Multiscale inversion of onshore-offshore electromagnetic data from southeast Brazil and Santos Basin
Resumen Abstract Índice Conclusiones
Santos Benevides, Artur
2025-A
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Resumen
In this work, magnetotelluric and Controlled Source Electromagnetic methods have been used to assess the deep and shallow geoelectrical nature of the Earth underneath Southeastern Brazilian onshore margin and the adjoining offshore Santos
basin (SB). We investigated the Southern Brasilia and Ribeira Orogenic Belts (BROB) and SB underlying lithosphere by three-dimensional magnetotelluric imaging using 174 amphibious MT dataset. 3D conductivity models reveal the presence
of a highly heterogeneous crust in BROB, whereas it is uniformly resistive below the SB continental shelf region. A resistive segmented layer (of about 60-80 km), and another moderately resistive lower layer (of ≈80-100 km) are key features of the lithosphere. Excepting the above, a steeply dipping sub-lithospheric conductor is associated with a confined asthenosphere upwelling and might be related with the surface deformation hills of BROB below the coast-parallel São Paulo-Rio de Janeiro dyke swarms. Deep lithospheric roots of ≈200 km were found beneath part of BROB, which is thinning out to be ≈75 km below the SB continental shelf and probably becomes thinner towards the deeper ocean coincident with the lithosphereasthenosphere boundary found in seismic tomography, speculating that the opening of the South-Atlantic Ocean probably uplifted the lithosphere underneath it. We also investigated the relationship between deep crustal structure and the deformation in the overlying sedimentary wedge in Santos basin, Brazil and evolution of the salt-related ‘Albian Gap’. We found beneath this wedge, the resistive continental crust is ≈35 km thick across the Cretaceous hinge line and thereafter thins seaward to ≈21 km over a lateral distance of ≈80 km defining a domain of highly extended and faulted crust. Our models show a mantle-associated basement high and evidence of significant dislocation of the overlying sedimentary wedge which spatially coincides with the Albian Gap and a previously proposed Moho high. This implies a coupled deformation of the basement and the sedimentary wedge. We propose that magmatism or lower crustal flow may have played a significant role in the inferred displacement at the Albian Gap. Finally, it is presented a CSEM processing workflow from a data set recently public available in Santos basin intesecting the marine MT data. A methodology involving joint inversion of MT and CSEM data incorporating a practical weighting scheme and constraints is employed resulting in better data fit and enhanced electromagnetic images that are validated through geological considerations.
1 Introduction
2 Electromagnetic induction in the Earth
2.1 Electromagnetic signal sources
2.1.1 Natural sources – MT
2.1.2 Active sources – CSEM
2.2 Maxwell’s equations
2.3 Diffusion equation
2.3.1 Skin depth effect
2.4 Impedance tensor
3 EM inverse and forward Problem
3.1 Inverse problem formulation
3.1.1 Inversion scheme – ModEM
3.1.2 Difficulties in inverse problems solution
3.2 EM forward modeling
3.3 ModEM-ON
3.3.1 Solver
3.3.2 Modified System of Equation
4 Geological setting of the SE Brazil and adjoining margin 21
4.1 Southern Brasılia belt
4.2 Ribeira belt
4.3 Santos basin
4.3.1 Stratigraphical setting
4.3.2 Petroleum system configuration
5 Deep lithosphere imaging from Amphibious MT data
5.1 Introduction
5.2 Amphibious Magnetotelluric Dataset of Southeastern Brazil
5.3 3D Modeling and Inversion
5.4 Results and Model robustness
5.4.1 Data fits
5.4.2 Depth penetration
5.4.3 Model resolution and sensitivity tests
5.5 Interpretation and Discussions
5.5.1 Crustal-scale conductivy anomalies
5.5.2 Lithospheric-scale conductivity anomalies
5.5.3 eLAB
5.5.4 MT and other geophysical anomalies correlation
5.5.5 Geophysical interpreted model
5.6 Conclusions
6 Deep structure of Santos basin from 3D MMT inversion
6.1 Introduction
6.2 Marine Magnetotelluric data
6.3 Dimensionality analysis
6.4 Regularized 3D resistivity inversion
