Earthquake Reaearch in China  2018, Vol. 32 Issue (3): 315-329
Regional Scale Seismological Investigation on the Continent Crust Using Airgun Sources—A Perspective Review
Wang Weitao1, Wang Baoshan1, Jiang Shengmiao1, Hu Jiupeng1, Zhang Yuansheng2     
1. Key Laboratory of Seismic Observation and Geophysical Imaging, Institute of Geophysics, CEA, Beijing 100081, China;
2. Lanzhou Institute of Seismology, CEA, Lanzhou 730000, China
Abstract: The investigations on the structure and temporal variations of the continent crust have been the long-lasting topics of seismology. The artificial sources play the most important role in these studies for their well-known locations and occur times among all the sources producing elastic waves, so that precise results are obtained. In past decades, we have successfully excited the airgun sources in on-land water bodies such as reservoirs, expanding their applications in marine seismology. Large volume airguns have been used to produce low frequency signals which can travel hundreds of kilometers after stacking, providing a source connecting the exploration seismology and traditional seismology at regional scale. In this review, we introduce the changes through which we turn the airgun into scientific instruments by experiments. We also present the advances on imaging the continental crust structure and monitoring the velocity variations using the highly repeatable signals emitted by airgun sources. The potential applications such as imaging the subsurface of urban areas and high resolution illuminating of mines are proposed and the challenges for further investigations are also discussed.
Key words: Airgun Sources     Crust velocity structure     Monitoring velocity variations    

INTRODUCTION

Since the beginning of seismology, researchers have obtained information on the structure, composition, and state change of the Earth by studying the generation and propagation of seismic waves. Most of our understandings of the earth's interior originate from seismic waves, which are "A Bright Light That Illuminates the Earth" (Chen Yong et al., 2005). Seismology falls into two major branches based on the seismic sources and investigated objectives: natural seismology and exploration seismology.Natural seismology uses the records of natural earthquakes from specialized seismic stations to study the Earth from a regional to global scale (tens to hundreds of kilometers). Exploration seismology mainly relies on seismic waves actively emitted by artificial sources, and uses densely-packed geophones as a receiving system to study fine shallow mineral resources at a local scale (hundreds of meters to a dozen of kilometers). Although they have different objectives and undergo independent development, they are based on common laws of elastic wave propagation and inspire each other. In recent years, they have many overlaps in terms of the objects and methods, especially in the study of continent crust.

Compared to the radius of the Earth, the continental crust is shallow, ranging from the near-surface sediments to the top of the uppermost mantle. The structure, state, and variations of the crust, especially on the continents where humans live, are the common objects of natural seismology and exploration seismology. Exploration seismology for resource exploration is to study subsurface at depths of several meters to several kilometers. However, as shallow energy resources have been reduced and the development of unconventional oil and gas (such as shale gas) in recent years, exploration seismology also focuses on deeper crust imaging. At the same time, natural seismology also adopts the observation system from the exploration seismology to use a large number of densely-distributed stations to study structure and state of the near-surface structure (Schmandt B. et al., 2013). Both of them have common research subjects in the continent crust. The key to their integration is to find a suitable source that can meet the requirements of the controllable position and time of the source in exploration seismology, while at the same time, can propagate far and deep enough to the scale of natural seismology.

Artificial seismic source is the main source of exploration seismology, and it is also one of the sources used by natural seismology. In traditional research, the most utilized source is the dynamite source using chemical explosion to generate seismic waves. The dynamite source can achieve blasting of tens of grams to several thousand kilograms. The excited seismic waves can spread hundreds of meters to several hundred kilometers, thus it has been playing an important role in the development of artificial earthquakes. However, with the increasing emphasis on environmental protection, the use of dynamite sources has been largely restricted. Moreover, the dynamite source is essentially a kind of instantaneous strong pulsed source. Blasting often causes damage in near source area, so it is difficult to produce repeating signals due to un-recovered exciting conditions. Seismologists turned to use the continuous vibrations as sources, impelling the development of sources such as Minisose, Vibroseis, and ACROSS (Yamaoka et al., 2001; Chang Xu et al., 2008; Wang Hongti et al., 2009). These sources are continuous non-impulse sources, and need further processing such as cross-correlation and convolution to obtain pulse response of the medium. However, the efficiency is low to convert such kind of source energy to seismic waves, and long-term continuous observation can also cause damage on the near source excitation site reducing signal repeatability.

