Coast Effects On Magnetotelluric Data From The Brazilian Southeastern Region

The coast effect on magnetotelluric (MT) data acquired<br>near the Brazilian SE coast in the state of São<br>Paulo is investigated using two-dimensional (2D)<br>modeling. The Atlantic ocean was modeled as a layer<br>of constant resistivity (0.3 ohm.m) and thicknesses up<br>to 2.0 km at 160 km from the coast, according to<br>bathymetry. Offshore Santos Basin was also included<br>in the model using parameters derived from boreholes<br>and seismic lines. The continental resistivity model<br>was constructed based on the electrical structure inferred<br>from previous MT studies. The modeling exercise<br>shows that the ocean-coast effects are very important<br>for periods greater than some seconds at distances<br>of tens of kilometers from the coast. Synthetic<br>data from the model are compared with a MT sounding<br>carried out in the Serra da Mantiqueira plateau.<br>Splitting between TE and TM modes observed at a<br>period of 30 s in the MT data can be explained as<br>being generated by electric currents induced in the<br>highly conductive Atlantic ocean (coast effect), about 90 km away.