Mechanism of Surface Vertical Deformation in Parts of the Underground Gas Storage Reservoir of Hutubi, Xinjiang, China

The Underground Gas Storage( UGS) in Hutubi( HTB),Xinjiang is the largest gas storage reservoir in China and it has significance for coordinating the gas supply and demand relationship,peak-load regulation,implementation of strategic reserves,national security, and social economic sustainable development. Therefore, the deformation monitoring and simulation analysis of UGS operation has important technical support and reference value for the stability and safe operation of the underground gas storage. In this paper,we use the elevation data obtained from 7 periods of second-order leveling surveys in the Hutubi underground gas storage area in 2013- 2015 to analyze the influence of gas well pressure on the vertical deformation of the underground gas storage reservoir.Research has shown that the absolute vertical subsidence rate is approximately in the range from 11. 8mm to 16. 1mm and the relative subsidence change is about 4. 3mm,near the surface deformation of Hutubi underground gas storage area except for the annual subsidence rate of- 2. 86 mm by the basic influence of uplift of the Tianshan Mountains.Groundwater over-extraction in the Hutubi area also has an impact on the vertical variation of the surface in this region. The land surface change per unit pressure of gas storage has an impact of about 0. 625mm- 1. 125 mm. 17 scenes Terra SAR-X radar images acquired from August,2013 to August,2014 are exploited by Small Baseline Subset( SBAS) In SAR method to obtain the surface deformation time series during the operation of UGS in Hutubi,meanwhile combined with the pressure data of injection / productionwells,the multi-point source Mogi model is used to simulate the UGS deformation field in Hutubi. The results show that the deformation characteristics of the whole UGS area is a discontinuous distribution with the peak deformation value of 10 mm and- 8mm in the satellite line of sight( LOS) during gas injection and production,respectively and the retrieved deformation sequences correspond very well to the gas injection / production pressure changes. Based on the multi- point source Mogi model, we simulate the deformation process of UGS,HTB,and with the adaptive forward search method,the radius and depth of point source are obtained. The simulated results indicate that when the average injection / production pressure of UGS,HTB is 18 MPa and 15 MPa, LOS deformation is up to 7mm and- 4mm,respectively,and surface deformation is related to the density of gas injection( production) wells. The UGS gas distribution is not uniform,indicating that the structure of underground gas storage is complex. Thus using a more elaborate geomechanical model and other deformation observation data will be helpful for better simulating the UGS internal structure and explaining the mechanism of deformation.

Evolution of an Ancient Large Lake in the Southeast of the Northern Tibetan Plateau

Nam Co is the largest (1920 km2 in area) and highest (4718 m above sea level) lake in Tibet. According to the discovery of lake terraces and highstand lacustrine deposits at several places in Nam Co and its adjacent areas, the authors confirm the existence of an ancient large lake in the southeastern part of the northern Tibetan Plateau. On the basis of the U-series, 14C and ESR dating, coupled with the levelling survey of lake deposits and geomorphology, the evolutionary process of the ancient large lake in the southeastern part of the northern Tibetan Plateau may fall into three stages: (1) the ancient large lake stage at 115-40 ka BP, when the ancient lake level was 140-26 m above the level of present Nam Co; (2) the outflow lake stage at 40-30 ka BP, when the ancient level was 26-19 m above the present lake level; and (3) the Nam Co stage since 30 ka BP, when the ancient lake level was < 19 m above the present lake level. During the ancient large lake stage, a large number of modern large, medium-sized and small lakes, including Nam Co, Siling Co and Zhari Namco, in the southeastern part of the northern Tibetan Plateau, were connected into a single large ancient lake, rather than several separate lakes connected by river channels. Its areal extent may have gone beyond the watersheds of the modern endorheic and exorheic drainage systems; so it may be called the 'ancient east lake', 'ancient south lake' and 'ancient west lake'. It might also be connected with other ancient lakes in the southern and western parts of the northern Tibetan Plateau to form a unified 'ancient large lake' on the northern Tibetan Plateau.

Deformation datum in source mechanism inversion

Based on the analyses of geodetic deformation datum, the concepts of real datum and relative datum of displacements associated with an earthquake are put foward. Introducting a deformation datum parameter into model parameter vector can enable the reference station to be set in the epicentral areas. Two improved inversionmethods, the deformation datum parameter (DDP) method and deformation difference (DD) method, are advanced in this paper. They are further applied to dislocation model inversion of the 1990 Gonghe Ms7.0 earthquake. The inversion results inferred by using the two methods are well consistent with each other, which basically reflect the source geometry and the slip of the causative fault.