Exploring And Monitoring Regional-scale Subsurface Structure Using Un-tuned Large Volume Airgun Array

Many seismic evidences show that the occurrence of earthquake is closely related to the deep earth structure. Besides deep earth structure, the occurrence of earthquake is more related to the variation of the structure and state of subsurface. It is generally accepted that earthquake is caused by stress accumulation and release. The variation of subsurface stress state is the key factor to the occurrence of earthquake. If we can continuously monitor subsurface stress state at high earthquake-prone area (fault zones, plate boundaries, transition zone between high and low velocity layers, etc.), we can make a better assessment of seismic risk in this area. But there is no means to measure the stress state at seismogenic zone directly. Laboratory experiments show that the acoustic velocity of rocks is increase with the load stress (acoustoelasticity), so we can monitor the variation of stress state of seismogenic zone by continuously measuring the velocity of seismic waves pass through it. Exploring and monitoring regional-scale subsurface structure is not only the requirement for disaster mitigation, but also the requirement for exploiting oil and gas.In order to explore and monitor regional-scale subsurface structure, we use two sources:un-tuned large volume airgun array and ambient seismic noise. We first introduce the un-tuned large volume airgun array method. Because earthquakes and traditionally artifical dynamite sources that commonly used in regional scale subsurface structure research both have their own shortcomings. Earthquakes have the shortcomings of limited spatial distribution, unpredictable occurence time, and errors in epicentre location. Dynamite sources have the shortcomings of bad repeatability and environmental damage. Both of them are not very suitable for exploring and monitoring regional-scale subsurface structure. People in our laboratory have always worked on developing new methods for regional-scale subsurface exploring and monitoring in these years. By field experiments, we found un-tuned large volume airgun array have the advantage of low frequency, long exploration distance, high repeatability, and no harm to environment, that is very suitable for subsurface exploring and monitoring.Understanding the source is the first step towards subsurface structure exploring and monitoring. Because the un-tuned large volume airgun array is fired in a land reservoir which often has a shallow depth and small area. The interaction between the airgun array and the water reservoir are very complex. The airgun array and the water reservoir composed a seismic source system. However, there are very few researches about this kind of seismic source system. According to Rayleigh-Plesset equation, momentum equation, and continuity equation in fluid mechanics, we built a single force source model for this kind of seismic source system.Then we introduce the application of un-tuned large volume airgun array in subsurface velocity monitoring. In order to monitor the subsurface velocity changes in western Yunnan province where earthquakes occurred frequently, we built a new type of seismic station called Transmitting Seismic Station (TSS) using an un-tuned large volume airgun array at Dayindian water reservoir (Binchuan county) in2011. Because of the great advantage of un-tuned large volume airgun array, Binchuan TSS can continuously generate long distance detectable and high repeatable seismic signals. Utilizing seismic signals generated by Binchuan TSS, we are able to monitor the subsurface velocity changes in western Yunnan Province. Although we are still in the stage of data acquisition and accumulation, we studied the general characteristics of seismic signals generated by un-tuned large volume airgun array fired in a water reservoir and their influencing factors (firing pressure, airgun towing depth, and water level of the reservoir). This work is the basis of our future work about subsurface velocity monitoring and also give some clues in improving the performance of this kind of TSS too.Then we introduce the application of un-tuned large volume airgun array in regional-scale subsurface structure exploration. With seismic data generated by un-tuned large volume airgun array experiments implemented in Shangguan water reservoir, we studied the one dimensional P wave velocity, S wave velocity, and Poisson’s ratio model of southern Yanshan uplift using forward waveform modeling method. We found that the crustal thickness of our study area is about33km. Rocks in the upper and middle crust have lower Poisson’s ratio values, which indicate felsic rocks in this area. Rocks in the lower crust and upper mantle have higher Poisson’s ratio values, which indicate mafic and neutral rocks in this area. We also found low velocity zones exist both in the upper and lower crust in our study area which were also high Poisson’s ratio zones. The low velocity zone in the upper crust may be caused by fluids in this area, however, the low velocity zone in the lower crust may be caused by partial melting.Finally, we introduce imaging subsurface velocity changes with coda wave interferometry. Coda waves are multiply scattered waves. Comparing to direct waves, they are more sensitive to small changes in the medium. Using coda waves, we can measure weak subsurface velocity changes. In water reservoir region, the variation of water level in the water reservoir will cause subsurface stress and strain change in the surrounding area, and this stress and strain change can be calculated theoretically. So water reservoir region is an ideal place for studying the relationship between subsurface velocity change and subsurface stress and strain change. With continuous empirical Green’s functions extracted from ambient seismic noise data recorded by seismic stations around Dayindian water reservoir, we measured and inverted the spatial distribution of subsurface velocity changes in this area using coda wave interferometry. We found that maybe only subsurface velocity changes in regions very near Dayindian water reservoir are caused by water level changes in the water reservoir, while velocity changes in other regions maybe caused by water table level changes.