Magnetotelúrico e Eletromagnético Transiente - CPRM

Abstract
This research work aims to contribute to the hydrogeologic knowledge of the eastern
margin of the Parnaíba basin through a geophysical investigation by using magnetotelluric (MT)
and transient electromagnetic (TEM) methods. Both methods give estimates of subsurface
electric resistivities. From the resistivity mapping of the subsurface it is possible to differentiate
sandstone layers (moderately resistive) from shale layers (more conductive relative to the first
ones) and the crystalline basement (more resistive), making possible a visualization of the basin's
structure.
The Parnaíba basin is located to the west side of the northeastern region of Brazil. It
occupies an area of about 600000 km 2 with an approximate 3400 m maximum sedimentary
thickness near its central position. It is an intracratonic basin where one finds Ordovician to
Tertiary sedimentary deposits and intrusive and extrusive rocks associated to magmatic events
from Jurassic to eo-cretaceous ages. On a hydrogeologic context, the Cabeças geologic
formation and the Serra Grande group (both predominantly sandstones) contain the most
important aquifers in the region.
The Serra Grande group and the Pimenteiras formation predominate in the study area.
They stand over a semi-arid region, particular to most of northeastern Brazil countryside where
water shortage is a long and well-known problem to the region. This fact also calls for the
importance towards a better understanding of the region's hydrogeology. Geophysical studies
presented in this dissertation are based on 28 MT and TEM stations sited upon three cross-section
profiles relative to the area's main structures. These three profiles are (see fig. 2.3):
· (1) the Monsenhor Hipólito profile, 58 km long with 9 MT/TEM stations, 4 to 12 km away one
from the other.
· (2) the Jaicós profile, 22 km long with 9 MT/TEM stations, each 4 to 6 km away from the other.
· (3) the Itainópolis profile, 41 km long with 10 MT/TEM stations, 4 to 6 km away one from the
other.
In all three profiles the MT frequency was sampled between 0.07-0.008 Hz and 334 Hz
whereas TEM measurements were taken between 33 Hz and 100000 Hz. In order to estimate the
impedance tensor and its related apparent resistivities and phases, a robust computer processing
technique given by Egbert & Eisel (2000) was used. To determine the geoelectric strike, Groom
and Bailey's (1989) technique was used. For static shift correction of MT data, we used the
TEM-method's apparent resistivity curves based on Sternberg's (1988) proposal. MT data
v