In recent years, we have conducted many experiments to explore various kinds of artificial sources in the study of continental crust (Wang Baoshan et al., 2010, 2012, 2016). Experimental studies demonstrate that exciting the airgun sources in the on-land water body is an effective way to study continental crust structures and changes. The airgun source is a kind of source excited in water. It releases high pressure air to stimulate seismic waves under water, and can be regarded as a pulse type source. Due to the easy recovery of the water body, airgun sources can produce highly repeatable signals across multiple excitations. In the time scale of multiple excitations, the airgun source is equivalent to a continuous pulse source. This feature makes it possible to increase the signal propagation distance by simply superimposing, and at the same time makes the airgun source optimal for repeating measurements.

Since the construction of the first Fixed Airgun Seismic Transmitting Station (FASTS) in Binchuan, Yunnan Province in 2011, a number of FASTS have been established across the country. The active seismic wave excitation by airguns in on-land water bodies provides the opportunity to investigate the shallow crust at the scale of tens to hundreds of kilometers (Wang Baoshan et al., 2016). Through these experiments, we analyzed the characteristics of airgun source excitation in on-land water bodies, and explored the application of detecting the shallow crust structure and monitoring subtle velocity variations.

1 LARGE VOLUME AIRGUN SOURCES EXCITED IN ON-LAND WATER BODIES-FROM INDUSTRIAL EXPLORATION EQUIPMENT TO SCIENTIFIC RESEARCH TOOLS

The airgun source is a widely used source in marine exploration. Its high-frequency impact signal can be used to study the fine structure of a small area. However, the ship-borne excitation mode and the limited propagation distance of high-frequency signals have limited its application in studying continent crust. To overcome this limitation, we used large volume airgun sources excited in the on-land water bodies to increase detection limitation (Chen Yong et al., 2007). In the experimental study to explore the conversion of airgun source from marine excitation to on-land water body excitation, the advantages of airgun source, including highly repeatable signals, environmental friendliness and low costs are maintained. At the same time, new features are presented, proving airgun source as an optimal source to actively study continental crust.

The excitation of the airgun source in the on-land water body realized the change of the excitation mode. The airgun source is a kind of transient shock source. The shock it releases includes high-frequency shocks generated by the instantaneous release of high-pressure air, and low-frequency shocks generated by the oscillation of the bubble formed by the shock. These characteristics of the shock signals are jointly controlled by the airgun's inflation pressure, trigger time and airgun capacity. Marine exploration imaging focuses on small-scale fine structures. Therefore, airguns of different capacities are often used to form a tuning airgun array. High-frequency shocks are emphasized and low-frequency shocks are suppressed by controlling the combination of airgun's capacity and firing time. It forms a relatively high-frequency seismic waves which contributes to obtain high-precision imaging results. When excited in terrestrial water bodies, high-frequency signals attenuate quickly, leading to a limited propagation distance, which is not expected for regional-scale crust exploration studies. Therefore, when the airgun source was introduced into the on-land water body, we chose large-capacity ones with a single-cylinder air chamber capacity of 2000 cubic inches, and used multiple airguns to fire in an untuned mode to further enhance its low-frequency signal. These helped to ensure long-range detection. The difference in the operation modes is shown in Fig. 1. The change of excitation mode has greatly increased the propagation distance of large-scale airgun source excitation signals in on-land water bodies. Experimental results show that four large capacity airguns of 2000 cubic inches excited at 15MPa pressure will generate seismic waves with the dominant frequency of 2Hz-8Hz. After 100 linear superpositions, the signal can propagate 300km-500km in many regions (Wang Baoshan et al., 2016). The dominant frequency range of the large-capacity airgun source signal allows it to retain a relatively high resolution of the subsurface medium while ensuring the propagation distance. Moreover, it's main frequency fall into the effective flat frequency range of seismic stations and is easily recorded by the receiving system. The transformation of the excitation mode extends the airgun's detection range to the level of 100km while maintaining its feature of artificially controllable active source. This turns it from an industrial exploration device into a seismological research tool.