6.4.1 Effect of model regularization weight
6.4.2 Effect of initial half-space resistivity models
6.4.3 Ground Truthing the sedimentary structure with well logs
6.4.4 Validating the deep crustal-mantle section: Depth sensitivity tests
6.5 Resistivity Structure of the Sedimentary Wedge
6.6 Deep structure
6.6.1 Crust
6.7 Conclusion
7 CSEM data processing from Santos basin
7.1 Overview
7.2 CSEM data acquisition description
7.2.1 Transmitters
7.2.2 Receiver
7.2.3 Bathymetry and seawater physical parameters
7.2.4 Transmitter outlook and source signature
7.3 Processing methodology
7.3.1 Loading the raw time series
7.3.2 Time series framing
7.3.3 Fourier Transform
7.3.4 Navigation parameters
7.3.5 Statistical analysis
7.3.6 Calibration (count conversion, gain, dipole, and antenna normalization)
7.3.7 Count conversion
7.3.8 Gain
7.3.9 Source Normalization
7.3.10 Orientation
7.3.11 Polarization ellipse parameters
7.4 Conclusion
8 MT and CSEM inversion in Santos basin
8.1 Model parametrization
8.2 Single Inversion
8.2.1 CSEM
8.2.2 MT
8.3 Joint MT and CSEM inversion
8.4 Joint inversion using constrained model
8.4.1 Constraints from MT model
8.4.2 Geological validation of the joint inversion
Bibliography
Study 1
Crust is highly heterogeneous beneath the Brasilia belt and a sub-lithospheric major
suture zone has been observed beneath Embu-Paraiba do Sul terrain of the Ribeira
belt. Two upper crust sutures (CTB and BSZ) have been marked from this study.
CTB lies in between occidental and oriental terrains, whereas BSZ is in between
Bras´ılia and Ribeira belt. MT inferred crustal conductivity anomalies distinguish
a wide antiformal below the Brasilia belt and a megasynform structure below the
Embu-Para´ıba do Sul Terrain of Ribeira belt, providing the evidence for both compressional
and extensional tectonic regimes beneath Southeast Brazil. Lithosphere
beneath Southeast Brazil is observed 130 km deep below the Southern Brasilia
belt thinning out up to ≈100 km underneath the Ribeira belt region, progressing
towards the ocean reaching up to 70 km below deep-water Santos basin. A steeply
dipping lithospheric conductor observed is associated with a confined asthenospheric
upwelling and the Cretaceous São Paulo-Rio de Janeiro dykes swarm. I suggest that
the observed lithospheric thinning may have triggered the delamination of the lower
crust and played a role in the surface deformation that shaped the Serra da Mantiqueira
and Serra do Mar.
Study 2
MMT data have been inverted in 3D to determine the deep crustal and upper mantle
structure beneath the sedimentary wedge across the central part of the Santos
basin. The resistivity structure of the crust and upper mantle is consistently imaged
by all the MMT models. Estimation from 3D marine MT data inversion is feasible
but requires a large amount of data and a judicious selection of optimal imaging parameters
as shown in this study. It was demonstrated clearly that the right choice
of initial 3D model will lead to models that are geologically valid. I compared the
resulting models with the available resistivity logs for 11 wells, collocated 2D seismic
data and gravity anomalies and found some remarkable consistency among all. In
the sedimentary section, the models consistently show two electrically conductive
(Cenozoic and Cretaceous) layers capped and underlain by resistive strata that are
heavily affected by faulting. Beneath the sedimentary cover, the highly resistive continental
crust is ≈35 km thick across the CHL until the 200 m isobath and thereafter
thins rapidly seaward to ≈21 km over a lateral distance of ≈80 km defining the zone
of stretched and heavily faulted continental crust. The changes in crustal thickness
correlate with changes in the sedimentary section implying basement control on
basin evolution or coupled partitioning of crustal deformation. It was found a ≈50
km wide zone of thinned crust inboard of the CFF. It spatially coincides with the
Albian Gap in the overlying sedimentary wedge. I suggest that the lower crust or
upper mantle is ductile beneath this part of the Santos Basin and that magmatism
may have played a significant role in the inferred vertical movement at the Albian
Gap