长周期大地电磁测深研究——青藏高原东部LMT响应函数及应用

STUDY ON LONG-PERIOD MAGNETOTELLURIC SOUNDING——THE LMT TRANSFER FUNCTION IN EASTERN TIBETAN PLATEAU

长周期大地电磁(LMT)是基于常规MT理论发展起来的电磁测深技术,青藏高原东部岩石圈较厚、视电阻率较低,应用LMT方法能够弥补常规MT仪器对低频信号响应的不足,获得有用的深部信息。文中概述了LMT的数据采集、处理及反演解释,并介绍了将LMT应用于东喜马拉雅构造结及其周围地区完成的长周期观测剖面——下察隅—玉树LMT剖面的岩石圈结构探测实例。实际观测表明,在重叠频段内LMT结果与常规MT具有良好的一致性;LMT数据处理实现的阻抗和倾子估计,在深部信息上具有更高的分辨率。LMT剖面结果揭示了青藏高原东部普遍存在地下低阻体,为局部地区部分熔融和地下流体存在的可能性提供了依据。LMT测深较好地弥补了常规MT方法在巨厚岩石圈和遇到低阻体时探测深度的局限性。

As a complement to conventional MT, Long-period Magnetotellurics （＂ LMT＂） has been developed at low frequency for soundings of deep electric structures. Due to the thick and low resistive crust and upper mantle in east Tibetan Plateau,the conventional MT could not provide sufficient infor- mation about deep structures of the study area that requested long period MT measurement to be complemented. This paper presents brief introduction of LMT in respect of the concept,data acquisition, processing,modeling,inversion and interpretation. Most important is that practical LMT sounding applied in eastern Tibetan Plateau gives the LMT transfer functions of impedance and tipper, which coincide with conventional MT. The result of the LMT profile illustrates the wide distribution of high-conductivity layers and provides more information about the base of the conductors beneath the eastern Tibetan Plateau,with higher resolution. Anomalous induction coefficients and 2-D model suggest extensive conductive bodies beneath Lhasa block and Qiangtang terrain which would be a possible evidence for partial melt and fluids at depth.