13dB lower than that at stations located on sedimentary layers. An underground nuclear explosion detonated by North Korea in 2006 is a good opportunity to examine the ability of CASN to detect weak signals. The records band-pass filtered between 1-5Hz show 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 the P waves is 16 nanometers. The magnitude calculated is mb4.3, the same as that given by NEIC. The result also shows the background noise level of a station is one of the most important factors for signal detectability. The good bedrock stations can play a significant role in 4D seismology.Nearly all the big cities in the Beijing area are located on sedimentary layers. The propagation and attenuation of seismic waves in sedimentary layers are different from that in the deep earth. It is important to know the features of the sedimentary layers when we want to obtain underground structure maps of the urban areas. A strong phase is found in the signal generated by airguns with a volume of 2570 cubic inches in Bo-Sea. Its velocity is about 1.7-1.8km/s and can be detected with offset up to nearly 100km. Note that the P wave can only be detected at offsets up to 21km. Even by increasing the the airgun volume to 8000 cubic inches, the P wave generated by airguns in a reservoir is only detected at offsets up to 30km. The result shows that the waveforms, signal-to-noise ratio and attenuation of seismic waves in the sedimentary layers are factors affecting the result of detection. The attenuation of seismic wave in sedimentary layers is much stronger than in rocks, and the variation is not linear, which means the detection range of the signal is not a linear relation with distance.Shear wave splitting is an possible method for earthquake stress-forecasting. I applied shear-wave splitting analysis to a seismic dataset generated by airguns in a reservoir. We found that the seismic data contains shear-waves (Sg and SmS). The shear-waves are converted at the water-solid interface from P-waves generated by the airgun source, and the energy of the converted shear-wave is equivalent to the energy released by a M_L 1.6 earthquake. We analyzed the data recorded by a seismic line deployed over the Yanshan uplift. The results shows that the predominant polarizations of the fast shear-wave are in the directions of NWW and NEE, which are affected by the characteristics of the local fault system. Using an airgun as a repeatable seismic source and recording the data at a fixed point, the variation of shear-wave splitting parameters can indicate the variation of local stress-strain fields, and hence provides a method for earthquake stress-forecasting.