日喀则城市活断层地球物理勘探方法和成果

日喀则地质资料匮乏,地球物理勘探资料更加稀缺,该地区在此之前没有开展过地震勘探的工作.本文针对日喀则地区活动断裂,采用夯源为人工震源的浅层地震勘探方法,结合小折射调查低速层,详细讨论工作中的关键性技术问题;提出在该地区地质条件下实施隐伏断裂勘探时的地震仪器选择、方案设计、参数选取、数据处理、断层识别的基本方法;查明拉堆—乃东断裂、抓各落断裂、毕定—甲舍拉断裂、甲岗—谢通门断裂的走向、产状、上断点埋深及其在地表的垂直投影位置等主要参数.为日喀则地区的深浅构造关系等研究提供基础资料,填补该地区地球物理勘探资料的空白.

Methods and results of geological prospecting along active faults in urban Xigazê

The Yarlung Zangbo River suture zone is characterized by the presence of active faults in southern Tibet. This site is prone to moderate-large earthquakes due to the particular tectonic setting. The Xigazê region is located in this suture zone; the unique geological character of this region increases the likelihood of earthquake occurrence. The three key active faults zones within this region include the Yarlung Zangbo River, Xaitongmoin-Namling, and Jaggang-Xaitongmoin fault zones. Scientists have conducted large number of research projects in relation with the geophysical and regional geological structures in the Tibetan plateau. The amount of geological data remains insufficient and geophysical prospect data is relatively less in the Xigazê area than at other study sites. Furthermore, shallow seismic prospecting work has not been implemented yet in this specific region.#br#This paper describes the application of a method that utilizes an artificial source of shallow seismic reflections. This method aims to detect active faults and survey low-velocity layers, in addition to shallow seismic refraction. The key technical problems are addressed, including details about the selection of seismic instruments, deployment design, parameters, data processing, and interpretation methods used to identify faults. The Geometrics Strata Visor NZXP seismograph was used to collect data and a 70 kg hammer was used as a seismic source, in addition to 60 and 100 Hz geophones. Seismic wave energy was maintained at constant high and low frequencies. The wave reflection range was between 10 and 120 Hz, and the high frequency energy attenuation gradually diminished. The seismic reflection observation system is comprised of a receiver arrangement in the front, the shotpoint in the rear, shooting in the downdip direction, reception in the updip direction, and continuous forward tracing. The spread geometry is determined by the receiver interval of 1 to 3 m, with a 2 to 9 m offset, 72 channels, and 6 to 9 folds. The acquisition parameters were set as 0.25 ms sampling intervals and 600 to 800 ms record lengths.#br#This region is located in the Yarlung Zangbo River drainage catchment, which is influenced by a low velocity layer. The study area was chosen to determine the degree of influence of the low velocity layer and to isolate its effects during data processing to improve the quality of the profile data. Data processing was completed using the Vista seismic reflection wave software system. For high-frequency random noise, one-dimensional filtering of 10 to 90 Hz frequency band widths was adopted, which applied a two-dimensional filtering to address the apparent velocity difference of interference wave (e.g. F-K). The main parameters such as the trend, orientation, and upper offset point were evaluated to ascertain the effects on the Ladoi-Nêdong, Biding-Jiashala, Zhuogeluo and Jaggang-Xaitongmoin fractures. By analyzing the seismic reflection profile, additional groups of apparent interface reflection characteristics were identified. Based on the characteristics of seismic reflection groups in each fault line, we determined eight breakpoints of fault interpretation.#br#The results show that the top of the bedrock reflective interface (i.e., the quaternary bottom), T_N, is common in the surveyed area. Furthermore, T_N has strong reflective energy in most of the profiles, significantly apparent seismic phase characteristics, and reflective interface that records changes in fluctuation. The two-way time of T_N is 0 to 220 ms, and the buried depth is 0 to 170 m. There are two main internal reflection interfaces in the Quaternary bottom layer. Unique identifiers were assigned, namely T_Q1 and T_Q2, from the top to bottom, respectively.