西藏那曲大地电磁数据反演与电性结构分析

西藏地处青藏高原,地质构造复杂,新构造活动强烈。西藏境内蕴藏丰富的地热资源,已发现的地热资源储量居全国之首。青藏高原受南北向强烈挤压,构造活动频繁,随着地质应力的变化,产生了一系列构造带,西藏那曲观测区处于西藏北部的班公错—怒江深大断裂的次级构造带上,主要受控于一组近东西向的断裂和南北向断裂所构成的断裂带,据资料显示,观测区内新构造运动极为活跃,主要表现为深大断裂至今仍有继承性活动,为研究观测区深部电性结构,分析地热在观测区的空间分布特征及形成机理,共部署了3条大地电磁剖面。通过数据维性分析,揭示了观测区浅部主要呈现1D/2D构造,深部2D/3D构造明显,因此进行大地电磁数据2D和3D反演,同时获得观测区3km以浅2D和3D电性模型,十分必要,本文采用连续介质反演方法进行大地电磁法2D反演,采用REBOCC 3D反演代码进行3D反演,综合观测区水文地质调查结果,分析观测区2D和3D电性结构特征。研究结果表明,观测区2D地电结构横向电性梯度带为断层反映,纵向分层明显,高阻层间存在低阻层表明观测区深部地层构造复杂,观测区3D地电结构局部高阻体代表侵入岩活动范围,低阻区域反映了观测区地热的空间分布特征,观测区地热形成于次一级断裂,地热活动受断裂构造控制,由地表河流下渗形成。

Magnetotelluric Data Inversion and Conductivity Structure Analysis in Naqu, Tibet

Tibet is located in the Qinghai Tibet Plateau and characterized by complex geological structure and new intense tectonic activities. Tibet hosts abundant geothermal resources, ranking the first in national reserves of geothermal resources identified. The Qinghai Tibet Plateau undergoes frequent tectonic activities due to strong north south extrusion. Change of geological stresses resulted in formation of a series of structural belts. The observation station in Naqu of Tibet is placed on the secondary tectonic belt of Bangongcuo Nujiang deep faulting in the northern part of Tibet, which are controlled by both near NW trending faulting and NS trending faulting. Existing data show that new tectonic movement in the observation area was active, manifesting that the deep huge faulting is still active. To study the deep electric structure of the observation area, three magnetotelluric profiles were investigated to analyze spatial distribution features and formation mechanism of geothermal resources. The shallow strata of observation area are mainly presented by 1D/2D structure and deep strata by 2D/3D structure. 2D and 3D magnetotelluric data inversion were also performed, yielding 2D and 3D electric models of for 3 km deep shallow strata. This study carried out 2D magnetotelluric inversion using continuous media inversion method and 3D magnetotelluric inversion using REBOCC 3D inversion code, and comprehensively interpreted 2D and 3D electric structures based on hydrogeological investigation results. Research results show that horizontal electrical gradient belt of 2D geoelectric structure in the observation area reflects a fault, with distinct feature of vertical layering; low resistance layers in the high resistance layers reflect a deep complex structure in the observation area; local high resistance bodies of 3D geoelectric structure represent active range of intrusive rocks and low resistance area reflects spatial distribution of geothermal resources in the observation area. Geothermal resources results from infiltration of surface rivers along secondary fractures, and therefore, geothermal activities are controlled by faulting structure.