Fig. 1 Schematic of the operation mode of airgun array (a) Airgun array for marine exploration.
(b) Airgun array used in on-land emission for regional scale seismology research

The excitation of the airgun source in the on-land water body realizes the change of the excitation environment from an infinite water body to a limited water body. Airgun is a source excited in water, and its excitation process involves the coupling of high-pressure gas, water body and solid earth. The characteristics of seismic waves generated by the airgun are also affected by this complex coupling process. As the airgun always excites at a certain depth below the water surface, the energy released by the high-pressure air impact is mainly controlled by three boundaries: the water surface can be considered a free boundary, and the energy transmitted to the water surface is completely lost; the bottom of the water body is the main energy absorption and transformation boundary, which absorbs the energy transmitted to it, converts it into seismic waves and spreads outward; the side boundaries of the water also control the conversion of impact energy to seismic waves. The changes of excitation conditions (i.e. from an infinite water body to on-land water bodies with boundaries) further affects the characteristics of seismic waves excited by the airgun.

The energy released from the airgun will lose at the free surface boundary of the water, so the depth of the airgun position will affect the seismic wave conversion efficiency. Fig. 2 shows the relative amplitude changes of a single airgun at different depths recorded at a seismic station 200m away from the excitation point in Qilianshan FASTS. The experimental results show that with the increase of the airgun depth, its energy loss decreases, and the seismic wave energy conversion efficiency increases. Taking into account the site constraints, the recommended depth of the air gun should be 15m under water or deeper.

Fig. 2 The amplitude variation of airgun signal with respect to the emission depth The waveforms are raw records of four single airgun shots at depth 12m, 15m, 18m and 21m respectively

In the infinite water body in the ocean, the side boundary and the bottom boundary can be regarded as infinite boundaries with respect to the impact scope of the high pressure bubble impact. However, in the limited on-land water body, these two boundaries are limited and are related to the total amount of water. Fig. 3 shows the time-frequency distribution of seismic waveforms and energy recorded near the source when the airgun source is excited in water bodies of different capacities. Smaller water bodies (such as wells, etc.) usually correspond to limited bottom and side boundaries, and when the bottom boundary is too small, it is not conducive to fully absorbing the downward energy of the air bubbles. A side boundary too close to the high pressure gas release point will produce a strong reflection, and may even destroy the oscillation process of the high pressure bubble, which is not conducive to the emission of lower frequency signals, causing excessive energy loss while increasing the complexity of the waveform (Chen Meng et al., 2014; Yang Wei et al., 2016). In a relatively large volume of water (such as a reservoir), the bottom boundary is relatively large and can absorb most of the downward-propagated energy. However, in actual experiments, the airgun excitation device is often fixed at a distance of several hundred meters from the solid boundary for maintenance. When the strong impact energy propagates to the solid boundary, it will interact with the lateral boundary to produce strong reflection and refraction, thus affecting the shape of the final seismic wave.

Fig. 3 Waveforms and the time-frequency diagrams of airgun shots in different exciting conditions. In each sub panel, the record of reference station is shown at the top with corresponding time-frequency diagrams as shown below. Both the amplitude and energy are normalized to unity (a) Binchuan downhole well. (b) Mabaoquan water pool. (c) Binchuan reservoir. (d) HTB bowel-shape artificial reservoir

After analyzing the influencing factors of the airgun's excitation in a limited water body, we built a bowel-shape water body in Hutubi, Xinjiang with regular boundaries optimal for converting the impact energy to seismic waves. The experimental results show that the energy of the seismic wave generated by a single airgun excitation is equivalent to a ML0.9 earthquake, which is slightly larger than the results under the same excitation parameters in the Binchuan natural reservoir, whose energy level is equal to ML0.7 earthquake (Wei Bin et al., 2016).

The excitation of airgun source in the on-land water body realized a change from a strong shock source detection style to a weak shock source one. The intensity of the signal emitted by the source determines its detecting range of the subsurface. The seismic waves released by major earthquakes with M≥7.0, can shake the entire earth and provide information on its deepest parts. In the research of artificial seismic sounding, in order to obtain a clearer seismic wave signal, a large-equivalent blasting is often used as a strong shock source to increase the signal intensity. The blasting dose varies from several hundred kilograms to several tons. However, the way to enhance the detection ability by simply increasing the intensity of the single source is now restricted. The large equivalent chemical blasting can seriously damage the environment. At the same time, the powerful dynamite source often causes strong damage to the blasting site, and makes it impossible to repeatedly perform in-situ measurements. The non-destructive detection mode, which obtains a large detection distance accumulating relatively weak single shots, is preferred.

