Data Acquisition and Analyses of Magnetotelluric Sounding in Lujiang-Zongyang Ore Concentrated Area

It is really challenging to perform MT data acquisition and processing in the Lujiang-Zongyang ore concentrated area, where severe and complicated noise is mixed with the useful data. Dense population, well-developed water systems, transport networks, communication and power grids, and some in-exploiting mines are main sources of the noise. However, to conduct MT sounding in this area is not only helpful to the study of geological structure and tectonics of this zone, but also brings valuable experience of data analysis and processing in real field work with heavy interference. This work has been accomplished by us in 5 survey lines with 500 sounding stations in total. In order to verify the consistency of the 6 V5-2000 data acquisition systems employed in our study, a consistency experiment was conducted in a test area with weak interference. Curves of apparent resistivity and phase obtained from these 6 instruments are plotted in Fig.1, in which acceptable consistency is showed, except for a few high noise frequencies. To determine the optimal interval for data acquisition in this noise-heavy survey area, a comparison experiment was implemented in a single sounding station for comparing the data quality by different intervals. We found 20 hours or more were required for each acquisition. The evaluation was based on degree of coherence and signal-to-noise ratio. With analysis of the MT data in both time and frequency domain, noise was categorized into several patterns according to the characteristic of various noise sources, and then corresponding filters were adopted. After removing flying-spot, cubic spline smoothing and spacial filtering to all the sounding curves, apparent resistivity profiles were obtained. Further studies including 2D and 3D inverse analysis are on processing. Fig 1. Consistency experiment of the apparatus (a) and (b) are apparent resistivity curves of yx and xy direction;(c) and (d) are phase curves of yx and xy direction;J1,J2,J3,J4,J5,J6 are marks of the 6 apparatus respectively.