MAGNETOTELLURIC STUDIES FROM 2 CONTRASTING BRAZILIAN BASINS - A REASSESSMENT OF OLD DATA

Before the mid-1980s large numbers of magnetotelluric (MT) data were c ollected and interpreted without addressing the possible effects of ne ar-surface (e.g. static) distortion contributions. In the intervening time, modelling and interpretation techniques have improved. There are many instances where extensive, processed tenser survey data of reaso nable quality exist on various back-up media. Before any complete rean alysis of these often large data sets, it is useful to investigate the degree of the distortion effects and the approximate degree of resolu tion of the data. Such assessment should help to clarify whether a ful l tensorial reanalysis is worth while. The present study provides cont rasting examples of the reappraisal of old data sets collected in two Brazilian basins, both with associated seismic reflection profiles and deep exploration well-logs. Both examples use 'simplified' data consi sting of only the off-diagonal tenser elements. These data are used in a limited way to assess static distortion and their intrinsic resolut ion capabilities. In the first example, from the basalt-covered Parana basin, a very high degree of static distortion is displayed. The appl ication of simple distortion correction techniques allow the data to d efine the main (vertical) geoelectric units of the basin. In the secon d example, from the Solimoes basin in northern Brazil, static distorti on, although less severe, can be readily identified. In this complex p etroleum province the resolution of structure, both vertical and later al, is of very low order. The MT data appear 'blind' to the detail of the Palaeozoic sedimentary rocks, and more detailed reanalysis is prob ably not warranted.