Airguns feature an intermittent pulse source that generates highly repetitive pulsed shocks at the same location, allowing the use of a single weaker energy to accumulate to greater energy. The energy released by a single shot of the airgun is weak and has little effect on the near-field venue. Experiments show that at 600m from the excitation point, the peak acceleration generated by a single vibration of the airgun is close to the level of the background vibration which is a lossless excitation for the environment (Liu Bideng, et al., 2011). This allows the use of the airgun source in environments that require stringent requirements for non-destructive excitations, such as detection of underground structures in cities and nondestructive detection of buildings. At the same time, due to the highly repeatable nature of the airgun, superposition of multiple excitations can be equivalent to a strong vibration excitation over the detection range. We performed a 900kg blast on land 300m away from the source of the Bingchuan airgun and compared the waveforms of the airgun signal superimposed on the same line with the blasting waveforms, as shown in Fig. 4. On the same line, the signal propagation distance of 110-time airgun superposition is better than that of 900kg of explosives. As far as the source is concerned, the superposition of several highly repeated excitations increases the equivalent intensity of the source. At the same time, multiple superpositions can also suppress incoherent noise signals, and highlight the coherent signals emitted by the airgun source, thus further increase the signal-to-noise ratio.

Fig. 4 Comparison of stacked airgun signals and 900km explosion recorded by the same profile (a) Section plots of 111 stacked airgun shots. (b) Section plots of records of 900km explosion

It is an important developing direction in modern seismology to use the accumulation of a weak seismic source to equivalent a strong source. The background noise cross-correlation method developed in the last 10 years (Shapiro N.M. et al., 2005) is a classic example of using coherent accumulation to retrieve high signal-to-noise ratio signals. The continuously excited Virbroseis and ACROSS sources also fall into this category. Compared to these sources, airgun single shots are pulse-like, and the increase in source energy can be achieved by simple superimposing, which help facilitate the subsequent analysis. At the same time, the signals emitted by airguns are mainly body-wave signals.Compared with the surface-wave signals obtained by the noise cross-correlation method, it can provide information of deeper Earth.

The excitation of airgun sources in the on-land water body provides an important reference for changing from one single strong shock to multiple coherent weak shots. This enables the airgun source to replace the traditional dynamite source under suitable conditions. At the same time, it overcomes the environmental limitation to use dynamite source, and expands the application scope of airgun source, i.e. it can be applied to both the exploration of shallow structures and regional scale researches on continent crust imaging.

2 APPLICATION OF AIRGUN SOURCE IN THE DETECTION OF CONTINENT CRUST STRUCTURES

The excitation time and conditions of large-volume airgun sources excited in on-land water bodies are all well controlled, a critical feature for exploration seismological sources. On the other hand, the propagating distance and depth can extend to the study of the regional scale that natural seismology pays attention to, which makes the airgun source widely used in the detection of continent crust structures.

The signals of large-capacity airgun excited in on-land water bodies can be transmitted several hundred kilometers after superposition. At the same time, the seismic waves emitted have the characteristics of repeatable signal and abundant phases, which can replace the traditional dynamite source in the research of continent crust structure whenever possible.

Fig. 5 Waveforms from stacked 200 airgun shots along a 200km line profile. All waveforms are vertical components and each trace has been band passed between 4Hz to 8Hz and normalized by its maximum amplitude (modified from Lin Jianmin et al., 2008)

The traditional dynamite source mainly produces P-wave signals. While the airgun source also produces abundant S-wave signals in addition to P-wave signals. Therefore, in the analysis of reflection and refraction profiles, P-wave and S-wave signals can be analyzed jointly. Using the airgun signals recorded on the 200km line in the south of the Yanshan uplift, we analyzed the characteristics of the seismic phase, and studied the P-wave and S-wave velocity structures and the Poisson's ratio of the crust in the area. The results show that there is a distinct low-velocity layer respectively in the upper and lower crusts of the region, and their causes are different (Lin Jianmin et al., 2008; Chen Jianxiong et al., 2011; Chen Meng et al., 2013).

