Geoelectric Directionality of a magnetotelluric (MT) survey in Parecis Basin, Brazil

Jean Marie Flexor, Hans Schmidt Santos


Eighty-eight broadband (0.001-1000 sec) magnetotelluric (MT) soundings have been made along a 500 km profile crossing the Parecis Basin (Mato
Grosso State and Rondˆonia State, Brazil) in a direction almost perpendicular to the main geological lineaments. The two-dimensional inversion (2-D) of MT data
(constrained by well-log resistivity data) allowed to estimate the distribution of the conductivity. For the inversion procedure, the impedance tensors were rotated by an
angle of 130â—¦ (- 10â—¦) ( E-W) measured clockwise from the geographic north according to the dominant geological trend estimated of the Parecis Basin. Accurate
interpretation of MT data requires understanding the dimensionality and directionality conditions that must be satisfied by the properties of the impedance tensor. However,
due to the usual presence of shallow, small-scale galvanic bodies which distort the responses, the regional scale structures can be masked. In order to verify if an average
geoelectric strike can be compatible with the basin"™s geological strike, a procedure of decomposition of MT impedance tensor for a set of 36 MT soundings along the
profile was applied. The results show the angular distribution of the geoelectric strike. The observed departures from an average angle value are explained both in terms
of geotectonic structures of the Parecis Basin and of their influences in the geoelectric anisotropy. From a three-dimensional model of the basement topography based
on downward continuation of gravity field constrained by spectral estimates of depth to magnetic sources, it is shown that the most regular strike angle distributions
correspond to the deepest sedimentary structures: Caiabis, Pimenta Bueno and Colorado grabens. In the interface regions, we expect anisotropy distorting strongly the
regional impedance tensor.

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