OVERVIEW OF GEOMAGNETIC DEEP SOUNDINGS (GDS) AS APPLIED IN THE PARNAÃBA BASIN, NORTH-NORTHEAST BRAZIL

Abstract

Geomagnetic Deep Sounding (GDS) is an electromagnetic method of geophysics, which is capable of imaging the Earth"™s interior in terms of electrical conductivity using natural geomagnetic transient variations. The method is particularly suited to map geological structures marked by large lateral conductivity contrasts. An overview of the methodology is presented for a magnetometer array study undertaken within and around the intracratonic Parnaíba Basin, north-northeast Brazil. The article describes the sequential steps of data processing, the results of numerical modeling, and the related geological/tectonic implications of the inferred conductivity distribution. In the initial stages of data processing, an advanced robust regression technique is applied to derive transfer functions used to diagnose the lateral conductivity distribution in the study region. The presentation of the transfer functions in the form of induction arrows helps to identify regions of enhanced conductivity. Contour plots and pseudo-sections of the anomalous vertical fields, estimated from the hypothetical event analysis on transfer functions, are essential to characterize the orientation and dimensionality of the electrical conductive structures of the region. The analysis of various frequency and polarization parameters indicates that an anomalous behavior of the magnetovariational field, with periods longer than one hour, is determined by currents induced in the seawater, and perhaps in the raised asthenosphere beneath the oceanic region. Yet, anomalous signatures observed at periods shorter than one hour are primarily controlled by concentrated currents in two inland conductive structures. They are the Parnaíba Basin Conductivity Anomaly (PBCA), that follows the trend of the Transbrasiliano Lineament in the eastern part of the Parnaíba Basin, and the LINK anomaly, that extends from the central part of the basin to the Marajó Graben. A non-uniform thin sheet modeling has mapped the lateral extent and estimated the depth-integrated conductances of PBCA and LINK to be of the order of 2000 S and 1000 S, respectively. The overall pattern of the inferred conductivity distribution helps to visualize the LINK anomaly as relics of a probable sedimentary channel connecting the Parnaíba Basin and Marajó Graben, and which could have acted as a gateway for sea transgressions during early stages of the basin evolution. The combined inversion and forward modeling of the GDS response functions provides the PBCA structural cross-section as an ensemble of a graben-like structure in the basement and a highly conducting block confined to the deeper central part of the basin, with an embedded resistive body in the middle. The existence of a broad conducting block confined to the central part of the basin is also consistent with magnetotelluric data, and the graben-like structure in the basement is corroborated by aeromagnetic data. The origin of graben-like structures in the basement could be possibly related to an extensional tectonism, whereas the resistive body is tentatively interpreted as a diabase dike or a recrystallized magmatic body intruded during a Cretaceous magmatic event. Carbon bearing sediments are suggested as an alternative to hydrated siliciclastic sediments to account for the high conductivity of the central block. A hydrothermal event associated with the Cretaceous magmatic activity may be the likely process to produce carbon through the pyrolysis of hydrocarbon-saturated Paleozoic sediments.

Keywords :Electromagnetic induction; Geomagnetic Deep Sounding; Magnetovariational fields; Electrical conductivity; Intracratonic basins; Magmatic activity.