When the airgun source is excited at a fixed point, the source position is fixed and the waveform is highly repeated. This feature allows us not only to obtain high signal-to-noise ratio data on a single line by superposition, but also make it possible to deploy multiple lines at greater ranges and higher density by stages using limited seismometers. At the end of 2015, we used 86 seismometers to set up an observation system to record airgun signals in the 1km×0.5km area in the Binchuan area. Owing to the repeatability of the airgun source, the experiment adopted a method of staged mobile deployment. Finally, 86 sets of three-component digital seismographs were used to obtain data from 543 observation points. The average distance between observation points is 30m, as shown in Fig. 6. In the airgun excitation test in the Mianhuatan Reservoir, Fujian, above-mentioned method is used to obtain a 250km survey line with 50 seismometers, and the distance between each point is 1km (Chen Huifang et al., 2016). Ultra-dense observation systems have long been an observation method used in exploration seismology. In recent years, they have also been adopted by Natural Seismology to develop new imaging methods (Schmandt B. et al., 2013). Using the repeatability of the airgun fixed-point excitation, it is possible to construct an ultra-dense observation system using limited observation instruments, which is of great benefit for obtaining fine structural features of the continental crust.

Fig. 6 Deployment of a dense array using limited number of seismometers benefit from the repeatability of airgun signals (a) Location of the airgun source and the dense array.(b) Locations of the recorders deployed in different time consisting the dense array

The record of the fixed point airgun source excitation on the line can be used for the study of wide-angle reflection and refraction profiles. The use of airgun sources with multi-point excitations, coupled with intensive observations, can provide necessary data support for 3D body wave imaging. The multi-point excitation of the airgun source can be achieved by performing fixed-point excitation in multiple on-land water bodies simultaneously or in stages, or by using a shipborne airgun source for mobile excitation in intracontinental rivers. In October 2015, we conducted a mobile excitation experiment of the airgun source along Wuhu-Anqing section of the Yangtze River, using a shipborne airgun source. During the experiment, the airgun source was excited more than 4, 000 times in the Yangtze River. The receiving system is a combination of fixed seismic stations and mobile seismographs. In the observation area, there are 11 observation lines (each about 200km long) composed of 1000 mobile seismic stations, and 2 small arrays composed of 100 mobile seismic stations. At the same time, there are 160 permanent stations continuously observing the area. Airgun excitation points and station distribution are shown in Fig. 7(a). Zhang Yunpeng et al. (2016) used the airgun signal recorded from the permanent stations for tomography and demonstrated the application of the mobile airgun source in three-dimensional body wave imaging, as shown in Fig. 7(b). The excitation and observation experiments of shipborne airgun sources in the Yangtze River provide abundant observational data for studying the crust structure, mineral distribution and genesis in the middle and lower Yangtze River. At the same time, this kind of experimental exploration makes it possible to use multiple excitations and mobile excitations of airgun sources in some areas with stable structures and less seismicity to obtain abundant body-wave travel time data and analyze the three-dimensional wave velocity structure in the continent crust.

Fig. 7 (a) Location of the stations and airgun shots in the Changjiang Project. The 20 fixed short points of airguns along the Changjiang River are shown as red stars. The 11 wide angle profiles are shown as yellow and green solid triangles while two dense arrays are shown as blue and red inverted triangles respectively. (b) The horizontal slice at depth 15km of the preliminary body wave tomography inferred from airgun signals recorded by the permanent stations (modified from Zhang Yunpeng et al., 2016)

In addition to the excitation in on-land water bodies, shipborne large-capacity airgun sources can be mobile-excited in the offshore area, and its signal can be received jointly by land stations and ocean bottom seismometers (OBSs). This method enables the detailed study of the subsurface structure of the land-sea transitional zone (Qiu Xuelin et al., 2007; Xu Huilong et al., 2012). As an artificial source, the excitation position and time of the shipborne airgun source are all controllable. Therefore, the observation system and the excitation position can be planned in advance, and the optimal combination of the source and the receiving point can be adjusted to the research purpose. Large-volume airgun sources excited in the sea can, to a certain extent, make up the limitation of the imaging research due to the lack of seismicity in the sea area and the unpredictable position and time of the earthquake. It is of great significance to study the structure of China's sea-land structure belt and explain its tectonic history.

When studying the velocity structure of the continent crust, the airgun source can be considered a traditional dynamite source. Therefore, a variety of imaging methods developed in both exploration seismology and natural seismology can be used. At the same time, compared to natural earthquakes, the excitation time and position of the airgun source can be accurately measured. This advantage can reduce the errors introduced by the source in imaging to obtain reliable subsurface structures.

3 THE APPLICATION OF AIRGUN SOURCE IN STUDYING THE CHANGE OF SUBSTANCE STATE IN THE CONTINENT CRUST

An important development of modern seismology is to monitor the changes in the composition, structure, and state of subsurface medium over time as 4-D seismology studies with time-varying information. From the perspective of seismology, the change of the physical properties of the subsurface medium in a certain area will lead to the change of the seismic waves' characteristics that passes through the area. Using repeated measurements using the same seismic source and observation system, it is possible to monitor the subsurface medium in 4-D by analyzing the changes in seismic waves and draw the "Underground Cloud Atlas".

The velocity/travel time of seismic waves is an easy-to-measure physical quantity. By measuring the seismic wave velocity variations, it is possible to address the subsurface medium state changes related to earthquake occurrence. The variation of the stress state will change the elastic properties, and then affect its elastic wave velocity. The corresponding velocity variation can be measured through seismic wave travel time by repeated measurements. Studies have shown that the wave velocity caused by the earthquake gestation process is very small—the relative variance in wave velocity/travel time is on the order of 10-4 (Wang Baoshan et al., 2008). As early as the 1970s, seismologists have attempted to use airgun sources to actively study velocity variance at 10km-20km in the shallow crust (Reasenberg P. et al., 1974). With the improvement of the observation instrument accuracy and the development of data processing methods, many researchers have tried to use a variety of sources to monitor the wave velocity variations associated with the seismic process. Niu Fenglin et al. (2008) conducted experiments in the drill wells in the San Andreas Fault using piezoelectric ceramic sources, and found significant variance in wave velocity before two minor earthquakes. Yamaoka K. et al., (2001) used ACROSS sources to continuously monitor the variations of subsurface medium wave velocity. Yang Wei et al. (2010) used ACROSS sources to monitor the variation of wave velocity in the Wenchuan earthquake fault zone. Brenguier F. et al., (2008) measured the long-term change of the seismic wave velocity in the fault zone of Parkfield area with background noise and found that it is closely related to the process of large earthquakes.

The above studies show that the state change of subsurface medium can be monitored by measuring seismic wave velocity/travel time variance. However, the wave velocity variance associated with the physical process of the earthquake is often very small, so highly-repetitive source signals are the basis for accurate 4-D monitoring. Airgun source is non-destructive to the environment and can excite highly repeatable waveforms, which makes it an optimal source in monitoring the change of material state in subsurface mediums.

The high-precision monitoring of the variation of the weak wave velocity in the subsurface medium benefits from the progress of the observation system and the data processing method. The time service accuracy of the seismic recording instrument is an important factor affecting the measurement accuracy of the travel time variation. Early studies on wave velocity variation were limited by the accuracy of the observation system. With the development of seismic observation instruments, the time service accuracy of seismic instruments can reach microsecond magnitudes. Fig. 8(a) shows the statistical characteristics of a 1600-times clock record for a REFTEK-130 data recorder at the Binchuan airgun testing site in Yunnan. REFTEK-130 generates a clock difference record every hour. The results show that within 1, 600 hours, the variance is kept within one microsecond (10-6 seconds) for most of the time, which provides reliable foundation for subsequent processing.

Fig. 8 (a) Distribution of clock error for 1, 600 hours records from REFTEk-130 seismic recorder. (b) Comparison of one week travel-time difference of the P-wave airgun signal shown as the red curve and the strain variations shown as the gray curve at the same period. The travel time differences are measured once per hour while the strain measurements are continuous

The coda wave is multiple-scattered in the medium and is more sensitive to small variations in the medium wave velocity. Therefore, it is widely used to monitor the wave velocity variations of subsurface medium (Wang Baoshan et al., 2008). However, when using coda to monitor the time-varying response, it only shows the average effect of velocity variance of the subsurface medium, and is difficult to determine the actual area where the wave velocity variations occur. Although it is possible to use the coda wave to locate the area of the wave velocity variations in laboratory, it is still difficult to achieve in field observations (Xie Fan et al., 2017). Thanks to the highly repetitive characteristics of the airgun source and the precise time service of modern instruments, the absolute travel time difference of different phases of seismic waves can also be used to measure wave velocity variations. By selecting different seismic wave phases and cross-correlating them with corresponding seismic phases obtained by long time superpositions, it is possible to obtain the travel time variation of the seismic phase with time. Using this method, the possible areas of the wave velocity variation can be obtained according to the corresponding path of seismic phase. It is also possible to analyze the sensitivity level of the different seismic phase to wave velocity variation.

The accuracy of the wave velocity variation measurement is the key to the application of travel time variation in 4-D seismological study. The earth's tide will cause weak stress changes in the subsurface medium. It can be used to benchmark the measurement accuracy by comparing earth tide variations and the wave velocity variation measurement. From October to November 2015, we conducted a continuous excitation test which was triggered every hour at the Qilianshan FASTS. The transit time variations were measured using the direct P-wave seismic phase at a seismic station 20km away from the source, shown by the red line in Fig. 8(b). The gray line in Fig. 8(b) shows the strain curve recorded on the Sunan cave strain station at 42.7km from the seismic station. There is a good correlation between the seismic wave travel time variations and strain changes, which reveals the possibility and effectiveness of using highly-repetitive airgun source signals to detect the weak internal stress change in the Earth.

The variations of seismic wave velocity or travel time are the most commonly used observational measurements in 4-D seismology studies. Changes in the subsurface medium not only reflect variations in the travel time of seismic waves, but also cause changes in the waveforms. Using the S-wave signal in the airgun excitation signals, Zhao Wenjia (2013) studied the variation of shear wave splitting over time. The results show that the fast wave polarization direction of the S-wave has a distinct diurnal variation, which may be related to the periodic loading of the earth tide.

With the development of 4-D seismology, a variety of repetitive sources, such as ACROSS source, airgun source and ambient noise, have been used to monitor the wave velocity variations associated with natural phenomena such as earthquakes and volcanoes. The characteristics of these sources vary, and the research subjects also have different focuses. Compared with other repetitive sources, the airgun source has high signal repeatability and far propagation distance thus can monitor the wave velocity variations of deeper subsurface medium. At the same time, the time and place of the excitation is well controlled, allowing us to conduct intensive observations at a specific time and place to improve the time resolution of velocity variation monitoring. Comprehensive use of various repetitive sources can form multi-scale and focused 4-D monitoring. And the results are of great significance for understanding the dynamic changes of subsurface medium in the continent crust.

4 THE POTENTIAL APPLICATIONS AND CHALLENGES OF USING AIRGUN SOURCES TO STUDY CONTINENTAL CRUST

Through a number of field experiments, large-volume airguns are successfully excited in on-land water bodies as an active source. It has been used to study the structure and medium changes in the continent crust. As a new kind of artificial seismic source, the airgun source has moderate excitation energy and a long propagation distance, which makes it a suitable seismic source connecting natural seismology and exploration seismology. Further applications are expected.

The airgun source can actively emit seismic waves to achieve a detection distance of 100km and can be used for deep mineral exploration. The mineral distribution can be imaged in a larger range and deeper depth using powerful active sources. This is of great significance for studying the distribution of deep minerals, and understanding the mineralization environment and mineralization mechanism.

Airgun source is a kind of continuous pulse source. Its single shot energy is small and environment-friendly. The energy would be strong after superimposing multiple shots, thus can be applied to areas with higher requirements for non-destructive detection like cities. The prevention of earthquake disasters in cities is very important in the context of China's rapidly developing urbanization process (Chen Yong et al., 2003). Although the concept of a digital city is well proposed, most of the digital geosciences information of the city was above the surface, and the digital image of the geological structure below the city is unclear. It is possible to use the non-destructive airgun source to construct urban subsurface three-dimensional maps where explosive dynamite sources cannot be used. The results would improve city digitalization and help reduce the hazards in natural disasters. Airgun source's non-destructive characteristics also have potential application prospects for nondestructive testing of high-rise buildings in cities.

Airgun source is an artificial source suitable for 4-D monitoring and can monitor the change of subsurface medium state. With the increase of humans' ability to remake nature, large-scale industrial activities, such as the construction of large-scale gas storages and underground storage of carbon dioxide, can cause changes in the stress state and physical properties of the subsurface medium. To dynamically monitor these changes via active seismic sources is of great significance to ensure the safety of energy storage and to study the effects of strong industrial activities on the nature.

Utilizing the characteristics of arigun source, especially when combined with advances of observation and processing technologies, more applications can be explored using airgun source.

The airgun source is a new type of artificial source, is has only been developed for six years, since first FASTS in Binchuan, Yunnan in 2011. There are still many technical and scientific issues that need to be resolved in terms of exciting technology, principles, and applications.

From the aspect of excitation technology, the existing airgun excitation technology systems are all large-scale systems. They can achieve a detection distance of 100km, but lack portability and mobility. And they require relatively deepwater bodies to be excited. The development of small-scale mobile airgun excitation system can reduce this restriction for short propagation distance from several kilometers to tens of kilometers, and will be of great significance to expanding the application range. Small airgun excitation systems are suitable for mobile excitation. It is of great significance to realize the low-cost multi-point excitation and construct the urban underground three-dimensional maps.

From the aspect of signal processing, the airgun source accumulates weak energy excitations to enhance signals. At longer receiving distances, single-shot signals tend to be submerged in noise. It is important adopt the signal processing technology in other disciplines (i.e. communication and GPS etc) to achieve detection and extraction of weak signals more effectively. The progress of weak signal extraction and detection technology will have practical significance for expanding the detection range of airgun signals and improving the time resolution of 4D monitoring (Zheng Chenglong and Wang Baoshan, 2015).

The excitation of the airgun source concerns the coupling of materials in gas, liquid and solid phases. It needs further work to combine the theory of bubble oscillations and the propagation of seismic waves to study the detailed mechanism generating seismic waves from airgun sources. Understanding the physical process of the airgun source excitation and achieving the simulation of the complex coupling process will be of great significance for further enhancing the airgun excitation efficiency and subsequent signal analysis. The simulation of the elastic waves produced by airgun source will help to better use waveform information for refined investigations.

From a research perspective, the understanding of the seismic source and subsurface medium structure is always progressed in turns. By observing via dense arrays and obtaining the fine velocity structure of the excitation region, the seismic wave's excitation mechanism of the airgun source can be promoted. In addition, it is also important to combine other seismic sources, such as background noise and natural earthquakes, to conduct comprehensive and multi-scale studies on continent crust structure and state changes.

5 CONCLUSIONS

The continent is a basic place for human habitation and activity. Seismologists in the 20th century used natural earthquakes to outline the structure of the internal Earth, and seismologists in the 21st century face the problem to depict the structure and status changes of the continent below our feet using artificially emitted seismic waves. The large-capacity airgun source excited in on-land water bodies is an important tool in the study of continental crust, and it is also a bridge connecting exploration seismology and natural seismology in term of source. In the past decade, the development of airgun source research shows its diverse application prospects and also exposes some challenges to be resolved. To better use seismological methods to serve the society, seismologists need conduct inter-discipline, innovative researches to open new era of continental crust investigations.

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利用气枪震源探测大陆浅部的地震学研究回顾与展望
王伟涛1, 王宝善1, 蒋生淼1, 胡久鹏1, 张元生2     
1. 中国地震局地球物理研究所 地震观测与地球物理成像重点实验室, 北京 100081;
2. 中国地震局兰州地震研究所, 甘肃 兰州 730000
摘要:气枪震源在大陆浅部探测中的应用日益广泛。首先描述了大容量气枪震源在陆地水体内的激发特征, 分析了其由海洋转入陆内水体激发实现的激发模式、激发环境和探测方法的转变; 然后介绍了气枪震源在探测大陆浅部速度结构及监测弹性波速随时间变化当中的应用; 最后简述了气枪震源在地震学研究中的潜在应用及面临的挑战。实践证明, 陆地水体内激发的气枪震源是一种绿色、环保、高效的人工重复震源, 其激发的信号可用于区域尺度的地壳结构成像, 在一定程度上可替代传统炸药震源。同时, 其激发的高度重复的信号可用于弹性波速随时间变化的监测, 测量精度可达到10-4。气枪震源是一种新型的、发展中的震源, 仍需要对其震源特性和信号处理技术开展深入的研究以拓展其在地震学研究中的应用。
关键词气枪震源    浅部探测    波速变化